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ASSESSMENT OF SOLE (Solea solea) IN GSA 17
(2005-2007)
Izmir, Turkey, 15-19 September 2008
G. Fabi1, I. Celic2, M. Despalatovic3, O. Giovanardi2, F. Grati1, I. Isajlović3, A. Jenic4, D. Pehar4, P.
Polidori1, S. Raicevich2, G. Scarcella1, N. Vrgoc3
1 CNR – ISMAR Ancona, Italy
2 ICRAM Chioggia, Italy
3 IOF Split, Croatia
4 FRIS Ljubliana, Slovenia
Scientific Advisory CommitteeSub-Committee on Stock AssessmentWorking Group on Demersal Species
The sole stock in GSA 17 is shared between the Adriatic countries (Italy, Croatia and Slovenia).
This resource is mainly exploited by rapido trawlers and set netters.
Rapido trawl and set nets (gill nets and trammel nets) are used by the Italian fleets, while the trammel net is the most used gear along the eastern coast.
Most of catches come from the Italian rapido trawl fleets (around 80%).
Catches strictly depend on the recruitment success, mainly including juveniles (age classes from 0 to 2).
Scientific Advisory CommitteeSub-Committee on Stock AssessmentWorking Group on Demersal Species
FISHERY
Sole exploitation
0
500,000
1,000,000
1,500,000
2,000,000
12 14 16 18 20 22 24 26 28 30 32 34 36 38 40
TL (cm)
Num
ber
of
ind.
Rapido trawl Set nets Otter trawl
Size at first sexual maturity
Date
Vessel
Species by size and kg
DATA SOURCE
Commercial landings (2005-2007)
daily auction documents
Scientific Advisory CommitteeSub-Committee on Stock AssessmentWorking Group on Demersal Species
samplings of landing
Rapido trawl survey (Fall 2005-2007)
SPRING 2005
FALL 2005
SPRING 2006
FALL 2006
Surface 38.880 km2
FALL 2007
Surface 44.880 km2
Surface 44.880 km2
Surface 44.880 km2
Surface 42.388 km2
0–30 m
30–50 m
50-100 m
Depth strata S’05 F’05 S’06 F’06 F’07
0-30 30 30 20 35 32
30-50 14 12 10 20 19
50-100 24 15 8 8 11
HR islands 0 5 4 4 0
TOTAL 68 62 42 67 62
N° of stations / Stratum
SURVEY DATA
Samplings station
Scientific Advisory CommitteeSub-Committee on Stock AssessmentWorking Group on Demersal Species
Abundance and Biomass indexes from surveys carried out in 2005 – 2007, were computed using the software AdriaMed Trawl Information System (ATrIS; Gramolini et al., 2005)
GIS Maps of the spatial distribution of the stock, of spawning females and of juveniles.
SURVEY DATA
Data analysis
Length frequency distributions were corrected using the selectivity parameters given by Ferretti and Froglia (1975)
0%
25%
50%
75%
100%
0 5 10 15 20 25 30
Total length [cm]
Pro
po
rtio
n r
eta
ine
d
0
500,000
1,000,000
1,500,000
2,000,000
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40
TL cm
Nu
mb
er
of I
nd
.
0
500,000
1,000,000
1,500,000
2,000,000
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40
TL cm
Nu
mb
er
of I
nd
.
Growth parameters were calculated by indirect methods using the length frequency data from the surveys.
As the demography of males and females did not show relevant differences, Linf, k and t0 were estimated for the whole population.
