New Model Accounts for Northern Shrimp’s Sensitivity to Temperature

by Catherine Schmitt

Winter is not the same without shrimp. Since 1938, Maine fishermen have spent the cold months from December through March working the icy waters, hauling in nets filled with small, tender, dark pink shrimp: Pandalus borealis, northern shrimp. Fishing historically took place statewide by hundreds of boats. Maine fishing families supplemented their incomes, diversified their vessels, and provided fresh, nutritious food in the depths of winter.

It was always a variable enterprise, as shrimp are short-lived (about six years max) and their populations naturally tend to fluctuate. But in the last ten years, this fluctuation has turned into a steady decline. The number of young shrimp reaching catchable size of 10-12 millimeters (what scientists call “recruitment”) and the number and biomass of spawning adults have reached historic lows.

This year, the fourth season in a row, regulatory agencies have closed the fishery. According to the latest stock status update from the Atlantic States Marine Fisheries Commission, “the Gulf of Maine northern shrimp stock status continues to be critically poor…the stock remains in a collapsed state.”

To date, fisheries scientists have based management decisions and harvest limits on an index of data from fishermen and from surveys conducted by state and federal agencies in Maine, New Hampshire and Massachusetts.

This spring, the Atlantic States Marine Fisheries Commission is set to adopt a brand new stock assessment model developed by Dr. Yong Chen and his postdoctoral research associate Jie Cao at the University of Maine. For the first time, management will account for shrimp’s complex life history, the seasonal nature of fishing, and environmental conditions.

The model was challenging for Chen and Cao, as the biology and ecology of shrimp make them difficult to assess. For one thing, there’s no reliable way to tell how old they are. Another thing is that they are hermaphrodites – they start out life as males and then, after about three years, they change into females. They make this transformation at varying sizes (lengths), depending on water temperature and their population density.

Shrimp mate in the summer, and then egg-bearing females move inshore in fall and early winter, when coastal waters are cold and, presumably, the larvae hatch in time to feed on the spring phytoplankton bloom. Larger females produce more eggs; some survive to mate again. Juveniles spend a year or more in coastal waters before migrating offshore.

Northern shrimp are sensitive to temperature changes. The Gulf of Maine population, which is believed to be separate, is at the southern extent of the species range, which stretches around the sub-polar northern hemisphere. According to ASMFC, Northern shrimp in other areas of the world (Greenland, Flemish Cap, Grand Banks) have also seen decreasing trends in abundance and growth, “providing additional evidence that environmental conditions are impacting northern shrimp across their range.”

In addition to affecting when males turn female, temperature influences other aspects of shrimp life. Because they are a cold-adapted species, warmer water is generally negative. Warmer temperatures may result in later inshore migration, poor egg quality, earlier hatching, faster growth but lower survival, ultimately leading to fewer shrimp reaching harvestable size.

In contrast to the animals they represent and the livelihoods they affect, fisheries models are simple and formulaic. Chen’s job is to make them better, and try to get as close as possible to what happens out in the ocean. ASMFC had rejected an earlier version of the model developed in Chen’s lab because it did not include the effects of temperature. He and his students kept working on it.

The new model incorporates some fifty different variables, including changes in length, sex, seasonal variations in growth and reproduction, predation, and fishing patterns. “The model aims to improve stock assessment by increasing biological realism in simulating key life history and fishery processes,” he wrote in the Canadian Journal of Fisheries and Aquatic Sciences with co-authors Cao and NOAA shrimp biologist Anne Richards. The research was funded by Maine Sea Grant and the Maine Department of Marine Resources, with additional funding from NOAA.

The model estimates the size of the shrimp population, as well as how many shrimp are of different lengths at the beginning of the fishing season. They tested the model using different scenarios, and found that temperature, seasons, and sex changes were important factors in a successful simulation. With these factors in place, they then tested the model against shrimp data from the last 30 years. The landings predicted by the model were almost identical to reported landings of shrimp. Past years of plenty were related to environmental conditions favorable for larval survival in the previous spring (cooler temperatures). Poor years, which have occurred recently, were explained by fishing pressure, declining populations of young shrimp, and high natural mortality.

