Research carried out at the University of Gothenburg shows that competition from older fish causes young salmon and trout to seek refuge in shallow water. Preserving such habitats may, therefore, be important for the survival of the young fish.
Using field studies in watercourses north of Gothenburg and laboratory experiments in Denmark and Scotland, scientist Rasmus Kaspersson at the Department of Zoology, University of Gothenburg, has studied the competition between different age groups of Atlantic salmon and brown trout.
Forced into shallow water
It has previously been believed that poor swimming ability forces young salmon and trout to remain in shallow habitats where the water flows at a lower velocity. Rasmus Kaspersson's work, however, shows that it is rather competition for habitats from the older fish that compels young fish to use shallow water. Rasmus Kaspersson's experiments show that young-of-the-year move to deeper parts of the watercourse as soon as the number of older individuals is reduced.
"This suggests that young-of-the-year actually prefer to live in deep, rapidly flowing water, where they can find food easier and are protected from predatory birds and mink," says Rasmus Kaspersson.
Population determines survival
In the natural world, however, older and younger individuals are both present, and shallow habitats then function as refuge for the younger fish. The weight and length of young-of-the-year increased when older individuals were removed from parts of the watercourses studied. Thus it seems that the population of older salmon and trout in a watercourse affects indirectly the number of young-of-the-year that reach adulthood.
More protected habitats required
The results presented in Rasmus Kaspersson's thesis show how important it is to preserve and restore shallow parts of Swedish watercourses with low-velocity flow. This will provide more protected habitats for the young fish.
- R. Kaspersson, J. Höjesjö. Density-dependent growth rate in an age-structured population: a field study on stream-dwelling brown trout Salmo trutta. Journal of Fish Biology, 2009; 74 (10): 2196 DOI: 10.1111/j.1095-8649.2009.02227.x
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