TEST OF SALT MARSH AS A SITE OF PRODUCTION AND EXPORT OF FISH BIOMASS

Stevens, P. W.^1,2 and C. L. Montague<sup>1

 1</sup>Dept. Environmental Engineering Sciences, Univ. of Florida, Gainesville, FL 32611
²Florida Integrated Science Center, USGS, Gainesville, FL 32653

Abstract

Salt marshes are among the most productive ecosystems in the world, and although they are thought to enhance the productivity of open estuarine waters, the mechanism by which energy transfer occurs has been debated for decades.  One possible mechanism is the transfer of saltmarsh production to estuarine waters by vagile fishes and invertebrates. Saltmarsh impoundments in the Indian River Lagoon, Florida that have been reconnected to the estuary by culverts provide unique opportunities for studying marsh systems with respect to aquatic communities. The boundaries between salt marshes and the estuary are clearly defined by a system of dikes that confine fishes into a known area, and the exchange of aquatic organisms are restricted to culverts where they may be easily sampled. A multi-gear approach was used to estimate standing stock, immigration/emigration, and predation monthly. Changes in saltmarsh fish abundance, and exchange with the estuary reflected the seasonal pattern of marsh flooding in the northern Indian River Lagoon system. During a six month period of marsh flooding, saltmarsh fishes had continuous access to marsh food resources. Piscivorous fishes regularly entered the marsh via creeks and ditches to prey upon marsh fishes, and piscivorous birds aggregated following major fish migrations to the marsh surface or to deep habitats. As water levels receded in winter, saltmarsh fishes concentrated into deep habitats and migration to the estuary ensued. The monthly estimates of standing stock, net fish migration, and predation were used to develop a biomass budget to estimate the annual production of fishes and the relative yield to predatory fish, birds, and direct migration to the estuary. Annual production of saltmarsh fishes was estimated to be 16.9 g·m^-2 salt marsh, which falls within the range of previously reported values for estuarine fish communities.  The relative yields were 21% to piscivorous fishes, 14% to piscivorous birds, and 32% to export.  Annual export of fish biomass was 5.2 g fish·m^-2 salt marsh representing about 1 - 2% of saltmarsh primary production. Saltmarsh fishes convert marsh production to high quality vagile biomass and move this production to the estuary providing an efficient link between salt marshes and estuarine predators.

Methods

Standing Stock -
Cast Net

Fish Migration -
Culvert Traps

Fish Predation -
Gill Nets

Bird Predation -
Bird Counts

Results

Patterns of Fish Use and Predator Abundance within the Salt Marsh

Standing stock of fishes, migration by fishes, piscivorous fish CPUE, and piscivorous bird abundance are shown in the adjacent figures. A generalized depiction of water level during the study period from datalogger records is also shown.  The average area (± SD) of 14 cast net deployments was 2.8 ± 0.2 m^-2.  Catch in each cast net deployment was divided by 2.8 to give the abundance or biomass·m^-2.  At the onset of marsh flooding, fishes immigrated into the salt marsh from the estuary and onto the marsh surface (Jul in Net Migration). Piscivorous bird abundance in the salt marsh increased dramatically following the fish immigration as birds preyed upon the now abundant fish on the marsh surface (Aug in Birds). During the fall and winter, fish remained on the flooded marsh and the net migration of fish was low (Aug – Nov in Net Migration). In late winter/early spring, water levels receded and fish concentrated into the deeper creek and perimeter ditch as the marsh drained (Dec – Feb in Standing Stock).  As fish concentrate, piscivorous bird abundance increased once again, although only slightly compared to the period following the initial fish immigration (Jan – Feb in Birds). Fish concentration into deep habitats was followed by a marked emigration of fish from the marsh to the adjacent estuary (beginning in Jan in Net Migration). This seasonal pattern of fish movement within the salt marsh and marked migration to the estuary during low water levels supports the hypothesis that saltmarsh fishes grow and reproduce on the marsh then migrate to the estuary contributing to the prey base of larger estuarine predators such as spotted seatrout, red drum, common snook, and tarpon.  Pronounced peaks in piscivorous fish CPUE occurred within the impoundment in Jul, Sep, Apr, and Jun. During low water levels in Dec - Mar, piscivorous fish CPUE was low despite the high concentration of fish in deep habitats.  Piscivorous fish may not have been able to take advantage of increased prey denisties during this period due to low activity as a result of low winter water temperatures.

Most abundant species within saltmarsh impoundment

Small Fishes

Cyprinodon variegatus
Poecilia latipinna
Menidia peninsulae
Paleomonetes pugio
Lucania parva
Gambusia holbrooki
Mugil cephalus
Fundulus grandis
Leiostomus xanthurus
Alpheus heterochaelis

Large Fishes

Lepisosteus platyrhincus
Mugil cephalus
Sciaenops ocellatus
Cynoscion nebulosus
Elops saurus
Leiostomus xanthurus

sheepshead minnow
sailfin molly
inland silverside
grass shrimp
rainwater killifish
mosquitofish
striped mullet
gulf mummichog
spot
snapping shrimp

Florida gar
striped mullet
red drum
spotted seatrout
ladyfish
spot