Scientists Enlist Marine Mammals to Probe Ocean – Sea Lion Foraging Patterns Give Insights on Warming

Scientists Enlist Marine Mammals to Probe Ocean –
Sea Lion Foraging Patterns Give Insights on Warming

May 1, 2006

Contact: Christina S. Johnson, csjohnson@ucsd.edu, 858-822-5334

SANTA CRUZ – Sea lions in Monterey Bay are anything but rare, just the opposite. They are probably the most commonly seen marine mammal, a permanent fixture of the marine landscape, one that unlike other wildlife, doesn’t seem to mind relatively close proximity to humans – or human infrastructure. On local buoys, piers and jetties, they sun themselves, frolic in boat wakes and bark pleadingly for fish scraps. In short, their presence is what people have come to expect when visiting the region, and this is why marine mammal biologists were so intrigued when in the winter of 2005, the usually ubiquitous sea lion basically vanished and did not return to the bay until the summer of 2005.

Tagged sea lion. Credit: M. Weise

Using tagging data, Sea Grant biologists have been able to piece together a scientifically sound explanation for the sea lions’ exodus. This explanation, besides being of scientific interest in its own right, illustrates the complexity of striving for truly ecosystem-based fisheries management.

“We think sea lions left because of a warm water anomaly that began in early 2005,” explained Sea Grant Trainee Mike Weise, who is a doctoral student with biology professor Dan Costa of the University of California, Santa Cruz. “It appears the warming pushed a lot of the sea lions’ typical prey species out of the bay. Things like squid disappeared. The fishery declined significantly. Sea lions left in search of food.”

Coastal waters off California, Oregon and Washington were unusually warm in the winter and spring of 2005, due to weaker than normal winds that usually drive upwelling of cold, nutrient-rich water from depth. Without nutrients to support phytoplankton growth, the food chain was thrown out of whack. As a result, there was a noticeable, some would say profound, redistribution of prey species. The entire colony of nesting pairs of Cassin’s auklets on California’s Farallones Islands, for example, abandoned their nests, most likely because of shortages of krill, their main food.

Sea lions, particularly adult males who do not help rear pups, have the biological luxury of being able to spend huge amounts of time foraging for food. Tracks of adult male sea lions collected during the Sea Grant project show that some males traveled as far as 300 miles from shore, swimming in giant loops that originated at the coast. They swam these loops repeatedly during the warm event, Weise said. “Sea lions are described as a purely coastal species. That’s why this offshore movement was so unexpected.”

Female sea lions who rear their pups on the Channel Islands tend to stay closer to shore to keep better tabs on their offspring. But, they also were observed to migrate farther northward along the coast than usual.

Not only are sea lions capable of traveling great distances for a meal, analyses of sea lion scat collected at Ano Nuevo Island shows the animals are also willing to shift their diet. During the warm event, sea lions were consuming greater amounts of sardine and rockfish, Weise said. Usually, squid is a main prey item.

Tagging data is being incorporated into sophisticated ocean climate models to improve reliability of forecasts and to validate physical assumptions about flow dynamics. Credit: Y. Chao, JPL

A sea lion’s diet is comprised almost entirely of commercially harvested species – squid, sardines, rockfish. One of the goals of the tagging project is to understand how oceanographic conditions influence sea lion foraging behaviors and what this means for commercial fisheries. “If we want to manage commercially important fish species that are primary prey species for large predators, we need to evaluate and incorporate into fisheries models how much, where, and when predators are taking the same fish,” Weise said.

So far, sea lion behavior mirrors what commercial fishers experienced during the warm event, a fact that underscores the link between fishing prosperity and ecosystem health. Squid landings in California declined by approximately 80 percent in 2005; sardines landings declined almost 35 percent, and the sardines that were caught were smaller than usual. “We believe adult male sea lions left the bay to feed on schools of larger sardines,” Weise said.

This year, researchers tagged 12 more male sea lions in Monterey Harbor. Some were equipped with a prototype tag that measures water temperatures, salinity and depth and uses GPS to locate an animal’s position to within 1-meter accuracy. Data from these animals will let researchers examine links between oceanic features, such as frontal boundaries, and sea floor bathymetry, such as submarine canyons, on sea lion foraging routes.

“We are getting an idea of where sea lions forage,” Weise said. “The next step is to overlay this information with maps of the distribution of fishing effort along the coast. Ecosystem-based management is about understanding the resource requirements for marine mammals. We need to know where they go, what they feed on, and how this changes when environmental conditions change.”