35
2
SUMMARY
Settlement is a process through which demersal littoral fishes go from a planktonic stage to a
benthonic stage. Understanding this process is fundamental in order to evaluate recruitment
processes and population dynamics. The analysis of settlement (pre-settlement phases including the
pelagic stages that are about to migrate to the bottom, and post-settlement phases that have
already been recruited to the bottom but don’t usually appear in conventional fishing gears) is rare in
the Mediterranean Sea and data exist only for few species within the frame of very specific
objectives. Locating settlement pulses is key to understand connectivity patterns and to spatially
delimit/evaluate MPAs. Sampling of these phases requires specific methods whose design is more
developed for tropical environments than temperate latitudes. In addition, pre-settled individuals
often escape conventional plankton nets, making it necessary to develop innovative methods of
sampling.
The objective of this study was to compare six different methods of sampling in terms of species
composition, capture efficiency, size-spectra and abundance in order to determine the best
combination of techniques to assess settlement processes in temperate littoral species. We used
three types of pelagic nets (bongo 40, beam trawl and WP2 ring of 80 cm) and two types of light
traps (Quatrefoil and Ecocean C.A.R.E) to sample pre-settled individuals whilst a type of beam trawl
of low impact for the bottom was evaluated to catch post-settled individuals.
We obtained at least three replicates per sampling gear for each of two nights in May and June 2012,
during the peak of the spawning season for summer species. Sampling was conducted in the Bay of
Palma, characterized by large littoral fish populations and known to be an area of both high self-
recruitment (Basterretxea et al. 2012) and high recreational fishing pressure (Morales-Nin et
al.2010). Individuals were identified and measured to the nearest 0.1 mm (SL).
Our results show significantly different size-spectrum for each method, with a continuum of sizes
from 2mm to 200mm SL with the smallest sizes taken by the bongo, followed by the ring net, the
neuston net, the quatrefoil, the Ecocean and finally the beam trawl. Consistent differences were
found in the two sampling periods. In general, the pelagic nets collected high abundance of pre-
settlement stages, the light-traps caught post-larval stages about to settle, and the beam trawl
sampled mainly larger juveniles but also recent settlers of key littoral species. These results prove
that an optimal strategy for sampling key littoral species around settlement is possible by selecting
the Ecocean C.A.R.E. for pre-settlers and beam trawl for post-settlers, which should guarantee
further insight into recruitment processes in littoral fishes of temperate water.
3
KEYWORDS: Fish Larvae ·Juveniles ·Settlement· Mediterranean· Light-trap · Plankton-net ·Beam-
trawl · Sampling
RESUMEN
El asentamiento es un proceso por el cual muchos peces litorales pasan de un ciclo de vida
planctónico a uno bentónico. Comprender este proceso es fundamental para evaluar los procesos de
reclutamiento y la dinámica de las poblaciones. El análisis del asentamiento (fases de pre-
asentamiento incluidos los estadios de vida pelágica que están a punto de migrar al fondo, y fases
post-asentamiento que ya han reclutado al fondo pero que no suelen aparecer en las pescas
convencionales) es muy infrecuente en el Mediterráneo y se ha realizado para pocas especies y con
objetivos muy concretos. El muestreo de estas fases requiere métodos muy específicos cuyo diseño
está más desarrollado para los ambientes tropicales que para latitudes templadas. Además, los
individuos en estado de pre-asentamiento suelen escapar a las redes de plancton convencional, por
lo que es necesario desarrollar métodos innovadores de muestreo.
El objetivo de este estudio fue comparar 6 diferentes métodos de muestreo en términos de
composición de especies, eficiencia de captura, espectros de tallas y abundancia a fin de determinar
la mejor combinación para evaluar los procesos de asentamiento de especies litorales en aguas
templadas. Se utilizaron tres tipos de redes planctónicas (bongo 40, patín neustónico y aro WP2 de
80 cm) y dos tipos de trampas de luz (Quatrefoil y Ecocean C.A.R.E) para muestrear los individuos
pre-asentados mientras que se evaluó un tipo de red bentónica de arrastre poco lesiva con el fondo
para los individuos post-asentados.
Se obtuvieron al menos tres réplicas por método de muestreo durante dos noches en Mayo y Junio,
en plena época de puesta para las especies estivales. Los muestreos se realizaron en la Bahía de
Palma, caracterizada por amplias poblaciones de peces litorales y conocida por ser un área de alto
auto-reclutamiento (Basterretxea et al. 2012) y de alta presión de pesca recreativa (Morales-Nin et
al.2010). Los individuos se identificaron y se midieron con una precisión de 0.1 mm (SL).
Nuestros resultados muestran espectros de tallas significativamente diferentes para cada método,
con un continuo de tallas de 2 mm a 200 mm SL con los individuos más pequeños capturados por el
bongo, seguido por el aro, el patín neustonico, las trampa de luz Quatrefoil, la trampa Ecocean y
finalmente, la red bentónica de arrastre. Diferencias consistentes fueron obtenidas entre ambos
periodos de muestreo. En general, las redes pelágicas recolectaron pre-asentados en alta
abundancia, mientras que las trampas de luz recolectaron post-larvas cerca del asentamiento. La red
de arrastre muestreó principalmente juveniles grandes, pero también individuos recientemente
4
asentados, incluidos especies litorales clave. Estos resultados demuestran que una estrategia óptima
para el muestreo de estas fases de desarrollo se puede conseguir usando la trampa Ecocean C.A.R.E.
para fases de pre-asentamiento y gánguil para recién asentados, lo que permitirá en un futuro tener
una mayor comprensión de los procesos de asentamiento en los peces litorales de aguas templadas.
PALABRAS CLAVES: Larvas de peces ·Juveniles ·Asentamiento· Mediterráneo · Trampas de luz ·
Redes planctónicas · Red bentónica · Muestreo
5
INDEX
1. Introduction
1.1. Background
1.2. Objectives
2. Materials and methods
2.1. Sampling area
2.2. Sampling procedures
2.2.1. Pre-settlement
2.2.2. Post-settlement
2.3. Fish Identification and measurement
2.4. Data analysis
2.4.1. Comparison of abundance and occurrence
2.4.2. Comparison of catches and catch per unit of effort
2.4.3. Comparison of sizes
3. Results
3.1. Taxonomic composition of samples
3.2. Catches performances
3.3. Size composition of samples
3.4. Multivariate analyses
4. Discussion
5. Conclusion
6. Acknowledgements
7. References
6
INTRODUCTION
Background
Most of the knowledge on population dynamics on fish in temperate areas comes from fishery
science, which has traditionally focused on commercial species. Already since its beginning (Hjort
1914), environmental processes affecting fish dynamics focused on the processes linked to their early
stages of life. Despite fisheries knowledge (and hence management) has mostly focused on
commercial fishing, the impact of recreational fishing on fish stocks can no longer be ignored (Cooke
and Cowx 2006; Lewin et al. 2006). In the Mediterranean, recreational fishing is a widespread activity
in constant expansion which greatly affects littoral fish populations (Cooke and Cowx 2004; Morales-
Nin 2005, 2010). The dynamics of these littoral fish populations depend both on fishing and, crucially,
on the addition of new individuals to the fished population.
