Heatwave Ushered in New Northern Shrimp Predator in Gulf of Maine
What can the 2012 heatwave tell us about the impacts of climate change on the collapse of northern shrimp? Rising temps are not the only reason the shrimp went away.
The northern shrimp (Pandalus borealis Krøyer) population in the Gulf of Maine collapsed during an extreme heatwave in 2012 and has not since recovered. Shrimp populations are known to be sensitive to temperature shifts. But a new study in PLoS ONE finds that warm water was not the sole stressor. Rising temps also triggered the untimely influx of a voracious shrimp predator, one that likely played a significant role in the collapse of northern shrimp in the Gulf of Maine.
Atlantic Northern Shrimp (Pandalus borealis) courtesy NOAA
Shrimp is an important species in the northern Atlantic ecosystem and once an important fishery for the Northeast. Valued at more than $20M during its peak in the 1970s, the fishery has experienced sharp declines and modest recoveries. Prior to the collapse of 2012, the landings averaged 7.9 million pounds (2003-2012), a dramatic drop from the 1969 high of 28.3 million pounds. The fishery has been shutdown since 2013 with no signs of recovery.
What happened in 2012 was, at the time, historic weather. A record heatwave that June met or surpassed 170 all-time US heat records. Fifty-six percent of the country was experiencing drought. The Gulf of Maine experienced a 2°C rise in mean sea surface temperatures and a drop in shrimp abundance by more than 50 percent. This heatwave presented an interesting lens for scientists to look at the impacts of a warming environment on a marine population. In this case, researchers from NOAA and the Maine Department of Marine Resources were able to look at how the warmer waters upset the predator-prey balance adding further stress to the northern shrimp population.
What’s Eating the Northern Shrimp?
The answer, not surprisingly, comes from analysis of what predators were eating during 2012. Data from the stomach contents of 20 potential northern shrimp predators helped identify eight species of interest including spiny dogfish, Acadian redfish, silver hake, white hake, Atlantic cod, windowpane flounder, Atlantic mackerel and longfin squid (Doryteuthis pealeii Lesueur).
Longfin Squid (Doryteuthis pealeii Lesueur) (image courtesy of NOAA)
The researchers then looked at information that broadly estimated fish population overlaps and compared that to local patterns from trawl surveys that simultaneously caught predators and northern shrimp. As expected, most predator-prey ratios dropped in response to or were in proportion to the declining shrimp populations.
“The only species that showed clear increases in tow-by-tow co-occurrence in 2012 was longfin squid (all four seasonal surveys),” the researchers said.
Longfin Squid Arrived Much Sooner
Longfin squid are a fast growing, short lived species. They are known to be highly opportunistic eaters with crustaceans being a large part of their diet. Also interesting is their behavior. Longfin squid undertake diel vertical migrations, that is daily trips up and down the water column, as do northern shrimp. Both tend to be near the bottom during the day and higher in the water column at night.
Nature has typically provided a seasonal buffer zone for these two species. Longfin squid are not usually abundant in the Gulf of Maine waters until summer. “By the time the longfin squid arrive, female northern shrimp would have migrated back offshore from coastal waters after hatching their brood, thus overlapping very little with longfin squid,” the study said.
Did Ocean Acidification Play a Role, Too?
In this case the researchers say no. Experimental evidence show that northern shrimp may be able to survive more acidic environments. And the Gulf of Main has a unique and complex carbonate system and in this case ocean acidification was not at play as a stressor on the northern shrimp population.
But rising temperatures changed that balance and brought voracious longfin squid to the shores of the Gulf of Maine earlier in 2012. And when the scientists looked at the abundance of longfin squid from Spring into Fall, they found a “much higher than usual spatial overlap with northern shrimp.”
“Dramatic events such as the GOM shrimp population collapse provide an opportunity to see through the haze a bit more clearly. Our results suggest that longfin squid may have been a major player in the collapse of GOM northern shrimp during an extreme marine heatwave event, and provide further evidence that changing species interactions will have major impacts as ecosystems reorganize due to climate change,” the researchers said.