Environmental effects on daytime sheltering behaviors of California horn sharks (Heterodontus francisci)

Abstract

California horn sharks (Heterodontus francisci) are relatively small (122 cm maximum total length; TL), nocturnally active sharks that have been observed refuging in shelters during the day; however, little is known about how they select their daytime resting shelters, and what factors may drive their degree of shelter fidelity. Depending on body size, thermal tolerance, and ability to avoid predators, horn sharks may need to balance tradeoffs when selecting a quality shelter. This study quantified vertical daytime distributions via dive surveys and shelter fidelity via acoustic telemetry during June – August 2016 and June – November 2017 in a temperate rocky reef (33˚26’ N, -118˚29’ W). Immature individuals ( 58 cm TL) were found in deeper (> 10 m), cooler habitats sheltering in larger caves and rock ledges. The sizes and relative abundances of sharks varied between land-associated and offshore-isolated reefs, indicating habitat type may be a likely driver of ontogenetic shifts in shelter behavior. No sharks returned to their shelters in 2016 coinciding with the period of El Nino conditions (increased temperature, shifts in community assemblage), yet shelter fidelity was 47% in 2017 under non-El Nino conditions. By quantifying sheltering behavior in a residential mesopredator, this study provides a framework for understanding how environmental conditions influence horn shark behaviors and their potential impacts on their community.