April 22, 2019 — Every year, humpback whales make their annual trek from tropical calving grounds to feed in the cold, nutrient-rich waters off the coast of California. Historically, they arrived to feast in June just as the Dungeness crab fishery was closing and gear was being pulled in for the season. But in 2012, they arrived a few weeks earlier than normal. In 2014, they were a month early. By 2015, the humpbacks arrived in April, a full two months earlier than the norm.
That shift has been dangerous for the whales. In March, it was the subject of a lawsuit settlement that prompted an early end to the Dungeness crab season. Some of our own research has determined the cause: climate change.
Humpback sightings from the Golden Gate Bridge are a hallmark of summer in San Francisco. The whales come to feed where the narrow straits of the Bay meet the ocean. Marine life abounds here, where nutrient-dense water feeds blooms of phytoplankton. They are then eaten by zooplankton, including krill, tiny reddish shrimp-like crustaceans which are a favored food for whales. Seasonal blooms of these tiny animals support the ecosystem bottom-up, producing a massive marine buffet for fish, birds and other marine mammals. Predators gorge themselves in the summer and fall when this area supports the most amount of food.
Where there are high concentrations of fish, there are people catching them, and the whales’ earlier arrival has driven more interactions with West Coast fisheries. Since 2014, the number of whales entangled in Dungeness crab pots has escalated dramatically, with 129 cases reported between 2015 and 2017, compared to around 10 each year in previous decades.