Small-scale patterns of nekton use among marsh and adjacent shallow nonvegetated areas of the Galveston Bay Estuary, Texas (USA)

Abstract

We quantified and compared nekton and infaunal densities among vegetated (edge Spartina alterniflora, inner Spartina alterniflora, Scirpus maritimus, Juncus roemerianus, and Spartina patens marsh) and shallow nonvegetated (marsh pond, marsh channel, cove, and shallow bay) areas of upper Galveston Bay and East Bay, Texas. In 2 seasons (spring and fall) of high nekton abundance, and over 2 yr, we collected 267 quantitative samples (upper Galveston Bay, 1993 = 127 and East Bay, 1994 = 140) using a 1 m2 drop sampler. The vegetated marsh surface consistently contained more species (i.e. higher species richness) and total numbers of decapod crustaceans than nonvegetated areas. In contrast, fish spedes richness and densities of total fishes on the marsh and in nonvegetated areas were not significantly different in most comparisons. Most numerically dominant species of nekton seemed to exhibit at least some degree of habitat selection. Within vegetation, 2 factors, elevation and proximity to open water, were most important in influencing the distribution of nekton. Low marsh edge dominated by Spartina alterniflora or Scirpus maritimus was apparently selected by most species that used the marsh surface including brown shrimp Farfantepenaeus aztecus, blue crab Callinectes sapidus, and daggerblade grass shrimp Palaemonetes pugio. White shrimp Litopenaeus setiferus and striped mullet Mugil cephalus also were concentrated in low edge marsh; although in one comparison, densities of these 2 species in edge and inner S. alterniflora were not significantly different. In contrast, gulf killirish Fundulus grandis and sheepshead minnow Cyprinodon variegatus were most abundant on inner S. alterniflora or S. patens marsh. Other fishes (gulf menhaden Brevoortia patronus, spot Leiostomus xanthurus, bay anchovy Anchoa mitchilli', blackcheek tonguefish Symphurus plagiusa, and Atlantic croaker Micropogonias undulatus) had higher densities over nonvegetated bottoms than on the marsh surface. Specific habitat types that these pelagic species seemed to favor were marsh channels (gulf menhaden, bay anchovy), marsh ponds (spot), and coves (Atlantic croaker, blackcheek tongue fish). Overall marsh-surface and adjacent nonvegetated habitat types contained much higher densities of most nekton than the shallow bay. Infaunal densities were estimated from sediment cores, and taxa (mainly annelids, crustaceans, molluscs, and insects) were most abundant in nonvegetated areas contiguous with marsh in the spring. Factors that influenced infaunal abundance are complex and may include predation, flooding patterns, elevation, and distance to edge. Our study has important implications for designing marsh-creation projects. Based on our results, we recommend creating a variety of marsh and contiguous shallow-water areas to enhance nekton biodiversify. To maximize fishery habitat, priority should be given to constructing low marsh edge by creating large areas of low marsh interspersed with a dense network of shallow channels and interconnected ponds.