The importance of environment and life stage on interpretation of silky shark relative abundance indices for the equatorial Pacific Ocean

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 silky indices and the PDO was found to differ by region and size category. The highest correlations by shark size category were for small (<90 cm total length [TL]) and medium (90–150 cm TL) sharks from the western region of the equatorial eastern Pacific (EP) and from the equatorial western Pacific. This 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.

The need for spatio-temporal modeling to determine catch-per-unit effort based indices of abundance and associated composition data for inclusion in stock assessment models

“…For example, purse seine data have been used to develop indices of relative abundance for silky sharks (Carcharhinus falciformis) in the eastern Pacific Ocean. However, these indices, particularly for juvenile sharks, appear to be biased due to movement of individuals in and out of the EPO, or in and out of the area fished by purse seiners, possibly due to changing environmental conditions (Lennert-Cody et al, 2019). Therefore, integrating additional data sets (e.g., purse seine data from the western and central Pacific Ocean and longline data for the whole Pacific Ocean) into a spatio-temporal analysis to extend the northern and western range of the data may help determine the influence of movement and improve the indices of abundance, since they provide a way to account for data beyond the spatial domain of interest (e.g., the EPO), but allow extraction of indices specific to the area of choice.…”

Adaptive spatiotemporal management to reduce shark bycatch in tuna fisheries

“…In 2016, Clarke et al (2018) estimated that silky sharks were at or below the biomass for maximum sustainable yield, although they raised concerns about the association of CPUE indices with oceanographic conditions and suggested they may not directly reflect the fluctuations in population size. This phenomenon has also been observed in the EPO for silky sharks, where the environment is believed to affect life-stage-specific silky shark relative abundance indices (Lennert-Cody et al, 2019). Furthermore, shark catch data from coastal artisanal fisheries are still very much lacking for silky sharks and most other elasmobranch species (Doherty et al, 2014).…”

“…This could be done through the creation of habitat models capable of capturing the patterns of distribution of target and nontarget species under different environmental scenarios by building, for example, on work by Lennert‐Cody et al. (2019), Lezama‐Ochoa et al. (2020), and Lennert‐Cody et al.…”

“…(2020), and Lennert‐Cody et al. (2021), who suggest that environmental conditions affect tuna and elasmobranch distribution in the EPO and that trends vary by area and size class.…”