Lennert-Cody, C. E., S. C. Clarke, A. Aires-da-Silva, M. N. Maunder,
P. J. S. Franks, M. Román, A. J. Miller and M. Minami, 2018:
The importance of environment and life stage on interpretation of
silky shark relative abundance indices for the equatorial Pacific Ocean.
Fisheries Oceanography, in press.
Recent large fluctuations in an index of relative abundance for the silky shark
in the eastern Pacific Ocean have called into question its reliability as a population indicator for
management. To investigate whether these fluctuations were driven by environmental forcing
rather than true changes in abundance, a Pacific-wide approach
was taken. Data collected by observers aboard purse-seine
vessels fishing in the equatorial Pacific were used to compute standardized trends
in relative abundance by region, and where possible, by shark size category as a
proxy for life stage. These indices were compared to the
Pacific Decadal Oscillation (PDO), an index of Pacific Ocean-climate variability.
Correlation between the silky indices and the PDO was
found to differ by region and size category. A positive correlation was identified between
This highest correlations by shark size category were the PDO and indices for small
(<90 cm total length (TL)) and medium (90-150 cm TL) silky sharks from
the western region of the equatorial eastern Pacific (EP)
and from the equatorial western Pacific. This positive correlation disappeared in the inshore EP.
Throughout, correlations with the PDO were generally lower for large silky
sharks (> 150 cm TL). These
results are suggestive of changes in the small and medium silky indices being
driven by movement of juvenile silky sharks across the Pacific as the eastern
edge of the Indo-Pacific Warm Pool shifts location with ENSO
events. Lower correlation of the PDO with large shark indices may
indicate that those indices were less influenced by environmental forcing and therefore
potentially less biased with respect to monitoring population trends.