ASSESSMENT OF SOLE ( Solea solea ) IN GSA 17 (2005-2007)

1. ASSESSMENT OF SOLE (Solea solea) IN GSA 17 (2005-2007) 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 Committee Sub-Committee on Stock Assessment Working Group on Demersal Species Izmir, Turkey, 15-19 September 2008

2. Size at first sexual maturity FISHERY Sole exploitation 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 Committee Sub-Committee on Stock Assessment Working Group on Demersal Species

3. Date Vessel Species by size and kg DATA SOURCE Commercial landings (2005-2007) daily auction documents Rapido trawl survey (Fall 2005-2007) samplings of landing Scientific Advisory Committee Sub-Committee on Stock Assessment Working Group on Demersal Species

4. 0–30 m 30–50 m 50-100 m SURVEY DATA Samplings station N° of stations / Stratum SPRING 2006 SPRING 2005 Surface 44.880 km2 Surface 38.880 km2 FALL 2007 FALL 2005 FALL 2006 Surface 44.880 km2 Surface 42.388 km2 Surface 44.880 km2

5. SURVEY DATA Data analysis 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. Scientific Advisory Committee Sub-Committee on Stock Assessment Working Group on Demersal Species

6. SURVEY DATA Growth parameters estimation Length frequency distributions were corrected using the selectivity parameters given by Ferretti and Froglia (1975) 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 Committee Sub-Committee on Stock Assessment Working Group on Demersal Species

7. 5 10 15 20 25 30 35 40 TL (cm) SURVEY DATA Demography Length-frequency distributions corrected by selectivity Age group distributions corrected by selectivity

8. N. ind. km-2 SURVEY DATA Spatial distribution of sole SPRING 2006 SPRING 2005 FALL 2007 FALL 2005 FALL 2006 Scientific Advisory Committee Sub-Committee on Stock Assessment Working Group on Demersal Species

9. SURVEY DATA Abundance and biomass indexes 2005 2006 2007 Scientific Advisory Committee Sub-Committee on Stock Assessment Working Group on Demersal Species

10. Data used in VPA flow tuned with trawl survey CPUE 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: 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): Scientific Advisory Committee Sub-Committee on Stock Assessment Working Group on Demersal Species

11. VPA-tuning flow 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. F vector Biomass (Tons) Scientific Advisory Committee Sub-Committee on Stock Assessment Working Group on Demersal Species

12. VPA-tuning flow 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 Committee Sub-Committee on Stock Assessment Working Group on Demersal Species

13. 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 Assessment Working Group on Demersal Species

14. N. ind. km-2 Recruits of sole Fall 2005 Size class range (TLmode+s.d.): Fall 2005 TL ≤ 18.5 cm Fall 2006 TL ≤ 17.5 cm Fall 2007 TL ≤ 20.0 cm Fall 2006 Fall 2007 Scientific Advisory Committee Sub-Committee on Stock Assessment Working Group on Demersal Species

15. N. ind. km-2 Recruits of sole vs bottom temperature and salinity °C °C Fall 2005 Fall 2006 S‰ S‰ Fall 2006 Fall 2005 Scientific Advisory Committee Sub-Committee on Stock Assessment Working Group on Demersal Species

16. Yield-per-recruit (Y/R) analysis Fbar F0.1 Fmax 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 Scientific Advisory Committee Sub-Committee on Stock Assessment Working Group on Demersal Species

17. Size frequency distributions from commercial fishery MLS = 20 cm 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 (20 cm) = 0.60 Fmax (16 cm) = 0.46 Scientific Advisory Committee Sub-Committee on Stock Assessment Working Group on Demersal Species

18. Management advice • 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. Scientific Advisory Committee Sub-Committee on Stock Assessment Working Group on Demersal Species

19. Management advice • 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 Committee Sub-Committee on Stock Assessment Working Group on Demersal Species

20. Management advice Thank you for your attention Scientific Advisory Committee Sub-Committee on Stock Assessment Working Group on Demersal Species