Study On The Influence Of Subsurface Environment On The Vertical Distribution And Longline Catch Rate Of Yellowfin Tuna In The Tropical Central And Western Pacific Ocean

Yellowfin tuna(Thunnus albacares)have high-speed and deep water swimming ability,with a unique physiological feature,which allows them to tolerate low ambient temperature and oxygen levels,display a distinctive depth distribution and vertical movement patterns.They usually remain in a surface layer of uniform temperature at night.They characteristically descend to a depth well below the thermocline to prey on the small nektonic organisms of the deep scattering layer(DSL)after dawn.The habitat and vertical swimming of yellowfin tuna have an impact on surface fishing vessel operations.In the area where yellowfin tuna is suitable for vertical habitation and compressed vertical space,the population of fish is relatively densely distributed,which is good for surface fishing vessels,and the catch rate is high;otherwise,the catch rate is low.The efficiency of longline gear differs depending on the depth of hooks and their relationship with the swimming depth of the fish.When the depth of longline fishing hook is consistent with the active water layer of tuna,CPUE is a good indicator of the abundance in fishery resources.Considering the environmental variables that influence the vertical distribution of tuna in CPUE standardization could improve the CPUE analysis results,otherwise it will lead to misunderstanding of the catch rate.The vertical distribution influence by the subsurface water temperature.Investigating the impact of subsurface water temperature on fishing catch rate can not only improve the fishing efficiency of the longline fishing,but also provide technical support for resource assessment.The Central and Western Pacific is one of the sea areas with the most abundant tuna resources and production.Under the background of the depletion of China's offshore fishery resources,it is urgent to further develop and utilize the fishery resources in this area.Most of paper study on the relationship between the spatiotemporal distribution of yellowfin tuna and the surface environment,and the research on the influence of subsurface environment on its vertical distribution drive and catch rate is few.The subsurface environment influence the longline catch rate by affect the vertical swimming of yellowfin tuna.From the perspective of the effect of water temperature on fish vertical movement,a longline catch rate mode was constructed by taking the subsurface environmental variable which influence the vertical water distribution of yellowfin tuna as input variables,to examine the relationship between the habitat space distribution of yellowfin tuna with the catch rate in the tropical central and western Pacific.Providing theoretical support for longline tuna fishing operations and resource conservationFirstly,the Argo data were adopted to mine the spatial information of vertical structure of water temperature,such as the thermocline,mixed layer,maximum temperature and gradient,vertical temperature difference of 7-9? from sea surface temperature,water temperature of water layer at 10-400 m interval 50 m,and 9-18?isolines.Analysis the temporal and spatial distribution pattern of key environmental variables that affect the vertical distribution of yellowfin tuna by plotting the spatial contours map.The GAM model was used to construct a non-linear response relationship between the subsurface environment variables and the CPUE.The key subsurface environment variables that affected the vertical movement of yellowfin tuna were excavated and identified,and the effect of the key subsurface environment variables on the CPUE was analyzed.Because the thermocline play an important role on the vertical distribution of yellowfin tuna,spatial statistical methods was used to analyze the impact of the spatial distribution of key subsurface environments such as the thermocline on the central fishing ground.The suitable water temperature for pacific yellowfin tuna is 17.5 ?;the tolerable limit water temperature is 10.5 ?.A temperature difference of 8? from the SST(note as delta 8?)affects the vertical distribution of Atlantic yellowfin tuna.Quantifying the suitable habitat space of yellowfin tuna with thermocline,delta 8 ? and 17 ? isotherm depth and other subsurface environmental variables to investigate the suitable horizontal and vertical spatial distribution of yellowfin tuna and its impact on longline catch rate.The main results of this study are as follows:(1)The seasonal changes of the key subsurface environmental variables of the fishing grounds showed that obvious seasonal zonal striped distributions were found in the upper boundary depth,temperature of the thermocline and Delta 8? which has the depth pattern by deep in winter and shallow in summer.From January to March,the upper boundary depth is deeper than 90 m to the north of the Equator,while shallower than 60 m to the south of 15°S.The opposite happens from July to September.The upper boundary temperature is more than 28 ? between 20°S-20°N and west of150°W.The depth of Delta 8? shows seasonal variation by shallower than 150 in the eastern equator Pacific with westward distribution,and deeper than 250 m to the north of 20°N and to the south of 20°S.The high value(>280m)of the lower boundary depth of thermocline shaped northeastern drifts from the equator to high latitude.And a shallow depth region occurred between the deeper depth regions in the eastern Pacific Ocean.Moreover,the same pattern was found in the lower boundary temperature of thermocline.Besides the high value shape,the lower boundary temperature greater than 17?.The lower boundary temperature is between 13-15?in the eastern Pacific.The isothermal depth of 12? is deeper than 400 m to the north of 15°N and to the south of 15°S.The isothermal depth of 12? is about 300 m in eastern Pacific like tongue East-West distribution.Between the two areas,the isothermal depth of 12 ? is shallow than 200 m.The topography of the lower boundary depth and the temperature of the thermocline and the isothermal depth of12? showed little seasonal variability on the annual mode.