Slow it Down! Boat Strikes Kill Sea Otters.

Boat strikes kill sea otters.
Anthropogenic (human-caused) sources of sea otter mortality include gun shots, entanglement, entrapment, and boat strikes.  Although these causes of mortality are relatively uncommon, this category is the most preventable source of sea otter deaths in California.

Of these human-caused sources of mortality, boat strike is likely the most unintentional and unnoticed, but is easily avoidable by employing safe boating practices. Boaters should always use caution and keep an eye out for otters, particularly when traveling at a fast speed just outside harbors, in and around kelp beds, or in other areas where sea otters may be present.

Injury to sea otter hind foot caused by boat propeller

Large 3-4 inch laceration on the right hind foot of this sea otter appears to be an old boat propeller strike wound which may have also broken some of the bones of the foot beneath it.

Researchers at CDFW’s Marine Wildlife Veterinary Care and Research Center in Santa Cruz recently examined two Southern sea otters that had trauma consistent with blunt force impact from a boat hull.  Both animals, an adult male and an immature male, were collected in or near Moss Landing on 30 May, 2013.  As with all boat strike cases, these animals were likely caught unaware at (or near) the surface when a fast-moving boat crossed their path.  Collision with a fast-moving boat typically causes broken ribs and other broken bones and often results in internal hemorrhaging.  If the boat propeller strikes the animal, serious lacerations can also occur.

From January 2003 to May 2013, 35 sea otters were recovered with trauma consistent with impact from a boat hull or propeller.  The majority of these sea otters were recovered in Monterey Bay and Estero Bay during spring to late summer (Figure 1).  April and May had the greatest occurrence of boat strike cases, which corresponds with increased vessel traffic, likely associated with the opening of recreational salmon (Apr) and rockfish (May) fishing, and favorable summer boating conditions.

 

Boat strikes kill sea otters.

Figure 1. Number of Southern sea otters with trauma consistent with impact from a boat hull or propeller, by month and year, January 2003-May 2013.

Of the boat-struck carcasses that were examined during this timeframe, the most affected demographic was adult males (n=13), followed by adult females (n=4), aged adult males (n=3) and females (n=3), immature males (n=3) and females (3), subadult males (n=3) and females (n=1), and pups (1 male, 1 female).  The frequency of suspected boat strike cases is concerning from a conservation perspective, as this cause of mortality is unnecessary and preventable.  Exercising vigilance and reducing boat speeds in sea otter habitat can greatly reduce unnecessary mortality in this threatened species.

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Seaotters.com is dedicated to raising awareness about California's threatened sea otters and the role research plays in the species recovery and conservation. It's a collaboration of the Monterey Bay Aquarium, U.C. Santa Cruz, U.C. Davis, U.S. Geological Survey, U.S. Fish & Wildlife Service, California Department of Fish and Wildlife, among others. Seaotters.com is also home of the world's first HD live stream of southern sea otters in the wild.