Oceanologists have found that Pacific humpback whales have become increasingly caught in fishing nets due to the heat waves, which simultaneously deprived them of their food sources and changed the reproduction cycle of toxic algae. This was reported on Monday by the press service of the University of California at Santa Cruz, citing an article in the journal Nature Communications.
"Marine predators usually live in the open sea, but the heat wave forced them to swim closer to the coast, as all their food" escaped "there. by which the whales moved to the shores of California, "explains Jarrod Santora, an oceanologist at the University of California at Santa Cruz (USA), whose words are quoted by the university's press service.
Recently, as Santora notes, oceanologists began to notice that heat waves occur not only on land, but also in the tropical and equatorial latitudes of the oceans. In particular, in 2014, a gigantic area with abnormally high water temperatures arose off the coast of California and other western regions of North America.
In total, it existed for almost three years, during which its borders gradually expanded to Alaska and the central Pacific regions. This led to a sharp increase in toxic algal blooms and a host of other consequences that caused serious rearrangements in the ecosystems of the Pacific Ocean.
In particular, thousands of seabirds and fur seals have died as a result of the disappearance of their usual food sources, and hundreds of humpback whales have been accidentally caught and entangled in crab and fishermen's nets. Santore and his colleagues managed to save a small part of them.
Faced with this strange phenomenon, Californian oceanographers tried to find out what exactly triggered it. To do this, they studied all the ecosystems that exist off the western shores of North America, and calculated how the heat wave was supposed to affect them.
These calculations showed that the whales began to rush to the trapping nets due to a coincidence generated by two effects of heat waves that were not directly related to each other. On the one hand, rising water temperatures have significantly reduced the abundance of krill and other zooplankton that humpback whales feed on, and have forced them to drift towards the coast of North America, where the cold California Current reaches the ocean surface.
In parallel, another event took place: the US authorities, fearing the consequences of another outbreak of toxic algae blooms, banned California fishermen and crabbers from going to sea from November 2015 to March 2016. In March, when this episode ended, the ban was lifted, and fishermen began to fish en masse from the sea, compensating for their losses.
At the same time, Santora notes, whales began to migrate towards the coast of California, moving in their direction with anchovies, a small fish that they switched to after the disappearance of krill. As a result, both humans and marine mammals were concentrated at the same time in a narrow strip of the ocean. This has resulted in thousands of whales becoming entangled in fishing gear, severely injured or killed.
Three years ago, when the heatwave ended, the krill and anchovy populations returned to their original state and the crisis was settled. Scientists hope that understanding its nature will help avoid another mass death of whales during the next heat waves due to the emergence of coordination between the actions of fishermen and the work of oceanographers.