Population indicators for porbeagle sharks in the Chilean swordfish fishery

This collaborative study of the Chilean swordfish fishery was carried out by NIWA and the Instituto de
Fomento Pesquero (IFOP) as part of the southern hemisphere porbeagle stock status assessment.
Objectives were, to explore the information available for all shark species, and potentially provide
porbeagle population indicators and biological information for the eastern Pacific region. The Chilean
swordfish fishery comprises industrial and artisanal longlining, and artisanal gillnetting. The longline
fishery operates mainly between 22°S and 35°S, while the gillnet fishery operates closer to shore and
slightly further north and south. IFOP collects effort and catch information for all species including
bycatch, along with some biological data, via an observer program that has operated since 2006.
We prepared, cleaned, and characterised the datasets, and plotted the spatial distributions of effort,
catch, and bycatch by fleet, year, and season. We used cluster analysis to identify potentially
different fishery components. Characteristics of artisanal and industrial longlining were similar and
they were combined for further analysis. We analysed catch rate, size, and sex ratio data for
porbeagle sharks using random forest analyses and generalised additive models.
Porbeagle catch rates were generally low, but much higher south of 35 °S. Spatial effects along with
moon phase (lunar illumination) were important for catch rates, with more porbeagles caught
further south and in brighter moonlight. Standardized CPUE indices were generated based on
generalised linear modelling, but were variable and uncertain. Continued data collection is
recommended to improve the estimates.
Biological analyses found variable sizes by year, with gillnet fisheries taking fewer small porbeagles.
Patterns by both size and sex were complex. Larger sharks were found further north, which contrasts
with results from elsewhere including Japanese, Argentinian, and New Zealand fisheries.
Differentiation by sex was both spatial and seasonal. Both catch rates and sizes were associated with
lunar illumination, which may be due to shark behaviour or to setting patterns in the swordfish
longline fishery.
This analysis has characterised the catches of porbeagle sharks and other shark species in the Chilean
swordfish fishery, and provided the first estimates of standardized catch rates for porbeagles in this
fishery. It has also analysed biological data associated with porbeagle sharks, and identified spatial
and temporal patterns, and operational effects, on the sizes and sexes of the sharks captured.