Scientific Advisory CommitteeSub-Committee on Stock AssessmentWorking Group on Demersal Species
SURVEY DATA
Growth parameters estimation
Length-frequency
distributions corrected by
selectivity
Age group distributions corrected by
selectivity
SURVEY DATA Demography
Spring 2005
0
50
100
150
200
250
N/k
m2
Fall 2005
0
50
100
150
200
250
N/k
m2
Spring 2006
0
50
100
150
200
250
N/k
m2
Fall 2006
0
50
100
150
200
250
N/k
m2
Fall 2007
0
50
100
150
200
250
0 1 2 3 > 4
Age
N/k
m2
Spring 2005
0
20
40
60
80N
/km
2
Fall 2005
0
20
40
60
80
N/k
m2
Spring 2006
0
20
40
60
80
N/k
m2
Fall 2006
0
20
40
60
80
N/k
m2
Fall 2007
0
20
40
60
80
N/k
m2
5 10 15 20 25 30 35 40TL (cm)
N. ind. km-2
SPRING 2005
FALL 2005
SPRING 2006
FALL 2006 FALL 2007
Scientific Advisory CommitteeSub-Committee on Stock AssessmentWorking Group on Demersal Species
SURVEY DATA
Spatial distribution of sole
0
10
20
30
40
50
Spring2005
Fall Spring2006
Fall Spring2007
Fall
Total Biomass
Spawning Stock Biomass
Recruits
Spring 2005 Fall 2005 Spring 2006 Fall 2006 Fall 2007
N. ind/km2 (CV %) 176 (12%) 305 (20%) 299 (15%) 327 (22%) 434 (24%)
kg/km2 (CV %) 23 (15%) 27 (15%) 36 (15%) 36 (16%) 40 (18%)
Scientific Advisory CommitteeSub-Committee on Stock AssessmentWorking Group on Demersal Species
0
10
20
30
40
50
Spring2005
Fall Spring2006
Fall Spring2007
Fall
kg/ k
m2
2005 2006 2007
SURVEY DATA
Abundance and biomass indexes
Data used in VPA flow tuned with trawl survey CPUE
Natural mortality:
The vector of natural mortality by age was calculated from Caddy´s (1991) method, using the PROBIOM Excel spreadsheet (Abella et al., 1997):
Age 0 1 2 3 4 5+
M 0.9 0.3 0.3 0.2 0.2 0.2
Abundance at age (N.ind./km2) from surveys.
Catch at age data considering ages 0 to 5+ (plus group) from commercial fishery in the period 2005-2007 of GSA 17 in number of specimens.
Biological parameter:
Growth: V. Bertalannfy (Linf = 39.6 cm; k = 0.44; t0 = -0.46).
Length-Weight relationship: W=a Lb (a = 0.007; b = 3.0368).
Maturation: logistic, age50% ~ 1.5 years:
Age 0 1 2 3 4 5+
Proportion of Mature 0 0.29 0.75 1 1 1
Scientific Advisory CommitteeSub-Committee on Stock AssessmentWorking Group on Demersal Species
Time interval = 1 year
Age: 0,1,..., 5+ (plus group)
VPA flow was processed by means of an Excel spreadsheet using catch-at-age from commercial landings, assuming Ma and abundance-at-age (start year) from survey data.
VPA-tuning flow
Scientific Advisory CommitteeSub-Committee on Stock AssessmentWorking Group on Demersal Species
0
500
1000
1500
2000
2500
3000
0 1 2 3 4 5+Age
To
ns
0.0
0.5
1.0
1.5
2.0
2.5
3.0
F
2005 2006 2007
2005 2006 2007
2005 2006 2007
2005 2006 2007
F vector
Biomass (Tons)
Analytical results and survey indexes showed similar patterns for total biomass, SSB and recruits.
The total biomass and SSB remained practically constant from 2005 to 2007, while a low recruitment occurred in 2006.
Fbar (0+, 1, 2) showed a decreasing trend in the three years reaching the minimum of 0.52 in 2007.
Biomass from VPA Trawl survey
Scientific Advisory CommitteeSub-Committee on Stock AssessmentWorking Group on Demersal Species
VPA-tuning flow
0
2000
4000
6000
2005 2006 2007
0.0
0.4
0.8
1.2
1.6
Total biomass SSB Recruits (0+) Fbar (0-2)
0
2000
4000
6000
2005 2006 2007
To
ns
0.0
0.4
0.8
1.2
1.6
F
0
10
20
30
40
50
2005 2006 2007
kg
/ km
2
0
20
40
60
80
100
120
140
1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007
LP
UE
kg
/da
y/ve
sse
l '
0
500
1000
1500
2000
2500
vess
els
*da
ys
LPUE EFFORT
This Fbar pattern was due to a decrease of the fishing effort by the main Italian rapido trawl fleets in GSA 17 in terms of both number of vessels and fishing time, as well as to the switching of rapido trawlers towards other resources (e.g. gastropods and bivalves).
