Multidecadal kelp forest canopy dynamics following the reintroduction of an apex predator

Background/Question/Methods

Giant kelp and bull kelp are canopy forming foundation species that create dense forests along shallow rocky reefs and provide energy and structure for vibrant kelp forest ecosystems. Sea otters are a keystone predator and serve as an important link in a trophic cascade by reducing herbivore pressure on kelp. Sea otters were reintroduced at several sites in Southeast Alaska in the 1960s and expanded into subregions over the past few decades at different rates with large increases in sea otter abundance occurring from the 1980s to 2010s. In some regions, such as Sitka Sound, there is anecdotal evidence of increased kelp cover in recent decades, following the collapse of the red urchin fishery, an important kelp herbivore. We used satellite observations of kelp canopy dynamics over four decades to relate patterns in kelp canopy to sea otter reintroduction over regional scales in Southeast Alaska. We focused on the timing of first observation of kelp canopy at a 30-meter resolution, indicating a possible shift from a barren state to a kelp state, and compared these results to regions around Oregon and Monterey Bay, California, areas with no sea otter population and a long history of sea otter presence, respectively.

Results/Conclusions

Kelp canopy area increased through time in the Southeast Alaskan subregions but differed in the timing of maximum ecosystem state change. We found the occurrence of kelp canopy was related to the timing and expansion of sea otters from reintroduction sites. Expansion of kelp canopy area around Northwest Prince of Wales Island reached a peak in the late-1990s and early 2000s, soon after rapid increases in sea otter population growth, while kelp canopy expansion occurred earlier and over a longer period in Sitka Sound, along with a more slowly increasing sea otter population regulated by hunting. Areas in California with persistent otter populations showed little expansion of kelp canopy expansion through time and a similar pattern was observed off the coast of Oregon. Understanding the regional impact of apex predator reintroduction is important for interpreting and predicting long-term ecosystem dynamics, fisheries, and resource conservation.