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

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 1,144 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.