Zooplanktivory by blue runner: an energetic subsidy to Gulf of Mexico fish populations at petroleum platforms

Abstract

The feeding habits of blue runner, Caranx crysos (Carangidae), were evaluated near offshore
petroleum platforms in the northern Gulf of Mexico (GOM). Fish and plankton samples were
collected at a mid-shelf platform located west of the Mississippi delta (Grand Isle 94B) from June
to August 1996 and at a mid-shelf platform (Main Pass 259A) located east of the delta from June
to September 1999. Meso- and macro-zooplankton comprised a large proportion of blue runner diets
at the two platforms. Decapod crustaceans, chaetognaths, amphipods, other invertebrates as well as
small fishes made up the majority of prey items. Variations in the diet over the sampling periods
may have been due to shifts in predator preference and prey availability. Stomach fullness revealed
that blue runner fed most intensely during night and pre-dawn hours with lower levels of feeding
during the day. Blue runner appeared to select for larger sized prey relative to size distributions
found in our plankton samples. Our hypotheses are that platforms are acting as large plankton
accumulators through hydrodynamic and illumination effects, which provide blue runner with food
and also extends foraging time.