Resource matching across habitats is limited by competition at patch scales in an estuarine-opportunist fish

I used field observations, assays, and experiments with the euryhaline cottid Cottus aleuticus to evaluate the extent to which average resource availability drives the large-scale distribution of these fish among upstream and estuarine habitats and how interspecific competition from a congener affects its performance in the estuary. Population densities of C. aleuticus were only consistent with resource densities across years during two of five study years, indicating a lack of resource matching at large temporal scales. On shorter temporal scales, fish growth rates that were two to three times higher in the estuary compared with the stream were inconsistent with the predictions of resource matching theory. A manipulation of C. aleuticus density showed that the estuary could support at least twice the number of individuals that occurred there; thus, the profitable estuary is underutilized. Interspecific competition with Cottus asper was partially responsible for this underutilization as indicated by a substantial reduction in growth and condition among C. aleuticus individuals in experimental manipulations that compared intra- and inter-specific effects. Observed spatial overlap between these two cottids combined with the results of the competition experiment suggests that C. aleuticus is more strongly limited in its ability to use estuarine habitats opportunistically by interspecific competition than by intraspecific competition.