On this week’s Alaska Fisheries Report with Terry Haines: Where have all the snow crab gone?
The Mystery of the Vanishing Snow Crab
(Sam Spade voice) Scientists and detectives have a lot in common. Both try to solve mysteries by following a trail of evidence. In 2017 the case Alaska’s fish scientists were working was The Mysterious Disappearance of the Pacific Cod in the Gulf of Alaska. They had a prime suspect. Its name was The Blob. The marine heat wave that swept into the Gulf between 2014 and 2016 was historic. And when bottom trawl surveys in 2017 showed an alarming 79% drop in cod biomass, it was easy to point the finger at The Blob. But the details were scarce. For one thing, why was cod the blob’s victim? Other species had seemingly been left unscathed, while some had even thrived during the heatwave.
Clues began to pile up as test results came in. For one thing, the warmer water raised the metabolic rate of young cod. A higher metabolic rate meant young cod needed more food to survive. At the same time the composition of vital forage species, especially plankton, the tiny critters at the base of the food web, changed dramatically. The high fat species that had dominated the blooms were replaced by high protein species. So at a time when they needed ribeye steak, tofu was on the menu. And scientists also learned that a relatively narrow band of water temperatures is required for the successful survival of cod eggs. The Blob brought in temps that virtually cooked the eggs in their beds.
The case isn’t closed yet, and a recent cooling trend has given cod in the Gulf a bit of a respite, but if water temperatures continue to trend upward in the Gulf, we can reasonably expect more assaults on the cod biomass going forward.
And now scientists have a new case: The Mystery of the Vanishing Snow Crab. Like the cod in the Gulf of Alaska, Bering sea snow crab seemed to be on the verge of a banner year when stock assessment surveys conducted in 2020 caused managers to set the lowest Total Allowable Catch ever, and declare the fishery overfished, which compelled them to begin formulation a rebuilding plan.
Here is Ben Daly from the Alaska Department of Fish and Game, from a presentation he gave at this year’s Comfish Alaska:
(The Big Question)
They had three scenarios in mind that might explain the lack of Opies:
(Where Did They Go?)
Obviously, fishermen would prefer that the survey missed them, or that they just walked away, or both.
(Survey Missed Em)
In others words, it’s highly unlikely that the crab cleverly lined themselves up between the survey lines to disappear in plain sight. Crabs do have legs, though and are known to use them. Is it possible that they just walked into the Northern Bering Sea, or into Russian waters?
(They Walked Away)
During the question period Duncan Fields pressed Ben Daly about possible causes:
Ah! Another suspect! Bitter Crab Syndrome. This disease, which affects a number of valuable crab species around the world, is caused by dinoflagellates, the same type of plankton that causes deadly red tides, and light up the ocean with bioluminescence. The dinoflagellete that causes bitter crab disease works its way into the crab’s bloodstream, where it somehow goes undetected by its hemocytes, which function like white blood cells. In fact, the parasites eventually replace the hemocytes, as they steal the crab’s energy from the inside, making them sluggish, eventually causing the flesh to become inedlibly bitter, and finally resulting in the death of the host.
Plankton populations are very much effected by sea surface temperatures. Their blooms can feed the ocean, or they can be deadly to its denizons. It’s possible that the most significant results of climate change will come about through the changes we will see in the life cycles of these tiny creatures.