LPUE and Effort of Ancona rapido trawl fleet Scientific Advisory Committee
Sub-Committee on Stock AssessmentWorking Group on Demersal Species
Fall 2005
Fall 2006Fall 2007
Size class range (TLmode+s.d.):
Fall 2005 TL ≤ 18.5 cm
Fall 2006 TL ≤ 17.5 cm
Fall 2007 TL ≤ 20.0 cm
Recruits of sole
Scientific Advisory CommitteeSub-Committee on Stock AssessmentWorking Group on Demersal Species
N. ind. km-2
Recruits of sole vs bottom temperature and salinity
Fall 2005 Fall 2006
N. ind. km-2
°C
Fall 2006Fall 2005
S‰PO RIVER INFLOW
0200400600800
10001200140016001800
1 2 3 4 5 6 7 8 9101112 1 2 3 4 5 6 7 8 9101112
2005 2006
m3 /s
S‰
°C
Scientific Advisory CommitteeSub-Committee on Stock AssessmentWorking Group on Demersal Species
0
0.02
0.04
0.06
0.08
0 0.2 0.4 0.6 0.8 1 1.2 1.4
Fishing mortality
Yie
ld-p
er-R
ecru
it (g
)
Y/R analysis applied with the age at first capture of 0.7 (around 16 cm TL) and using as SRR the Beverton & Holt model (Pilling et al., 2008), showed that the resource is fully exploited / slightly overexploited.
F0.1 = 0.26 Fmax = 0.46 Fbar = 0.52
Yield-per-recruit (Y/R) analysis
Scientific Advisory CommitteeSub-Committee on Stock AssessmentWorking Group on Demersal Species
FmaxF0.1
Fbar
0
500,000
1,000,000
1,500,000
2,000,000
12 14 16 18 20 22 24 26 28 30 32 34 36 38 40
TL (cm)
Num
ber
of in
d.
Rapido trawl Set nets Otter trawl
Size frequency distributions from commercial fishery
MLS = 20 cm
Fmax (20 cm) = 0.60
0
0.02
0.04
0.06
0.08
0 0.2 0.4 0.6 0.8 1 1.2 1.4
Fishing mortality
Yie
ld-p
er-R
ecru
it (
kg)
More than 40% of soles landed by rapido trawlers had a TL<20 cm (Minimum Landing Size; EC reg. 1967/2006).
The increase of the length at first capture from 16 cm to 20 cm TL produces, according with Y/R analysis, a ≈20% increase of Fmax.
Fmax (16 cm) = 0.46
Scientific Advisory CommitteeSub-Committee on Stock AssessmentWorking Group on Demersal Species
Management advice
Scientific Advisory CommitteeSub-Committee on Stock AssessmentWorking Group on Demersal Species
• A further reduction of the rapido trawlers’ fishing pressure of 10% would be recommended, also considering that the exploitation is mainly orientated towards juveniles and the success of recruitment is strictly related to environmental conditions. Hence, in the case of both increasing fishing effort and yearly bad recruitment there could be a high risk of stock depletion.
• A two–months closure for rapido trawling inside 6 nm offshore along the Italian coast, after the biological fishing stop (August), would be advisable to reduce the portion of undersized specimens (mainly YOY).
• For the same reason specific studies on rapido trawl selectivity are necessary. In fact, it is not sure that the adoption of a larger mesh size could correspond to a decrease of illegal catches, considering that the mesh opening used by the Italian rapido trawlers is already larger (48 mm or more) than the legal one.
• SSB remained constant in the 3 years maybe because in late fall - winter the main spawning area is only partially exploited by the Croatian set netters and Italian fleets. Therefore, the safeguard of spawning areas to prevent a possible future exploitation might be crucial for the sustainability of the Adriatic sole stock.
• Finally, a set of specific rules for rapido trawl fishery would be advisable (e.g.: size and number of gears, mesh size, towing speed).
Spatial distribution of spawning females and fishing grounds of the
Italian fleets
Scientific Advisory CommitteeSub-Committee on Stock AssessmentWorking Group on Demersal Species
Management advice
Scientific Advisory CommitteeSub-Committee on Stock AssessmentWorking Group on Demersal Species
Management advice
Thank youfor your attention