Vertical Distribution And Integrated Habitat Index For Albacore(Thunnus Alalunga) In The Southeast Pacific Ocean

Vertical distribution and habitat characteristics of pelagic species are important information for the understanding of ecosystem structure. They also provide important information for fishery management of target species, and for the development of effective measures to mitigate bycatch. Based on fishery data and environmental data collected by China’s tuna longline fishery observers in the southeast Pacific Ocean, the vertical distribution of albacore(Thunnus alalunga), the relationship between albacore vertical distribution and environmental factors, and the integrated habitat index of albacore were evaluated in this thesis. The main results and conclusions for these three questions are as follows:(1) Vertical distribution of albacore.Based on depth data of longline hook with TDRs and capture position, collected by on board scientific observer from September 2013 to January 2014, the shoaling rate of longline hook and vertical distribution of albacore were analyzed. The differences in depth distribution of albacore and bycatch species were compared. The results showed that the relative shoaling rate range of longline hook was 8.9%-17.1%, and the average relative shoaling rate was 13.5%. The depth range of albacore capture was between 74.3m and 296.5m, with mean depth of 214.3m. The mean depth and depth distributions of albacore were significantly different from that of bycatch species(P<0.05), including bigeye tuna(Thunnus obesus),skipjack tuna(Katsuwonus pelamis), blue shark(Prionace glauca), swordfish(Xiphias gladius) and so on, except for yellowfin tuna(Thunnus albacares) and striped marlin(Tetrapturus audax).(2) Relationship between vertical distribution of albacore and environmental factors.Based on fishery data and environmental data, collected by longline observers, the relationship between albacore vertical distribution and the environmental factors was analyzed. The results showed that the optimal swimming depth, water temperature, salinity, and dissolved oxygen range for albacore was found to be 190-220 m,19-21℃, 35.0-35.6, and 5.4-5.6, respectively. The common swimming depth, water temperature, salinity, and dissolved oxygen range was 160-250 m,15-21℃, 34.8-35.4, and 5.0-5.6, respectively. The catch rates of albacore in and out of the thermocline showed significant difference.(3) Integrated habitat index for albacoreBased on catch data of albacore and environmental data(water temperature, salinity, and dissolved oxygen) at 61 fishing sites, also collected by longline observers, Integrated Habitat Index(IHI) models were developed by the quantile regression method. IHI Model was developed for each 40 m-depth water stratum from 60 m to 300 m, and for the entire water column to predict the spatial distribution of albacore. The Wilcoxon test was used to test the difference between the predicted CPUE and the observed CPUE for single water stratum and the entire water column. The pearson correlation coefficients were used to define the models’ predictive power.Data from the other 23 fishing sites were used to validate the IHI models. The Wilcoxon test was used to compare the predicted CPUEs and the observed CPUEs for these sites. The results showed:(1) The predicted CPUE and the observed CPUE had high relevance, and the IHI models had strong predictive power;(2) The influential environmental factors in the IHI models among the depth strata were different;(3) The optimal inhabit depth of albacore was from 180 m to 220 m;(4) The albacore IHIs in the areas of 15°S-18°S, 104°W-110°W, and 18°S-22°S, 107°W-117°W were relatively high. The results suggest that fishing gear should be deployed at a depth of 180-220 m to increase CPUE of albacore and mitigate bycatch.