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Shrimp Down, Lobster Up: Is There a Connection?

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northern shrimp
Northern shrimp, like those shown here being hauled aboard a shrimp boat, are in decline. So much so that the 2014 season in Maine was shut down. TheGreenGrok takes a look. (NOAA)

Something fishy's goin' on with the the shrimp off the coast of Maine.

Maine Shrimpers Cut off

You may have heard, the 2014 shrimp season in the Gulf of Maine, the most southern region where northern shrimp reside, has been called off. On December 3, state regulators in the Atlantic States Marine Fisheries Commission's Northern Shrimp Section, who set annual catch limits for shrimpers, took a step they hadn't taken since 1978: they declared a moratorium.

Citing a "collapsed" shrimp stock [pdf], the group's technical committee noted that the numbers were so low that there was  "little prospect of recovery in the near future" [pdf]. Indeed, as the figure below shows [pdf], the Gulf of Maine's shrimp stock has declined precipitously since 2007 and is at its lowest level in at least 30 years.

shrimp mortality
Estimates of the northern shrimp stock as a function of year. The dotted biomass threshold line denotes the level below which the fishery is considered to be overfished [pdf] and the dotted biomass limit line indicates numbers at which "the capacity of self-renewal of the stock" is endangered. (Source: Atlantic States Marine Fisheries Commission)
What Could Be Behind the Dwindling Shrimp?

These days when a fishery collapses -- for instance, Bluefin tuna, orange roughy, and Canadian cod -- it is usually because of overfishing. An inspection of the annual northern shrimp haul [pdf] since the 1960s suggests that indeed overfishing may be at least one of the culprits behind Marine's shrimp collapse.

Between 2007 and 2011, according to state data, Maine's share of the northern shrimp haul increased 20 percent while the fishery biomass declined precipitously. Higher catch rates could have depleted the population. But the rates of shrimp landings in 2010 and 2011 were nowhere near the highs of the late 1990s, and while the fishery in the 1990s was depleted, the biomass was about 10 times higher than now and the fishery was not closed. Seems that something else in addition to overfishing is at play here.

(Source: Atlantic States Marine Fisheries Commission
(Source: Atlantic States Marine Fisheries Commission

Some suggestions for that "something else" can be found in the report of the Atlantic States Marine Fisheries Commission [pdf],

"increasing water temperatures and a decline in phytoplankton abundance (a food source for shrimp) are factors which likely have and will continue to contribute to the poor recruitment in the stock. The increased abundance of northern shrimp predators (spiny dogfish, redfish and silver hake) may play a role in declining biomass. Northern shrimp stocks in other areas of the world (Greenland, Flemish Cap, Grand Banks) have also seen decreasing trends in abundance and recruitment, providing additional evidence that environmental conditions are impacting northern shrimp across their range."

Note the reference to increasing water temperatures. As scientists have been gathering for some time now (see also here), global warming could very well be one of the culprits in Maine's shrimping woes.

But there's another part to this story, a somewhat different kettle of fish, but still relevant.

A Lobster Tale 

If Maine shrimp is a delicious coastal treat and a small but important food fish, Maine lobster is world-famous and a point of state pride and identity, even -- and an economic force for the state.

My family often summers in the Vacation State on Mount Desert Island. It's a wonderful locale to be sure but the last two years have had an added attraction: cheap, abundant lobster.

State statistics capture the situation: in recent years Maine's lobster haul has skyrocketed, resulting in plentiful, inexpensive crustaceans. 2012 was a record-breaker for highest lobster landings and value recorded, beating out previous records in 2011 and 2010 (see graphic [pdf]). Good news for lobster-loving tourists like moi, but not so great for fishers whose livelihoods depend on the higher prices that smaller catches nab.

lobster landings
(Source: Atlantic States Marine Fisheries Commission

It's a Bit of a Puzzle and Maybe an Ironic Solution

While populations of Maine shrimp are dwindling, Maine lobsters are thriving. (Canada's lobsters are also doing quite well.) So why the glut of Maine lobsters? Theories vary. One holds that it's due to a decline in the populations of cod, sharks and other predators. Another that warming ocean temperature temperatures speed up the metabolic processes in lobsters allowing them to grow faster. If the latter is correct, it'd be an ironic twist of fisher fate -- global warming decimating Maine's shrimp fishery while enhancing its lobster fishery.

But before all you lobsters out there start cheering on global warming, there's one more twist to this story to consider.

For Lobster Populations, It's Location, Location, Location

It turns out that the northeastern United States has three so-called lobster stock areas [pdf]:
  • The Gulf of Maine, the northernmost area, runs from the state's northern border to the north side of Cape Cod,
  • The Georges Bank area lies east of Cape Cod, and
  • The Southern New England area runs along the coast south of Cape Cod to just north of North Carolina.
While lobster stocks from the Gulf of Maine and Georges Bank areas have been thriving in recent years, Southern New England lobster populations have been dwindling. So much so that the Long Island Sound was closed to lobster fishers last fall.

The Southern New England decline appears to be driven by environmental and biological changes [pdf] typified by the spread of a bacterial shell disease, rising ocean temperatures (which may be a root cause for the spread of the shell disease), and a loss in recruitment as spawning has shifted to deeper (and colder) waters. (A similar situation occurred in 1999 with similarly identified culprits but without a shutdown.)

So Are Warm Waters Good for Lobsters?

It's a pretty strange situation, don't you think? Warming temperatures are leading to a thriving lobster population in the Gulf of Maine while decimating lobsters further south. How so? Bob Steneck a marine biologist at the University of Maine explains things like this:

"Anything above 20º C[elsius] is extremely stressful for lobsters. While warmer waters off the coast of Maine in recent years have probably aided the boom in lobster numbers, putting us right in the temperature sweet spot for this species, we're getting closer and closer to that point where the temperature is just too stressful for them, their immune system is compromised and it's all over."

And this is leading many Maine lobster fishers (and lobster lovers like me) to worry. With continued warming, could the disease that's been helping reduce lobster populations to the south begin to do the same in more northerly waters? The Pine Tree State is a beautiful state, sporting some of the country's most spectacular wilderness, but without its lobster and shrimp, Maine just wouldn't be the same. Could it be that going without lobster on a Maine vacation is but one example of the climate adaptations that may be in store?

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