Aspects of porbeagle shark bycatch in the Argentinean surimi fleet operating in the Southwestern Atlantic Ocean (50-57 degrees S) during 2006-2014

In the southern Southwestern Atlantic Ocean (SAO) off Argentina, the porbeagle shark,
Lamna nasus, is incidentally caught by trawl fleets operating south of 44°S. The surimi fleet has
the most frequent and abundant bycatch. After the implementation of the National Plan of Action
for the Conservation and Management of Sharks, porbeagle shark data gathering and research
were proposed as priority actions. The wide spatial distribution of the porbeagle shark and the
unbalanced fishery-dependent data produce high variability in the bycatch trend, creating the need
for analyses that include spatio-temporal, environmental and operational variables not considered
in previous studies. Therefore, the aims of the present study were 1) to quantify the historical L.
nasus bycatch in the Argentinean surimi fleet, 2) to determine the bycatch trend using spatiotemporal,
environmental and operational variables and 3) to analyze the length and sex structure
of the porbeagle shark bycatch.
Bycatch data were recorded by scientific observers on the Argentinean surimi fleet
operating at the southern limits of the Southwestern Atlantic between 2006 and 2014.The annual
L. nasus bycatch was estimated, taking into account the on-board observed coverage. We used a
Delta model to standardize the bycatch of porbeagle shark considering spatio-temporal,
environmental and operative variables. Generalized additive modeling was applied to explore the
variation in the length, sex and proportion mature of the bycatch.
A total of 9965 fishing hauls were analyzed of which 11% had a positive L. nasus bycatch.
Estimated annual L. nasus bycatch by the Argentinean surimi fleet ranged from 10 to 117 tons.
The standardized catch rate was stable with some variability until 2011, and increased between
2012 and 2014. The length structure, sex ratio and proportion mature varied by month and latitude.
Catch rates were higher during summer and autumn. During this period, mature females with fork
length between 180 and 200 cm predominated.
The levels of porbeagle bycatch by the Argentinean surimi fleet demonstrate the need to
maintain continuous observer monitoring of the fleets operating in the Southwestern Atlantic Ocean
and to adopt further precautionary management measures to mitigate the bycatch.
The variability in the porbeagle bycatch trend may reflect effects associated with unbalanced
datasets, the wide distributional range of this species, or changes in the targeting strategies of the
fleet that are not apparent in the available data, impeding its use as a population indicator. Further
work, such as the investigation of potential changes in fishing strategy, could substantially improve
results. Additionally, regional and collaborative studies are needed to understand the spatial
distribution and population trend of porbeagle shark in the Southern Hemisphere.