Understanding growth patterns of coastal largemouth bass: integrating bioenergetics modeling and life history theory

Abstract

Relative to inland freshwater populations, largemouth bass (Micropterus salmoides) in coastal systems typically have a small maximum size, high condition, and low annual survival. Much attention has focused on determining whether the size structure of largemouth bass in these systems could be enhanced. However, high condition factors of fish in coastal systems suggest not only that there may be little potential for increased growth rates, but also that these fish may possess an alternate energy allocation strategy favoring high lipid reserves. In this study, I examined the role of energetic constraints and differing life-history strategies on growth and condition of largemouth bass along a freshwater-estuarine gradient within the Mobile-Tensaw River Delta, Alabama (Mobile Delta). Growth of age-0 largemouth bass was fastest downstream and declined linearly with distance from Mobile Bay. However, after age-2 the relationship between growth and proximity to Mobile Bay switched, with faster growth observed upstream. Bioenergetics simulations suggested that these patterns were due to a complex interaction among size-specific metabolic costs of salinity, maximum summer water temperature, and prey energy content. Additionally, freshwater riverine inputs influenced the relationship between age-specific growth and proximity to Mobile Bay, likely through its effects on prey availability, salinity, and temperature. Histological assessments revealed the probability of maturing at younger ages was greater downstream than upstream for both sexes. Annual survival rate was similar between regions (51%), suggesting regional maturation differences were largely growth rate dependent. A dynamic state-variable model suggested that estuarine environments should select for an increased energy allocation toward energy reserves at the cost of length when compared with a strategy suited for a freshwater environment. High lipid reserves decreased starvation risk and allowed females to switch energy allocation toward ovary development prior to spawning to ensure reproductive output in the face of poor energy availability. These results suggest slow growth and high condition of coastal largermouth bass are due to energetic constraints and an adapted energy allocation strategy. Further, it appears there is little potential to enhance the size structure of largemouth bass in the Mobile Delta, even under the best environmental conditions expected for this system.