Analysis of the at-haulback mortality of striped marlin (Tetrapturus audax) in the western Indian Ocean by Chinese longline fishing using a GLM

Striped marlin (Tetrapturus audax) is an oceanic pelagic migratory fish. The
stock status of striped marlin in the Indian Ocean is now considered to be overfished
and subject to overfishing. Quantifying the level of at-haulback mortality caused by
tuna longline fishing is critical to reducing fishing pressure and protecting the fate of
billfish stocks. This study was based on data from 2,482 longline fishing operations
carried out by Chinese observers in the western Indian Ocean from 2012 to 2019. The
dataset includes information on the survival status of 774 striped marlin and their
corresponding details. We used a generalized linear model (GLM) to analyze the level
of at-haulback mortality and its potential influencing factors. The results indicate that
the distribution of 774 striped marlin had a lower jaw-fork length range from 130 to
220 cm, and 51.5% of the samples died at the time of haul-back. The observed
at-haulback mortality rates showed significant differences among quarters, hook type,
bait type, longitude, and environmental variables; the GLM model revealed that
quarter, sea-surface temperature (SST), hook type, lower jaw-fork length (LJFL),
chlorophyll (CHL), and longitude had significant effects on at-haulback condition
when the fish were retrieved on board, with the quarter and sea surface temperature
having the most significant effects. The interaction term between hook type and lower
jaw-fork length also had a significant effect on at-haulback mortality, with the model
predictions showing that mortality increased with LJFL when using circle hooks, but
decreased when using Japanese tuna hooks. There has been limited observational
analysis of hooking mortality rates for striped marlin, and the present study may
provide an important reference for the conservation and management of striped marlin
stocks in the Indian Ocean.