Growth- and size-selective mortality in pelagic ­larvae of a common reef fish

Abstract

Larvae of most aquatic species experience high mortality in the plankton and where traits are naturally variable, mortality can be selective. We tested the hypothesis that reef fish lar- vae with faster growth rates and larger size-at-age will be more likely to survive the pelagic larval period. We examined the otolith microstructure of larval bluehead wrasse Thalassoma bifasciatum collected across the Straits of Florida to obtain daily growth rates and relative size-at-age. Using a cross-sectional approach, we compared these traits between 2 larval age groups (young larvae: 18 to 26 d old, survivors: 27 to 36 d old) to determine whether mean traits varied with age. In contrast to expectations, survivors had slower early growth and were smaller-at-age than the young lar- vae, suggesting that faster growing, larger individuals were selectively removed from the popula- tion over time. To examine the role of water masses, we estimated maximum upstream larval posi- tions. Differential current speeds across the Straits of Florida resulted in substantial overlap in back-estimated upstream locations of larvae at younger ages, suggesting that differences in early growth environments were unlikely the cause of the observed patterns. Instead, the most parsimo- nious explanation is the selective loss of fast-growing, large-at-age larvae in the plankton. Shifts in the direction of selective pressures across stages may contribute to the maintenance of high plasticity in the early life history traits of this species.