A team of researchers is working to solve a problem with fish ladders aimed at boosting salmon populations. California sea lions have learned to snatch fish as they migrate, harming salmon conservation efforts. The team used mathematical models to determine the best methods for halting this behavior, including the removal of sea lions that have learned the skill. The findings are in a paper that was just published in the journal Proceedings of the Royal Society B: Biological Sciences.
The Bonneville Dam between Oregon and Washington requires constant wildlife management. The salmon that pass through the dam are on the Endangered Species list and fully protected by law. The problem is that California sea lions (Zalophus californianus) are also protected—under the Marine Mammal Protection Act. California sea lions migrate between a few areas and spend their springs on the Columbia River, the location of the Bonneville Dam. This wasn’t an issue until a few individual sea lions learned to grab salmon off of the ladders designed to let the fish migrate over the dam. As the behavior has become more common, wildlife officials have begun to remove individual sea lions. It’s unclear, however, if this makes a real impact on preventing clever sea lions from poaching endangered salmon.
Scientists from the NOAA Fisheries West Coast Region conducted a study to investigate the effects of these sea lion culls and whether or not they’d actually save endangered salmon. The team used mathematical models that are already used by researchers to track the spread of disease. In this case, the team of scientists was using the models to track and predict social transmission of salmon poaching behaviors. The behavior is learned, not innate, and so it spreads between sea lions as time goes on.
The models showed that sea lion culls would have worked—had they been conducted right away. If the first few individuals had been removed before “teaching” the other sea lions how to grab salmon off of ladders, the problem may not have spiraled out of control so quickly. Unfortunately, the results of the study show that it might be too late now. Future studies will investigate alternative methods of sea lion control.
Schakner et al. Epidemiological models to control the spread of information in marine mammals. Proceedings of the Royal Society B: Biological Sciences (2016).