Most littoral fish species have a complex bipartite life cycle (Vigliola 1998), divided into a larval
pelagic stage and a more sedentary demersal adult phase (Leis et al. 1991). Generally, the pelagic
phase is thought to be characterized by high levels of dispersal, although in the last decade many
studies have modified this concept. Fish populations seem not to be as open as once thought due to
evidenced mechanisms for high levels of self-recruitment, even at the scales of few kilometres (Jones
et al. 2005). In many demersal species, the shift from pelagic to the benthic stage occurs within hours
(usually at night, Robertson et al. 1988; Holbrook and Schmitt 1997, 2002) and is referred to as
settlement (Levin 1994). Recruitment, which can be defined in several terms, is here referred to as
the demersal phase, after settlement, when individuals progressively incorporate to the fishable
stock. Early life history patterns of fish, such as larval supply, settlement and recruitment are known
to significantly influence adult population dynamics (Victor 1983; Doherty and Fowler 1994).
Depending on the habitat and species, the different mortality processes associated to each early life
stage will shape the final number of fish recruiting to the stock. The benthic period between
settlement and the entry into the fishery remains the least understood of all life stages (Milicich et al.
1992; Planes 2002). Further, spatial and temporal patterns of settlement into benthic habitats,
known to be influenced by many environmental and ethological factors (Wilson and Meekan 2001),
are still poorly understood, especially in temperate waters.
Indeed, studies that have assessed settlement processes in the Mediterranean have generally
focused on a limited number of species including labrids, gobiids and sparids (García-Rubíes and
Macpherson 1995; Macpherson and Raventos 2005; Carreras-Carbonell et al. 2007). Moreover, these
studies are generally based on visual censuses conducted by scuba divers. Estimates obtained by
7
visual census can be highly variable (Ordines et al. 2005) and they generally underestimate fish
populations density and diversity (Willis 2001). Moreover, the presence of scuba divers is known to
affect fish behaviour (Murphy and Jenkins 2010), and this method might not be efficient in detecting
smaller individuals (Franco et al.2012) that are about to settle or just settled into benthic habitats.
Other studies have used the analysis of otoliths to study mortality events and settlement processes
(Raventos and Macpherson 2001). Even though this method allows getting some information about
temporal patterns of settlement, it cannot provide information about spatial patterns. Indeed, the
information connecting the area of larval origin (spawning grounds) and area of settlement is always
biased because validation tends to be in individuals that are either too young (larvae which will suffer
a strong mortality and dispersal before settling) or too old (juveniles that may have travelled a large
distance until the area of collection). In conclusion, the methods generally employed to study early-
life processes lead to biased information on fish assemblages as they don’t allow sampling efficiently
individuals about to settle and that just settled in nursery grounds. For this reason, innovative
sampling methods need to be developed to understand settlement processes in littoral fish
populations of temperate waters.
Conventional net-based sampling methods at the pelagic phase include plankton and neuston nets of
different mesh sizes, and are useful to sample and quantify early life stages of fish (Doyle et al. 2002;
Madenjian and Jude 1985; Sabatés et al. 2003). However, it has been shown that these methods
have the tendency to subsample pre-settlement fish larvae (post-flexion larvae) (Leis 1982, Morais et
al. 2009; Chícharo et al. 2009) which might be able to avoid nets (Brander and Thompson 1989). For
this reason, only in few instances (large nets, night/fast tows) they are of any use to analyse fish close
to settlement.
Other sampling methods such as light traps have been used extensively in the last decades to sample
fish larvae and juveniles (Doherty 1987;Floyd et al. 1984) in both marine (McCormick and Hoey 2004;
Rankin and Sponaugle 2011) and freshwater (Marchetti et al. 2004; Vilizzi et al. 2008) environments.
Various structural (Nakamura et al. 2009; Secor and Hansbarger 1992; Vilizzi et al. 2008) and light
source modifications (Kehayias et al. 2008; Marchetti et al. 2004) have been tried out over the years
to improve sampling efficiencies. However, most studies have been conducted in tropical waters,
where fish diversity and abundance are much higher than in the temperate waters, where many
designs have shown to be of low efficiency to capture post-larval stages (Hickford and Schiel 1999).
Quatrefoil light traps (Floyd et al. 1984, Secor and Hansbarger 1992) have proved to be effective in
structurally dense habitats and at various depths (Humphries and Lake 2000; Humphries et al. 2002).
Recently, new designs such as the C.A.R.E (Collect by Artificial Reef Eco-friendly) light-trap developed
8
by Ecocean (Lecaillon 2004) has shown to collect post-larvae that are about to settle in coral reefs
habitats, by using their natural attraction for a substrate in this critical period (Carassou et al. 2009).
Some recent studies done in the Mediterranean have also demonstrated its efficiency in temperate
waters (Lecaillon et al. 2012). Even though light traps are known to underestimate early larval stages
and taxonomic diversity (Chícharo et al. 2009; Hickford and Schiel 1999) by attracting individuals that
have considerable swimming abilities or/and positive phototaxis, they allow to collect
developmental phases close to settlement (pre-settlers). After settlement, methods for fish
collection range from manual collection techniques in rocky environments (Fontes et al 2010;
Raventos & Macpherson 2005; Strydom 2008) to experimental fishing trawls which can efficiently
sample a surface of sea bottom (e.g. Deudero 2008). However, some of these methods are labour
intensive, and some may damage the seafloor if delicate habitats are to be sampled.
