A snapshot analysis of age distributions of fishes in urban and forested Virginia streams

摘要

An understanding of the spatial variation in the population structure of lotic fishes is vital to their conservation. Population level approaches may be more suitable than community level approaches for identifying stream fish response to urbanization. Lithophilic (clean mineral substrate) spawners are disproportionally affected by common habitat disturbances, and are thus expected to exhibit great demographic variation along gradients of disturbance. We related age distributions of six stream fishes, exhibiting four different types of lithophily (speleophily, saucer-pit nesting, gravel mound nesting, and simple broadcasting/ nest association), to land cover and instream habitat variables. Fishes were collected from 18 urban or forested reaches of three 2nd-4th Strahler-order tributaries of the New River, Virginia. Individuals were assigned to age classes based on length-frequency histograms verified by sagittal otoh'th analysis. Chi-square tests and multiple polytomous logistic regression were used to relate population structure to land cover types and associated instream habitat variables. Age distributions of broadcast lithophils were unbalanced (containing relatively higher proportions of adults than juveniles) in urban reaches, whereas those of nest-constructing spawners were always balanced. Mixed responses were observed between the two speleophils. Differences in the directional effect of urbanization on population structure may be attributable to species' tendency to: a) modify available substrate, b) to provide parental care to their brood, and c) life-history traits other than spawning mode (e.g. age at maturation). Although nest association may confer greater reproductive success to participants, this activity was not beneficial enough to give associates balanced age distributions in urban reaches. These results suggest that source-sink dynamics may operate to prevent populations of various fishes in urban reaches from being extirpated. Future research should focus on differences in population dynamics of stream fishes among land cover types.