Species recovery and recolonization of past habitats: lessons for science and conservation from sea otters in estuaries

Abstract

Recovering species are often limited to much smaller areas than they historically occupied. Conservation planning for the recovering species is often based on this limited range, which may simply be an artifact of where the surviving population persisted. Southern sea otters (Enhydra lutris nereis) were hunted nearly to extinction but recovered from a small remnant population on a remote stretch of the California outer coast, where most of their recovery has occurred. However, studies of recently-recolonized estuaries have revealed that estuaries can provide southern sea otters with high quality habitats featuring shallow waters, high production and ample food, limited predators, and protected haul-out opportunities. Moreover, sea otters can have strong effects on estuarine ecosystems, fostering seagrass resilience through their consumption of invertebrate prey. Using a combination of literature reviews, population modeling, and prey surveys we explored the former estuarine habitats outside the current southern sea otter range to determine if these estuarine habitats can support healthy sea otter populations. We found the majority of studies and conservation efforts have focused on populations in exposed, rocky coastal habitats. Yet historical evidence indicates that sea otters were also formerly ubiquitous in estuaries. Our habitat-specific population growth model for California’s largest estuary—San Francisco Bay—determined that it alone can support about 6,600 sea otters, more than double the 2018 California population. Prey surveys in estuaries currently with (Elkhorn Slough and Morro Bay) and without (San Francisco Bay and Drakes Estero) sea otters indicated that the availability of prey, especially crabs, is sufficient to support healthy sea otter populations. Combining historical evidence with our results, we show that conservation practitioners could consider former estuarine habitats as targets for sea otter and ecosystem restoration. This study reveals the importance of understanding how recovering species interact with all the ecosystems they historically occupied, both for improved conservation of the recovering species and for successful restoration of ecosystem functions and processes.

Predators, Disease, and Environmental Change in the Nearshore Ecosystem: Mortality in Southern Sea Otters (Enhydra lutris nereis) From 1998–2012

“…Ultimately, a key component of southern sea otter population recovery efforts will be range expansion. To the extent that some potential future habitats (e.g., estuaries) may also have lower risk of shark bite mortality (Hughes et al, 2019;Mayer et al, 2019), this conservation approach may address several of the emerging challenges identified by our analyses. As management actions are taken to enhance range expansion, we recommend continued population monitoring and careful assessment of potential threats in currently unoccupied habitat.…”

Habitat Features Predict Carrying Capacity of a Recovering Marine Carnivore

“…S4). We caution that some estuarine areas outside the current range contain a diversity of substrate types, hydrological features, and human activities, and not all of these may be suitable for sea otters (Hughes et al 2019); thus, further examination of the quality and distribution of different features within estuaries is warranted, particularly in areas modified by human activities.…”

Informing sea otter reintroduction through habitat and human interaction assessment

“…San Francisco Bay, Drakes Estero, and Morro Bay) were historically occupied by sea otters, based on archaeological remains discovered in Native American shell middens and anecdotal accounts of sea otter estuarine habitat use (Schenck 1926, Odgen 1941, Broughton 1999, Jones et al 2011, Hughes et al 2019. The potential for California estuaries as future sea otter habitat has recently been considered (Hughes et al 2019). Sea otters may also thrive in estuaries along the Oregon coast, with relative prey availability likely acting as the primary determinant of population potential.…”