Variation in Octopus bimaculatus Verrill, 1883 Diet as Revealed through δ13C and δ15N Stable Isotope Analysis: Potential Indirect Effects of Marine Protected Areas

Abstract: Anthropogenic impacts on marine environments can be far-reaching and variable, and in an effort to mitigate these impacts, marine protected areas (MPAs) have been established globally. Indirect effects of MPAs on marine food webs and consumer behavior can be revealed through the diet of generalist predators, like octopuses, since their diet is a reflection of both preference and prey availability. Octopuses (Octopus bimaculatus Verrill, 1883) and invertebrate prey species were collected around Santa Catalina Island, CA in the summer of 2012, 2013, and 2014, and muscle tissues were analyzed for δ13C and δ15N. The δ13C and δ15N signature of octopuses caught within the MPA area and the non-MPA area were compared within diet space. Estimated contribution of prey species to octopus diet was calculated using a Bayesian mixing model. Octopuses caught in MPA areas had significantly different isotopic signatures than octopuses caught outside MPA areas in 2012 and 2013, but not 2014. Prey contributions to diet were highly variable between areas and years. Bivalves were a consistent contributor to MPA octopus diet, and the large snail Megastrea undosa (W. Wood, 1828) made up a relatively large proportion of the diet of octopuses from all areas and years. Additionally, there were more moray eels, a nocturnal predator outside MPA areas but more diurnal predators inside MPA areas. These results suggest octopus foraging behavior could be influenced by variation in predation risk reveal an indirect impact of diurnal predator recovery in the MPA. To fully understand the impact of anthropogenic change on marine environments, we must assess changes in the entire community and the interactions that drive community function.