Effects of drifting prey abundance on food consumption and growth of brook trout in Shenandoah National Park

Abstract

The relation between prey density and brook trout Salvelinus fontinalis food consumption has not been quantified in natural streams. This relation may be most critical in the summer when southern Appalachian trout have demonstrated poor growth. It is not Known whether the poor growth is due to the low food densities or to excess temperatures and increased metabolic costs. Purposes of this study were to quantify the relation between prey density and food consumption in brook trout, identify how diel feeding and prey size selectivity by brook trout affect the relation, and to determine if slow growth in the summer was caused by low daily rations or other factors. Daily consumption was estimated in May, July and September of 1989, for four streams in Shenandoah National Park. Sampling was performed at 6-hour intervals over 24 hours. Adult and yearling trout did not consume more food at one specific time throughout the summer, but juvenile trout (< 100 mm) fed significantly less at night in both July and September. All sizes of fish selected for prey longer than 4mm. The best predictor of daily ration (corrected for temperature) over the summer was the number of prey longer than 4mm/m³. Models parameterized to quantify the relation between prey density and food consumption showed that adult trout were significantly more likely to have a higher daily ration per gram body weight (temperature and size corrected) than yearling trout at low ( < 1.0 prey longer than 4mm/m³) prey densities. Daily ration of juvenile trout was independent of prey abundance. The prey density-consumption relation was more consistent within streams than among streams, which suggests that habitat or population characteristics may influence the prey density-consumption relation. Trout had the most energy available for growth in July followed by May and then September. Daily ration was most dependent on temperature in May and on prey density in July and September. Juvenile trout did not decrease daily ration as much from July to September as did yearling and adult trout. All sizes of trout met maintenance costs in both early and late summer, yet yearling and adult trout did not grow in late summer although stream temperature was optimum for growth. Growth was negatively correlated with daily ration in the late summer, suggesting that activity costs may be more important in determining growth in late summer than prey densities. Activity costs varied substantially among streams. The effect of food consumption and habitat on activity costs needs to be defined to improve understanding of the determinants of growth in stream environments.