Chasing fish and catching data: recreational spearfishing videos as a tool for assessing the structure of fish assemblages on shallow rocky reefs

Several citizen science initiatives, taking advantage of growing numbers of re creational SCUBA divers, have produced information for conserving marine biodiversity. In contrast, the potential benefits of involving recreational spearfishers in research activities has been utterly overlooked. A noteworthy aspect of modern breath-holding spearfishing is the use of full HD microcameras for recording the fishing action. When using the sit and wait fishing technique, the spearfisher remains still on the bottom, waiting for the prey to approach. These videos may provide information on fish species richness and relative abundances. We explored the potential of recreational spearfishing footage for assessing the structure of fish assemblages on shallow rocky reefs. We assessed how estimates of fish species richness, composition and relative abundance from spearfishing videos compare to those from underwater visual censuses (UVC), at 2 sites in the NW Mediterranean. Species accumulation curves show that the 2 sampling methods provided comparable estimates of species richness for any sample size. Although the structure of fish assemblages differed between UVC and videos, the relative abundance of only a few species, mostly characterized by a sedentary habit, was greater in UVC than videos. In addition, in order to show the potential use of information from spearfishing videos, we modelled the occupancy of a species (the wrasse Symphodus rostratus) at increasing distances from a commercial port, explicitly incorporating imperfect detectability as a function of video length and habitat characteristics. While the occupancy of this species did not vary with distance from the port, its detectability depended on video length. Our study suggests that videos recorded by re creational spearfishers may be useful for monitoring fish assemblages and for testing hypotheses at large spatial and temporal scales.