Precocious, selective and successful feeding of larval billfishes in the oceanic Straits of Florida

The oligotrophic open ocean of low latitudes is larval fish habitat for a diversity of resident and migratory species. It is hypothesized that these waters, relative to coastal regions, yield reduced predation mortality, but little is known regarding the feeding and feeding environment of these larvae and the extent to which a nutritional tradeoff may exist, whereby lower predation mortality is accompanied by poor feeding conditions. Monthly sampling of larval billfishes (Istiophoridae) across the Straits of Florida over 2 yr allowed for an investigation of the temporal, spatial and ontogenetic variability in the feeding of sailfish Istiophorus platypterus and blue marlin Makaira nigricans. Consumed prey were numerically dominated (90%) by 2 crustaceans: a copepod (Farranula; mainly F. gracilis) and a cladoceran (Evadne; mainly E. tergestina), with relative proportions displaying marked spatial variability. These prey were consumed throughout early larval ontogeny, from first feeding through piscivorous lengths (>5 mm), until piscivory became exclusive near 12 mm. High daytime feeding incidence (0.94) and rapid digestion (∼3.5 h) support generally frequent and successful feeding by billfish larvae. Prey selectivity was illustrated by preference for Evadne over Farranula and a near absence of calanoid copepods from diets despite high environmental concentrations. Gut fullness exhibited a distinct sunset peak and also differed significantly with larval length and year, but not with season or location. A gut evacuation rate was used to estimate a daily ration of 29 to 75% of gut-free body weight, varying greatly with model selection but also with daylight length. Although potentially unique to the Straits of Florida and larval billfishes, these results contradict the general presumption that the subtropical open ocean is nutritionally constraining for larval fish.