| Skipjack tuna(Katsuwonus pelamis)is a species with high economic value in the pelagic purse seine fishery,which enjoys abundant fishery resources.Kiribati is one of the important tuna fisheries in the Western Central Pacific Ocean(WCPO).Its fishing area of fishing vessels overlaps the Exclusive Economic Zone(EEZ)in which China is an important fishing country.Its fishery also contains a certain amount of bycatch species such as yellowfin tuna(Thunnus albacores)and bigeye tuna(Thunnus obesus).But there are few research reports on the status and causes of bycatch as well as the strategies related to avoiding it.Therefore,based on the catch data of the WCPO purse seiner Zhongtai No.3 in the waters off Kiribati between June 2019 and January 2021 and the ocean satellite remote sensing environmental factors of the fishing grounds,this study analyzed the status and causes of bycatch in this area.The study also investigated the strategies related to avoiding bycatch in combination with real-time environmental variables at fishing ground.Results provide a reference for the sustainable development and utilization of the Kiribati skipjack fishing grounds.The conclusions are as follows:In the first sction,we analysize the bycatch status of Skipjack purse seine fishery in Kiribati sea area.Based on the catch data of Zhongtai 3,a purse seine vessel in the sea area near Kiribati,the temporal and spatial changes of fishing vessel operation and fishing conditions,as well as the dynamic relationship between main catch(skipjack)and bycatch were analyzed.The results showed that,from June 2019 to January 2021,(1)the operation range of fishing vessels is 7.23° S-3.41° N and 152.23° E-179.82° E,and there is no significant difference in the spatial distribution of fishing grounds among months;(2)a total of 344 nets were operated,including 265 nets of log school and 79 nets of free school in which the the largest number of nets occurred in November 2020(30 nets);(3)catch for log schools occurs more than free schools which the proportion of log schools in month was more than 50% except for July 2019;(4)the success rate of log school is higher than that of free school,and the empty net rate of log school is less than 20% every month in comparison with the free school whose empty net rate fluctuated greatly.The results of catch conditions showed that(1)total catch has no obvious trend with months,but catch in June is highest in every year;(2)the total catch of log school in each month is greater than that of free school except for August 2019;(3)bycatch species are mainly yellowfin tuna and bigeye tuna.The correlation analysis of the monthly catch for the log school showed that the monthly variation of the catch and CPUE of skipjack and its bycatch(yellowfin tuna and bigeye tuna)are relatively consistent.In addition,we also found the proportion of larval in by catch was high.Connectivity of habitat distribution for skipjack,as well as the maine bycatch speciese,yellowfin tuna and bigeye tuna in the Kiribati skipjack seine fishing ground was studied in Sention 2.Based on the data of the high-yield area of these three species from June to December during 2019-2020 and the environmental factors(Sea surface temperature,SST,Sea surface height,SSH,and Sea surface salinity,SSS and Temperature at 75 m depth,T75),this study used the maximum entropy model(Max Ent)to simulate the habitat hotpot distribution for these three species.Besides,the study also evaluated the importance of environmental factors.Results showed that the Max Ent provides a good prediction for the potential habitat distribution of these three species with the AUC value greater than 0.8.But some models for individual months gave a poor prediction.From June to December during 2019-2020,SSS was the primary factor influencing the distribution of hotspot areas for skipjack tuna,yellowfin tuna and bigeye tuna habitats,with an average contribution of 47.28%,41.98% and 40.10%,respectively,but the contribution varied with month.The results showed there was large overlaps between potential hot spot habitat areas between skipjack,yellowfin tuna,and bigeye tuna along month.In section 3,the relationship between CPUE and temporal-spatial factors and realtime environmental variables was explored.Based on the catch data of Zhongtai 3in the sea area near Kiribati from June 2019 to January 2021,this study used the CPUE of skipjack and its bycatch species(yellowfin tuna and bigeye tuna)in the log schools used as the index of catch status.Combined with the generalized additive model(GAM),the influence significance and differences of real-time environmental variables in fishing ground(water temperature and salinity at different depths and thermocline depth),temporal and spatial(year,month,longitude and latitude)factors on the CPUE of skipjack,yellowfin tuna and bigeye tuna were analyzed.We found that CPUE was affected by time factors(year and month),spatial factors(longitude and latitude)and various real-time environmental factors in fishing grounds,but the interpretation rates for these three fish species were low(skipjack: 26.4%;yellowfin tuna: 41.3%;bigeye tuna: 27.8%);The model suggested that the significance and mode of influence for each factor on three species CPUE was different.There were no real-time environmental variables at the fishing ground that affected these three species CPUE at the same time.The salinity at 150 m depth had a significant effect on skipjack CPUE(P<0.01);The temperature and salinity of 50 m and 100 m depth had significant effects on CPUE of yellowfin tuna(P<0.05);The depth of the upper boundary of thermocline significantly affected the CPUE of yellowfin tuna and bigeye tuna(P<0.01).In section 4,the reason for the length differences of skipjack,yellowfin tuna and bigeye tuna in the Kiribati skipjack seine fishing ground based on real-time environmental variables in fishing ground.This study used GAM model to analyze the relationship between the length differences of skipjack,yellowfin tuna and bigeye tuna in the Kiribati skipjack seine fishing ground and real-time environmental variables in fishing ground.The results showed that longitude and latitude have significant effects on the body length of skipjack,yellowfin tuna and bigeye tuna catch.In latitude,when the fishing operation was closer to the equator,the body length of skipjack catch was larger but the body length of yellowfin tuna catches decreased.For bigeye tuna,when the fishing operation was closer to the equator,the body length of catch decreased.In longitude,with the longitude shifting eastward,the body length of skipjack and bigeye tuna catch increased,but the body length of yellowfin tuna catch first increased and then decreased.Among the real-time environmental variables at the fishing ground,the water temperature of 100 m and 150 m depth and the thermocline depth significantly affect the catch body length of skipjack,yellowfin tuna and bigeye tuna,which were common factors.The response of the catch body length of the three species was different with the change of real-time environmental variables at the fishing ground.From the relevant results of GAM model,the relevant strategies to avoid bycatch in the Kiribati skipjack seine fishing ground can be put forward based the real-time environmental variables of the fishing ground,including(1)in order to avoid the simultaneous capture of too many yellowfin tuna and bigeye tuna,it is necessary to pay attention to the variation of the thermocline,the depth of the upper boundary of the thermocline increases,and the possibility of yellowfin tuna and bigeye tuna catch increases;(2)in order to avoid catching more larval in bycatch,the environmental factors such as location,net depth and water temperature should be comprehensively considered during operation.The operation can be concentrated in the waters near the equator of the Western Pacific,with a water depth of about 100 m,a water temperature of about 28 ℃ and a deep thermocline. |
