INTERACTION BETWEEN YOUNG-OF-YEAR FATHEAD MINNOWS AND BROOK STICKLEBACKS - EFFECTS ON GROWTH AND DIET SELECTION

The hypothesis that brook sticklebacks Culaea inconstans and fathead m innows Pimephales promelas are competitors was tested in field enclosu res containing young-of-the-year individuals from both species. Growth was monitored for 6 weeks, then final weights and gut contents were e xamined. Fathead minnows were relatively unaffected by brook stickleba cks. Neither dietary composition nor growth was affected in enclosures containing only fathead minnows or a combination of fathead minnows a nd brook sticklebacks at similar density. The presence of fathead minn ows did, however, stimulate growth of brook sticklebacks; this was pos sibly related to the relatively higher mortality of fathead minnows wi thin the jointly occupied enclosures and the resulting reduced absolut e density of fish. The diet of brook sticklebacks also became more div erse in the presence of fathead minnows. In enclosures containing only sticklebacks, the fish consumed primarily copepods. These data demons trate that growth of brook sticklebacks may be sensitive to both intra - and interspecific competition. This apparent sensitivity of growth t o the presence of fathead minnows may account for the observed spatial distribution and migratory habits of brook sticklebacks.