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.

Abstract. 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 (less than 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 (greater than 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.

Preprint (pdf)