About 300 species inhabit the littoral zone in the Mediterranean, 100 of which live above 50 meters
deep, principally Sparidae, Labridae, Bleniidae and Gobiidae (Whitehead et al. 1986). These species
settle few meters below the surface in well-defined habitats, normally in spring and summer (Álvarez
et al. 2012; Macpherson and Zika 1999; Tsikliras et al. 2011). The larval duration of these species is
usually highly variable, ranging from weeks to few months, but generally of few weeks during warm
months (Raventos & Macpherson 2001). Even though behaviour might also affect larval dispersal
(Leis et al 2006), this process depends mainly on physical parameters such as wind patterns, coastal
topography and oceanographic conditions. In a recent study, it was shown that the Bay of Palma
(Mallorca, Balearic Islands) is not only a heavily exploited area by anglers but also a relatively isolated
bay with high levels of self-recruitment (Basterretxea et al. 2012). Further, these authors suggested
through a numerical study that the close-by Marine Protected Areas may have different degree of
connection with Palma Bay via larval export. However, numerical models need validation and pre-
settlement phases are key to analyse the strength of settlement along coastlines with high
topographical interference on currents. Furthermore, the most fished littoral species in the area (e.g.
several sparids and serranids) have small home-ranges (March et al. 2010, 2011; Palmer et al. 2011)
in their adult phase. Therefore, they may depend upon the return and settlement of planktonic
larvae to balance adult mortality losses and self-maintain themselves. As the fishing pressure in the
bay of Palma is particularly high (Morales-Nin et al. 2010), it is obvious that understanding the factors
that affect settlement processes variability in these complex coastal environments is primordial to
implement adequate fisheries management measures.
9
-2 0 2 4
3638
40
Majorca
Spain
Algeria
200 m
Palma Bay
C
C
C
Q
Q
Q
Q
Q
Q NN
N
S
b)
a)
Objectives
As it has often been recommended, a combination of sampling methods need to be used when
studying early life stages processes, in order to maximise sampling success (Chícharo et al. 2009;
Vilizzi et al. 2008). Therefore, the objective of this study is to compare six different sampling methods
(three pelagic nets, two light traps and one bottom trawl) in terms of different criteria (size spectra,
abundance, species composition) in order to design in the near future an optimal design for analysing
pre and post-settlement phases of coastal fish in temperate areas.
MATERIALS AND METHODS
Sampling area
The study was carried out in Palma Bay, Mallorca Island (NW Mediterranean, Fig. 1). Palma Bay is
located in the southern part of the island. Bottom habitats from 0 to 35 m are dominated by seagrass
meadows of Posidonia oceanica and rocky bottoms (March et al. 2011). Moreover, it is considered
to be the area with the highest larval retention within the southern part of the archipelago
(Basterretxea et al. 2012). This area also undergoes the highest pressure by recreational fishers that
target several littoral sedentary species (Morales-Nin et al. 2005).
Figure 1. Sampling area and field design. In (a), the general location of the sampling site is shown, where S=site for the
beam trawl. In (b), detail of the positions in which traps and nets were deployed. N=net sampling (see text). C, Ecocean light
trap; Q, Quatrefoil light trap. The star in b depicts the oceanographic Buoy.
10
Sampling procedures
From February to April 2012, when the number of littoral species in reproductive period is lower
(Álvarez et al. 2012), a preliminary sampling using different gears was conducted to determine the
best sampling sites and to confirm the feasibility of the sampling for each technique.
Pre-settlement
Pre-settling fish were sampled in the water column using five different methods, including 3 types of
plankton-nets and 2 types of light-traps. Sampling was conducted around a fixed oceanographic buoy
(Fig. 1 b), which provides real-time free information about atmospheric (wind, pressure,
temperature) and oceanographic conditions (currents, swell, temperature, turbidity and
conductivity) (http://www.socib.es/?seccion=observingFacilities&facility=mooring&id=37).
Pre-settlement sampling was performed during two nights in May (24-25th) and June (13-14th) 2012,
coinciding with the peak of summer-spawning species (Álvarez et al. 2012). To optimize the catch
efficiency of the light traps, sampling was performed as close as possible to the new moon period,
weather permitting.
The first plankton net was a WP2 ring net with a circular opening of 80 cm and a mesh-size of 1 mm
(Fig 2), theoretically built for the collection of relatively large larval fish. Each sampling night, one
circular tow around each replicated station N (Fig.1b) was performed just below the surface. A
flowmeter (General Oceanics, GO) was mounted in the center of the net opening to determine the
volume filtered.
The second net was a neuston-net, with a frame of 1.5 m high by 1 m wide equipped with 780 µm
meshes, in which three vertical levels were discriminated: 0-0.5 m, 0.5-1 m and 1-1.5 m (Fig 3). Two
flowmeters were mounted on the middle and lower nets openings. No flowmeter was mounted on
the upper net opening as part of it was sometimes out of the water, thus making flow data not too
accurate. The towing operation was as for the ring-net, with three samples collected each night
around stations N (Fig.1b).
11
Figure 2. Ring net.
Figure 3. Neuston net.
12
Thirdly, a 40 cm bongo-net equipped with 335 µm meshes and flowmeters was used. (Fig. 4). Oblique
stratified hauls were performed from 15 m to the surface during approximately 12 minutes (equal-
time stops at bottom, 10 m and subsurface).
Figure 4. Bongo net.
Numbers of fish collected in each net were standardized to number of individuals per cubic-meters of
tow.
Two light traps were used. The first type was a Quatrefoil light-trap (Fig.5, Floyd et al. 1984),
illuminated with a green chemical light stick, as recommended by literature (Kawamura et al. 1996).
Six traps were anchored with a 2-3kg weight attached to a 35m rope, and positioned at the surface
around the oceanographic buoy and at distances over 200 m apart in order to avoid light
contamination among traps (Fig.1b).
The second type of light-trap was the Ecocean C.A.R.E (Collect by Artificial Reef Ecofriendly)
developed by the Ecocean society. Each trap consisted of a buoyant water-tight block containing a 12
V battery and a 55W 90 LED light, under which a 2m conical net of 2mm mesh size with a narrow
mesh funnel in the middle is attached vertically (Fig. 6).
13
Figure 5. Quatrefoil light-trap with green light stick.
In brief, this trap is based on the pre-settlers tendency to search for a substratum (the illuminated
net) around settlement, which impels them to explore the illuminated mesh. These traps were
anchored around the oceanographic buoy and were no closer than 200 m with any of the other traps
(Fig. 1b).
Figure 6. Ecocean light-trap.
14
The light-traps were tied with a 5m rope to a 10L buoy and anchored to the bottom. They were left
overnight for a minimum of 7 hours and harvested the next morning before sunrise. Captures were
referred to effective sampling hours, whereby an effective sampling hour was assumed to be
comprised between 30 minutes after sunset and 30 minutes before sunrise.
