Estuaries as habitat for a freshwater species: Ecology of largemouth bass (Micropterus salmoides) along a salinity gradient

Abstract

Similar to the freshwater systems largemouth bass inhabit, the largemouth bass is a popular recreational sportfish in estuarine environments, like the Mobile-Tensaw Delta. However, catch rates of large (= 2.3 kg) largemouth bass are often low in these coastal systems, and coastal influences on this freshwater predator are not well understood. In Chapter II, I investigated the potential effects of marine influence on largemouth bass along a downstream-upstream gradient in the Mobile-Tensaw Delta, Alabama. While salinity remained negligible upstream, the timing and absolute magnitude of peak salinity at our most downstream site varied among years. Mean length and catch rates did not vary predictably from downstream to upstream. A greater per gram diet biomass of vertebrates was generally consumed both downstream and upstream; however, invertebrates were consistently consumed throughout this system. Condition (Wr) was generally high for all largemouth bass but was greater downstream than upstream in all years. Few largemouth bass age-4 and older were present, and survival was generally low across years throughout the study area. Growth to age-1 was greater downstream versus upstream in all years, with no differences occurring for older fish. Largemouth bass growth at all ages was greater for fish transplanted into freshwater from the Mobile-Tensaw Delta had a greater growth potential, but survival remained low in the absence of coastal influences. In Chapter III, I combined three approaches to explore movement of adult largemouth bass in relation to salinity and angler displacement: external tagging, acoustic telemetry, and fish releases at tournaments. Movement patterns of downstream fish included remaining in protected channels near the release location, moving upstream as salinity increased (< 2‰), or moving into the main river channel. Fish upstream generally remained near the release site. Recaptures of largemouth bass tagged externally during regular sampling were typically found in the original tagging site (86-100% across years), while largemouth bass from a tournament tagging effort dispersed from the release point in < 23 days. In summary, among-year abiotic variability was great, and salinity did not solely drive yearly patterns. Experimental and modeling approaches to better quantify the effects of interacting abiotic variables as well as those caused by angling on population characteristics of coastal largemouth bass.