Diet of Red Drum and Snook in Florida Bay

     In this study, we examined the dietary composition of red drum (Sciaenops ocellatus) and snook (Centropomus undecimalis) in Florida Bay.  Surprisingly little quantitative information exists on the diets of adults of these species despite the fact that they are top-level predators and that they are among the more popular species sought by recreational fishers in the inshore waters of Florida (Schmidt et al. 2000).    Most of the trophic information available pertains to juvenile fish.  This is in part due to interests throughout the state in determining their nursery grounds and in rearing these species in captivity either for direct market use or for stock enhancement.  Ecological changes in Florida Bay warrant a closer evaluation (Boesch et al. 1993)

     Adult red drum concentrate on bottom-associated crustaceans and fish.  Although adult snook are primarily piscivorous and feed on many species of fish (e.g., clupeids, engraulids, carangids, gerreids, and sciaenids), blue crabs (Callinectes sapidus) and pink shrimp (Farfantepenaeus duorarum) are often found in their diet (Fore and Schmidt 1973).   The seasonal and areal differences in diet we found for red drum could be due to differences in prey selection or differences in availability.  It is very difficult to determine which of these factors is responsible for the observed differences in diet.  It is reasonable to assume that the red drum diet in Florida Bay reflects the abundance of potential prey in the places and times of feeding.

     Fish caught from more northerly inshore regions differed from those caught in the southern part of Florida Bay, whether from inshore or offshore.  Part of these differences may be due to the communities associated with different salinity regimes.  Mangrove and oyster-reef communities dominate the more northern sites, whereas seagrass communities dominate more southerly sites.  The lack of seagrass in northern sites is not due to seagrass die off.  Further, the northwest coastal area is significantly influenced by freshwater input from the Shark River Slough, flowing through Whitewater Bay and out around Cape Sable.  The area within Florida Bay is frequently influenced by hypersaline conditions particularly during droughts and during the dry season of December through May.   The relatively lower preponderance of pink shrimp in the diets of fish from area 4 is likely due to this difference.  Although Whitewater Bay is one of the primary nursery habitats for pink shrimp and an important recruitment site for juveniles, pink shrimp tend to be more abundant in areas structured by Thalassia and Halodule (Sheridan 1996).

     Occurring at higher frequencies than expected were species associated with oyster beds, including the snapping shrimp Alpheus heterochaelis (which burrows in oyster clumps and rubble), the mud crab Eurypanopeus depressus and other xanthid crabs in the genus Panopeus.   Again, because of the preponderance of oyster reef in area 4 compared to other areas, abundance of these reef-associated species is to be expected.  Also in the fall, blue crabs occurred less frequently than expected while Farfantepenaeus sp., Panopeus sp. and xanthid crabs occurred at higher frequencies than expected, based on the time of year.  Seasonal differences in the diet of red drum occurring in the fall can be explained for the most part as being associated with the fall and winter migrations of pink shrimp from estuarine nursery grounds inshore to offshore spawning grounds (e.g., Bielsa et al. 1983, Gitschlag 1986). 

     The snook diet was not as well resolved as that of red drum because fish prey digest very quickly and there are few hard parts to give clues as to the identification of the prey.  Therefore we had to pool large portions of the snook diet into the category of unidentifiable fish parts.  Comparatively, red drum prey mostly on crustaceans which have relatively indigestible and more easily identifiable exoskeletons.  The lower resolution of the snook diet could be a factor in the lack of observed seasonal and areal dietary differentiation. 

     The stable isotope analyses of snook tissues gave patterns of seagrass-mangrove-pelagic isotopic signatures, with little areal differentiation.  There are two possible explanations for this observation.  Either snook move about within a wide variety of habitats, including mangrove, seagrass, and reef, where they remain long enough to develop distinctive habitat-associated isotopic signatures or their movement is limited and their diet is specialized depending on their specific microhabitat association.  Nevertheless, they encounter a diverse array of food types (mostly mobile fishes) derived from offshore and estuarine habitats.  Snook are commonly observed on reefs, in mangroves and on seagrass habitats, but the regularity with which they move among these habitats is unknown.  We know that the crustacean component of the diet is not highly mobile, but components of the fish prey could be.  The top fish prey (Table 13) in the snook diet were a mix of highly mobile pelagic-feeding species such as Oligoplites sauris and clupeids, highly mobile benthic-feeding species such as mullet and mojarras, and low mobility benthic-feeding species like pinfish and Fundulus.  Thus, there is insufficient information to discern which of these two possibilities is most likely.

     The oral and body morphology of adult snook and red drum give clues to their modes of predation.  The mouth and body form of snook is that of a mid-water fish eater. The data presented in this study indicate that pink shrimp and blue crabs are abundant in the snook diet.  Since both these prey are adept swimmers, it is likely that snook prey on them while they are swimming in the water column.  Other dominant prey in the snook diet do not live directly on the bottom, but above it.  In contrast, adult red drum have a bottom-associated body form, with a rounded dorsal aspect and a flattened ventral.  They also have a relatively small mouth that is ventrally located and designed primarily for sucking prey off the bottom.  Most of their prey live directly on the bottom, including xanthid crabs, spider crabs, snapping shrimp, toadfish and gobies.  It is unlikely that there is direct competition between red drum and snook.  Even though there is a high degree of niche overlap (60%), particularly for pink shrimp and blue crabs, red drum and snook prey upon these species in spatially distinct ways.   Pink shrimp and blue crabs buried in the sediment (the typical daytime pattern for pink shrimp) would be unavailable to snook, while being a prime target for red drum.  Conversely, swimming shrimp and crabs would be readily available to snook, but not easily captured by red drum.  This is a good example of why one cannot use niche overlap to imply competition without a priori knowledge of the behavioral ecology of the predators and their prey. 

     This research was conducted as part of The Critical Ecosystem Studies Initiative, which  supports studies conducted to provide physical and biological information, simulation modeling, and planning that are critical for achieving South Florida ecosystem restoration.   Funding was provided byThe U. S. Department of the Interior National Park Service, The National Oceanic and Atmospheric Administration, and NOAA's South Florida Ecosystem Restoration Prediction and Modeling Program.