A Study On Bycatch Of Cetaceans By Purse Seine Fishery In The Western And Central Pacific Ocean

The Western and Central Pacific Ocean is one of the major producing areas of tuna in the world,and the tuna catch in 2020 is about 2.6 million metric tones,accounting for about 52% of the world tuna catch,and the tuna purse seine is its main fishing method.In addition,increasing attention has been paid to fisheries interactions with cetacean populations,however the magnitude of cetacean interaction with fisheries is still poorly known.Based on fishery and bycatch data and annual reports released by the Secretariat of the Western and Central Pacific Fisheries Commission(WCPFC),we get some results as follows.1.Overview of tuna purse seine bycatch in the Western and Central Pacific OceanThe results showed that a total of 9 881 cetaceans classified as 30 species interacted with purse seine fishery,belonging to 2 orders,5 families and 19 genera.In addition,eight bycatch data(include 295 individuals)for cetaceans were recorded in different taxonomic ranks due to the complexity of at-sea operations,etc.The annual bycatch rate ranged from 1.781 per hundred sets to 4.214 per hundred sets.The interactions were mainly distributed in the sea waters around Papua New Guinea,Solomon Islands,Micronesia,Marshall Islands,Nauru,Kiribati,and Tuvalu(10°S~5°N,130°E~180°W).The main cetaceans that interacted frequently with purse seine were false killer whales(Pseudorca crassidens),rough-toothed dolphins(Steno bredanensis),shortfinned pilot whales(Globicephala macrorhynchus),bottlenose dolphins(Tursiops truncatus),spinner dolphins(Stenella longirostris),bryde’s whales(Balaenoptera edeni),sei whales(Balaenoptera borealis)and indo-pacific bottlenose dolphins(Tursiops aduncus),of which the rough-toothed dolphins and bottlenose dolphins were vulnerable to purse seine.In addition,there were significant differences(P < 0.05)in bycatch and net bycatch rates between odontoceti and mysticeti.The annual BPUE of baleen whales varied little and was at a low level,with an average BPUE of 0.28 per hundred sets.While the annual BPUE of toothed whales ranged from 1.508 to 3.845 per hundred sets,and the trend was more consistent with the trend of BPUE of cetaceans throughout the operational area;The net bycatch rate of toothed whales was significantly higher than that of baleen whales,with an average net bycatch rate of 0.002 per hundred sets for baleen whales,and in addition,no mortality of baleen whales was recorded in 2013,2015 and 2020.The highest and lowest net bycatch rates for toothed whales occurred in 2013 and 2020,at 0.766 and 0.060 per hundred nets,respectively.With an overall decreasing trend between 2013 and 2017,and to 0.087 per hundred nets in 2017,and then followed by an increasing trend in 2018 and 2019.Furthermore,the bycatch rates of bottlenose dolphins,short-finned pilot whales and sei whales were positively correlated with tuna catch rates.On the contrary,the bycatch rates of sperm whales(Physeter macrocephalus)and risso’s dolphins(Grampus griseus)were negatively correlated with tuna catch rates.2.Impact of environmental factors on bycatch of cetaceans in purse seine fisheriesFor the Delphininae and Mysticeti,the optimal GAM model deviance explained 40.9% of BPUE,and the largest contribution was made by chlorophyll with 19.3% explanation,accounting for 47.2% of the total bias explanation.Longitude,sanility,sea surface temperature,chlorophyll and CPUE of skipjack and yellowfin could influence BPUE of cetaceans;For Globicephalinae,the optimal GAM model deviance explained 11.0% of BPUE,and the largest contributor was sanility with 4.8% of the total deviation explanation,accounting for 43.6% of the total deviation explanation.For sperm and beaked whales,the optimal GAM model deviance explained 67.1% of the BPUE,with the largest contribution from chlorophyll at 23.3%,accounting for 34.7% of the total bias explained.The result of F-test showed that longitude,sanility,sea surface temperature and chlorophyll had an effect on BPUE.3.Management of cetaceans bycatch by purse seine fisheries in the Western and Central PacificThe results show that,according to the annual reports submitted by related countries(regions),there is a trend of refinement in the collection of data on cetacean bycatch since the conservation management measure CMM 2011-03 came into effect in 2013.But there may be some problems.A total of 3,225 cetaceans were recorded as bycatch during this period,which is somewhat different from the bycatch data published by WCPFC.Probably because the data published by WCPFC include cetaceans inside and outside the purse seine,and reports submitted by some countries(regions)only analyze cetaceans inside the purse seine.Another important reason is that the United States in the early years and the Philippines used word ‘instances’,and Japan used word ‘times’ to describe cetacean bycatch events in their annual reports.But in each event of ‘instance’ and ‘times’,we don’t konw how many individuals were incidentally catched.We suggested that in the management of tuna purse seine fisheries,training for crew’s ability to increase the understanding of cetacean habitat and life history should be strengthened.The results of this study may improve our understanding of cetacean bycatch and facilitate to develop more adequate mitigation measure for cetaceans in the Western and Central Pacific Ocean.