A Multi-Model Approach To Assess Atlantic Bonito(Sarda Sarda,Bloch 1793) In The Eastern Atlantic Ocean

The Senegalese coasts have high productivity of fishery resources due to the frequency of intense up-welling system that occurs along its coast,caused by the maritime trade winds making its waters nutrients rich.Fishing plays a primordial role in Senegal’s socioeconomic plans and food security.However,a global diagnosis of the Senegalese maritime fishing sector has highlighted the problems that this sector suffers.Among these concerns,certain significant stocks need to be evaluated,which are a priority target for artisanal fishing.If no efforts are made in this direction,most stock will be overexploited or even in decline.It is in this context that this research was initiated.Therefore,this dissertation aims to apply a multi-modal approach（LBB,LBSPR,CMSY,JABBA,and JABBA-Select）to assess the stock of Atlantic bonito,Sarda sarda（Bloch,1793）in the Senegalese Exclusive Economic Zone（SEEZ）.Artisanal fisheries heavily harvest Atlantic bonito in this area.Gillnets,purse seines,longlines,and sleeping nets are the main gears used in artisanal fisheries to harvest this small tuna.Available catch,effort,and size data from Atlantic bonito over 15 years（2004-2018）were used to calculate the nominal and standardized CPUE,size-frequency distribution,and length at retentions（50%and 95%selectivity）of the species.These relevant results were employed as input parameters for stock assessment models mentioned above to define the stock of this species in this region of the Atlantic Ocean.The retention length at 50%and 95%selectivity for each year and gear was calculated from the cumulative length frequency distribution.Optimal model,standardization CPUE results showed a significant trend for gillnets,ranging from 4.6 kg/trip in 2005 and 92.65 kg/trip in 2018,while sleeping nets showed lower CPUE values,ranging from 2.12 kg/trip in 2004 up to 0.83 kg/trip in2018.For the length-based analysis models（LBB and LBSPR）,approximately 55159specimens caught by four principal fishing gears between 2004 and 2018 are sampled and considered in the analyses.From these samples,Atlantic bonito size frequency,length distribution,and retentions length at 50%and 95%selectivity were determined and used as input parameters into these length-based models.The LBB model indicated an Atlantic bonito healthy stock status with B/B_MSYvalues ranging from 1.3 to 1.6 and B/B0 values varying from 0.47 to 0.61 of the main scenarios performed（BON＿AFG＿CL,BON＿GN＿Length,and BON＿PS＿Length）.The LBB results,however,indicated that longlines and sleeper nets seriously impact this species’stock in this area,demonstrating,respectively,a slightly overfished and overfished stock status.The results estimated by LBSPR showed that the current SPR of the stock is above the target reference point（SPR of 40%and the limit reference point of SPR of 20%）.For LBSPR,the biomass relative B/B_MSYis above the reference value（1）.These models indicated a healthy stock status for this species in the SEEZ.Notwithstanding these results,some fishing gears（sleeping net,purse seine,and longline in this case）would deserve particular concern in establishing management measures as they harvest more juveniles and individuals of bonito that do not reach their first sexual maturity.The data moderated catch-based models（catch-only-based CMSY model and its most recent version（CMSY++）,JABBA,and JABBA-Select）used Atlantic bonito catch and standardized CPUE data main inputs parameters to provide essential reference points valuable to understand this stock species level.As length-based models,the catch-based models also indicated that the SEEZ Atlantic bonito stock is in sound condition.Catch-only-based CMSY model and its most recent version（CMSY++）The CMSY model results demonstrate that the SEEZ Atlantic bonito stock is in a sound condition in the final year of the main scenarios analyzed（BON,BON-bt,BON-GN-bt,and BON-PS-bt）with sustainable relative stock biomass(B₂₀₁₈/B_MSY=1.13 to 1.3)and fishing pressure levels(F₂₀₁₈/F_MSY=0.52 to 1.43).However,the CMSY results showed that longlines and sleeping nets negatively affect this species’stock in this area,showing a slightly overfished and overfished stock status,respectively.The MSY results evaluated by JABBA ranged between 4,885 and 6,520 tons.In the JABBA-Select reference and base case scenarios,the MSY values ranged between 3,796to 4,671 and 4,179 to 4,434 tons,respectively.The B/B_MSYand F/F_MSYresults for the JABBA model ranged between 2.01 to 2.14 and 0.47 to 0.33,respectively.The B₂₀₀₄/K range was 0.55-0.62 and 0.80-0.86 for B₂₀₁₈/K.The estimated B/B_MSYand F/F_MSYfor JABBA-Select ranged from 1.91 to 1.92 and 0.52 to 0.54,respectively,from 0.76 to 0.69for B₂₀₀₄/K and 0.77 to 0.76 for B₂₀₁₈/K.The Kobe phase plots results of the base case scenarios ranged from 75%to 89%probability in the green area,indicating sustainable fishing pressure and an Atlantic bonito healthy stock size capable of producing high yields close to the MSY.Based on the stock assessment results,scientific advice for temporary management measures was proposed in this dissertation to avoid the overexploitation of the Atlantic bonito in the SEEZ.As a recommendation,this study suggests an improvement of the selectivity parameters of longlines and purse seines and a temporary prohibition of using sleeping nets in the fishery for the Atlantic bonito stock in the SEEZ based on the results of the length-base models.Besides,this study recommends holding the total annual catch to a tonnage less than or equal to 4,625 tons to maintain the stock in sound condition until2030 based on catch-based model projections results.Although these actions are temporary,they can be essential to reduce or avoid intense pressure on the Atlantic bonito stock in the SEEZ.However,it is necessary to conduct an exhaustive study integrating all the results from the different models to provide coherent and solid scientific information that leads to appropriate decision-making for rational and sustainable exploitation of Atlantic bonito in the SEEZ and the whole Eastern Atlantic Ocean area.