Rapid consumption of kelp crab: implications for sea otters in washington state

As animal populations approach environmental carrying capacity, competition for food increases, generally leading to decreased individual energy intake rate. Energy-intake rate can therefore be used as one metric of population status relative to carrying capacity. Focal observations of Sea Otter (Enhydra lutris) foraging behavior have been used throughout their range to estimate energy-intake rates and infer population status. In Washington State, previous research has demonstrated that handling times for Kelp Crabs (Pugettia spp.) by Sea Otters are 1.5 to 2 times faster than those observed in California and British Columbia, resulting in higher energy-intake rate estimates for Sea Otters in Washington. We investigated potential causes for the difference in handling time by: (1) comparing Sea Otter handling times of Kelp Crab and non-Kelp Crab prey items in Washington, California, and British Columbia; (2) comparing the handling times of Kelp Crabs by a subset of Sea Otters in California, which are Kelp Crab specialists (2003–2012, n = 244 Kelp Crab captures) to those of Sea Otters in Washington (2015–2018, n = 541 captures) and British Columbia (2013–2017, n = 359 captures); and (3) comparing the biomass-to-width ratios of Kelp Crabs from Washington and California. We did not observe consistent differences between regions in Sea Otter handling times of non-Kelp Crab prey. Mean Sea Otter handling time of small Kelp Crabs (carapace ≤1 Sea Otter paw width) in Washington (32.7 s) was significantly faster than in British Columbia (52.0 s, P < 0.0001) and all of California (40.6 s, P < 0.0001), but was not significantly different from that of Kelp Crab-specialist Sea Otters in California (31.7 s, P = 0.313). Mean Sea Otter handling time of large Kelp Crabs (≥1 Sea Otter paw) in Washington (64.7 s) was significantly faster than in British Columbia (87.7 s, P= 0.003), in all of California (104 s, P < 0.0001), and in the subset of Kelp Crab-specialist Sea Otters in California (91.6 s, P = 0.007). Kelp Crabs in Washington had a larger biomass-to-width ratio than Kelp Crabs in California: in Washington, a Kelp Crab with a 20-mm maximum carapace width had a 3.8% greater predicted biomass than a Kelp Crab in California of the same width, and a 27.1% greater biomass for a 60-mm carapace. Our results suggest that Sea Otters in Washington are Kelp Crab specialists with behavioral differences allowing them to consume Kelp Crabs faster, a difference that may affect the inference of Sea Otter population status from energy-intake rates in Washington.