Nutritional Condition of Spot Larvae Associated with the Mississippi River Plume

Abstract

We examined morphological criteria to determine nutritional condition, assessed recent feeding activity by examining gut fullness, and examined recent growth patterns from otoliths to determine if the Mississippi River plume front enhances feeding opportunities for larvae of spot Leiostomus xanthurus. A greater percentage (35%) of starved larvae, as determined by morphological criteria, occurred at the plume front than well inside (19%) or well outside (15%) the plume. Considerable variability in the incidence of starvation occurred within and between stations, especially at the plume front. The majority of spot larvae, regardless of capture location, had only a small volume (<0.050 mm3) of food in their guts. The proportion of larvae that had 0.050 mm3 or more food in their guts was equal at the plume front, well inside, or well outside the front. A low correlation (r = −0.56) between food volume and percent starvation, although statistically significant (P < 0.05), may be explained by the high variation in both variables and the different time scales of each. Gut fullness is a measure of feeding success on a scale of hours, whereas nutritional condition is a measure on a scale of days. There was minimal association between instantaneous growth rates, gut content volumes, and the degree of starvation. Larvae that exhibited the highest recent growth rate past 3 d of life) were considerably larger for any given age than those with low rates. Our inability to demonstrate consistently that larvae have a nutritional advantage when associated with the Mississippi River plume may reflect the transitory and dynamic nature of the plume front.