Print PageDisentangling the multiple effects of human activities and managing for desirable ecosystem states, resilience, and service production is a primary goal of ecosystem-based management initiatives globally. The near extinction of sea otters (Enhydra lutris) is one of the most dramatic examples of human-induced impacts to the structure and functioning of temperate nearshore marine ecosystems. Simultaneously, over-exploitation of commercial and recreational fisheries has also contributed substantially to ecosystem change in marine ecosystems. To understand the direct, indirect, and relative effects of these drivers on ecosystem service production, we performed a comprehensive assessment of relationships between the loss of this top predator, fishing pressure and nearshore rocky reef food web structure along the west coast of Vancouver Island and Haida Gwaii in British Columbia, Canada. In regions with and without sea otters and high and low recreational fishing pressure, we performed subtidal surveys of rocky reef habitats to quantify community structure, hook and line surveys to estimate relative fish abundance, and stomach content and stable isotope analyses to determine reef fish trophic niche space.
Results/Conclusions
Sea otters exhibit strong top-down control of grazing by herbivores, such as sea urchins, and thus indirectly increase kelp population spatial occupancy and productivity. By facilitating kelp populations, otters indirectly affect trophic structure and promote diversity through provisioning of habitat and resource subsidies. Our results indicate that the impact of fishing on food web structure is moderated by the presence of sea otters and their indirect positive effects on fish productivity. Ultimately these results have implications for coastal communities, economies and ecosystem-based management initiatives in British Columbia and worldwide.
