GROWTH AND MORTALITY IN SUCCESSIVE COHORTS OF FISH LARVAE WITHIN AN ESTUARINE NURSERY

The young of many coastal fishes appear on estuarine nursery grounds in a progression of successive cohorts, few of which encounter identical environmental conditions. Using in situ manipulations, I explored the effects of natural environmental variability and use of a nursery habitat by prior cohorts on growth and mortality rates in successive larval cohorts of the mummichog Fundulus heteroclitus, a cyprinodontid fish common in salt marshes along the east coast of North America. Variation in mean daily specific rates of growth (G = 0.027 to 0.143) and mortality (Z = 0.007 to 0.118) within and among cohorts was associated with the tidal cycle. Growth rate was positively associated and mortality rate negatively associated with the duration of tidal flooding experienced by groups of fish larvae. Tidal flooding may have affected growth and survival by controlling the renewal rate of prey resources (primarily harpacticoid copepods) in intertidal aquatic microhabitats where the fish larvae are normally found. Overall high survival (up to 90 % after 2 wk) of larvae in experimental cohorts was attributed to the exclusion of predators from field enclosures. Successive cohorts of larvae were not nutritionally independent and use of the nursery habitat by a prior cohort usually had a negative ef f ect on survival of larvae in the next cohort. These findings challenge the general tenet that fish larvae are not food-limited in nurseries and suggest how variation in the physical environment can restrict food availability even in a highly productive estuarine habitat. The spawning pattern of F. heteroclitus seems to combine features from along a continuum of reproductive strategies exhibited by other marine fishes and may contribute to maintaining a large and stable population in a fluctuating estuarine environment.