Fish-performance ecoassay of urbanizing streams in the San Antonio River Basin, Texas

Abstract

Bluegill (Lepomis macrochirus, a sunfish species) were used to perform an ecological assay of stream health in urbanizing watersheds of the upper San Antonio River Basin, Texas. Ecoassays were conducted during summer 1999 and 2000, and during winter 2000-2001. In each of the three ecoassays, caged bluegill were exposed continuously for 10 - 22 days, to in-stream conditions at mid- and lower-watershed sites both on Leon Creek and Salado Creek. While in the cages, fish were fed freshly chopped earthworms or whole, live mealworms. In-stream temperature, dissolved-oxygen concentration (DO), pH, and conductivity were measured near each set of cages and recorded every 15 min. Observed weight-change rate (treatment medians, -0.56 to 3.01 %·day⁻¹) and average daily mortality rate (0 to 2.96 %·day⁻¹) were used as in-stream measures of fish performance. Complementing these performance measures were laboratory estimates of metabolic capacity obtained via automated routine respirometry, using some surviving fish from each stream/site/year/season treatment. Respirometry was performed with fish immediately after they had completed the in-stream phase of the ecoassay, and in water collected from their stream site and at the same time. Although fish performance varied from place to place and from time to time, those differences were not clearly associated with "stream." Bluegill had significantly greater rates of weight-change during summer and during the year 2000. Average daily mortality rate tended to be greater at the downstream sites, less in summer than in winter, and more with decreasing absolute minimum DO. Rates of routine metabolism and marginal metabolic scope were higher during summer and in 1999. On a per-treatment basis, the median of estimated metabolic scope-at DO = 3 mg·L⁻¹, at median in-stream temperature, and logarithmically transformed-accounted for 51 % of the variation in the median rate of bluegill weight-change. Leon and Salado creeks provided habitat of apparently equal quality for bluegill, during this study. Urbanization of these streams does not seem to have irrevocably compromised their water quality, in that bluegill performance was comparable to that observed in "healthy" systems elsewhere. Main differences I observed in fish performance were associated with ephemeral differences in stream temperature and DO. The cage-study approach, combined with respirometric assay of metabolic status, provides a practical methodology for assessment of stream and watershed health.