Dramatic increase in sea otter mortality from white sharks in California

Abstract

Although southern sea otters (Enhydra lutris nereis) are not considered prey for white sharks (Carcharodon carcharias), sharks do nonetheless bite sea otters. We analyzed spatial and temporal trends in shark bites on sea otters in California, assessing the frequency of shark bite wounds in 1,870 carcasses collected since 1985. The proportion of stranded sea otters having shark bites has increased sharply since 2003, and white shark bites now account for >50% of recovered carcasses. The trend was most pronounced in the southern part of the range, from Estero Bay to Point Conception, where shark bite frequency has increased eightfold. Seasonal trends were also evident: most shark‐bitten carcasses are recovered in late summer and fall; however, the period of elevated shark bite frequency has lengthened. The causes of these trends are unclear, but possible contributing factors include increased white shark abundance and/or changes in white shark behavior and distribution. In particular, the spatiotemporal patterns of shark‐bitten sea otters match increases in pinniped populations, and the increased availability of marine mammal prey for white sharks may have led to more sharks spending more time in nearshore waters utilized by both sea otters and pinnipeds.

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

“…1B), where spatial gaps in giant Macrocystis pyrifera and bull Nereocystis luetkeana kelp forest canopies (Fig. Although the presence of white sharks in this region is seasonal and migratory (Boustany et al 2002, Weng et al 2007, Jorgensen et al 2010, by 2014, bites from white sharks had become the single greatest cause of sea otter mortality (Miller et al 2014, Tinker et al 2016. Shark-biterelated mortality, once limited in magnitude and geographic extent, has dramatically increased since the early 2000s (Tinker et al 2016).…”

“…Unlike other protected marine taxa (e.g. 1D) may expose sea otters to fatal bites from white sharks Carcharodon carcharias (Ames and Morejohn 1980, Wcisel et al 2014, Tinker et al 2016. 1B), where spatial gaps in giant Macrocystis pyrifera and bull Nereocystis luetkeana kelp forest canopies (Fig.…”

“…sea turtles, cetaceans, pinnipeds, or seabirds), all sea otter life history stages are confined to a narrow coastal corridor (Fig. Shark-biterelated mortality, once limited in magnitude and geographic extent, has dramatically increased since the early 2000s (Tinker et al 2016). 1D) may expose sea otters to fatal bites from white sharks Carcharodon carcharias (Ames and Morejohn 1980, Wcisel et al 2014, Tinker et al 2016.…”

“…Each year, about 20% of the California sea otter population dies, and roughly half of these animals are retrieved along the shore (Estes et al 2003, Gerber et al 2004, Tinker et al 2016. These data provide useful information, particularly in documenting acute mortality associated with white sharks and infectious diseases (Thomas and Cole 1996, Kreuder et al 2003, Tinher et al 2016.…”

Megafaunal Impacts on Structure and Function of Ocean Ecosystems

“…Some of these taxa may have powerful influences on benthic communities, suggesting that loss of large sharks has triggered trophic cascades (39). White shark attack rate on sea otters in central California has increased markedly over the past three decades from just several carcasses recovered per year in the mid-1980s to almost 100 per year at present (137). This sharp increase in shark-inflicted mortality on sea otters may have resulted from the attraction of sharks to increased numbers of pinnipeds in the area over this same period.…”