California Sea Grant College Program

Coral reefs support an abundance of organisms despite being surrounded byoceanic waters characterized by low nutrient levels. Over more than a century ofresearch, scientists have debated whether life on coral reefs is self-sustaining orwhether reef organisms extract nutrients from the open ocean that in turn subsidizeorganic production within the reef system. This dissertation focuses on one guild ofcoral reef consumers - fish that feed on zooplankton from the water column. Pairingtwo independent metrics of fish diet - gut content analysis and stable isotope analysisfollowed by a mass-balance mixing model - I provide direct evidence that zooplankton from the open ocean comprise a significant proportion of the diet of fish inhabitingboth offshore and nearshore reefs.

In a study of feeding behavior of the planktivorous fish Dascyllus flavicaudus,I document that this species feeds selectively on certain taxa within the zooplankton assemblage. Oceanic copepods (Oncaeidae and Corycaeidae) were over-represented in fish gut contents relative to their abundance in environmental zooplankton samples. Non-random feeding by D. flavicaudus resulted in a 2 to 6-fold increase in the contribution of oceanic prey to fish diet beyond that expected under random feeding.

The natural spatial variability in the zooplankton assemblage on coral reefs hasthe potential to affect not only fish diet but fish growth. I examined the relationshipbetween zooplankton abundance, fish feeding and fish growth using a field experiment where juvenile fish were transplanted to reef habitats spanning a range of ambient zooplankton densities. The resulting spatial patterns in fish growth support the hypothesis that spatial variability in the abundance of zooplankton prey cansignificantly affect fish growth. Fish transplanted to locations with turbid watersexhibited low growth rates, suggesting that changes in land use practices which alterwater quality may have deleterious effects on planktivorous reef fishes and that factorssuch as turbidity can act to de-couple fish growth from zooplankton abundance.

This dissertation provides a landscape-scale perspective of planktivorous fish as links between oceanic and coral reef food webs and highlights the effects of fish behavior and reef habitat on cross-ecosystem exchange.