(2)The influence of subsurface environment variables on CPUE analysis showed that the environmental factors and spatiotemporal variables have significant effects on the spatial distribution of yellowfin tuna catches in longline fishing.The results revealed that the cumulative deviances obtained using the selected GAMs were 48.3%.The CPUE of the longline yellowfin tuna in the tropical central and western Pacific has risen rapidly after 2012.The high catch rate appears in the northern hemisphere summer,and the space is near 10 °S,160 oW.The thermocline has significantly nonlinear effect on the CPUE.The CPUE decreased as the value of temperature and depth of the upper boundary of the thermocline become higher.The strong associated relationship between the upper boundary temperature and depth with CPUE were 27-28 ? and 70-90 m,respectively.High catch rates were observed where the lower boundary depth of thermocline at 250-280 m.Besides this range,the hooking rates decline.The CPUE fluctuated while the lower boundary temperature of thermocline lower than 18?,and then increase as the temperature value become higher.As the thickness of the thermocline increases,the CPUE value gradually increases.The strong associated relationship between CPUE with the thickness and low temperature of thermocline were 13-16 ° C and 40-70 m.The effect of the 13 ° C isotherm depth on the CPUE is a non-linear oscillation increase.The effect of 16 ? isotherm depth on CPUE is non-linear decreasing.The effect of 18 ? isotherm depth on CPUE is dome-shaped.The strong associated relationship between CPUE with the 13 ?,16 ? and 18 ? isotherm depths were at 300 m,250 m and 220 m,respectively.The nonlinear effects of the relative depth between delta 8? and the lower depth of thermocline likes a dome,and the strong associated relationship located at 70 m.The results suggest that the influence of delta 8? isolines and thermocline were cross.The longline catch rates increase as the relative depth increase to 70 m,while the depth greater than 70 m,the catch rates became low.The catch rates reach maximum when the vertical habitat was compressed to consistent with hooking depth.The catch rates could be changed by adjust the depth of hooks.(3)Relationship between the temporal-spatial distribution of longline fishing grounds of yellowfin tuna and the thermocline characteristics in the Central Pacific Ocean showed that the fishing grounds distributed where the upper boundary temperature of the thermocline was higher than 26 ?,but CPUE was lower than Q3 while temperature higher than 30 ?.The fishing grounds located between the two high value shape of the lower boundary depth of thermocline,if the depth was >300 or<150m,the CPUE tended to be low.The lower boundary temperature of the thermocline in the fishing grounds was lower than 13 ? all year in the equatorial zone.Conversely,if the temperature was higher than 17?,the hooking rates are very low.Frequency analysis and the empirical cumulative distribution function(ECDF)were used to calculate the optimum range of thermocline characteristics.The optimum depth range of the upper boundary temperature and depth were 27-29.9 ?and 70-109 m.The optimum depth range of the lower boundary and the lower boundary temperature were 250-299 m and 11-13.9 ?.(4)Spatial analysis of suitable habitat for yellowfin tuna in the tropical Central and Western Pacific Ocean showed that the delta 8 ? isothermal depth distribution showed obvious seasonal variation,roughly in the zonal striped distribution on the annual mode.The spatial distribution of the delta 8 ? isothermal depth is deep in winter and shallow in summer.The CPUE is highest in areas where the 17 ? isothermal depth ranged from 180 to 279 m,the average depth was 228 m.if the depth of 17 ? isothermal is greater than300 m or shallower than 150,the CPUE tends to be low.Similarly,the highest CPUEs were observed in areas where the delta 8 ? isothermal depth ranged from 120 t-239 m,the average depth was 186 m.The horizontal distribution of high hook rates was concentrated in the area between5°S and 10°N.The vertical distribution of Thunnus albacares in the Central and Western Pacific Ocean was affected by the 17 ? and delta 8 ?isothermal depth.and the foraging depth of Thunnus albacares was affected by the low boundary temperature and depth of the thermocline.Frequency analysis and the empirical cumulative distribution function were used to compute the optimum range of subsurface factors.The optimum ranges for 17 ? and delta 8 ? isothermal depths were 160–300 m and 160–240 m,respectively.The optimum depth difference ranges were 0–70 m for 17 ? isothermal depths and 30–109 m for delta 8 ? isothermal depths.