Northern shrimp, Pandalus borealis

Background Pandalus borealis is one of about 20 Pandalus species world-wide (Komai 1999). While officially known as the northern shrimp (Williams et al. 1989), it is also recognized under several other common names, including pink shrimp (Shumway et al. 1985), deep-sea shrimp (Ito 1976), as well as a prawn rather than shrimp. More local names, such as Maine shrimp (Czapp, 2005), also exist.

Until recently it has been regarded as a widely ranging circumarctic species (Williams and Wigley 1977) that extends into northern Pacific and Atlantic regions, here referred to as P. borealis sensu lato. Western populations from the Bering Sea and further south in the Pacific can now be considered a separate species (Squires 1992) based on distinct traits of larvae and adults, here referred to as P. borealis sensu stricto. However, this change has not yet been universally adopted in the literature (Bergström 2000).

Within the North Atlantic, P. borealis is known from northern Europe in the eastern Atlantic, and in the western Atlantic from Greenland to about 40ºN latitude (Squires, 1990), just south of the Gulf of Maine. The species is typically found at depths ranging from 10-500 m (Squires, 1990; Pohle, 1988), but is also known from deeper waters (Bergström 1992). Usually these schooling shrimp live on or near soft bottom with high organic content but do exhibit diurnal vertical migration (Shumway et al. 1985). They are found at temperatures ranging from 2 – 14ºC, but occurrences in areas below 0ºC are known (Squires 1968).

In Atlantic waters, P. borealis is one of three recognized species of Pandalus that overlap geographically and bathymetrically, but is the only one amongst these with a major commercial fishery (Williams and Wigley 1977). Attaining a total length of over 18 cm in females, it is also the largest of these species (Bergström 1992). This shrimp is an important link in marine food chains, feeding on sea-bed invertebrates and plankton, such as krill, amphipods, and copepods, while being consumed in turn by such fish as cod, halibut, redfish, silver and white hake (Squires 1965; 1990).

Life cycle Like its congeners, Pandalus borealis has a protandric life cycle, starting life as males that, through transitional stages, change to females at about 4 to 7 years of age. In this species some individuals may also develop directly into ‘primary females’, in which male characteristics never appear. Environmental temperatures have a considerable effect on sex determination in terms of timing and duration (Squires 1990). The ability to vary timing of sex change is seen as a strategy to maximize reproductive success (Bergström 1992).

Generally, mating and oviposition of fertilized eggs occur within 36 hours during late summer to fall (July-October), with developing embryos being carried by the female on pleopods until hatching the following spring (March – June) (Bergström 2000). The colder the environment, the earlier the spawning and the longer the incubation period. The latter ranges 5-11 months. During that time ovigerous females migrate from deeper waters offshore to shallow areas inshore (Haynes and Wigley 1969) where spawning occurs. Fecundity may vary from 300-4300 eggs (Squires 1990), averaging about 2000 for a typical-size female (Shumway et al. 1985).

Pelagic larvae hatch as a stage I zoea, followed by an additional four or more zoeal stages and a megalopa, before entering the juvenile stage (Squires 1993, Wienberg 1982). Larval development may take 45-120 days, depending largely on temperature (Stickney and Perkins 1977). Larval dispersion should largely be determined by local current conditions but larvae tend to stay in shallow water, being able of some vertical movement (Shumway et al. 1985). Juveniles tend to remain inshore for more than a year before migrating offshore as they mature. In the Gulf of Maine age at 50% maturity is about 3.5 years in females (Idoine 2001).

