Gaps in kelp cover may threaten the recovery of California sea otters

Abstract

Despite more than a century of federal protection, the California sea otter Enhydra lutris nereis remains threatened under the U.S. Endangered Species Act (ESA), and the population has not appreciably expanded its range in two decades. Here, we examine a novel dataset of 725 sea otter live strandings from 1984-2015 to gain insights into demographic and environmental factors underlying threats to sea otter recovery. Using multinomial logistic regression to evaluate spatiotemporal patterns of stranding causes, we demonstrate that increases in stranding rates, particularly outside the range center, are related to a substantial increase in shark bites. By contrast, trauma linked to human activities has declined dramatically, and now accounts for less than 5% of stranding cases. Within the range core, where the sea otter population seems regulated by prey availability, symptoms of energetic stress represent more than 63% of all strandings and are strongly associated with high sea otter density. Conversely, in range peripheries, the majority of strandings are caused by shark bite and neurological disease. Notably, these threats are virtually absent where nearshore habitat is characterized by at least 10% kelp canopy cover. Our analyses reveal that declining kelp cover may therefore constrain the population's spatial expansion and recovery in two key ways. Absence of kelp intensifies density-independent threats in the range peripheries, and likely limits dispersal of reproductive females, which depend on kelp canopy for nursery habitat. These results highlight the significance of both top-down and bottomup processes in population dynamics, and inform an ecosystem-based approach to conservation planning.

Non‐trophic impacts from white sharks complicate population recovery for sea otters

“…Sea otters, however, have been slower to recover than pinnipeds and have reoccupied only 13% of their prior range (USFWS, ). Though the original causes of the sea otter population decline (hunting, fishery interactions) have been solved, the recent advent of mortalities from white shark bites suggests new threats to recovery (Nicholson et al, ; Tinker, Hatfield, Harris, & Ames, ).…”

“…Though it is increasingly clear that shark bites on otters are incidental and nonconsumptive, the emerging impact of this dynamic is only recently explored. To begin, four studies (Ames, Geibel, Wendell, & Pattison, ; Ames & Morejohn, ; Nicholson et al, ; Tinker et al, ) have documented the extensive occurrence of stranded sea otters that bear bite wounds from white sharks, but have not (even partially) been consumed. Beyond this, an extensive study of white shark stomach contents (Klimley, ) revealed zero sea otter parts.…”

“…Most strandings of shark‐bitten otters occur in sandy, shallow habitats near the northern and southern range peripheries (Nicholson et al, ; Tinker et al, ). A recent analysis showed that shark bites occur almost exclusively where Macrocystis and Nereocystis kelp canopies cover <10% of the available habitat (Nicholson et al, ). This makes sense as kelp forests provide shelter for sea otters, potentially protecting them from visual ambush while they rest at the ocean surface.…”

“…Though it might not have been significant historically, several factors indicate that white sharks may be currently limiting the recovery of California sea otters. The sea otter population is not spatially homogeneous, but densest at the range center on the Monterey Peninsula (Nicholson et al, ). There it is either approaching, or already at, carrying capacity (Gagne et al, ; Smith, Newsome, Estes, & Tinker, ).…”

“…Here, we integrate independent data streams (Ferretti et al, ; Jorgensen et al, ; Nicholson et al, ; Tinker et al, ) to more cohesively explain the observed patterns of shark–otter interactions and to inform sea otter as well as ecosystem conservation. We hypothesized seasonal trends in shark‐bitten otters would mirror trends in mistargeted shark prey, expecting that they will be contextualized by prey foraging trends but not directly coincident with this targeted feeding season.…”