Summer Thermal Thresholds of Fish Community Transitions in Connecticut Streams

Abstract

Thermal tolerances have been studied for individual fish species but few have investigated how stream fish assemblages respond along a temperature gradient and which thermal ranges act as a threshold, triggering discernible community change. The purpose of this study was to define summer temperature thresholds of fish community transitions in Connecticut streams. The program Threshold Indicator Taxa Analysis suggested that the coldwater class had a June–August mean water temperature < 18.29°C, the coolwater class 18.29–21.70°C, and a warmwater class > 21.70°C. Significant indicator species of coldwater streams were Slimy Sculpin Cottus cognatus and Brook Trout Salvelinus fontinalis. Significant indicator species of warmwater streams were Cutlip Minnow Exoglossum maxillingua, Smallmouth Bass Micropterus dolomieu, Rock Bass Ambloplites rupestris, Brown Bullhead Ameiurus nebulosus, Redbreast Sunfish Lepomis auritus and Yellow Bullhead A. natalis. The narrow 3.41°C temperature range between the coldwater and warmwater thresholds was designated as a coolwater transition zone, with potential for the presence of both coldwater and warmwater species and lack of species uniquely associated with this thermal range. Our approach based on a robust set of water temperature and fish community data should be applicable to other temperate regions and will be useful for informing development of thermal criteria, application of multimetric indices, and planning for anticipated effects of climate change.

Received July 2, 2013; accepted October 8, 2013

Published online January 30, 2014

Appendix: Temperature–Fish Species Relationships Based on TITAN

TABEL A.1 Threshold Indicator Taxa ANalysis (TITAN) change points of fish species in response to June–August mean water temperature (°C). The observed change points (CP) corresponds to the value resulting in the largest indicator value (IndVal) z-scores for each taxon either as an increase (+) or decrease (−) to the temperature gradient. Percentiles (5%, 50%, 95%) correspond to change points from 500 bootstrap replicates. Purity is the mean proportion of correct response direction (z− or z+) assignments; reliability (Rel) is the mean proportion of P-values < 0.05 among 500 bootstrap iterations.

TABEL A.2 Threshold Indicator Taxa ANalysis (TITAN) change points of fish species in response to July mean stream temperature (°C). The observed change points (CP) corresponds to the value resulting in the largest indicator value (IndVal) z-scores for each taxon either as an increase (+) or decrease (−) to the temperature gradient. Percentiles (5%, 50%, 95%) correspond to change points from 500 bootstrap replicates. Purity is the mean proportion of correct response direction (z− or z+) assignments; reliability (Rel) is the mean proportion of P-values < 0.05 among 500 bootstrap iterations.

TABEL A.3 Threshold Indicator Taxa ANalysis (TITAN) change points of fish species in response to maximum daily mean stream temperature (°C). The observed change points (CP) corresponds to the value resulting in the largest indicator value (IndVal) z-scores for each taxon either as an increase (+) or decrease (−) to the temperature gradient. Percentiles (5%, 50%, 95%) correspond to change points from 500 bootstrap replicates. Purity is the mean proportion of correct response direction (z− or z+) assignments; reliability (Rel) is the mean proportion of P-values < 0.05 among 500 bootstrap iterations.