Coastal Estuaries as Habitat for a Freshwater Fish Species: Exploring Population‐Level Effects of Salinity on Largemouth Bass

Abstract

Estuaries are systems where marine and freshwaters interface. The largemouth bass Micropterus salmoides is a freshwater predator commonly found in the oligohaline portions of estuaries in coastal North America. As a popular recreational sport fish in estuaries, largemouth bass are caught in large numbers, but large (≥2.3‐kg) individuals are rarer than in inland (freshwater) systems. A pervasive factor affecting estuarine inhabitants is variation in salinity, which we hypothesized would represent a stress influencing largemouth bass performance. To test this, we sampled fish monthly (2002–2004) across an upstream–downstream gradient in the Mobile–Tensaw Delta of Alabama. Salinity remained negligible upstream but increased seasonally downstream. Catch per effort did not differ predictably across the upstream–downstream gradient, as would be expected if salinity influences abundance. Largemouth bass diets included more fish than invertebrates both upstream and downstream, but fish were more abundant in the diet upstream. Marine invertebrates remained an important diet component at all sites. Condition was generally high for all largemouth bass but was consistently greater downstream than upstream. Length at age 1 was greater downstream than upstream, but there were no differences for older fish. Few largemouth bass age‐4 and older were present, and survival was generally low (<57%). Although growth was greater for a population of largemouth bass from the Mobile–Tensaw Delta established into a freshwater impoundment, survival remained low in the absence of coastal influences. Our results suggest that the largemouth bass in the coastal Mobile–Tensaw Delta have different growth patterns and perhaps different life history strategies than those in freshwater, which may be the result of adaptation and could have important implications for management.