Light trap efficiency was estimated as fish/hour and total fish number.
The operational procedure of pre-settling sampling (for one given night) consisted on i) light-trap
deployment, ii) net towing (3 methods x 3 sampling points) during the night and iii) trap-recovery at
dawn.
All larvae samples were preserved in a 4% Formaline-seawater mixture buffered with sodium
tetraborate.
Post-settlement
Post-settlers were collected during two consecutive nights right after each pre-settlement sampling.
Settled fish were collected at night in shallow water (3-6 m depth, Fig. 1) over Posidonia seabeds
using two beam trawls with a mouth diameter of 80 and 90 cm respectively. Both nets were
composed of 2 parts separated by a no-return conical mesh. The first part had a mesh size of 1 cm
whilst the second one had a mesh size of 0.5 cm. The total length of both nets was 3 meters.
Figure 7. Beam trawl.
Sampling was carried out on the nights of the 31-1st of May and 26-27th of June 2012. The boat
described an ellipse of 400-700 m at a speed of approximately 1.1 knots during at least 20 minutes
with both trawls deployed at a time, one on each side of the boat. For each sampling tow, the
15
catches of both towed nets were combined for most analyses as they were considered to be
replicates and preliminary analyses showed that a very similar composition. The number of tows was
three in May and five in June due to weather constraints. Each paired tow was conducted at close-by
positions, thus they are pseudoreplicates.
Samples were frozen at -20°C. Abundances were standardized to square-meters of tow and as
number of fish h-1 of effective tow.
Fish Identification and measurement
Fish were brought in the laboratory and identified to the lowest taxonomic level. All individuals
were enumerated, photographed, and measured to the nearest 0.1 mm standard length (SL) with the
software ImageJ. When samples contained more than 50 individuals of the same species, 50
individuals were randomly photographed and measured. A Leica M165C Stereomicroscope equipped
with an AVTMarlin F-080B camera and a Leica MZ16 stereomicroscope equipped with a Leica DC300
camera were employed to identify and photograph smaller individuals. All Individuals longer than 20
mm were photographed next to a ruler with an Olympus E-PL1 camera fixed to a tripod.
Data analysis
Scorpaenids (Mainly Scorpaena porcus) and Syngnatus acus individuals were excluded from the
analysis as they were mainly formed by adult specimens that appeared in relatively large quantities
in the beam trawl and therefore are not key for the goals of this study.
Comparison of abundance and occurrence
For each method, the relative abundance (% N) and the relative occurrence (%O) of each species
were calculated according to the following formulas:
Where:
16
The five most abundant (five highest %N) and the five most frequent species (five highest %O) have
been marked in the corresponding table (see Table 1 for details).
Comparison of catches and catch per unit effort (CPUE)
The capture efficiency of each method was explored through the comparison of total catches,
standardized catches (to volumetric, surface or time units) or catch per unit effort (catches per hour
of gear work. Due to the presence of zero captures in some gears and the limited number of
replicates (usually 3) per sampling date, comparisons of the above descriptors were performed
through visual inspection of quartile ranges per method and date.
To determine standardized catches, the distance covered during each sampling was first calculated
from the flowmeters readings for the three plankton nets, using the following formula:
Where:
The volume filtered (m3) was then calculated using the following formula:
For the beam trawl, the covered distance was determined using the coordinates of the route and the
towed area (m2) was calculated using the following formula:
Comparison of sizes
The multimodal distribution of sizes by some methods even after severe transformation and the lack
of balance prevents from the use of several parametric techniques. Therefore, comparisons of
pooled sizes were done through non-parametric tests (Mann-Whitney (M-W), Kruskall-Wallis (K-W)
followed by multiple comparisons of mean ranks following Siegel & Castellan (1988)). In some
comparisons the date of sampling (of the two possible) was not considered as a factor due to
insufficient sample size for some methods and a descriptive approach was adopted.
17
In order to analyze the multivariate contribution of sizes, taxa and yield by each method, a series of
matrices (presence-absence data, fourth-root transformed abundances and percentage of captures
standardized by sample) were first built on the samples vs size-structured taxonomic composition.
Each selected taxon (see Table 1) was subdivided into 5 length classes.
The smallest size class corresponded to newly hatched larvae (<3 mm SL). The second class
corresponded to larger larvae with some degree of swimming abilities (3-6 mm SL). The third class
corresponded to individuals with high probability of being close to settlement (6-12 mm SL) following
several works in temperate areas (Harmelin-Vivien et al. 1995; Vigliola et al. 2001; Ishihara and
Tachihara 2011). The fourth class corresponded to early settlers (12-30 mm SL) and the last one to
larger individuals. Samples with no data were discarded. We first compared distance-matrices (Bray-
Curtis (B-C) dissimilarity) through permutation-based rank correlation analyses, and found that no
matter how the matrix was built, the structure remained exactly the same (Rho values >0.99 in all
cases, permutations n=1000).
This indicated that the size-structured taxa composition was the structuring variable, and relative or
absolute abundance gave almost no information to the final ordination. We thus tested the possible
differences between sampling methods using a permANOVA (analysis of variance based on
permutation) approach through the Adonis function in the package vegan (Oksanen et al. 2011) from
R. We selected a fourth-root transformed B-C dissimilarity matrix and used the factors “method” and
“sampling date” as fixed, including an interaction term. However, due to the fact that the difference
in days was not exactly the same in the case of the beam trawl, the exact relationship between
samples was further explored through unconstrained techniques. With that purpose, an UPGMA
cluster analysis followed by the analysis of species contribution to within-group similarities was
analyzed (SIMPER routine in Primer v.6, Clarke and Ainsworth 1993). Briefly, the groups defined
through cluster analysis were analyzed for contribution to within-group similarity, classifying the
characteristic groups as those having a higher similarity/SD ratio (Clarke and Warwick 1994).
18
RESULTS
Taxonomic composition of samples
In total, the 62 samples captured 1894 larval and juvenile fish from 24 families, from which 10 genus
and 27 species could be discerned (Table 1). The bongo net caught 1467 individuals (77.46% of
total), the ring net 146 individuals (7.71%), the neuston net 118 individuals (6.23%), the beam trawl
91 individuals (4.80%), the Ecocean light-trap 68 individuals (3.59%) and the Quatrefoil trap 4
individuals (0.21%). Six families were caught by only one sampling method: one Paralepididae, one
Cepolidae and 8 Trachinidae were sampled by the bongo net, some Scorpaenidae (abundance not
determined), two Ophididae and one Pomacentridae were sampled by the beam trawl. Only 2
families, Sparidae and Mullidae, were caught by all 6 methods. Generally, the most abundant species
-as the 5 highest %N- were also the most frequently -as the 5 highest %O- caught (in dark grey in
Table 1). Gobiidae was the most abundant family in the 4 nets (bongo, ring, neuston, beam trawl),
whereas Sparidae was the most abundant in the light traps.
