Explaining California’s sudden humpback entanglement surge

Every year, humpback whales make their annual trek to feed in the cold, nutrient-rich waters off the coast of California.

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.

When a whale becomes entangled, the gear typically gets wrapped around their head, mouth, fins or flukes. At best, the whale will drag this heavy gear around with them for months or even years. However, some entanglements prevent the animal from being able to eat, sever their flukes or even cause them to drown.

What’s driving the humpbacks to an earlier arrival? As ocean temperatures rise and climate patterns shift, the timing of krill and algae blooms change in response. On their migration route, whales typically stop first in southern California before making their way north. In 2015, one of the strongest El Nino events ever recorded warmed coastal Pacific waters drastically. This was coupled with an unusual warm-water patch known as “the Blob,” the result of a stable high- pressure system over the Pacific. Both events depressed krill production and drove the whales north earlier between 2014 and 2016.

At the same time, the warming influence of the nutrient-poor patch of Blob warm water mixed with seasonally rich California coastal waters caused spikes of phytoplankton growth from Point Conception, California north through British Columbia, mostly toxigenic diatoms belonging to the pseudo-nitzschia genus. These algae produce domoic acid, a neurotoxin deadly to humans and marine life if consumed in shellfish. The 2015-2016 Dungeness crab fishing season opened in California at the very end of the season and was limited to small tracts of west coast fishing grounds unaffected by the threat of domoic acid.

To avoid the outbreak, many fishermen gathered in these smaller areas, causing a higher concentration of traps. As climate shifts drove the whales north early, they swam right into the tangle of gear.

Researchers from our lab at San Francisco State University’s Estuary and Ocean Science Center and Point Blue Conservation Science confirmed the link between humpback whale migration timing and increased entanglements in fishing gear. Early arrival driven by higher temperatures brought more whales to the area during the fishing season. Before the Blob, whales arrived after the majority of active pots were taken out of the water. Starting in 2014, we saw the timing of their arrival overlapping with active fishing efforts and entanglements skyrocketed.

The end of the Blob may not mean the end of humpback entanglements. Even though sea surface temperatures dropped slightly following the dissolution of the Blob, humpbacks continue to arrive early and entanglement rates continue to be high. A recent study out of Oregon State University concluded that other whales rely on memory to find prior feeding grounds, regardless of food abundance available that year. If humpbacks also rely on their past migrations, cueing the migration to central California early, the problematic overlap with the fishing season may continue.

Recognizing the threat to whales and fisheries, in September of 2015 the California Department of Fish and Wildlife, the National Marine Fisheries Service and the California Ocean Protection Council convened the California Dungeness Crab Fishing Gear Working Group. Fishermen, gear specialists, marine mammal entanglement experts, researchers and representatives of state and federal agencies came together to develop fishing gear and practices that are safer for whales and other large marine animals.

Unsatisfied with the pace of progress as entanglement rates continued to be high, the Center for Biological Diversity sued the California Department of Fish and Wildlife in October 2017. The lawsuit claimed that the number of whales entangled violated the Endangered Species and Marine Mammal Protection Acts. This led to a settlement in late March 2019 requiring the California Department of Fish and Wildlife to close the Dungeness crab fishery nearly three months early — April 15 this year and April 1 through 2021 — with the caveat that the season will close earlier if too many whales are found entangled.

Challenges abound in conservation of any species. As climate change picks up the pace, projecting the future for any animal is a challenge, much less a 30-ton marine animal that relies on stable ocean temperatures to produce the massive quantities of krill they require. Whales are integral to the marine environment near San Francisco. Fishermen, researchers, and agencies working together to develop new gear and methods, and economically supporting implementation of these creative strategies, is the way forward to protect these ocean giants when they visit our waters to feed.

This was written by Kaytlin Ingman, Karen Backe and Ellen Hines. It is one of an occasional series about the Bay and sea around us through the eyes of researchers at San Francisco State University’s Estuary and Ocean Science Center. Hines is associate director and professor of geography & environment at the center. Ingman and Backe are recent graduates.

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