Fishery Among all shrimp species of the pandalid family, the fishery for Pandalus borealis is the largest one (Balsiger, 1981) and thus of considerable economic importance. In the northwestern Atlantic primary fishery areas for P. borealis s.s. are concentrated off Greenland, Labrador, Scotian Shelf, Gulf of St. Lawrence, Bay of Fundy and Gulf of Maine, as far south as Gloucester Mass (Parsons and Fréchette 1989; Koeller et al 1999). Yields are higher for more northern fishing areas, the Davis Strait fishery peaking at about 41,000 tonnes in the 1980’s. Areas around Iceland and further east along the coast of Norway are also important fishing zones (Holthuis 1980). Worldwide, the Pandalus borealis s.l. fishery has been steadily increasing from a few thousand tonnes in the 1950’s to over 350,000 tonnes in recent years, with Canada reporting the largest catches (http://www.fao.org/figis/servlet/species?fid=3425). However, stock biomass in some fishing areas, such as the Gulf of Maine, have experienced periods of significant decline in the 1970’s and late 1990’s (Idoine 2001) that included a fishery closure during the former period (Clark et al. 2000).

Temperature limits The northern shrimp is a cold water stenothermal species with a preferred temperature range of 1-6ºC (Shumway et al 1985). Based on the current northwest Atlantic distribution of P. borealis, SSTs range from a February minimum of -2.1ºC to an August maximum of 23.1ºC, considerably outside the preferred temperature range. However, in their review of temperature ranges for P. borealis, Shumway et al. (1985) reported northern shrimps in waters with temperatures from -1.6ºC to 12ºC, with Squires (1990) reporting temperatures as high as 14ºC. Larvae indicate possible tolerance of still higher temperatures, as laboratory work on specimens acclimated at 10ºC showed 89% and 77% survival at 16º and 18ºC after 22 hours exposure, respectively (Stickney and Perkins 1977). This may be an adaptation of this phase, as larvae hatch and are more prevalent in shallow waters. Nevertheless, while low SSTs agree with reported temperature ranges, the max. SST’s exceed what is known for the species. Likely, the shrimp occur in deeper locations or further offshore within those areas or are not found there at those times of year. The former appears to be the case at southern limits, where Squires (1990) reports records at 40º latitude as offshore near the continental shelf.

Impacts: Temperature, together with depth, substratum and salinity, are all major physical determinants of the distribution of P. borealis (Bergström 2000), but temperature seems most closely correlated with changes in abundance (Shumway et al. 1985). Temperature also appears to have an effect on sex ratio, and therefore reproduction, but this does not seem to be well understood, may vary by location (Shumway et al. 1985) or be related to differences between Atlantic and Pacific taxa.

A 4ºC rise in global temperature will impact the future distribution of P. borealis in the western Atlantic. Results from all four models under both scenarios agree in a predicted loss in the southern range of the species. The affected waters are south and north of Cape Cod, including portions of the Gulf of Maine, with the CSSR model showing the widest affected area. These losses may extend further offshore beyond areas not covered in this study. Two models predict potential habitat losses elsewhere. Under scenario A2 for CCSR and GFDL models, this includes the southwestern Gulf of St. Lawrence near Prince Edward Island. Within the Gulf these are areas with relatively warmer waters. While those may not represent prime shrimp habitat, it nevertheless indicates that there is potential for a fisheries impact within the Gulf of St. Lawrence. The CSSR model predicts impacts elsewhere, including the Scotian Shelf and offshore on portions of the Grand Banks off Newfoundland under scenario A2. As well, both scenarios of the CCSR model predict possible impacts in areas off Labrador. Although the latter CCSR scenarios are perhaps less likely to occur than the others, it indicates that regions other than southern ones can be potentially impacted under a 4ºC warming scenario. No northward gain of habitat into arctic waters being present-day occurrence is predicted in our study, but this is not necessarily so as the study did not extend to these areas. In the southern range of the species increasing water temperatures resulting from global warming consistently predict a loss of habitat, areas where P. borealis is not widely abundant today. A northern distributional shift will likely result in suboptimal fishery conditions of P. borealis in the adjacent Gulf of Maine. In that area, significant declines in landings have already been linked to increases in water temperature (Dow 1981). Studies on Pacific pandalid shrimp have also corroborated impacts of climate change, having found that populations declined to near extinction in shallow water and low population levels in deeper water, following an abrupt water column warming after 1977 (Anderson 2000). It is reasonable to assume similar effects in the Atlantic, and particularly the Gulf of Maine, under the proposed warming scenario.

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