The bongo net caught 20 families. Whilst 6 families could not be identified further, 8 genus and 10
species were recognized. The ring net sampled 14 families, with 5 not identified further, and 4 genus
and 5 species detected further. The neuston net caught 15 families, of which 6 families could not be
identified further and 5 genus and 5 species recognized. The Quatrefoil light-trap allowed sampling 2
families, one of them being further identified at specie level. The Ecocean light-trap caught 4
different families, 2 not identified further and 2 discerned at specie level. The beam trawl sampled
11 different families, 2 of which could not be identified further, and with 3 genus and 15 species that
could be discerned (Table 1).
Catches performances
Table 2 shows two main aspects of the catching performance of each gear. Pelagic nets tended to
catch a higher number of individuals, with the bongo net showing the highest values. The beam trawl
and light traps captured less efficiently in terms of CPUE (as per hour). Further, Quatrefoil light trap
showed several zero catches in the different deployments (up to 83% of samples), therefore the
ECOCEAN light trap clearly performed better than the Quatrefoil. Variability in catches between
month sampling periods was evident, but rather consistent between methods: pelagic nets tended to
catch more in May than in June (Table 2). Light traps, aiming at larger size fractions, tended to catch
more in June, although numbers were low to further interpret these results.
19
Nets Traps Trawl Spawning season/Ichtyoplankton presence in Mediterranean
Order&family genus/species Bongo Ring Neuston Quat. Ecocean B.trawl J F M A M J J A S O N D
Aulopiformes
<<z Paralepididae Lestidiops jayakari 0.07;16.67
Clupeiformes Clupeidae Sardinella aurita* 0.75;66.67 1.37;33.33 5.93;50 Engraulidae Engraulis encrasicolus * 1.5;100 2.74;66.67 5.08;66.67
Gadiformes Gadidae Gaidropsarus mediterraneus 1.10;6.67
NI 0.68;16.67 Gobiescociformes
Gobiescocidae NI 2.04;66.67 0.68;16.67 Perciformes
Blenniidae NI * 1.91;83.33 0.68;16.67 3.39;50 17.65;83.33 Callionymidae Callionymus spp.* 2.11;100 0.68;16.67 5.08;50 Carangidae Trachurus mediterraneus 0.55;50 0.68;16.67
Trachurus trachurus * 26.47;66.67 Cepolidae Cepola sp.*** 0.07;16.67 Gobiidae Gobius ater 2.2;13.33
Gobius NI * 1.10;6.67
Pomatoschistus spp. * 16.48;33.33
NI * 46.01;100 54.79;83.33 30.51;66.67 2.20;13.33
Labridae Coris julis 0.48;16.67
Symphodus melops 4.40;6.67
Symphodus ocellatus 2.2;13.33
Symphodus roissali * 9.89;33.33
Symphodus rostratus * 9.89;20.00
Symphodus tinca 2.20;13.33
Symphodus spp. * 2.93;83.33 3.42;50 0.85;16.67 3.30;13.33
NI 1.84;33.33 1.69;33.33 Mullidae Mullus barbatus 0.68;16.67 0.85;16.67 1.10;6.67
Mullus surmuletus * 25; 8.33 14.71;16.67
Mullus spp. 0.41;50 Pomacentridae Chromis chromis * 3.30;20.00
Serranidae Serranus hepatus 0.48;33.33 0.85;16.67
Serranus scriba * 10.99;40.00
Serranus scriba/cabrilla 1.7;33.33 Sparidae Diplodus annularis * 15.38;53.33
Diplodus vulgaris * 8.79;26.67
Pagrus pagrus 0.68;16.67
NI * 11.52;100 23.97;100 28.81;83.33 75;25 39.71;50
Trachinidae Trachinus draco 0.55;66.67 Myctophiformes
Myctophidae Hygophum sp.*** 0.07;16.67
Ceratoscopelus maderensis 0.34;33.33
Myctophum punctatum 0.85;16.67
NI 1.10;6.67
Ophidiformes
Ophididae Parophidion vassali*** 2.20;13.33
Pleuronectiformes
Bothidae Arnoglossus sp 0.61;50 0.85;16.67 Soleidae NI 0.14;33.33 3.39;33.33
Scorpaeniformes
Scorpaenidae Scorpaena porcus **
Stomiiformes
Gonostomatidae Cyclothone spp. 0.82;50 0.68;16.67 2.54;33.33 Sternoptychidae Maurolicus muelleri 0.07;16.67 0.85;16.67
Syngnathiformes
Syngnathidae Hyppocampus sp.*** 0.07;16.67 0.68;16.67 1.69;33.33
Syngnathus acus*** **
YSL NI * 21.13;100 1.37;16.67 0.85;16.67 NI NI * 1.7 ;66.67 6.16;83.33 1.69;33.33 1.47;16.67
highest % O highest % N Both highest %N and %O Tsikliras et al. 2010 Álvarez et al. 2012 Fishbase
Table 1. Relative abundance and relative occurrence (in each cell, %N; %O) for each taxa caught in each gear type over the sampling period.
The 5 most representative taxa in either %N, %O or both are represented in different grey colours (see table foot). Information on the
presence of early stages in the plankton is derived from a combination of i) spawning season information ( Tsikliras et al. 2010, in dark grey)
ii) icthyoplankton presence in this area ( Alvarez et al. 2012, light grey), and iii) Fishbase data (fishbase.org, grey). For Tsikliras et al. 2010,
data for a given species or family taken at different sites were combined. * : Selected taxon for the multivariate analysis. **: abundance
not available. When information was not available at genus level, data at species level was combined, and marked with ***. Gobius ater
information not available in any of the 3 resources (in red). NI: not identified, YSL: Yolk-sac larvae.
20
Table 2. Descriptors (25 and 75% quantiles) of catches and capture efficiency per sample of the different
methods. CPUE is catch per unit effort of a particular method. *: Scorpaenidae and Syngnatus acus excluded
from the analysis.
