Slow it Down! 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.
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.
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.