Study On Cetacean Bycatch In Tuna Fisheries Of The Western And Central Pacific Ocean

Cetacean play key roles in the world’s ecosystems.However,it is well established t hat many populations are at risk from anthropogenic disturbances.Global cetacean byca tch,the incidental catch of unused or unmanaged cetacean species,is a major threat to th e world’s oceans.Assessments of fisheries interactions with cetacean species are crucial for quantifying anthropogenic threatening processes and informing management action s.This study is based on the Ecological Risk Assessment for the Effects of Fishing meth od to assess impact of tuna fishery on ceatacean in the Western and Central Pacific Ocea n and Southwest Indian Ocean.The ERAEF framework involves a hierarchical approach that moves from a comprehensive but largely qualitative analysis of risk at Level 1,qua litative methods are used to sort out the cetacean species that are non-Target bycatch in t he tuna fishery in the Western and Central Pacific Ocean.Then through a more focused and semi-quantitative approach at Level 2,semi-quantitative risk assessment of cetacean species affected by tuna purse seine and longline fisheries in the WCPO is carried out u sing the productivity sensitivity analysis（PSA）method.Finally,there are highly centrali zed and completely quantitative"model-based"approach at Level 3,the generalized add itive model（GAM）is used to analyze and quantitatively study the relationship between cetacean bycatch in tuna longline fisheries and the space-time and environmental factors,INLA-SPDE is used to analyze and predict spatiotemporal changes in cetacean bycatch hotpots in the WCPO,PBR is used to solve the problem of setting the cetacean mortality limit.Through qualitative,semi-quantitative and quantitative methods,the cetacean spe cies affected by fisheries were systematically analyzed,the environmental factors during the process of cetacean bycatch were explored,and the hotpots of cetacean bycatch wer e predicted,which has scientific guidance and practical significance for the sustainable d evelopment of fisheries and the protection and monitoring of cetacean species resources.The main results are as follows:（1）According to the observer data during 2013-2020 obtained from Western and Central Pacific Fisheries Commission（WCPFC）,the potential risk to cetacean species from direct interaction with two type tuna fisheries was assessed using a semi-quantitative based on a Productivity Susceptibility Analysis（PSA）.Data Quality Scores and sensitivity methods were incorporated to identify potential sources of uncertainty relating to the quality of the attribute data and the selection of attributes.The results demonstrate that the number of both individual and species of cetaceans affected by purse seine fisheries are higher than those by tuna longline fishing,but there is greater uncertainty in the risk assessment of tuna purse seine fishery.In tuna purse seine fishery,only Dwarf sperm whale（Kogia sima）and pygmy sperm whale（Kogia breviceps）are classified as posing potentially low risk,while in tuna longline fishery,10 species are considered as posing potentially low risk,such as Blaninevill’s beaked whale（Mesoplodon densirostris）,Bryd‘s whale（Balaenoptera edeni）,Cuvier’s beaked whale（Ziphius cavirostris）,Minke whale（Balaenoptera acutorostrata）,Pygmy sperm whale,Sei whale（Balaenoptera borealis）,Short-beaked common dolphin（Delphinus delphis）,Dusky dolphin（Lagenorhynchus obscurus）,Fraser’s dolphin（Lagenodelphis hosei）and Long-beaked common dolphin（Delphinus capensis）.For cetaceans that may be underestimated,it is necessary to further strengthen monitoring and assessment of relevant stock dynamics with other cetaceans that are classified as moderate and high risks.Documenting the quality of data used to produce the productivity score allows research efforts to focus on areas where data are lacking,or of poor quality,and allows future iterations of the PSA process to incorporate higher quality data as it becomes available.（2）Cetacean bycatch and its relationship between the spatiotemporal and environmental factors in tuna fisheries in WCPFC were analyzed by using the generalized additive model（GAM）.The results showed that a total of 260 cetaceans,including19 species belonging to 4 families and 2 orders,were incidentally caught in tuna longline fishery.Main cetacean bycatch species were identified as False killer whale（Pseudorca crassidens）,Bottlenose dolphin（Tursiops truncatus）,Risso’s Dolphin（Grampus griseus）、Short-finned pilot whale（Globicephala macrorhynchus）,Rough toothed dolphin（Steno bredanensis）.While the results also showed that cetacean were incidentally caught in tuna purse seine fishery,including 30 species of 2 orders,5 families.Main cetacean bycatch species were identified as False killer whale（40.76%）,Rough-toothed dolphin（Steno bredanensis）（15.50%）,Short-finned pilot whales（Globicephala