Seasonal Occurrence and Food Resource Use of an Assemblage of Nearshore Fish Species in the Bothnian Sea, Sweden

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Abstract
Temporal distribution, food selection, body size and growth periodicity of the fishes Pungitius pungitius, Gasterosteus aculeatus, Pomatoschistus microps, P. minutus, Gobius niger and Phoxinus phoxinus were studied in southern Bothnian Sea (.apprxeq. 5.permill. S; > 15.degree. C during 1.5 mo.). The hypothesis was tested that competition for food acts as important structuring factor of nearshore fish assemblages. In spite of fluctuations in species numbers and relative abundances during the season, species diversity remained relatively stable. No differences in mean food-niche widths were detectable between adults of each species; between juveniles such differences occurred. No correlations were found between mean food overlaps and food-niche widths or species diversity. No evidence could be found that would identify food competition as important structuring factor. In contrast, for a nearshore fish assemblage on the Swedish west coast, competitive interactions for food played, at least temporarily, an important role. Extreme abiotic conditions appear to keep the Bothnian fish populations below the abundance level at which food shortage could become an important selective force.

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