Posted on behalf of Wendee Holtcamp, blogging for Nature aboard the research vessel Thomas G. Thompson
Good news! After a couple of days of bending metal, the crew fixed the Multi-Core and the mud team is once again collecting sediment as we continue traversing transects in the Bering Sea.
The fish team also seem excited. Their keenness is due to the ‘NP’ (Nunivak-to-Pribilof) transect line we just started and the fact that last year they caught more pollock here than anywhere else.
“So far we haven’t found many fish this year,” University of Alaska doctoral student Elizabeth Siddon told me.
What is happening with pollock is a question Siddon’s research hopes to address. She will use fish collected from this project to study how environmental variability – including salinity, temperature, and zooplankton abundance – influences the fish community composition in the Bering Sea. What makes young pollock more and less abundant and what explains the current decline in pollock numbers?
Scientists divide the Bering Sea into three domains: coastal, middle, and outer, all on the continental shelf. A long finger of water that stays less than 2 degrees C on the ocean floor – the “cold pool” – characterizes the middle domain. It reaches further south on cold years, forcing many organisms to avoid it.
Winter sea ice extent reached a 30-year high in 2008, and the trend continued in 2009 and 2010. This causes a large cold pool when the ice melts, and the cold spell may be causing low larval fish catches in the MOCNESS (Multiple Opening/Closing Net and Environmental Sampling System).
Between 2000 through 2005, average Bering Sea temperature was unusually high. The extreme fluctuations created a good scenario in which to test the Oscillating Control Hypothesis (OCH), a broad framework for understanding the complex food web of the Bering Sea. Originally developed to understand what drives pollock production, the hypothesis also makes predictions about everything from benthic invertebrates to seabirds.
It works like this: In cold years, sea ice melts late, when ample daylight spurs algal growth at the edge of the melting ice. Algae thrive in the icemelt layer, which has lower salinity. Since zooplankton don’t fare well in too-cold water, the algae lives, dies, and sinks to the ocean floor rather than getting eaten. Sparse zooplankton likewise limits the population of ‘forage fish’ – small species as well as young pollock. Overall, cold conditions cause pollock production to be limited by food supply – aka bottom-up regulation of the food web. Colder conditions also favour benthos; the king and snow crabs caught by fishermen in Deadliest Catch hang out in Bering’s northerly regions. The Bering Sea floor contains a wealth of other benthic life such as brittle stars, polychaete worms, bivalves, and sea cucumbers.
In warmer years, the sea ice melts when it’s still mostly dark. Winter storms mix the sea, delaying the algae’s growth. By spring, the top layer warms, becoming stratified by temperature rather than salinity. The late-blooming algae feed the zooplankton, which prosper, leading to a thriving fish community. This, in turn, allows apex predators, like larger fish and piscivorous seabirds, to prosper. The oscillations kick in after a few warm years because pollock become so abundant that they start to eat their own young, and their population starts to exert top-down control on the food web. The OCH predicts that when the temperature shifts to cold again, you get a combination of top-down and bottom-up regulation of the food web.
A warm spell in the 1970s led to super-abundant pollock in the Bering Sea, driving an active fishery. The population is now declining, but no one knows exactly why. The pollock may be oscillating back towards bottom-up regulation, they could be declining from competition with arrowtooth flounder, or they could be overfished – although Alaskan fisheries are renowned for their sustainability.
Siddon’s catches may answer some of these questions. She also studies bioenergetics, looking at how fat reserves help fish larvae survive winter. In fact, recent data on bioenergetics has led George Hunt, author of the OCH, to tweak his original theory; that paper has just been submitted for review.
Previous posts:
Wild Weather, Damaged Equipment & the Oscillating Control Hypothesis
Setting sail for climate change research
Image: Wendee Holtcamp