Everybody Loves Shrimp

A catch of Northern shrimp, Pandalus borealis, being hauled aboard of fishing boat.  Credit: wikimedia commons - NOAA FishWatch

A catch of Northern shrimp, Pandalus borealis, being hauled aboard of fishing boat.  Credit: wikimedia commons - NOAA FishWatch

Last week the advisory committee for the Gulf of Maine Northern Shrimp Fishery announced the closure and cancellation of this year’s shrimp fishing season because the Gulf of Maine shrimp stock has collapsed.

What exactly does that mean, “the stock has collapsed”?

A stock is a distinct subpopulation of shrimp (or organism); the whole population can contain several stocks or subpopulations between which there is little migration.  A stock is considered collapsed when its size falls below a threshold number.  When this happens population growth is negative (essentially, there are not enough young born to replace the deaths).

A brief history of the Gulf of Maine Northern Shrimp Fishery

Estimates of the population size of the Gulf of Maine Northern shrimp stock based on a few long term scientific trawl surveys.  Credit: 2013 ASMFC Stock Assessment Report

Estimates of the population size of the Gulf of Maine Northern shrimp stock based on a few long term scientific trawl surveys.  Credit: 2013 ASMFC Stock Assessment Report

In the early 1970s the shrimp population was healthy.  In 1978 it crashed, leading to the closure of the fishery.  The shrimp population then showed recovery and the US fishery was reopened (the Canadian fishery in the Gulf of Maine remains closed to this day).  The population remained relatively stable through most of the 80s and 90s, peaking in the late 90s.  However, since then the population has begun decreasing.  Last year the shrimp population was so low, that fishermen in the Gulf of Maine caught less than 50% of the catch quota across the fishing season (something that is nearly unheard of in the fishing industry, particularly if the fish populations are healthy).  Just illustrate the dramatic crash of the population, shrimp landings (pounds per hour of trawling) in Maine for the last 4 years (2010-2013) were: 401, 347, 399, and 110 (2013 ASMFC Stock Assessment Report)

All estimates of the stock size suggest there are no signs of rebound, and thus we find ourselves without a northern shrimp fishery in the eastern United States.

So what happened?

The Northern Shrimp, Pandalus borealis, on the seafloor.  Photo credit: Penobscot Bay Blog

The Northern Shrimp, Pandalus borealis, on the seafloor.  Photo credit: Penobscot Bay Blog

The northern shrimp, Pandalus borealis, is a circumpolar species, meaning it is found around the North Pole in the North Pacific and North Atlantic Oceans.  The Gulf of Maine is the southernmost region in which these shrimp can be found in the North Atlantic, and it is thought that temperature is the main determinant of this southern limit.  These organisms thrive in cold water and the Gulf of Maine historically represented the warmest temperatures at which these organisms can thrive.  However, ocean water temperature have been increasing.  Recent research shows that, in the Gulf of Maine (where the water is already relatively warm – relative to Greenland that is), increasing water temperature is having a detrimental effect on reproduction.  The current warmer water of the Gulf of Maine is decreasing the likelihood of a young shrimp larvae surviving through early development to metamorphosis and settlement on the seafloor.  Following the trends in recent in warming in the Gulf of Maine is the decline in the local shrimp population.

So this is all a result of climate change right?  One fisherman interviewed in response to the fishery closure might say so.  He was quoted as saying that overfishing can’t be the cause of the population crash because they didn’t even reach their quota last year.  He’s right in his suggestion that the population was showing signs of trouble before this year.  But here’s the problem with his suggestion that overfishing isn’t a part of the problem.  Standard fishing practice sets a minimum catchable size for the “fish”.  The goal is to ensure that we are not catching the young fish before they have the opportunity to reproduce, maintaining a stable population size.  This effectively ensures that we are catching only larger individuals who are capable of reproducing (and hopefully already have).  Applying this strategy to Northern shrimp poses a potential problem.

Typical life cycle of the Northern shrimp Pandalus borealis.  Details shown are for more northerly species which grow slower and live longer than the Gulf of Maine stock.  Credit: Fisheries and Oceans Canada

Typical life cycle of the Northern shrimp Pandalus borealis.  Details shown are for more northerly species which grow slower and live longer than the Gulf of Maine stock.  Credit: Fisheries and Oceans Canada

The Northern shrimp is a sequential hermaphrodite.  All individuals develop first into males and then later into larger females.  Most individuals, particularly in the Gulf of Maine stock, live long enough to reproduce only once as a female.  If we target larger individuals, we are selectively targeting females.  If the population is healthy and robust, then, in theory, we should be removing only a small portion, small enough as to not have a large impact on total reproductive output. 

However, when the population is struggling, this strategy can lead to compounding impacts year after year.  When a fish population is struggling, like with Northern shrimp, there is a time delay before we are able to observe it and a further delay before fishing regulations are altered in response.  The result: fishing pressure was likely too high for the declining population before regulations were adjusted, exacerbating any other environmental changes affecting the shrimp population.

Size difference between female and male Northern shrimp, Pandalus borealis.  Credit: Fisheries and Ocean Canada - C. Nozeres

Size difference between female and male Northern shrimp, Pandalus borealis.  Credit: Fisheries and Ocean Canada - C. Nozeres

In the same interview, the local NH fisherman went on to say that if the shrimp stock doesn’t rebound after this one year of shrimp fishing moratorium, then it will prove that overfishing isn’t the cause.  He is neglecting to consider the biology of this organism in this statement.  Shrimp take 2.5 years to develop into mature males and another year to transform into mature females.  This means any young shrimp that hatch out this year will not begin reproducing for at least another 2 more years.  There is a 2 year time delay between when the newly hatched shrimp can contribute to the population.  At least a 2 year time delay before we will be able to observe any signs of population growth.

So what caused the population crash? 

Likely, many things contributed:  Rising ocean temperatures decreased reproductive and young shrimp larvae success.  Overfishing removed reproductive adult shrimp from the population, decreasing the population reproductive potential.  Trawling practices of several fisheries damages the benthic habitat of these shrimp.  Removal of top predators allows intermediate predators to thrive, predators which may prey on young shrimp.

The question now - is the removal of fishing pressure enough for the shrimp population to rebound from its low abundance in the face of many recent environmental changes?

Resources:

Looking for guidance on sustainable fisheries or alternative seafood options?  Check out Monterey Bay Aquarium's Seafood Watch

Fisheries Science 101: an overview of how fisheries are assessed and information about the Northern Shrimp trawl surveys

Background Information on the Northern Shrimp, Pandalus borealis

Official Reports on the Gulf of Maine Northern Shrimp Fishery from the Maine Department of Marine Resources (including historical/current landings)

Canadian Northern Shrimp Fishery: biology, historical and current fisheries, and conservation measures

 

References:

Primavera, J. Honculada. "Socio‐economic impacts of shrimp culture." Aquaculture Research 28.10 (1997): 815-827.

Richards, RAnne, et al. "Climate change and northern shrimp recruitment variability in the Gulf of Maine." Marine Ecology Progress Series 464 (2012): 167-178.

Whitmore, Kelly, et al. "Assessment Report." (2013).