Since 1983, sports fishermen from the Drammen River in Norway have been saving the scales of Atlantic salmon, caught as they return from years at sea to spawn in fresh water. A team of researchers including Jennifer McCarthy of the University of Massachusetts Amherst is using these scales to solve the mystery of why most of these endangered fish never survive their ocean stay.
“North American populations of Atlantic salmon have crashed, and European stocks are also declining,” says McCarthy, a graduate student of natural resources conservation. “Growth rings on the scales of fish from European stocks indicate that late summer conditions in nursery areas in the Norwegian Sea are less favorable for the survival rate of young salmon.”
Climate change may be one of several factors affecting this temperature sensitive species, bringing warmer water to the nursery areas and decreasing the numbers of small fish that salmon depend on for food.
Additional researchers include Kevin Friedland of the National Marine Fisheries Service and Lars Hansen of the Norwegian Institute for Nature Research. Results were published in the May 2008 issue of the Journal of Fish Biology.
Atlantic salmon typically spend from one to five years in freshwater rivers before heading for the ocean, and McCarthy says they seem to be doing well during this phase of their life cycle. After entering salt water, a process known as smolting, they usually undergo a period of rapid growth and feeding, but tracking them is next to impossible in their new environment.
McCarthy used the salmon scales like the rings of a tree, analyzing a record of growth that shows how well the salmon are faring. After examining scales from over 2,850 individuals, McCarthy found a consistent record of narrow rings and slow growth occurring four to five months after smolting, corresponding to the late summer, when the young salmon would be living in ocean nursery areas.
“Scale records showed good growth in the freshwater environment and immediately after smolting in May or June, which was a surprise,” says McCarthy. “Conventional wisdom has held that the months immediately following the transition to salt water are the most difficult for young salmon, since they are smaller and more vulnerable to prey.”
According to McCarthy, growth patterns show that current efforts to increase salmon populations may not be enough. “Conservation efforts focused on improving the quality of rivers and stocking them with fish are working well in the fresh-water phase, and thousands of salmon tagged by researchers are making it out to sea,” says McCarthy. “Unfortunately, only a few of these fish ever return to spawn, pointing to ocean conditions as a major factor in declining European populations.”
Continued research by Kevin Friedland will analyze climate data, including ocean temperatures and currents, to determine how big a role climate change is playing in salmon mortality by altering conditions in the nurseries and affecting ocean plankton and populations of smaller fish.
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