Seasonal patterns of adult and subadult white shark presence at coastal aggregation sites in central California

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Context Gaining insights into seasonal aggregations of marine megafauna and how patterns vary among demographic groups is pivotal for evaluating anthropogenic risk exposure and modeling populations and ecosystem dynamics. In California, adult and subadult white sharks recurrently aggregate on the coast near pinniped colonies in fall and winter months, facilitating comprehensive long-term field studies. Aims In this study, we used over 15 years of passive acoustic telemetry data to compare the seasonal dynamics of coastal habitat use for white sharks tagged in central California among four demographic groups (adult females, adult males, subadult females, and subadult males). Methods Acoustic tags were deployed on 355 white sharks at coastal aggregation sites and monitored across a coastal array of underwater receivers from 2006 to 2022. The main aggregation sites of the Northeast Pacific (Año Nuevo, the Farallon Islands and Tomales) were continuously monitored, with an expansion of the acoustic network to the south in the latter years of the study. Key results White sharks were tracked for an average duration of 594 ± 552 days (mean ± s.d.), with total track durations ranging up to 3235 days. Notably, adult male sharks exhibited the highest residency to central California coastal aggregation sites and demonstrated earlier seasonal peak densities in late October. Adult female presence peaked in early December. Adult sharks displayed distinct seasonal gaps in detection where they have been shown with satellite tags to migrate offshore, with females displaying much longer average detection gaps than for males (averaging 1.5 years vs 0.7 years). In contrast, subadults exhibited higher coastal affinity with more consistent and widespread detections across a higher number of coastal sites throughout the year, often extending beyond the main aggregation areas outside of the peak aggregation season. Conclusions We hypothesize that the observed differences between demographic groups are attributed to sex- and size-specific foraging and reproductive strategies. The extended receiver network also showed expansive coastal movements and identified potential undescribed aggregation sites. Implications Insights from our extensive acoustic dataset represent a significant advancement in assessing the timing of anthropogenic interactions and modeling both ecosystem and population dynamics.

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