When the Atlantic Salmon comes home – more than once!
When will you come back? That’s a question we could ask the Atlantic Salmon spawning in the waters of New Brunswick’s Miramichi River. For the past 20 years, it has been returning more frequently to conceive its progeny there. Once, twice and sometimes more throughout its adult life. This repeated homecoming is related to the abundance of food according to two scientists at Fisheries and Oceans Canada.
"For other ocean food chains, scientists had already shown that animals depend on the abundance of their prey in this way. But this is the first time we have been able to demonstrate this dependency in the case of the Atlantic Salmon. The profusion of small fish in the Southern Gulf of St. Lawrence explains why increasing numbers of salmon are coming more than once to breed in the Miramichi River," Gérald Chaput, a Fisheries and Oceans Canada biologist for the Gulf Region, explained.
In a May 2012 article in the International Council for the Exploration of the Sea's Journal of Marine Science, Mr. Chaput and his fellow biologist Hugues Benoît also established for the first time that increasing numbers of Atlantic Salmon (Salmo salar,to use thescientific name) cut short their stay in the sea by a year or more before coming back to spawn in the fall because they stock up on energy in an "oceanic food market" close to the Miramichi River. This spares them the exhausting and sometimes fatal ocean migrations they need to undertake in order to feed.
A return home
It is not by chance that the Miramichi River nowadays receives the largest Atlantic Salmon population in eastern North America. "At first, we thought that the fishery closures were responsible for the increase in numbers of salmon since the beginning of the 1990s. These closures were beneficial, but there has also been a decrease in the number of large predators of salmon prey. For example, the decline in cod resulted in a drop in predation on salmon prey populations and enabled these populations of small fish to thrive in the southern Gulf of St. Lawrence," according to Gérald Chaput.
"Since then, salmon, being opportunists, have been taking advantage of this small prey to build up their reserves in the sea in the spring before returning to spawn in the river in the fall," Mr. Chaput added. Salmon only undertake this procession once a year. (See box Did you know?)
According to Mr. Chaput: "There is no special reason for salmon mortality in the sea except for commercial fishing. However, as soon as salmon cannot find enough food for their development, they become more vulnerable to predation. Changes in ocean currents and water temperatures may also push them towards feeding grounds where there are more predators."
"In fact, the increase in spawners is consistent with the low mortality rates noted and the proximity of abundant food of an adequate size for salmon. Some salmon may thus return to the river to spawn after spending only a few months replenishing their reserves in the Southern Gulf," Mr. Chaput indicated.
Hands in the water
Concretely, to estimate salmon populations that return to the Miramichi River to spawn, Fisheries and Oceans Canada scientists place trap nets in the estuary. "This monitoring program, which started in the 1950s, takes place from mid-May to late October every year. Once a fish has been live-trapped, it is sampled, that is, the scientists note characteristics such as its length, weight and scales, which then enable them to determine its age and the number of spawning events before releasing it and allowing it to continue its way upriver. These data enable us to assess the number of fish that have returned," Gérald Chaput explained.
Fine, but how do we know if the fish has returned home for the first time or is an old hand at this aquatic transhumance? "By observing its scales, a bit like a tree's growth rings," says Mr. Chaput.
"The scales tell us the fish's whole history because it is successive mineral deposits that make the scales grow. So they can tell you the age of the fish, and a whole lot more. During the breeding period and during the winter's fast, that is, after spawning, a fish draws on the energy reserves it accumulated during its time in the sea. This stage leaves marks on the scales like a hole, which is in fact the erosion of the mineral layer. By counting the number of holes, we can find out the number of spawning events and even determine whether they were successive," the biologist explained.
A long-term analysis
While Chaput and Benoît's discoveries may seem simple, they had to analyze and match up data compiled over more than forty years by marine and freshwater biologists concerning both river fish populations and observations made out in the open ocean. "Our work will certainly affect how the consequences of fishery closures are assessed and the steps needed to restore Atlantic Salmon populations," the scientist said.
"For our scientific article, we combined our expertise in marine and freshwater ecosystems. If we had not combined our experience and knowledge, we might perhaps not have managed to make a hypothesis to explain the increasing abundance of the Atlantic Salmon, which comes back to spawn several times in its birth river," Mr. Chaput said.
In a time of changing marine and freshwater ecosystems, scientific collaboration is critical to increasing our knowledge of aquatic population dynamics. The future depends on this, the researcher believes.
Did You Know?
After they are born, young Atlantic Salmon (11 to 15 cm long) spend two to four years in a river before hurtling into the ocean, where they may migrate long distances to feed and grow considerably before returning to breed in their native river. As has been shown by the research of Gérald Chaput and Hugues Benoît, Miramichi River salmon tend to remain in the southern Gulf of St. Lawrence for one to three years before they first return home in the fall, when they are 50- to 90-cm-long adults. After this reproductive stage, the fish fast deep in the river during the winter and return to the sea in the spring to replenish their reserves in the Gulf and continue to grow.
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