Why does our local cold-water shrimp fishery undergo such huge ups and downs? Part of the variability in harvest from year to year is due to the shrimp’s life cycle. Pandalus borealis or northern shrimp are hermaphroditic: typically, they first mature as males at about 2 1/2 years of age, and then transform to females a year later. The females develop eggs, which they typically spawn (extrude from their bodies) in offshore waters beginning in late July. Eggs are then carried by females, tucked along their undersides.
Egg-bearing adult females move inshore in late autumn and winter where they are the target for the Gulf of Maine fishery (at this point, the adults are about four years old). Eggs are released into the water while the shrimp are inshore, usually during January, February and March. While some females may survive to spawn again, it is believed that most northern shrimp do not live beyond five years in the Gulf of Maine, so the continuing health of the population in terms of the numbers of eggs produced is entirely dependent on the breeding success of one or two adult female year-classes.
Once released, the eggs and larvae must encounter productive conditions to lead to a strong new year-class of juvenile shrimp. Newly hatched larvae ideally feed on the early pulse of phytoplankton that typically occurs each winter before the big spring bloom. The timing of the hatch is sensitive to temperature.
Analysis by Anne Richards of the National Marine Fisheries Service shows that the start of the hatch has shifted earlier due to the influence of warmer sea surface temperatures on egg development. Because of the earlier start date, the total hatch period is longer, resulting in a greater overlap with the phytoplankton pulse (which is not related to temperature, but more strongly influenced by levels of nutrients and wind-driven mixing).
“Temperatures are warming, and we’re getting these huge year classes” said Richards. “This is counterintuitive because shrimp are an Arctic species. They are at the southern edge of their range in the Gulf of Maine, so you’d expect them to do better in colder water.” But shrimp appear to be doing well, possibly because of the more extended time window for newly-hatched larvae to feed on phytoplankton blooms. An earlier hatch could be bad for shrimp if the phytoplankton aren’t there to feed on, which so far hasn’t happened. Richards also notes that developing larvae do better in cooler temperatures, as do adults, seeking refuge along the bottom of deep, cool muddy basins in the southwestern Gulf when surface waters heat up in summer.
Juveniles remain in coastal waters for a year or more before migrating to deeper offshore waters. The exact extent and location of these migrations is variable and unpredictable, and depends on wind, currents, and unknown factors. “They’re pretty cryptic,” said Margaret Hunter of the Maine Department of Marine Resources, who supervises data collection for the annual stock assessment. “We think they stay inshore for a year and over time make their way offshore, because we do see them in our summer survey.”
The annual summer survey provides a snapshot of the shrimp stock offshore, with each year’s shrimp being tracked over time. Shrimp that hatched in 2009 were found in the 2010 survey, and will be available for harvest in another few years. So the abundance of shrimp in 2009-2010 season was the result of a strong 2005 year class; the 2006 class didn’t fare as well, so the prediction is not as good for the 2010-2011 season.
“This summer we saw a decline in the number of females, the larger shrimp, which was not unexpected because the 2006 class has been weak all along,” said Hunter. Surveys are coupled with a computer model that generates the number of harvest-size shrimp. These computer-generated population numbers are translated into various scenarios for the fishery each winter. “The management plan goal is to maintain fishing mortality at a level comparable to the relatively stable period from 1985-1995,” said Hunter.
Another question is what else is eating northern shrimp besides us, and what impact this predation has on population levels. According to Brian Smith, a fisheries biologist at the Northeast Fisheries Science Center who works with the food web dynamics program, many fish species eat P. borealis. However, the primary predators of northern shrimp are all generalists. They consume not just P. borealis, but also other shrimp and other non-shrimp prey such as small fish. “You’re talking tens of shrimp species in the region” notes Smith. “Predators are selecting feed by habitat or habit, time of day, season, location in the water column, rather than searching for a particular shrimp species.” Pollock, as an example, are big shrimp eaters, and are found with many shrimp species in their stomachs. Fishermen, on the other hand, aim exclusively for P. borealis, our preferred Gulf of Maine food shrimp.
The annual summer shrimp survey provides incredibly valuable information for fishermen and fishery managers, allowing people to make management and business decisions with at least a few months lead time before the winter fishing season. Unfortunately, we still face considerable uncertainty in predicting and understanding what the northern shrimp fishery will look like more than a few years in the future, given predictions for changing temperatures and timing of phytoplankton blooms. Research into the synchrony between the hatch and the phytoplankton bloom and other environmental factors is shedding some light on possible future conditions.
This article is made possible in part by funds from Maine Sea Grant and the Oak Foundation.
Heather Deese holds a doctorate in oceanography and is the Island Institute’s senior programs director – marine initiatives. Catherine Schmitt is communications coordinator for Maine Sea Grant.