This agrees with the ASMFC stock status report, which blamed the fishery’s “highly uncertain future” on “increased fishing pressure, coupled with failed recruitment, the lowest abundance indices on record, and unfavorable environmental conditions.”

ASMFC counted warming winters as the primary factor in the decline. Average winter sea surface temperature, as measured daily at Boothbay Harbor since 1906, has increased 2.5 degrees Celsius. Shrimp are moving inshore later, eggs are hatching a month earlier. “Long term trends in environmental conditions have not been favorable for northern shrimp in the Gulf of Maine. This suggests a need to conserve spawning stock biomass to help compensate for what may continue to be an unfavorable environment.”

This is Chen’s second model adopted by ASMFC; his lobster model has been used since 2005. At the state level, Chen and students in his lab at UMaine have worked with DMR to improve management of lobster, urchins, and scallops.

Chen and his students will lead a workshop in April to show the ASMFC’s shrimp technical committee how to use the new model.

Northern shrimp was never a big part of Maine’s fishing portfolio. But Chen said that, socially, its really important. “It’s good timing for fishermen, providing a winter season for lobstermen, and nice for local communities. In the past, we focused on volume, big fisheries. We need to refocus on small, low volume, high-value fisheries.” Chen believes shrimp can be such a fishery, that they still have a chance to find good habitat in colder parts of the Gulf. “I’m hopeful, confident that we can have a fishery, but we have to change our strategy.”

At its November 10, 2016 meeting, the ASMFC Northern Shrimp Section voted to extend the Gulf of Maine commercial shrimp fishing moratorium for another year, and set aside up to 53 metric tons of shrimp for a small research fishery, in order to continue the time series of winter samples that used to be collected during the commercial fishery. According to DMR marine resource scientist Margaret Hunter, they are also collecting data on the performance of a size-sorting grate intended to prevent the capture of the smallest shrimp. Gulf of Maine shrimp trawlers have been required to have a fish excluder grate in their net since 1990. The new compound grate was developed by fishermen working with New Hampshire Sea Grant, but has never been thoroughly tested.

“The particular grate the trawlers are using is a compound grate, which is a single grate with two sections. One section has small bar spacings that allow the passage of small shrimp through the bars and out an escape opening in the net; the next section has larger bar spacings that allow the larger shrimp to pass through to be retained in the codend; anything too large to pass through either section of the grate is probably a fish and will exit out another escape opening,” said Hunter (a diagram and more information about the project can be found at http://www.maine.gov/dmr/science-research/species/shrimp/winter2017.html).

Fishermen, too, want a fishery. More than 60 applied to participate in this year’s Gulf of Maine shrimp research program; ten trawlers and five trappers were selected. The trawlers from Massachusetts, New Hampshire, and Portland began the week of January 16, and the Midcoast trawlers and all the trappers began the week of January 30.

Hunter provided data from the first three weeks. The New Hampshire boat caught 99 lbs the first week and 190 lbs the second week. The Massachusetts boat caught 225 lbs the first week and 425 lbs the second. The three Portland boats caught their limits (1200 lbs per trip) or close to it (with the exception of some trips that met with bad weather). The two Port Clyde boats caught 320 and 500 lbs their first week. The three Midcoast trawlers got their limits. The catches for the trappers for their first week were mixed with one getting his limit.

“These results are similar to last year’s, with highest catches in the Portland to midcoast Maine area. The shrimp are beautiful, averaging about 40 count per lb, mostly egg-bearing females so far, but egg hatch seems to be running earlier than last year. The shrimp size is consistent with a four-year-old, which we expected. That’s about all I can tell you so far,” said Hunter.

Prices paid at the Portland Fish Exchange varied from $6 to $12 per lb (whole), averaging about $8. At markets including Delano’s Seafood in Waldoboro, Cantrell’s in Topsham, and Harbor Fish in Portland, a few lucky customers gladly paid the price.