Mangrove prop-root habitat as essential fish
habitat in northeastern Florida Bay

George D. Dennis and Ken J. Sulak

Florida Integrated Science Center
U. S. Geological Survey, Gainesville, Florida

Presented at the Florida Bay Science Conference, April 2001, Key Largo, Florida.

Introduction

       The red mangrove prop-root fringe is the primary interface between the freshwater Everglades system and marine Florida Bay.  This habitat has been proposed as a nursery area for several fish species, but little is know about habitat use under varying salinity conditions. Changes in freshwater flow into northeastern Florida Bay may affect the use of the prop-root habitat.

       Proposed restoration of historical freshwater flow in South Florida has the potential to change fish distribution and abundance especially forage fishes that typically can not migrate from changing environmental conditions.  Forage fishes are known to vary with freshwater input being more abundant with greater freshwater input (Lorenz, 1997). In addition the species composition can change dramatically with salinity (Ley et al., 1999). Though most fish in this system are euryhaline some common species are very sensitive to salinity change and move away from changing conditions (Serafy et al., 1997). While data exists on the relationship between salinity change and forage fish abundance in northeastern Florida Bay there are little data on the effects on higher trophic levels.

       Increased forage fish abundance should increase productivity of the system by providing additional resources to next higher trophic level, especially wading birds and piscivorous fish.  If the increased forage fish abundance is available to predators there should be a direct positive relationship between forage fish and piscivorous fish abundance.  Thus coupling greater freshwater input with greater abundance of higher trophic level species. Smith and Robblee (1994) found a significant positive, though complex, relationship between sport fishery catch and freshwater inflow.

       We assessed the use of the prop-root habitat in northeastern Florida Bay where previous water diversion had potentially altered salinity conditions and recent rediversion has increased freshwater input.

Objectives

• Determine the spatial and temporal distribution of piscivorous fishes and their forage fish base in northeastern Florida Bay that would be affected by water diversion from the C-111 canal to Taylor Slough.
• Identify indicator taxa that would reflect changing salinity conditions.

Study Area

Four sites in three embayments, Little Madeira Bay, Trout Cove, and Manatee Bay, were compared across a nearshore salinity gradient and to a control site on the leeward side of Key Largo.

Methods

       We used diver visual census to identify and enumerate fish using the prop roots in Little Madeira Bay, Trout Cove and Manatee Bay.  This method has the advantage of being non-destructive but limited to areas with adequate water clarity. Another potential bias that has not been examined is the influence of the diver on fish behavior.  Some fish tend to flee or avoid the diver lending to potential density (or diversity) underestimation. We developed a remotely operated video platform (RVP) to assess the diver bias inherent in the method.

       The low juvenile abundance lead to secondary sampling of larval fish to see if recruitment or habitat limitation might explain the low abundance.  Light traps were used to sample in the prop roots and compared to nearshore and bay locations in Madeira Bay, Little Madeira Bay, and Trout Cove.  A sampling site on the leeward side of Key Largo near potential recruit sources was used as a control site.

Early Juvenile/larval Fish Light Trap

Results

       Preliminary analysis indicates that some species such as cichlids, sheepshead, and grunts may be underestimated due to flight response elicited from the diver. Other species, such as mangrove snapper and gerreids, may be adequately enumerated.  One surprising observation was the very limited number of forage based fishes in the prop roots. The RVP confirmed the relative absence of this important trophic group was not due to diver bias. In addition, the number of juvenile fishes was very low when compared to mangroves in the Bahamas and Puerto Rico.

       Gray snapper (Lutjanus griseus) were the most common piscivores. Their abundance was related to water depth and fringe width.  There was no relationship with density of prop roots. There was no difference in abundance between wet and dry season.  Fewer were observed inshore (lower salinity), but this was confounded by the poorer habitat quality there.

       The forage base was dominated by pelagic species, such as Atherinomorus stipes and Menidia spp.  Pelagic forage fishes were more common in the wet season.  There appears to be a shift in species dominance from Menidia spp. in low salinity conditions to Atherinomorus stipes at higher salinities.

       In contrast to previous studies (Thayer et al., 1987; Ley et al. 1999) benthic forage species, such as Floridicthys carpio, Lucania parva, Poecilia latipinna, were relatively rare in the prop root fringe. This may be due to different sampling methods rather than changing hydrological conditions.

       In comparison to the Key Largo site early juvenile/larval abundance was very low in northeastern Florida Bay. Among the northeastern Bay sites the prop root sites had significantly more early juvenile/ larvae than nearshore (but away from mangroves) and bay sites. Preliminary analysis suggests that northeastern Florida Bay is relatively depauperate in early juvenile/ larval fishes. Whether this is due to poor circulation from potential offshore recruitment sources or the altered salinity regime still needs to be explored.

As fringe width increases abundance increases

As water depth increases abundance increases

Conclusions

       Visual survey is relatively unbiased by diver avoidance.  Thus could be used in areas with good water clarity to monitor prop-root fish abundance and composition.

       Fish are very patchy in prop-root habitat such that not all areas are of equal value. Some areas may more essential than others requiring special protection. These need to be better identified.

       Inshore areas have poor habitat quality, shallow depths and narrow fringe, that is not conducive to harboring large numbers of higher trophic level species. Additional freshwater is unlikely to increase this capacity.

References

Ley, J.A., C.C. McIvor, and C.L. Montague. 1999. Fishes in mangrove prop-root habitats of northeastern Florida Bay: distinct assemblages across an estuarine gradient. Est. Coastal Shelf Sci. 48: 701-723.

Lorenz, J.J. 1999. The response of fishes to physiochemical changes in the mangroves of northeast Florida Bay. Estuaries 22: 500-517.

Serafy, JE, K.C. Lindeman, T.E. Hopkins, and J.S. Ault. 1997.  Effects of freshwater canal discharge on fish assemblages in a subtropical bay: field and laboratory observations.  Mar. Ecol. Prog. Ser. 160: 161-172.

Smith III, T.J. and M.B. Robblee. 1994.  Relationships of sport fisheries catches in Florida Bay to freshwater inflow from the Everglades. Bull. Mar. Sci. 54(3): 1084.

Thayer, G.W., D.R. Colby, and W.F. Hettler. 1987. Utilization of the red mangrove prop root habitat by fishes in south Florida. Mar. Ecol. Prog. Ser. 35: 25-38.

Acknowledgments

       Funds for this project were provided by the U.S. Department of Interior, South Florida Ecosystem Restoration Program, Critical Ecosystems Studies Initiative. We would like to thank the Everglades National Park for providing permits, laboratory facilities, and accommodations.  Additional support was provided by the NOAA/UNCW National Undersea Research Center and the NOAA Florida Keys National Marine Sanctuary.