Estimates of Shark at-vessel, Post-release Mortality, and Retention Ban Effects on Stopping Overfishing

Sharks are among the most threatened groups of exploited fishes,
comprising common bycatch across many fisheries. Management efforts
intended to safeguard threatened species have increasingly focused on
retention bans to reduce bycatch mortality. However, the population
effects of such measures remain unevaluated across species. We combined
available data from 160 studies providing estimates of at-vessel or
post-release mortality for 147 taxa caught by different fishing gears to
create random-forest regression models and predict mortality rates for 341
shark species incidentally captured by longlines or gillnets.
Smaller-bodied species inhabiting shallow waters were more likely to
suffer at-vessel mortality compared to their deep-water counterparts,
which were more likely to suffer post-release mortality. We then use
results for longlines to simulate the effect of retention bans in reducing
fishing mortality to sustainable levels. Our metric consists of the ratio
between the proportion of each species’ population caught and discarded
(PMAX) under a retention ban divided by the fishing mortality (F)
predicted to achieve maximum sustainable yield (FMSY). Our calculations
show that a retention ban yielded an average ~three-fold higher PMAX
compared to FMSY, with 18% of the species having PMAX/FMSY < 2,
72.3% having 2 < PMAX/FMSY < 5, and 9.7% having PMAX/FMSY
> 5. For threatened species, median PMAX/FMSY = 2.28 and
non-threatened ones had median PMAX/FMSY = 2.77. Our study shows that
retention prohibitions could reduce shark mortality, but must be combined
with additional measures to stop overfishing, especially for
low-productivity species.