Day/night patterns of habitat use by dogfish sharks (Squalidae) at photic and subphotic warm-temperate reefs: evidence for diel movements and size- and sex-segregation

Abstract

Dogfish sharks (genus Squalus) demonstrate complex distribution patterns that may increase their vulnerability to selective overfishing. This study investigated the day/night reef-use patterns in a population of dogfish (presumably Squalus acutipinnis) on shallow photic (13–35 m) and deep subphotic (51–99 m) nearshore rocky reefs in South Africa. Using baited remote underwater stereo-video systems, immature male dogfish were commonly recorded at subphotic deep reefs. At shallow photic reefs, dogfish were essentially absent during the day; however, a significant increase in the abundance of large male dogfish was observed at night. The size class of dogfish that moved onto photic reefs at night was not recorded at deep subphotic reefs, suggesting that they make use of a different habitat during daytime. The observed differences in depth use by cohorts of small and large male dogfish, and the absence of females on the reefs, provided strong evidence for size- and sex-segregation within the surveyed population. While the potential biotic and abiotic drivers were not directly tested, the results suggest that reef-use patterns may be linked to photic or temperature preferences and/or to competition- and mating-avoidance strategies. This new information about the use of nearshore rocky reefs by dogfish in South Africa raises important questions relating to the distribution and habitat use of females and the daytime habitats of mature males. With dogfish extensively caught in longline and trawl fisheries in South Africa, further research is needed to address the current knowledge gaps.