Life history variation along a salinity gradient in coastal marshes

Coastal habitats are susceptible to changes in the environment associated with alterations in salinity. A field study was conducted on natural populations of the sailfin molly Poecilia latipinna, the western mosquitofish Gambusia affinis and the least killifish Heterandria formosa collected from coastal marsh sites along a salinity gradient, to investigate the influence of salinity on body condition and reproductive life history traits. In brackish marsh sites male P. latipinna had the best body conditions, while females had similar body conditions across all 3 marsh types. Female P. latipinna had greater reproductive allotment and fecundity in brackish marshes, where this fish was most abundant. Specimens of G. affinis collected from fresh marsh sites had less favorable body condition, and females had lower reproductive allotment and fecundity than those collected from higher salinities. While G. affinis was more abundant in freshwater marshes than in higher salinity marshes, this higher abundance did not correspond with a better body condition or higher reproductive effort, suggesting that G. affinis may be stressed in freshwater. There was no difference in the conditions between (both male and female) H. formosa from fresh and intermediate marshes. Female H. formosa from fresh marshes had a similar reproductive allotment and lower fecundity than those from intermediate marshes. The 3 species exhibited different life history patterns along the salinity gradient, and some of these patterns conflicted with expectations based on species abundances. When assessing habitat quality along an environmental gradient, measures of abundance should be accompanied by more sensitive indicators of environmental stress.