macrorhynchus）（7.31%）,Bottlenose dolphin（Tursiops truncatus）,Spinner dolphins（Stenella longirostris）（4.68%）,Bryd’s whale（4.51%）and Sei whale（4.31%）.The rate of deviance explained about BPUE（Bycatch per unit effort）amounted to 71.7%in tuna longline fishery,and longitude was the maximal contribution factor with contribution rate of 34.3%.The results of stepwise GAM showed that the relative importance of the factors affecting the BPUE listed in the order of:longitude,sea surface temperature,Chlorophyll-a concentration,latitude,CPUE（Catch per unit effort）.The hot spots of cetacean bycatch are mainly determined by spatial and environmental factors,and the relative abundance of cetaceans was mainly affected by Sea surface temperature,and the distribution of cetaceans was affected by chlorophyll a concentration.Meanwhile,the rate of deviance explained about BPUE（Bycatch per unit effort）amounted to 65%in tuna purse seine fishery,and free school sets（Set＿una）was the maximal contribution factor with contribution rate of 27.8%.The results of stepwise GAM showed that the relative importance of the factors affecting the BPUE listed in the order of:Species,CPUE,longitude,log sets（Set＿log）,SST,SSS,a FAD sets（Set＿afad）,the deviance explained are respectively 15.8%,12%,4.2%,1.8%,1.4%,1.2%and 0.5%.The concurrent cetacean bycatch of tuna purse seine fishery is mainly concentrated in 10°N-10°S,130°E-140°E.The interaction between the cetacean and the tuna fishery may be driven by the distribution of prey,resulting in the similar performance of the free school sets group and tuna purse seine sets on dolphin group of cetacean bycatch.（3）Based on integrated nested Laplace approximations（INLA）and stochastic partial differential equations（SPDE）to understand and predict the spatial changes in cetacean catch might be the key to mitigation.Results indicate that cetacean bycatch is greatly affected by Temporal and spatial elements,and there are seven main hotpots in Western and Central Pacific Ocean,which are 1)5°N-10°N,155°E-160°E;2)10°S-15°S,185°E-190°E;3)10°N-15°N,185°E-190°E;4)10°S-15°S,200°E-205°E;5)5°S-10°S,217°E-220°E;6)5°S-10°S,226°E-230°E;7)35°N-40°N,165°E-170°E.In addition,there is a positive correlation between the phenomenon of cetacean bycatch and sea surface temperature（SST）,and the cetacean bycatch increases with the increase of SST between 20-30℃,while the concentration of chlorophyll-a（Chl-a）.the number of hooks and the catch of fishing effort have little impact on the cetacean bycatch.The inter-annual variation of the cetacean bycatch hotspots is relatively weak,but considering the impact of temporal factor on the cetacean bycatch,more detailed records of the date are needed.While there are two main hot spots in tuna purse seine fishery:1)7°N-15°N,132°E～140°E;2)0°-8°N,154°W-142°E.The situation of cetacean bycatch varies among different species,among which the bycatch rate of cetacean in Rough-toothed dolphin and False killer whale is high.While the population bycatch rates of Blaninevill’s beaked whale,Cuvier’s beaked whale,Blue whale（Balaenoptera musculus）,Bryd‘s whale,Striped dolphin（Stenella coeruleoalba）,Pacific White-sided dolphin（Lagenorhynchus obliquidens）,Fraser‘s dolphin Killer whale（Orcinus orca）,Northern right whale dolphin（Lisodelphis Borealis）,Sperm whale（Physteter macrocephalus）and Ginko-toothed beaked whale（Mesoplodon ginkgodens）are low,and the cetacean bycatch rates of the other 16 populations are between 0.715（±0.321）and1.615（±0.661）.The predicted cetacean bycatch hotspots can limit the fishing activities at the space with high risks area.Results also suggested that fishing fleets may shift from the actual bycatch hotspot to neighboring areas and/or different periods,which could be an efficient strategy to mitigate cetacean bycatch as well as maintaining sustainable development for tuna longline fishery.（4）Potential Biological Removal（PBR）is the mortality limit to maintain the sustainability of the population.PBR was calculated as the product of a minimum population estimate(N_MIN),one-half of the maximum net productivity rate(R_MAX),and a recovery factor（F_R）.The setting of the three parameters values will lead to the deviation or error of the estimation of the mortality limit.Results indicate Mortality limits were evaluated based on whether at least 95%of the simulated populations the criteria which that populations starting at 30%of carrying-capacity（K）recovered to at least MNPL after100 yr.Simulations of populations that experienced mortality equal to the mortality limit indicated that using approximately the 5th percentile of the abundance estimate for N_MINand value of 0.5 for F_R met the criteria during bias trials.It is suggested that the values of N_MIN and F_R can be adjusted according to the accuracy of population abundance estimation and different conservation objectives to determine the mortality limit,thus providing scientific basis.