Stable isotope mixing models elucidate sex and size effects on the diet of a generalist marine predator

We applied a 2-step clustering algorithm and Bayesian stable isotope mixing model to examine intraspecific differences in the contribution of prey sources to the diet and foraging habitat of harbor seals Phoca vitulina in the Salish Sea, USA. We analyzed stable isotopes of carbon and nitrogen collected from 32 seals and 248 prey samples representing 18 of 25 of the most common seal prey items identified in seal scat. Stable isotope analyses identified significant harbor seal sex- and size-based differences in diet and foraging habitat use. In comparison to males, female harbor seals had a higher contribution of prey items that were more 13C-enriched. This result may indicate that females derived more of their δ13C value from nearshore versus offshore food webs, an explanation supported by movement data on this population. However, large seals of both sexes displayed a greater offshore signal in their diet, indicating that seal mass effects on foraging habitat use were somewhat independent of sex. Our work contributes to understanding trophic linkages between these generalist consumers and their prey. The foraging differences that we detected between male and female harbor seals present complex challenges for fisheries management and for the design of marine reserves. Many marine reserves in the Pacific Northwest are located in close proximity to seal haul-out sites. By lowering the energetic costs of foraging of females, these reserves may ultimately have the unintended effect of increasing individual fitness, population growth rate, and influencing future predator-induced mortality on endangered species

A regional analysis of coastal and domestic fishing effort in the wider Caribbean

Although regulated fishing effort is relatively well documented for fisheries in developed states, developing countries are dominated by artisanal fisheries that are characterized by large numbers of small boats, fishing in dispersed and remote locations. These factors make quantifying artisanal fishing effort difficult. In this study, we examined the distribution and density of fishing effort across a region dominated by coastal, artisanal fisheries: the wider Caribbean. We used generalized linear regression models to predict missing data needed to compute fishing effort metrics and to explain variance in average boat length of a fishery and the number of small-scale boats in a given country. Clear intra-regional differences between mainland and island fisheries, and between northern and southern Caribbean fisheries, are evident in the results. To map artisanal fisheries based on the minimal data available, we created a free, automated Fishing Effort Envelope Tool (FEET). Through the use of this tool, we mapped all fisheries in the Caribbean to the extent possible given current data. Further, this mapping process also allowed us to identify hotspots of high density coastal fishing and data gaps that may mask areas of even higher fishing pressure. The potential ecological consequences of the scale of artisanal fishing are profound, and have greater implications for developing regions worldwide