Seasonal dynamics inNephrops norvegicus(Decapoda:Nephropidae) catches off the Catalan coasts
(Western Mediterranean)
Jacopo Aguzzia,∗, Francesc Sardaa, Rosario Allueb
a Institut de Ciencies del Mar (CSIC), Passeig Mar´ıtim de la Barceloneta 37-49, 08003 Barcelona, Spainb Direccio General de Pesca i Afers Maritims, Departement d’Agricultura i Pesca, Generalitat de Catalunya,
Gran Via de les Corts Catalanes 612, 08007 Barcelona, Spain
Received 23 June 2003; received in revised form 8 April 2004; accepted 16 April 2004
nvironmental factors modulating emergence behaviour in the species. Other factors affecting behaviour, such as meproduction, were also taken into account.
2004 Elsevier B.V. All rights reserved.
eywords: Nephrops norvegicus; Seasonal variations; Periodogram analysis; Form estimate analysis; Light intensity; Sex ratio;editerranean
. Introduction
A well developed multispecies fishery exists in theestern Mediterranean and has been extensively de-
∗ Corresponding author. Present address: Neuroscience Institute,orehouse School of Medicine 720 Westview Drive, S.W. Atlanta,A 30310, USA.E-mail address:[email protected] (J. Aguzzi).
scribed in several studies (e.g.,Lleonart, 1996). Ofall the locally exploited fisheries resources, the Nway lobster,Nephrops norvegicus(L.), is one of themost economically important (Sarda, 1998) so locafishing activity, though basically multispecific, isten redirected toward this species. This tendencheightened by the expertise of local fisherman,ordinarily exploit grounds on the lower shelf andper slope (200–500 m), the maximum depth rag
294 J. Aguzzi et al. / Fisheries Research 69 (2004) 293–300
distribution for this species in the Catalan area (e.g.,Maynou et al., 1998; Abello et al., 2002). In the Span-ish Mediterranean, where fishing harbours generallyhost around 10–15 commercial fishing vessels (ratedat approximately 500 HP), an average of 20–30 mt ofN. norvegicusis taken per port yearly (Maynou andSarda, 2001). N. norvegicuslandings are not partic-ularly high compared with catches of other exploiteddemersal resources, such as fishes, but they account foraround 15–20% of total earnings by the local demersalfishery (Bas et al., 2003).
Daily fluctuations in catches have been extensivelystudied in the Atlantic as a result of diel changes inemergence behaviour in response to light intensitycycles (e.g.,Moller and Naylor, 1980). The link be-tween emergence, catch levels, and light intensity cy-cles was explained byChapman et al. (1975). Theyhypothesized that the individuals leave their burrowsunder conditions of optimum light intensity, rangingfrom night-time to daytime, for an increasing depthof distribution. In the Atlantic, on a bigger temporalscale, seasonal fluctuations in the catchability of thisspecies have also been recorded (e.g.,Hill and White,1990; Farina et al., 1994) but not in the Mediterranean.Although the influence of light cycles on the deep-water fishery of other commercially exploited species
ast (We cation.
in the western Mediterranean area, such as the prawnAristeus antennatus(500–1000 m), has been clearlydemonstrated (Tobar and Sarda, 1992), no data on thisaspect are available forN. norvegicuson a seasonalscale.
In the Mediterranean region seasonal changes inspecies catchability are a source of bias in trawl-basedassessments of demersal stocks (e.g.,Sarda, 1991), inturn affecting the economies of local fishing commu-nities (Maynou and Sarda, 2001). An understanding ofbehavioural rhythms and their links to environmentalvariables is essential for purposes of fishery manage-ment (Naylor, 1992). The aim of the present paper isto look at variations inN. norvegicuscatchability on aseasonal time scale by analysing fishing fleet landingsdata. Studies of this kind bear directly on planning sus-tainable short-term and intermediate-term exploitationof local stocks.
