This study examined whether temporal variability in a finfish community in a southeastern Connecticut estuary is consistent with long-term warming of surface waters, a trend documented at other estuaries in the region. Specifically, we sought to determine (i) whether fish species composition has changed over recent decades in a manner consistent with a warming trend; and (ii) whether seasonal fish distribution in the estuary is related to the thermal structure of the water column. This study was conducted in the Thames River estuary, Connecticut, bordering Long Island Sound. Field sampling consisted of bottom trawl (5.1-cm mesh net) surveys conducted during August-October in 1974-75, 1992-1998, and 2003-2016. In addition, hydroacoustic surveys were conducted with a 200 kHz DT-X BioSonics Inc. echosounder (dB > -70) every 2-3 weeks during April-November in 2012-2016. A conductivity-temperature-depth meter (YSI Inc.) was used to develop vertical temperature profiles of the water column on days when hydroacoustic surveys were conducted.
We found a marked decrease since the 1970s in the abundance of finfish species characterized as cold-water species, e.g., winter flounder (Pseudopleuronectes americanus), and an increase in the abundance of warm-water species, e.g., scup (Stenotomus crysops) (n = 204 trawls). In addition, some subtropical species absent from trawls in the 1970s and early 1990s, such as moonfish (Selenes setapinnis), were relatively abundant in 1998-2016 (0.3-2.0 CPUE/yr). Hydroacoustic surveys conducted from 2012 to 2016 (n = 54) showed that finfish biomass (all species) was generally most abundant during August-September, when surface water neared its annual maximum temperatures (22-24 °C) or had just begun to cool. Overall, fish biomass at the spatial scale of the study site was not correlated with surface or bottom water temperatures; however, correlations become evident when relationships are analyzed at smaller spatial scales may, e.g., demersal fishes vs. bottom temperature. Our results, especially those from bottom trawl surveys, support the hypothesis that temperature-related shifts are underway in the finfish community of the Thames River estuary; however, it is unclear how such a shift will influence local trophic dynamics or what the implications are for fisheries management. Similar shifts in fish species composition have been documented in at least two other large estuaries in the region, suggesting that driving factors likely involve larger-scale oceanic-atmospheric processes.