Size composition of samples
The size of fish captured by each method covered the whole spectrum from around 2mm to over 200
mm SL (Fig.8). When pooling all data of sizes, significant differences in size were detected between
methods (K-W, H5,1158=594.3, p<0.001). In general, there was a continuum of sizes, with the smaller
median sizes attributable to the bongo (median=2.7 mm, the only method having significantly
different mean ranked sizes from any other method, post hoc rank-tests, not shown), followed by the
ring net (median=3.8 mm), the neuston net, (median=4.85 mm), the Quatrefoil trap (median=9.7
mm), the Ecocean trap (median=16.3 mm) and the beam trawl (median=46.9 mm) (Fig. 8 top).
Further, the spectra of sizes is relatively maintained among methods in both May and June samplings
(Fig B bottom), although small differences exist probably due to pulse-effects (see further). The
Quatrefoil trap only captured one individual in May so it was not plotted in the monthly comparison.
Method/sampling
Quartiles of total
catch (N. ind.)
Quartiles of
standardised catch
Quartiles of
CPUE (ind h-1
)
% cero
catches
Pelagic
nets
NEUSTON NET ind.100m-3
May 4-55 13-15 22-275 0
June 4-16 15-52 22-74 0
RING NET ind.100m-3
May 28-58 76-139 141-232 0
June 8-9 20-22 40-49 0
BONGO NET ind.100m-3
May 193-441 162-374 965-3188 0
June 91-253 93-295 496-1687 0
Bottom
net
BEAM TRAWL* Ind. 100 m-2
May 4-11 0.7-2.2 9.5-22 0
June 3-6 0.5-13.2 9-18 0
Light
traps
QUATREFOIL ind h-1
May <0.1-<0.1 <0.1-<0.1 <0.1-<0.1 83
June 0-1 0-0.1 0-0.1 80
ECOCEAN ind h-1
May 2-11 0.2-1.3 0.2-1.3 0
June 10-26 1.2-3.2 1.2-3.2 0
21
The comparison of sizes among methods for four of the most abundant families collected by more
than 2 methods showed similar trends (Fig.9). Significant differences between methods were
detected for Sparidae (K-W; H5,245=174.8,p<0.001), Labridae (K-W; H2,101=62.3, p<0.001), Gobiidae (K-
W; H3,363=125.2, p<0.001) and Serranidae (M-W; Z1,36=-4.6, p<0.001). Hence, in general these groups
size-graded between the smallest sizes taken by the bongo and the largest taken by the bottom
trawl, irrespective of the group compared. An example of individuals from the family Sparidae
caught by each sampling method is given in figure 10.
Figure 8. Distribution of mass for each size distribution according to an empirical kernel. In the top figure both
sampling days are pooled and disaggregated densities plots (without the Quatrefoil samples) are shown in the
lowest panels. The x axis is in log10 scale. Beam trawl: Scorpaenidae and Syngnatus acus excluded from the
analysis
22
SL
(mm
)
23457
10
20
406080
100Labridae
2
3457
10
20
40
60
Gobiidae
2
3457
10
20
40
6080
100
Sparidae
23457
10
20
40
6080
100Serranidae
SL
(mm
)a
b,db,d
b,c,d c,d
c
a a
b
ab b
c
a
b
13222 41
3 25
22
66 6
29
23972 32
20
27
10
Figure 9. Size differences among capture methods for four abundant families. Both sampling dates and replicates are
pooled due to low N. Within each family, a common letter among methods indicates no significant differences (after
multiple K-W comparisons).
Figure 10. Plate of Sparidae caught by each method with size scale bars of 2 mm for all methods except for the beam trawl
(scale of 10 mm).
ECOCEAN
23
Multivariate analyses
From the initial comparison of distance matrices, it was clear that the size-structured taxa
composition was the structuring variable, and relative or absolute abundance of these groups gave
almost no information to the final ordination (e.g. the beam trawl not only caught different sizes
than the bongo net, but also in different abundances).
The multivariate permutation-based two-way ANOVA with interaction (method and day) showed
that there were significant differences in the interaction, so that at least for one method there were
significant differences in structure between day 1 and day 2. (Table 3)
Table 3. Permutation ANOVA results on the B-C dissimilarity matrix of species, size classes and abundances.
Scorpaenidae and Syngnatus acus excluded from the analysis.
FACTOR Degrees of
freedom
Sum of
squares
Mean
Squares F. model R2 Pr(>F)
METHOD 5 5.715 1.1430 4.6297 0.4045 <0.001
DAY 1 0.570 0.5707 2.3117 0.0404 <0.01
METHOD:DAY 4 2.163 0.5408 2.1904 0.1531 <0.001
Residuals 23 5.678 0.2468 0.4019
Total 33 14.127 1.000
The above analyses had, nevertheless, several drawbacks in order to understand what was going on
in the structure of the matrix. Firstly, the Quatrefoil only contributed with two positive samples in
one of sampling dates. Secondly, the beam-trawl could be responsible for the interaction term due to
the high structure of the habitat at small scale and the fact that the tows were separated by dozens
of meters (vs. the pelagic-based methods). Therefore, we decided to perform an unconstrained
analysis of the data instead of a priori comparisons in order to better understand how the
multivariate matrix was performing. The cluster analysis (Fig.11) evidenced a clear distinction
between beam trawl, pelagic nets and light traps, all these methods being largely different. However,
a finer distinction could not be made clearly, besides it was confirmed that the beam trawl
composition between May and June samples was quite different. In general, therefore, it is clear that
the day of collection is less important than the method of collection for at least the three large
groups presented in Fig.11.
24
May
June
Beam trawl Pelagic Nets Light traps
A B C
100
60
0
20
40
80Bra
yC
urt
is s
imila
rity
Fig. 11. Group average cluster on the transformed BC similarity matrix. Three main groups are distinguished at around 4%
similarity. Beam trawl: Scorpaenidae and Syngnatus acus were excluded from the analysis. Each point for each sampling
period/method is assumed to be a replicate.
The contribution of size/species classes to the groups separated by the cluster revealed that group A
corresponding to the beam trawl collected mainly individuals over 30 mm SL corresponding to
common species, being the typifying fraction D.vulgaris> 30 mm SL, followed by other common
species including Serranidae and Labridae. However, the abundances for a given taxon and size-class
were relatively low, not exceeding average values of 2 individuals/haul. Recent settlers for key
species were captured, including D. annularis. Group B, corresponding to pelagic nets, evidenced
much higher abundances in many cases (up to 10 individuals of a given taxon/size class) but mainly
corresponding to pre-settlement stages (Table 4). These plankton and neuston nets did not manage
to capture individuals in the pre-defined pre-settlement category except for non-settling, elongated
species (Clupeiform) and some Gobiidae. Group C, corresponding to the light traps, captured a low-
abundance set of taxa, of which some of them clearly fell within the possible pre-settling stages,
including basically unidentified sparids. Other taxa that did not appear in other gears such as
Mullidae and Carangidae were also captured.