2. Materials and methods
Seasonal fluctuations in commercial landings inCatalonia were studied by analysing the register of his-torical landings data for the port of Barcelona compiledby local fishermen’s associations. Local commercial
Fig. 1. Fishing area geographic location off the Catalan co
stern Mediterranean). The grid indicates the fishing ground lo
J. Aguzzi et al. / Fisheries Research 69 (2004) 293–300 295
trawlers exploit one of the largest fishing grounds inthe region, the Serola bank (Sarda and Lleonart, 1993)(Fig. 1). Landings data expressed as the total quantity(kg) of N. norvegicuslanded per month for the periodfrom 1992 to 2000 were selected for periodogram andform estimate analyses to detect the presence of any sig-nificant rhythmicity and phase (i.e., months of peaks inlandings), respectively. The periodogram analysis wascarried out using the PERIO (Aagaard et al., 1995),screening time series for inherent periodicities between4 and 14 months. In the resulting plot, peaks indicatingthe significant periodicity on thex-axis, are indicated bypoint-values crossing the 95% upper-limit confidenceinterval. Obviously, for those outputs presenting morethan a significant peak, only the one corresponding tothe smaller periodicity is considered, the others beingmultiple repetitions of this peak. To assess the phaseof the recorded rhythmicity, a form estimate analy-sis was performed on the time series of data. Monthlylandings data were averaged for corresponding monthsover the entire period considered (i.e., 1992–2000).The resulting mean values were plotted with theirstandard deviations over a time interval spanning a cal-endar year. Peak significance was assessed by com-puting a yearly mean value (adapted fromHammondand Naylor, 1977). According to this method, two ad-jacent peaks are distinguished as different only if sep-arated by three or more values below the yearly mean.To assess the influence of seasonal environmental lightintensity on landings, the monthly mean landing val-u calr t theh
Monthly fluctuations in landings at the port ofBarcelona are indicative of clear seasonal rhythmic-ity for most of the years considered (Fig. 2). In gen-eral, total landings exhibited peaks in May-July, withthe lowest value in September–October. In some yearslandings rose again in November–December (1992,1993, 1995, and 1998) and then decreased again pro-gressively until the next spring–summer peak. In otheryears (e.g., 1997 and 1999), no clear seasonal pat-tern was observed. Periodogram analysis showed a sig-nificant six monthly-based periodicity in the data set(Fig. 3), the 12-month-peak being a multiple in theplot. Form estimate analysis of this six monthly-basedlandings rhythm using the monthly mean landingsvalues revealed the presence of a major significantpeak in spring–summer spanning from May to Au-gust (Fig. 4), whose maximum value occurred in June(1944.5 ± 665.2 kg). This maximum was followedby a sharp decrease in landings to a minimum inSeptember (387.2± 186.7 kg). The subsequent risein landings hence occurred in November (902.3±524.1 kg), therefore marking the six monthly period-icity in landings, as disclosed by the periodogramanalysis (seeFig. 3). However, this increase did notoccur in all the years in the time series considered (seeFig. 2), and for this reason it did not appear as a sig-nificant peak in the resulting form estimate analysisplot.
thea sun’szi thes tioni ne–J
TG omm mmer(
S
SA
N
es were plotted jointly with monthly astronomiecords for zenith height of the sun (i.e., the angle aorizon).
To evaluate seasonal variations in the sex ratatalan commercial landings, fishery inspectorsex records from random subsamples from a siay’s catch by the trawler Madove in March, Jueptember, October, and November 1999 and Julyust, and October 2000. These months were choseause the proportion of females in the catches decrn the Western Mediterranean in autumn during theearing season (Sarda, 1995; Orsi Relini et al., 1998).he samples were grouped by season as autumn–wSeptember, October, and November 1999 and Oc000) and spring–summer (March, July, and Aug999). AG-test (Gadj) (Sokal and Rohlf, 1981) waserformed to assess significant deviations of theatio values from 1:1.
The landings rhythmicity was consistent withstronomical data for seasonal changes in theenith height (angle at the horizon) (Fig. 4). Land-ngs rose during the summer, when the angle ofun at the horizon and hence light intensity penetranto the water column were at their maximum (Juuly).
able 1-test outputs (Gadj) computed from sex ratio values obtained frales and females sampled in months grouped by spring–su
S–S) and autumn–winter (A–W)
eason Gadj F/N
–S 1.299 0.517–W 7.380 0.0454∗
: total number of individuals; F: females.∗ Significant difference (P < 0.05).
296 J. Aguzzi et al. / Fisheries Research 69 (2004) 293–300
Fig. 2. Total monthly landings (kg) at the port of Barcelona for 1992–2000.
J. Aguzzi et al. / Fisheries Research 69 (2004) 293–300 297
Fig. 3. Periodogram analysis of the total monthly landings data from 1992 to 2000. S.D. = standard deviation units.
Fig. 4. Monthly mean total landings (kg) at the port of Barcelona forhorizon) and to the different stages of the reproductive and moult cyc(914.3 kg) used to assess peak significance;partial temporal overlap ampopulation phenomenon, not all individuals undergo any given stage
1992–2000 in relation to the monthly position of the sun (angle at theles.monthly position of the sun; yearly mean value of landingsong the different stages of the reproductive and moult cycles (being a
at the same time).
298 J. Aguzzi et al. / Fisheries Research 69 (2004) 293–300
The proportion of females in the catches decreasedfrom spring–summer to autumn–winter (Table 1). TheG-test revealed a significant difference from the ra-tio of 1:1 in the latter season as a result of this de-crease.
