Analysis Of At-haulback Mortality And Influencing Factors Of Swordfish(Xiphias Gladius) In The Indian Ocean Based On Chinese Scientific Observers Data

Swordfish(Xiphias gladius)is the top predator in the food web observed in pelagic fisheries.In addition,it is also a very important bycatch in tuna longline fishery.The 19 th Working Party on Billfish of the Indian Ocean Tuna Commission(IOTC)conducted a stock assessment of swordfish,showing that although the population is not currently overfished and overfishing,but the risk of overfishing and overfishing is high,which is great significance to the development of Conservation and Management Measures.Studying athaulback mortality of swordfish is very important for assessing the impact of longline fishing on the sustainable development of bycatch resources.It can also provide a basis for how to effectively reduce the negative impact of fishing bycatch on marine ecosystems.At present,reports of swordfish at-haulback mortality are mostly concentrated in the Atlantic Ocean,while information on this matter in the Indian Ocean tuna longline fishery is still very limited.This information is essential for the development of Conservation and Management Measures,such as minimum landing sizes(MLS).This study is based on data from the project of Chinese distant-water fishery observer.A generalized linear model was used to analyze the effects of various factors,such as quarter,hook type and Lower JawFork Length(LJFL),on the at-haulback mortality of Indian Ocean swordfish.The results of this study are:1)The results of the study showed that the overall mortality rate of swordfish was64.0%,which is close to the at-haulback mortality rate of 67.5% for North Atlantic longline swordfish,but it is significantly lower than other studies in the Atlantic,which is about 85.0%and higher than the average level of 43.2% of swordfish caught by longline fishing in Hawaiian waters.The average female LJFL was 166.3 cm(SD=32.5 cm),with an observed at-haulback mortality rate of 64.3%.Male LJFL was on average 155.1 cm(SD=25.6 cm),which was smaller than that of females.The observed at-haulback mortality rate was 63.6%,and there was no significant difference between the sexes.2)Quarter,longitude,hook type,and Lower Jaw-Fork Length(LJFL)had a significant impact on the at-haulback condition when fish were retrieved onboard,of which hook type and longitude have the greatest impact.The predictions of the probabilities of swordfish dying at-haulback if caught by circle hooks and others were 28.0% and 34.1%,respectively,and swordfish caught by Japanese tuna hooks had a significantly higher probability(60.0%)of dying.The capture position will also have a significant influence on the at-haulback mortality of swordfish.The prediction of the probability of death in different longitudes varies between 39.0% and 75.0%.And the predicted results show that the probability of death is smaller when the capture location is further east in the Indian Ocean.3)The interaction term of hook type and LJFL had a significant impact on at-haulback mortality.When using a circle hook,the greater the LFJL,the greater the mortality rate;when using Japanese tuna hooks and other hook types,the greater the LJFL,the lower the mortality rate.As the LJFL increases,the probability of the swordfish dying at haulback decreases when using Japanese tuna hooks and other hook types.However,there is a lower level of prediction of the probability of swordfish caught by other hooks dying at haulback than the swordfish caught by the Japanese tuna hooks.As the length of the LJFL increased,the probability of dying at haulback increased when using a circle hook.For swordfish with a smaller LJFL,the probability of death if caught by circle hooks was significantly lower.4)Taking the first sexual maturity of female swordfish in the Indian Ocean at 170 cm and the first sexual maturity of male swordfish at 120 cm as the dividing points,the athaulback mortality of immature samples were 66.7% and 50.0%.Taking the Atlantic minimum landing size of 125 cm as a reference,in this study,at-haulback of swordfish less than 125 cm was 80.7%,which was lower than that of the Atlantic Ocean(swordfish less than 125 cm)of 87.8%.This study provides reference information for understanding the at-haulback mortality of Chinese tuna longline fishing fleets in the Indian Ocean and its influencing factors,and provides the basis for the development of the IOTC minimum landing size for swordfish,which will help the future conservation and management measures of swordfish resources in the ocean.In future research,factors such as bait type and hook width can be added to conduct in-depth research on the at-haulback mortality.It is also possible to carry out a more comprehensive discussion on the conservation and management measures of swordfish in combination with the natural mortality and fishing mortality.