November 3, 1997
By Jennifer McNulty
The number of Atlantic salmon in American rivers has dropped dramatically in recent years, making the scientific quest to understand the mysteries of the life cycle of the remaining fish a high priority.
One of the scientists at the forefront of that investigation is Marc Mangel, a professor of environmental studies at UCSC. Mangel is a key member of the small international community of researchers dedicated to probing the biological and environmental factors that trigger the salmon's patterns of migration and spawning.
To further the probe, Mangel recently started a three-year project, funded by the National Marine Fisheries Service, to explore how oceanic conditions affect patterns of maturation, migration, and survival of the Atlantic salmon.
"Atlantic salmon used to be all over New England, but now they're largely absent from American rivers because of overfishing, habitat destruction, and pollution," said Mangel. "In North America, now they're found only in Maine and Canada."
Ultimately, understanding the life cycle will enable fishery scientists to predict abundance, which is critical information when regulators set harvest quotas. "This research has great applied value," said Mangel. "Aquaculturists need to understand how individual fish grow and interact with environmental conditions to make accurate predictions."
Atlantic salmon, which measure between 18 and 36 inches at adulthood, are born in freshwater, migrate to sea, and return to freshwater to reproduce. Unlike Pacific salmon, many Atlantic salmon survive to spawn again. Although juvenile Atlantic salmon in North America typically first head to sea at the age of two or three years, migration may occur as late as the age of eight, depending on the latitude of the river, and Mangel and his colleagues want to understand what determines the age of this important migration.
Mangel will be working with scientists at the Northeast Fisheries Science Center in Woods Hole, Massachusetts, which is the regional research arm of the National Marine Fisheries Service. His first task will be to analyze data collected by his colleagues on the survival of salmon at sea relative to their growth rate.
"Using their data, I'll be able to make inferences about growth rate and how it affects return of salmon to the rivers," said Mangel. "Generally, the bigger a fish is, the higher its chances of surviving, but there are trade-offs." For example, fast growth may suppress the immune system as the fish's energy is directed toward building tissue. Speedy growth may also indicate that the fish has enjoyed a large food supply, which may carry the increased risk of a correspondingly large population of predators. Finally, ocean temperatures affect growth rates, too.
Mangel, whose office is decorated with framed art prints of salmon, will spend the second phase of the project developing mathematical models of individual fish growth that incorporate all three variables: growth, risk, and environmental conditions.
The work has value for aquaculture companies because the physiological changes associated with reproduction are linked to the quality of the product. For example, maturing Atlantic salmon stop eating around July in anticipation of returning to freshwater and reproducing in November. Members of the aquaculture industry are eager to know how to determine which fish are preparing to reproduce.
Mangel's previous research has revealed details of the Atlantic salmon's life cycle: Born in April, the fish's first opportunity to head out to sea comes 13 months later in May; subsequent opportunities occur every 12 months. Because bigger fish are more likely to survive the journey and prosper at sea, Mangel wanted to understand the mechanisms by which the fish "decide" when to migrate. Through a series of experiments and models, he has found that August is the critical time when salmon essentially "ask themselves if they'll be ready next May or not," factoring in the anticipated decline in food supply that comes with winter.
"If the answer is yes, it triggers physiological processes that help prepare the fish for departure in May," said Mangel. "If the answer is no, they prepare to stay over again by eating a lot through September and then slowing their metabolism so they'll be able to run on their reserves as they hide through most of the winter."
Mangel's new collaboration with the National Marine Fisheries Service, which has provided $45,000 for the first year of the project, will add further detail to the picture of the Atlantic salmon's life cycle.
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