25
Table 4. Mean number (total captures, Mean), mean within-group similarity (Av-S), index of relative
importance (S/SD) and percentage contribution to similarity (%S) for the three groups derived from
the cluster analysis. The size classes are in bold. Scorpaenidae and Syngnatus acus excluded from the
analysis.
A B C
Average similarity: 19.35 Average similarity: 26.66 Average similarity: 18.47
Taxa SL-classMeanAv.SS/SD%S SL-classMeanAv.S S/SD%Sim SL-classMeanAv.SS/SD%S
D.vulgaris >30 1.00 3.2 0.51 16.54 - - - - - - - - - -
Pomatoschistus sp. 12-30 1.9 2.32 0.34 11.98- - - - - - - - - -
S.roissali >30 0.63 2.32 0.34 11.97- - - - - - - - - -
S.scriba >30 1.25 1.77 0.34 9.14 - - - - - - - - - -
D.annularis >30 1.13 1.69 0.34 8.72 - - - - - - - - - -
S.roissali 12-30 0.5 1.65 0.34 8.54 - - - - - - - - - -
S.rostratus >30 1.13 1.5 0.33 7.76 - - - - - - - - - -
C.chromis >30 0.38 1.39 0.34 7.17 - - - - - - - - - -
Gobius spp >30 0.38 1.34 0.34 6.94 - - - - - - - - - -
D.annularis 12-30 0.63 0.7 0.19 3.6 - - - - - - - - - -
Gobiidae - - - - -
3-6 10.6 6.56 1.14 24.63 - - - - -
Diplodus sp. - - - - -
3-6 4.6 4.24 0.64 15.89 - - - - -
E.encrasicolus - - - - -
3-6 0.9 1.93 0.48 7.23 - - - - -
Callionymidae - - - - -
<3 1.5 1.52 0.56 5.69 - - - - -
YSL - - - - -
<3 9.4 1.47 0.48 5.53 - - - - -
Gobiidae - - - - -
<3 8.4 1.43 0.49 5.36 - - - - -
Symphodus spp. - - - - -
3-6 1 1.40 0.55 5.24 - - - - -
Gobiidae - - - - -
6-12 1.2 1.30 0.46 4.88 - - - - -
Blennidae - - - - -
3-6 0.8 1.01 0.37 3.78 - - - - -
Sparidae NI - - - - -
3-6 2.8 0.99 0.39 3.73 - - - - -
Symphodus spp. - - - - -
<3 2.1 0.82 0.40 3.06 - - - - -
NI - - - - -
<3 1.4 0.52 0.34 1.96 - - - - -
S.aurita - - - - -
6-12 0.5 0.51 0.26 1.90 - - - - -
E.encrasicolus - - - - -
6-12 0.4 0.46 0.21 1.73 - - - - -
Blenniidae - - - - - - - - - -
12-30 1.5 6.99 0.58 37.83
Sparidae NI - - - - - - - - - -
6-12 3 4.73 0.44 25.58
T.trachurus - - - - - - - - - -
12-30 0.9 2.05 0.29 11.10
M.surmuletus - - - - - - - - - -
>30 1.3 1.98 0.30 10.70
T.trachurus - - - - - - - - - -
>30 1.3 1.91 0.29 10.32
26
DISCUSSION
Most of the information available on early-life stages of temperate littoral fish focuses either on
spawning and pelagic larval stages or on juvenile forms, whereas data on individuals around
settlement is rarely found in literature. This is mainly due to the fact that settlement processes are
hard to evaluate because they have highly specific temporal (Raventos and Macpherson 2005) and
spatial (García-Rubíes and Macpherson 1995) scales. While sampling larvae during their pelagic
phase is relatively easy as larvae remain weeks to months in the water column (Macpherson and
Raventos 2006), sampling individuals close to settlement is complicated as this transition period is
not only prone to high mortality rates (Almany and Webster 2006; Doherty et al. 2004; Planes 2002),
but also happens in pulses and spatial patches, over few days or less, and generally at night
(Holbrook and Schmitt 1997; Irisson and Lecchini 2008).
The table 5 gives a general overview of the 6 sampling methods that we compared in order to find
the best combination to sample littoral fish around settlement.
Table 5. General overview of the sampling techniques used
Pelagic nets Light traps Bottom net
Bongo Ring Neuston Ecocean Quatr. Beam trawl
Developmental stage of fish young larvae (pre-flexion/flexion)
older larvae (post-larvae)
Recent/older post-settlers
Settlement period Pre Pre Pre Pre Pre post
Simultaenous sampling possible no no No yes yes no
Ease to get min.3 replicates/day 3 3 4 1 1 2
Ease of installation 3 2 4 1 1 1
Cost (incl. logisitics)* 3 2 4 1 1 1
Unwanted species 4 3 3 1 1 2
Abundance 1 2 2 3 4 2
Taxa richness** 1 3 1 3 4 2
Ease of identification 4 3 3 2 2 1
Rel. mean occurrence of zero
catches (%) 0 0 0 0 80-83 0
1: excellent, 2: good, 3: acceptable, 4: poor *based on the boat size required and the number of people required **based on the average number of different genus/gear type, excluding NI and LSV and zero catches.
in red: the methods we consider the most appropriate to sample individuals around settlement
27
From Table 5 it is clear that sampling around settlement is best achieved through the combination of
the Ecocean trap and the beam trawl. It is also true that depending on the species of interest the
Ecocean will not yield good results because only species with phototactic behavior are selected. To
this respect, the small-scale beam trawl may be of higher interest, when performed at night, in order
to describe the community of settlers. However, for connectivity analyses the Ecocean trap may be
highly valuable as it indicates the endpoint of a transport plus behavioural process before the high
bottom-associated mortality or displacement takes place. It must also be bore in mind that the
specific design of the beam trawl survey (depth intervals, bottom-type, frequency) will have a larger
influence than the design of the trap location, as mortality processes at the bottom are higher and
complex during the first settlement days (Doherty et al. 2004) and are particularly prone to high
differences in the structural and community composition at a small scale (Almany 2003).
