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Description
The recovery of apex predators can improve ecosystem structure and functioning but may also lead to conflicts as predators and humans compete for resources. While the reestablishment of sea otters has benefitted eelgrass habitat, there may also be negative impacts from otter predation on economically important crab species. Understanding the potential influence of otters on crabs is of particular importance for California fisheries as otters continue their range expansion into other estuaries that act as important nurseries for Dungeness crab. Chapter 1 assesses how exposure to otter predation affected the abundance and size of juvenile Dungeness crabs in Elkhorn Slough. Using annual otter census data from 2012 and 2016, we asked how Dungeness crab abundance and size varied in relation to sea otter foraging density. Additionally, we used bi-weekly otter census data from 2016 to consider whether the relationship between Dungeness crabs and sea otters varied between habitats. Our results show Dungeness crab abundance declined between years and with increasing sea otter density. However, our results using data from the bi weekly otter censuses indicated that Dungeness crab abundance was positively correlated with sea otter density in areas close to the mouth of the estuary. Despite this, the size of crabs was negatively correlated with sea otter density in unvegetated habitat. Chapter 2 examines how otters may impact economically valuable crab fisheries if otters were to reoccupy California estuaries outside their current range. We used a comparative analytical approach to assess variation in Cancrid crab abundance and size across four California estuaries with and without otters to ask how biotic and abiotic factors may contribute to observed variation. We also compared trends in the growing southern sea otter population to Dungeness crab landings in California. We found that crab abundance and size were negatively associated with otter presence. While abiotic factors varied among estuaries, these factors explained little of the observed difference in crab abundance or size. Although we found evidence that otters have some localized effects on Cancrid crabs within estuaries, we found no evidence that southern sea otters, at current population sizes, have negatively impacted Dungeness crab landings.