4. Discussion
The present study looked for the occurrence ofrhythmic seasonal fluctuations in commercial catchesof N. norvegicusoff the Catalan coast. Analysis ofcommercial data revealed an increase in landings inspring–summer. This suggests that light intensity af-fects the fishery of this species, even on deep-waterslope grounds off Catalan coasts, where illuminationis several orders of magnitude lower than in shal-lower Atlantic shelves, where this phenomenon wasalready reported (e.g.,Farmer, 1974a; Hammond andNaylor, 1977). In fact, in the Catalan area light inten-sity decreases by several orders of magnitude from theshelf down to the slope, illumination levels in centralhours of the day being 2.5�Ei m−2 s−1 at 100–110 mand 2.10−7 �Ei m−2 s−1 at 400–430 m, respectively(Aguzzi et al., 2003).
Catches ofN. norvegicuscan be considered directlyproportional to the number of individuals emergingfrom their burrows at a given time of day (Main andSangster, 1985; Newland et al., 1992). As suggestedby Chapman (1980), only 10–30% of the individualsi thisb m”l1 ins ighto ngthd rea.T mn( nm forl e inl s arei s.no thef thea ngere
Besides light intensity and photoperiod duration,other factors may influence the seasonal emergence be-haviour of animals and help explain the observed sea-sonal variations in the landings. Marked behaviouralchanges at the onset of sexual maturity have often beenreported for coastal species of decapods that are impor-tant fishery target species, such asMaja squinadoandCallinectes sapidusin the Atlantic (e.g.,Hines et al.,1995; Freire et al., in press). For the much more deep-water distributedN. norvegicusresource in the WesternMediterranean, the influence of the reproductive cycleon the basal level of emergence activity can be inferredfrom sex ratio in catches, but unfortunately no directscientific evidence is available to date. Apparently, theinhibition of emergence behaviour by females duringthe season of berry results in a drop in their share incommercial catches both in the Atlantic (e.g.,Rice andChapman, 1971; Chapman, 1980) and in the Mediter-ranean (Sarda and Lleonart, 1993; Orsi Relini et al.,1998). As reported here (Fig. 3), a sharp decrease inlandings took place in September in most years. Thisdecrease coincides with the season of berry in femalesand could plausibly be caused by a greater tendencytowards concealment at that time. Variations in the sexratio recorded in the present study provide indirect sup-port for this. The spring–summer increase in landingscoincides with the condition of females being free ofeggs (Sarda, 1995). After the eggs have hatched, fe-males may have an emergence rhythmicity similar tot on,t wlg
in-fl tingi er( hee yt1 e-cu thebo g.,Fi be-h mes ur-r bal
n a population leave their burrows on a daily basis,ehaviour being driven by the interplay of “optimu
ight intensity fluctuations and hunger state (Oakley,979). Our results indicated a rise in total landingspring–summer. At that time maximum annual hef the sun at the zenith and largest photoperiod leuration are achieved at the latitude of the study ahe increased light penetration into the water colu
Chapman et al., 1975) and day light phase duratioay respectively induce more individuals to emerge
onger periods of time, with a consequent overall risandings in spring–summer. These considerationn agreement with feeding behaviour of the specieN.orvegicus, as a visual feeder (Oakley, 1979), relies onptimum light intensity to feed, so an increment in
eeding activity of slope animals was recorded inrea during spring–summer time as a result of loxcursions from burrows (Aguzzi et al., 2004).
hat of their male counterparts in the slope populatihus increasing their vulnerability to demersal traears.
Other aspects relating to reproduction may alsouence emergence behaviour and duration, resuln an overall increase in catches in spring–summFig. 3). Females moult in spring–summer after tggs have hatched (Sarda, 1995) and are then reado mate in the hours immediately following (Farmer,974b). The males moult earlier, in the months preding this (Sarda, 1995). Mating dynamics are poorlynderstood, but partners are likely to meet outsideurrows (Farmer, 1974b; 1975). Additionally, becausef the strongly territorial behaviour of this species (e.armer, 1975; Chapman, 1980), fighting displays with
ntruders may take place before mating. All theseavioural traits may considerably increase the tipent by individuals of both sexes outside their bows in spring–summer, with a corresponding glo
J. Aguzzi et al. / Fisheries Research 69 (2004) 293–300 299
increase in the population’s vulnerability to fishinggears at that time.
Acknowledgement
The authors wish to thank Dr. J.B. Company, Dr. F.Maynou, Dr. J. Lleonart, Dr. G. Rotllant, Dr. P. Abello,and Mr. J.A. Garcia for their useful comments dur-ing preparation of the manuscript. Special thanks alsogoes to the crews of the commercial trawlers La Blavaand Madove for their support during sampling. Thepresent work was carried out in the framework of a na-tional research project funded by the Spanish Intermin-isterial Council for Science and Technology (CICYT)(NERIT: MAR98-0935).
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