Food habits of mangrove fishes: a comparison along estuarine gradients in northeastern Florida Bay

Gut analyses were performed on 1,081 fishes from four species resident in mangrove habitats in northeastern Florida Bay to determine: (1) if the diets varied systematically along a complex salinity gradient; and (2) if diets in the upstream areas were of lower overall quality than those from portions of the gradient with less salinity variability. Fishes commonly consumed amphipods, isopods, shrimp, nematodes, eggs of unknown origin, fish, insects, and algae. Diets of two of the four species differed significantly along the estuarine gradient. This systematic variation in diet along the longitudinal gradient may distinguish locations in terms of habitat quality. The tidewater mojarra (Eucinostomus harengulus) and goldspotted killifish (Floridichthys carpio) ingested more algae, a relatively low quality food, upstream in areas of high salinity variability. These same species ingested more benthic invertebrates, relatively high quality foods, downstream in areas of lesser salinity variability. If we accept the premise that gut contents of fishes are a good indicator of habitat quality, then these upstream locations afford fishes a relatively poor quality habitat. We propose that highly variable amounts of submerged aquatic vegetation in upstream locations provide reduced (and variable) abundances of benthic invertebrates for fishes at these sites. Our results, combined with those of others cited herein, provide support for the hypothesis that water management practices in the catchment of northeastern Florida Bay may be, at least in part, responsible for lowered productivity there. We suggest that gut contents of resident fishes can be used as one measure of habitat quality as part of a comprehensive monitoring and restoration program for the region. Comparisons along physico-chemical gradients can be used to determine how species respond to features of their physical environment such as salinity (Odum, 1983). In estuaries, complex gradients in salinity and associated factors occur from the region of freshwater introduction (e.g., upstream) to the region of marine influence (e.g., downstream). Fishes respond to these gradients by movement to preferred zones or by adaptation to local salinity conditions. Physiological tolerance of a wide range of salinities is a common adaptation in many species of estuarine fishes. Additionally, these euryhaline fishes may display other behavioral adaptations such as dietary flexibility. Diets have been studied for fish communities associated with whole estuaries (Darnell, 1961) and sub-habitats such as seagrass beds (Brook, 1977; Livingston, 1984) and small brackish ponds (Harrington and Harrington, 1961). In general, estuarine fishes are characterized by: (1) omnivory, (2) sharing of common resources, and (3) flexibility to exploit temporary peaks in prey populations such as shrimp and mosquito larvae (Harrington and Harrington, 1961; Hettler, 1989; Salini et aI., 1990). Therefore, food habits of estuarine fishes are likely to reflect the types and variability of resources available in their environment (Beumer, 1978; Livingston, 1984). Because gut residence times are short for omnivorous fishes, the gut contents represent an integration of the best quality foods available in the habitat within their foraging range. Differences in gut contents of a given species reflect differences in food availability, and thus, by inference, of habitat quality (McIvor and Odum, 1988; Rozas and Odum, 1988). In a recent study in northeastern Florida Bay, submersed vegetation and benthic