Intercohort competition effects on survival, movement, and growth of brown trout (Salmo trutta) in Swedish streams

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Abstract

The effects of density-dependent intercohort competition on growth and mortality in stream-resident brown trout (Salmo trutta) were tested by experimentally reducing the densities of age-1 fish and fish older than age 1 in six small streams. When densities of age-1 fish were reduced, abundance of age-0 and age-1 fish increased the following year, while age-1 fish experienced a reduced mean size. Reduced densities of fish older than age 1 resulted in increased apparent survival of age-0, age-1, and age-2 fish in the subsequent year. Mean size of age-2 fish increased as well. Many older immigrants (age >2) were found in the treatment sections where densities previously had been reduced. Data from the Swedish Electrofishing RegiSter (SERS) showed that mean body size of age-0 brown trout was negatively related to both age-0 and age >0 densities. The database also verified the inverse relationship between age-0 abundance and abundance of older cohorts. Our results indicate that intercohort competition regulates territorial fish populations, even when simple single populations in restricted environments are considered.

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