Analysis of At-Haulback Mortality and Influencing Factors of Indian Ocean Swordfish (Xiphias gladius)

Abstract

The at-haulback mortality of swordfish (Xiphias gladius), a highly migratory fish, in the Indian Ocean during tuna longline fishery is a concern of the Indian Ocean Tuna Commission Fisheries Management. We obtained the data of 1144 swordfish recorded in 1925 operations in the Indian Ocean by Chinese tuna longline observers from 2012–2018. A generalized linear model was used to analyze the at-haulback mortality of swordfish and the potential influencing factors. The overall mortality rate of swordfish was 64.0%, and the average female size was 166.3 cm (SD = 32.5 cm), with an observed at-haulback mortality rate of 64.3%. The average male size was 155.1 cm (SD = 25.6 cm), which was smaller than females. The observed at-haulback mortality rate was 63.6%. No significant difference was observed between the sexes. Quarter, longitude, hook type, and Lower Jaw-Fork Length (LJFL) had a significant effect on the at-haulback condition when fish were retrieved onboard. Longitude and LJFL had a significant effect on the at-haulback mortality of swordfish. When the capture position was closer to the east, the at-haulback mortality decreased and LJFL increased. The interaction term of hook type and LJFL had a significant effect on at-haulback mortality. When using the circle hook and other hook types, the greater the LFJL, the greater the mortality rate; the opposite was true for Japanese tuna hooks. We provide information for understanding the at-haulback mortality of swordfish bycatch by Chinese tuna longline fishing fleets in the Indian Ocean and its influencing factors, which will help the future conservation and management of swordfish.