The northern shrimp: in time for dinner!
An international team of researchers has demonstrated that northern shrimp, wherever they are found, from the Gulf of St. Lawrence to the Gulf of Maine, all the way to the Barents Sea, have adapted themselves to give their larvae the best conditions for survival. A strategic thinker, the shrimp is!
Since the 1990s, researchers at Fisheries and Oceans Canada have tried to gain a better understanding of the population dynamics of northern shrimp (Pandalus borealis) in the Gulf of St. Lawrence. Abundant in the northwest Atlantic, this crustacean is an important part of the region's economy. The fishing industry reported landings of 36,000 tons of northern shrimp in 2009, with a value of $36 million. Since the quantities harvested by fishers are strongly linked to the abundance of shrimp populations and, by extension, to the annual recruitment of juvenile shrimp, researchers have strived to understand what influences these factors. At the Maurice Lamontagne Institute (MLI) in Mont-Joli, Quebec, Patrick Ouellet and Louis Savard are part of team that has undertaken a series of projects aimed at documenting the relationship between inter-annual and inter-regional variations in the northern shrimp's reproductive cycle and the oceanographic conditions in which its larvae grow. Since the species is not confined to the Gulf of St. Lawrence, the project is carried out in collaboration with researchers from Nova Scotia, Newfoundland and Labrador, the United Kingdom, the United States, Denmark and Iceland. This will enable them to compare populations that are isolated and remote from one another.
Before looking at study results, here is an overview of the northern shrimp's reproductive cycle. Individuals are born and grow to sexual maturity as males. Between their fourth and fifth year of life in the Gulf of St. Lawrence, they will change sex and becomes females. These shrimp will lay their eggs at the beginning of fall and carry their fertilized eggs under their abdomen until the following spring, when their larvae will hatch. The ocean water temperature influences the eggs' incubation time; colder waters will delay hatching, giving larvae time to reach a certain level of maturity before they emerge. For example, in the Gulf of St. Lawrence we can see an increase in the time it takes larvae to hatch from east to west, corresponding to the temperature gradient in the deep waters where adult shrimp are found. Larvae then migrate towards the surface, where they feed on plankton for the first few weeks of their lives. This early phase can be crucial to their survival and, therefore, to the recruitment of future populations.
The researchers asked themselves whether adult shrimp were synchronizing the hatching of larvae with phytoplankton blooms. This is known as the "match/mismatch" hypothesis. A "match" means a hatching that is synchronized with a phytoplankton bloom, increasing the chance that larvae will find enough of the food that is essential to their survival, growth and development. By contrast, if the hatching time is not synchronized with phytoplankton production, the larvae will face a famine: many will die and recruitment will drop off in the coming years. This is the "mismatch."
Given that water temperature has an influence on the incubation period and hatching time of shrimp larvae, it might seem that phytoplankton cannot have this effect. Phytoplankton blooming time is relatively stable in time, probably because the length of days and luminosity are more important contributing factors to blooms than water temperature is. As well, the abundance of northern shrimp varies from year to year not only in Gulf waters, but also in the northwest Atlantic; this detail is a key element in current research. Indeed, if the match/mismatch hypothesis can explain inter-annual variations in shrimp populations, how do we expand it to describe what occurs in the wider territory the species occupies, in which deep water temperatures vary?
Accordingly, the research team verified the match/mismatch hypothesis all across the north Atlantic, in 12 populations of northern shrimp from the Gulf of Maine in the south to the Barents Sea in the north. Based on data gathered from monitoring shrimp populations and oceanographic conditions, they succeeded in demonstrating a remarkable synchronism between the time the larvae emerge and the phytoplankton bloom in all 12 of the areas studied. "The availability of reliable long-term data on ocean water temperatures, the quantity of chlorophyll, ocean ice, the dates the shrimps' eggs hatch, and the abundance and ages of populations has allowed us to perform these analyses," reported researchers Ouellet and Savard. "We have been able to demonstrate this for the same species in several areas with different conditions," they added, which is an especially innovative feat. The synchronism between the moment the larvae hatch and the phytoplankton bloom is seen as an adaptation in the northern shrimp's reproductive cycle based on local water temperature conditions, such that mating time, egg laying and incubation correspond to phytoplankton bloom in a given region.
Once they had confirmed the match/mismatch hypothesis, the researchers wanted to see if annual variations in the characteristics of phytoplankton bloom had an impact on larva survival and the recruitment of juvenile shrimp. To do so, they selected four out of the 12 groups they had initially studied. They found that the duration of the bloom (number of days) correlated positively with larva survival. That said, it seems that larva survival was affected when near-surface water temperature fell below average in certain years, independent of the duration of phytoplankton bloom. Thus, despite the synchronism between blooms and the hatching of shrimp larva, water temperature affects the survival and growth rate of juvenile shrimp.
In conclusion, these research efforts showed that northern shrimp were able to adapt over time to the oceanographic conditions in which they evolve. The era of climate change that awaits us will certainly have an impact on the ocean environment these crustaceans call home. Can the fragile equilibrium between water temperature, incubation time, larva hatching time and phytoplankton bloom be maintained? The pressure of natural selection might once again bear down on the northern shrimp, forcing it to adapt.
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