Water depth and physical structure are important components of habitat complexity for stream fishes. Experiments in a semi-natural stream, containing four depth and structure treatments, quantified the effect of these two habitat components on the distribution, growth, and survival of five fish species common to coastal streams of Washington State. When fishes were permitted to move freely among the various habitat types, most species and age-classes were underrepresented in shallow pools lacking structure. In some cases water depth or physical structure alone appeared to explain these distributions. However, assemblage- and species-level responses were strongly influenced by the combined effects of depth and structure. A subsequent 30-d experiment revealed that mortality (likely due to bird predation) of water-column species using the simplest habitat type was as much as 50% greater than in the other treatments. However, there were no differences in survival of the benthic species, coastrange sculpin (Cottus aleuticus), among the different treatments, nor were there significant differences in growth among treatments for any species. These results indicate that predation risk may help to explain the importance of both deep water and physical structure to fishes in coastal streams in Washington. The results of our habitat-selection experiment also support the growing view that a community-level approach may be more effective than the single-species approach in evaluating the effects of human activities on stream fishes.
We removed fish from pools in two Arkansas streams to determine recolonization rates and the effects of isolation (i.e., riffle length, riffle depth, distance to large source pools, and location), pool area, and assemblage size on recovery. To determine pool-specific recovery rates, we repeatedly snorkeled 12 pools over a 40-day recovery period. Results indicated the effects of isolation on percent numerical recovery, but no effects of pool area or assemblage size. Numerical recovery of assemblages in pools separated from neighboring pools by short riffles occurred by day 30 whereas more isolated pools had not reached 70% numerical recovery by day 40. Recovery also was more rapid in downstream pools and in pools that were closer to large source pools. Finally, recovery patterns differed among species and size-classes, with large fish (<100 mm total length) recolonizing pools more rapidly than small fish. This is the first study to quantify species- and assemblage-level recolonization rates at the scale of individual pools and the effects of isolation on recovery. The findings of this study have potentially important implications for research aimed at understanding the ecology of stream fishes and predicting the consequences of land-use activities.
Recent research has suggested that the within-habitat dynamics of fish populations and assemblages can be affected by the spatial distribution of habitats within streams. In this study, we determined the extent to which pool isolation (length of riffles connecting adjacent pools) influenced fish movement in two Arkansas streams. We marked individuals from 12 pools assigned to two treatment categories: pools separated by long riffles (>50 m) and those separated by short riffles (~10 m). Repeatedly snorkeling pools for 3 days in 1995 and 1997, we discovered substantial emigration (>20%) and significant effects of riffle length. Total emigration from short-riffle pools was three times higher (29%) than movement from long-riffle pools (10%). Further, marked fish in short-riffle pools moved upstream and downstream with equal frequency, whereas fish in long-riffle pools moved twice as often downstream. Collectively, these results indicate significant effects of habitat spacing on short-term movement patterns by fish. In streams, where fish are distributed within a mosaic of habitats of varying quality, such movements may allow individuals to assess spatial variability in resource conditions (e.g., food, predators). Because land-use activities can alter habitat spacing, these findings have important implications for fish conservation in degraded streams.
R6sumC: Des recherches r6centes semblent montrer que la dynamique des populations et des assemblages de poissons au sein des habitats peut Ctre influencCe par la r&partition spatiale des habitats dans les tours d'eau. Dans la prCsente Ctude, nous avons dCtermin6 dans quelle mesure l'isolement des fosses (longueur des radiers reliant des fosses contigu&s) influait sur le dkplacement des poissons dans deux tours d'eau de I'Arkansas. Nous avons marquC les individus pr6levCs dans 12 fosses rCparties en deux catkgories exp6rimentales : fosses sCpar6es par des radiers longs (>50 m) et fosses &par&es par des radiers courts (
Grouping is an evolutionary strategy that allows individuals to optimize foraging success in habitats of varying quality and when under the risk of predation, but group foraging can lead to competition between group members. The effects of group size, habitat, predation, and competition on foraging success also can change as animals grow. Our study explored how these concurrent factors influenced the foraging success of group members at two different life stages. In a wild population of coho salmon (Oncorhynchus kisutch), we first tested four hypotheses concerning the effects of group size on foraging success. We then analyzed our field observations with structural equation modeling to test causal relationships between group size, habitat, predation risk, competition, and the foraging success of individuals during two periods of development. We found support for the hypothesis that the relationship between group size and foraging success was parabolic during both study periods, revealing an optimal range of group sizes that maximized individual foraging success, which was conserved as fish aged. Predation risk had a positive effect on group size, particularly for older fish, and distance to cover and water depth were indicators of risk for both age groups. As group size increased, so did competition, but only for young fish; competition only had a weak positive effect on foraging success for older fish. Our results reveal that the relative direct effects of predation risk and competition on foraging success were weak compared with the direct effect of group size. Our study provides new insights and theoretical implications for understanding how selective forces-that is, group size, predation risk, competition, habitat, and ontogeny-act concurrently to affect foraging success in wild populations.
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