Today, the USGS Western Ecological Research Center announced the results of the 2012 spring population survey for the southern sea otter (Enhydra lutris nereis) -- a federally listed threatened species.
The annually conducted California survey is coordinated by WERC scientist Brian Hatfield for the U.S. Fish and Wildlife Service, which is the office responsible for managing the southern sea otter’s recovery. The survey is the result of hard work by scientists and volunteers from USGS, California Department of Fish and Game’s Office of Spill Prevention and Response, Monterey Bay Aquarium, University of California at Santa Cruz, and other institutions.
The 2012 data suggests that California’s sea otter population is continuing its pattern of tepid recovery. Some geographic areas might be reaching equilibrium density for sea otters, while other outlying geographic areas are seeing a rise in otter numbers.
What’s the future of sea otter populations in California? To clarify some common misconceptions about sea otter biology and population trends, I spoke with Tim Tinker, the lead sea otter biologist at WERC:
What does it mean when you say “we may be reaching the ‘equilibrium density’ for sea otters” in parts of the central coast? What does ‘equilibrium density’ mean?
Tim Tinker: To understand what scientists mean by equilibrium, let me use the “seafood buffet” metaphor. Think of a buffet table that constantly gets replenished, but there’s only so much room for a certain number of people to get food from.
If you feel crowded, you’ll waste more time and energy jostling with other people for food, and pretty soon you decide to head to a smaller but less crowded buffet next door. And if that gets filled up, you try the next one down the street, until you find a spot that works out for you and your fellow diners.
That’s what’s probably happening with sea otters in areas like from Seaside to Cayucos, where there is an abundance of food but sea otters are likely reaching equilibrium for that area -- it's a full table. A sea otter can only devote so much energy and time towards hunting before its health suffers and becomes more likely to get sick, so they’ll start spreading out over time to new areas.
But sea otters are slow to spread out between areas, so range expansion and population numbers will take time.
Is food the limiting factor for sea otter population growth?
Tim Tinker: Well, food is ultimately the limiting factor for any predator -- IF everything else in the environment was perfect – no pollutants, no new stress factors. But that’s obviously not the case.
What scientists are trying to do is to identify all the factors that regulate sea otter numbers, including food availability and sea otter population density. The ‘equilibrium density’ for Monterey Bay, might be different than say, Big Sur -- because they have different amounts of prey and resources, and different pressures from naturally occurring disease and predation.
In some cases there are clear cases of human caused mortality that impact population trends -- direct mortality such as boat strikes or gunshot, or indirect mortality such as that caused by land-sea pollution, as we recently saw when microcystin toxins from inland waterways made their way into the sea otter’s habitat. But the difficult part to understand is where do natural processes end and man-made influences begin? That question forms the central theme of our research.
You’re coming across a number of stranded sea otters with white shark bite marks. I suppose we can’t stop sharks from biting sea otters?
Tim Tinker: No, the interactions between species are natural processes that take place, although scientists are curious to understand why there is an increase in these attacks.
And what is interesting about these attacks is that we think they are ‘tasting bites’ – it appears that the sharks aren’t eating the otters, but for some reason they are still biting them and spitting them out. Unfortunately, this still results in a mortal wound for many sea otters.
So why is there an increase in this behavior? That’s something we hope to work with our shark expert colleagues on.
-- Ben Young Landis
Top Image: Scientists from the California Department of Fish and Game, UC Davis and Monterey Bay Aquarium perform an exam on a wild sea otter in Big Sur while it is under anesthesia. Image credit: Joe Rubin/Capital Public Radio