Trophic interrelationships of selected fishes on the continental shelf of the northern Gulf of Mexico
The present study surveys the trophic interrelationships of 26 demersal fishes inhabiting the continental shelf of the northern Gulf of Mexico. Volumetric stomach content analyses were carried out on 4,550 specimens. Fishes were collected at 128 stations between Brownsville, Texas and St. Andrew's Bay, Florida in depths of approximately 3 to 200 meters. Within each species, fish were grouped by size, depth, and geographical location in order to compare variations in food habits due to these factors. Food habits of the individual species are discussed emphasizing trends in diet by food categories, transitions associated with growth, and variations associated with geographical location. Feeding periodicity is discussed for those species where data were available. From this detailed information, trends in the life history and food habits of continental shelf fishes are proposed. Larger individuals of a species are indicated to spawn in deeper waters. Larval and juvenile fishes subsequently enter the water column, espesially the supra-benthic zone, where they undergo a planktonic stage as they are transported by currents toward shallower waters. They eventually settle to the bottom to lead ademersal existance gradually moving offshore to complete the life cycle. This trend in life history patterns is reflected in the ontogenetic food habit transitions. Larvae and juveniles feed largley on zooplankton. The importance of zooplankton decreases with ont genetic development except in certain planktivorous species. As the importance of zooplankton decreases, benthic organisms increase in importance. Some species remain benthic feeders throughout their life cycle while in others , the contribution of bottom animals decreases, and they are replaced in the diet by larger macrocrustaceans and macromobile organisms. These higher preditors feed in the water column on actively swimming prey. From these considerations, a conseptual model of the tropic interrelationships in the benthic zone on the continental shelf is proposed. Two food chains are postulated: a planktonic chain involving conversion of energy fixed by phytoplankton to zooplankton for utilization by higher consumers, and a benthic chain involving conversion of energy fixed in organic deritus to deritus feeders for utilization by consumers in the sediment and eventually consumers in the water column. Zooplankton including larvae and juveniles of higher consumers is eaten largly by small fishes and planktivorous adults. Eggs and larvae of many demersal fishes leave the benthic zone assuming a planktonic existance and escaping pred- ation from this lower zone. Organic detritus enters the benthic food chain largly through the assimilation by micro-bottom animals and benthic consumers as well as browsers from the water column. Larger macrocrustaceans and macromobile organisms readily utilize these benthic and pelagic browsers. trophic energy is lost from the benthic zone to larger pelagic fishes acting as top predators.