Warm Water or Cold Water...The Northern Shrimp Shows its Preferences!

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The northern shrimp (Pandalus borealis) is the primary shrimp species in the Canadian Atlantic. In the Gulf of St. Lawrence, it has been fished since the mid-1960s, and, with the development of the commercial fishing industry for this prized crustacean, it has been and continues to be necessary to protect the resource. That is why, in 1998, Fisheries and Oceans Canada signed an agreement with a group of Quebec and New Brunswick fishers fishing for northern shrimp in the Gulf of St. Lawrence (called "Group B fishers"). This agreement included joint funding by Fisheries and Oceans Canada and the fishers for a comprehensive research program on the biology and ecology of the northern shrimp.

Some of the research in connection with this program was done at sea. However, a number of other studies were conducted in a controlled environment. In the tank room at the Maurice Lamontagne Institute (MLI) in Mont-Joli, the natural living conditions of the northern shrimp were reproduced in part. In each tank, parameters were varied to determine their influence on population dynamics. Among other things, biologist Louise Savard co-ordinated, in collaboration with Yvan Lambert and Patrick Ouellet, a component of this research that looked at the effects of water temperature on the development of northern shrimp at various stages in their life.

It should be remembered here that P. borealis has an amazing life cycle, with very special characteristics. First of all, toward the month of April, the larvae of the northern shrimp hatch, after eight months of incubation. Surprise! When they hatch, all the larvae are males, and the shrimp remain males for the first years of their life. The northern shrimp is a protandrous hermaphrodite species. In other words, it first reaches sexual maturity as a male, then changes sex and spends the rest of its life as a female. It is at the age of four or five years that the male shrimp changes sex. Thus, during the winter months, a few months after it mates, the male moults many times, its weight and size increase and it undergoes many morphological changes, finally becoming a female. It is interesting to note that, once a male begins the sex change process, it must complete it entirely: this process cannot be stopped. In the fall after the sex change, the shrimp can reproduce as females. Nearly all the females will be inseminated and will carry their eggs under their abdomen until the following April.

In the experiments conducted by the MLI team, the researchers therefore studied the effects of temperature on these various stages in the shrimp's life cycle. Specifically, egg-bearing females were collected at sea and placed in tanks at specific temperatures, i.e. 2, 5 and 8° Celsius. These temperatures were not random choices: they are the temperatures found along the western Atlantic coast, the natural living environment of the northern shrimp.

When the larvae hatched, it was thus possible to evaluate the effects of temperature on the length of incubation and the status of the progeny. The results show that, in warmer water (8 ° Celsius), the incubation time of the eggs is shorter and the survival rate of the eggs is lower. It would appear that the warmer water precipitated the hatching, leading to an increase in the frequency of larval malformation. The same type of experiment was conducted on shrimp at the juvenile stage, measuring the influence of temperature on their growth. Here, the effect was the reverse of that on the eggs: the warmer water temperatures had a positive effect on the juvenile shrimp. Faster growth rates were observed in the juveniles that were kept at 5 and 8° Celsius. However, with regard to the sex change, the effect of temperature on this important stage in the life cycle of the northern shrimp has not yet been determined, and the researchers have not yet been able to clearly identify the determining parameters in this process.

The link between the results of this research and the survival and reproduction behaviour of the northern shrimp in its natural environment is striking. In the Gulf of St. Lawrence, the adult shrimp live at a depth of 150 to 400 metres, in waters that are at a temperature of approximately 4° Celsius. However, when they are inseminated, the females migrate, slowy following the bottom up toward the coast and a colder layer of water at a depth of nearly 150 metres. Since the shrimp carry their eggs under their abdomen, outside the body, it seems that this move to colder water during incubation may serve to control the incubation time, thus preventing the hatching of abnormal larvae and allowing the emergence to occur when the springtime conditions are optimum. Once the incubation period ends and the larvae have hatched, the female shrimp return to deeper water.

With global warming and the as yet unknown temperature changes that will result in ocean waters, it is difficult to predict what the impact will be on shrimp populations and, consequently, on the resilience of stocks to commercial fishing. Although the research project on the effects of temperature on the growth of northern shrimp has significantly increased our understanding of the crustacean's population dynamics, the results show that it is important to integrate study of habitats into research on individuals at various stages in their life.