Population Resource Dynamics Of Blue Sharks In The Indian Ocean

The blue shark(Prionace glauca)is a major bycatch species in the tuna longline fishery,and the working party on ecosystems and bycatch(WPEB)of the Indian Ocean Tuna Commission(IOTC)has held several meetings to assess the stock status of the Indian Ocean blue sharks using an integrated age-structure model(SS3)and data-limit models(JABBA,ASPIC,SAR,CMSY).The model was used to estimate common biological reference points for fisheries management and to simulate various fishing strategies for the management of the blue sharks esources.In 2021,the results of SS3 assessment were applied to show that the current strock status of the blue sharks in the Indian Ocean is in a healthy state(B>Bmsy,F<Fmsy).results suggesting the stock is currently not overfished nor subject to overfishing,but with the trajectories showing consistent trends towards the overfished and subject to overfishing quadrant of the Kobe plot.As commercial fishing effort increases,there will be an impact not only on the evolution of the individual size of the fish,but also on the sexual maturity of the individual,resulting in earlier maturation of the individual.Therefore,excessive fishing pressure may force the blue sharks to change its reproductive strategy to maintain population stability.High discard rates result in reported landings that are much smaller than actual catches,and changes in hook mortality due to changes in fishing strategy.In addition to the lower reporting rate of nominal shark bycatch data,reported bycatch shark catches are thought to represent only those species that remain on board,and do not account for discard mortality,which has been an important parameter in the model.Understanding and mastering the reproductive biology and hooking mortality of blue sharks is an important foundation for stock assessment.Although some studies on reproductive biology and hooking mortality have been conducted in the past decades,they have mainly focused on waters outside the Indian Ocean,and the estimates of reproductive biology parameters and hooking mortality are in a relatively wide range.In order to obtain more accurate evaluation results,the selection of parameters should be more accurate.For example,the length of the primary sexual maturity fork in reproductive biology.In fact,because the accuracy of parameters is very important,this paper studies the reproductive biology of blue sharks.parameters and hook mortality,and incorporate the updated parameters into the JABBA-Select model to assess the Indian Ocean blue shark stock status.In order to formulate more effective protection and management measures to alleviate the further decline of population resources.Therefore,the second chapter of this paper is based on 412 blue sharks collected by scientific observers of tuna longline fishing trips in the Indian Ocean(10°N～35°S,24°E～90°E).Analysis of its reproductive biological characteristics to conduct a preliminary study.The results show as follows: 1)there was no significant difference in the distribution of fork length between males and females(P>0.05).The average forklength of male and female blue sharks was 189 cm and 182 cm respectively.2)The ratio of female to male blue shark was about 0.59:1,which was significantly different byχ2 test(P<0.05),but the ratio of male to female of fetus was consistent with 1-1 by χ2test;3)There was a significant linear correlation between the clasper length and the forklength of the male blue shark,Similarly,the shell gland widths also increases with the forklength of the female blue sharks;4)The number of Pups of gravid blue sharks was20 to 54,Pearson correlation analysis showed that litter size was related moderately to maternal size(P=0.002<0.05,Multiple R=0.645);In limited records,male pups were slightly larger in number and length than female pups’;Fork length of 50% maturity of female blue shark was almost 162.23 cm,Most sexually mature females have shell glands larger than 3cm wide,When the fork length reaches 177.04 cm pregnancy.When the clasper length reaches 14.1～15cm,the males begin to mature;The findings of the Indian Ocean blue shark analyzed in this chapter can provide basic reproductive biology information for assessing the population status of the Indian Ocean blue sharks.The third chapter of this paper is based on data of 778 blue sharks from 1272 fishing sets recorded by Chinese tuna longline observers in the Indian Ocean(17°N～11°S,40°E～90°E)from 2012 to 2019,a generalized linear model(GLM)was applied to analyze the at-haulback mortality(i.e.,the mortality rate when fish was captured and hauled back to the deck)and potential influencing factors of blue shark.The results was shown as follows: 1)The overall at-haulback mortality of blue shark was 13.4%,and that of males was higher than that of females;2)Among the potential influencing variables,hook type,hook size,individual size,capture depth and capture latitude had significant effects on the survival status of blue sharks(P <0.05),and hook type and capture latitude had the greatest effects;3)The interaction between hook type and latitude,gender and fork length also had significant influence on at-haulback mortality(P <0.05).When using olecranon hooks,the more northerly being caught,the lower mortality rate was observed;for the other three hook types,the more northerly being caught,the higher mortality rate was observed.The larger the female blue shark was,the lower the mortality rate was observed;however,there is no clear pattern between the mortality rate of male blue shark and their individual size.This study can provide scientific information for reducing the uncertainty of the fishing mortality estimated for blue shark and promoting the sustainable use of resources in this fishery.The fourth chapter of this paper is based on the Bayesian residual yield model JABBA-Select,considering fishing selectivity and life history information,the assessment of stock status shows that the probability of the stock is in a healthy state is 95.4%.