It must be emphasized that our work is not designed to describe differences in size-related taxa-
composition, as at least a whole year of data would be needed. However, it has been shown that
once the spawning season started, centered on the spring-summer period, pelagic larval abundance
remains relatively constant over the summer period (Álvarez et al. 2012). This means that sampling
only over 2 days, once in May and once in June, should been enough to compare sampling methods
in term of at least sampling efficiency and size distribution within a relatively stable environment
characterized by the spawning of the majority of the species present in the littoral realm. This is
confirmed by our results, as the taxonomic composition, abundance and size-spectra of each
sampling methods was generally agreement with the expected patterns of spawning of individual
taxa over the sampling period (Table 1).
The little differences existing between May and June samples could be due to: i) The inherent
variability due a progression of the reproduction period for summer species and the end of the
reproduction period for the spring species (Álvarez et al. 2012), which leads to some variation in
sizes. ii) Changes in oceanographical conditions, which, given a particular spawning configuration,
may produce different results. For example, during the week preceding sampling, the average water
temperature, which is known to affect the water column structure as well as fish physiology, was
19.37°C in May and 23.44°C in June. This temperature differences probably trigger the spawning of
few species and accelerates the development of larvae. Indeed, the amount of pelagic larvae caught
in the plankton tended to be higher in May than in June (Table 2).
The results of this study reveal a stable size-structure for the 6 methods, as each technique sampled
over a well-defined size-spectrum. Still, efficiency to sample early fish stages around settlement
differed greatly amongst methods and not all are adequate to get information about settlement
28
processes, as recently suggested in another comparative study conducted in tropical areas (Carassou
et al. 2009).
Conventional net-based sampling methods are active gears whose size structure and taxonomic
composition depend on mouth diameter, towing speed and mesh size (Barkley 1972). Many authors
have shown that plankton and neuston nets underestimate early larval stages (Chícharo 2009; Choat
et al. 1993). The present study lead to the same conclusions on littoral assemblages , as all three
pelagic nets, including small (bongo 40) to mid-range nets (neuston nets), fail at sampling highly
mobile larger post-larvae (post-flexion) even when towed at night, at different depths (horizontal
and oblique hauls) and with various mesh sizes (335 µm, 780 µm and 1000 µm).
Another drawback of net towing is that it damages more fish larvae than other methods (Chícharo
2009). The same occurs in our study, where samples from pelagic nets are hard to identify as many
individuals are either damaged or too small (Table 4). Moreover, sorting is time consuming because
other zooplankton is abundant, especially in the bongo nets.
Oppositely, light-trap devices are passive sampling methods which rely on the swimming ability and
the phototactic behavior of larval fish to approach voluntarily the trap and explore the provided
artificial sustratum. In this study the Quatrefoil light-trap has shown little efficiency at collecting
larval stages, which could be caused by unwanted predation as suggested by Vilizzi (2008) or because
efficiency relies on the chance that individuals find the slot in order to enter the trap (Lecaillon and
Lourié 2007). Additionally, both the intensity and wavelength (color) of the light source of light
aggregation devices are known to influence light trap efficiency (Gehrke 1994) and could have played
a role here too.
The Ecocean C.A.R.E has an innovative design that combines the traditional light-trap strategy with
an artificial reef. This latter characteristic is supposed to attract demersal species in their settlement
phase whilst they are in search for a shelter. These traps have proved to work in tropical regions and
our results demonstrate that they are also efficient at capturing post-larvae in temperate waters.
These traps caught an important number of Trachurus trachurus individuals, probably because
Carangidae are both phototactic and exhibit a pelagic-demersal exploratory (related to feeding)
behavior in their juvenile phase (Palomera pers comm.). However, the trap didn’t catch any
phototactic pelagic species such as Clupeiforms, which can be abundant in other light-trap designs
(e.g. Beckley and Naidoo 2003) as they get caught whilst swimming. This result was expected as it is
one of the advantages described by Lecaillon (2004) and this design is conceived not to collect purely
phototactic fish (Lecaillon and Lourié 2007) but also based on the exploratory behavior of demersal
species before settlement.
29
The presence of Mullus surmuletus in the light traps was not expected as they were not sampled by
the pelagic nets. Individuals as big as 65mm TL have been observed in the water column in previous
studies (Deudero 2002), and we speculate that this species settles at a larger size than most of other
littoral species.
Another advantage of light traps is that they permit simultaneous sampling at different locations,
thus they are particularly appropriate to investigate spatial distribution of pre-settlers.
The beam trawl also provides a very useful sampling gear to link the pre-settlement phase to the
benthic phase as it captures settled individuals including the ones of small size, that are likely to have
recently arrived to benthic nursery areas, such as Diplodus annularis. Moreover, the beam trawl
used in this study has shown to be of little impact for the bottom, with no substrate collected in the
nets.
Both the Ecocean C.A.R.E and the beam trawl are easy to handle and install, even on a small boat
(<5m). On the contrary, larger towed nets need larger boats (we used a 12m modified recreational
boat) in order to operate the large structures and/or to maintain the appropriate towing angle). One
of the good points of the present work, which will lead to a long time-series through the combined
use of both Ecocean CARE and the beam trawl, was the fact that individuals around settlement from
some of the key groups for recreational fishing in the area, such as the sparids, labrids or serranids,
were well represented in one or both methods. Indeed, sampling of species such as D. annularis is
not as easy as sampling other sparids such as D.sargus which concentrate at the shoreline (Harmelin-
Vivien et al. 1995). D. annularis is one of the top species in biomass and abundance associated to
seagrass meadows in the Mediterranean (Deudero et al. 2008) and therefore future analyses of
settlement processes will benefit from this combined sampling.
CONCLUSION
In conclusion, a combination of the Ecocean light trap and the beam trawl seems a promising tool to
sample key littoral species around settlement and should provide useful information to study
mortality processes and connectivity in littoral fishes of temperate water. A future time series using
these combined methods will guarantee further insight into recruitment processes in littoral species
in temperate areas.
30
ACKNOWLEDGMENTS
First of all, I would like to thank the IMEDEA for allowing me to conduct my project in their facilities
and to use their resources.
I am particularly thankful to the group of Fish Ecology, especially my supervisor, Ignacio Catalan, for
being of great support, always available to give me an advice or answer a question, and Itziar Álvarez
for her precious help.
I would also like to thank my colleague Noemi Colinas for helping me with the practical side of my
project whenever it was needed.
31
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