Early life stages of resident nekton in intertidal marshes

Abstract

Variability in early life stages of species that are permanent residents of the estuarine nekton is poorly understood, especially in systems with extensive areas of emergent vegetation (e.g., salt marshes and mangroves). Sampling small mobile nekton in these shallow intertidal habitats presents a difficult methodological challenge. Simulated aquatic microhabitats (SAMs) were used to collect the early life stages of resident nekton that remained on the emergent marsh surface after it was exposed by the tide and could not be adequately sampled by traditional methods. Where the intertidal is a prominent areal component of the estuary, a large portion of young nekton could be overlooked using other common survey methods (e.g., plankton tows or block nets). Populations of young fishes and natant crustaceans were monitored for a year at 3-d to 6-d intervals from both low and high intertidal elevations within each of two marsh sites on Sapelo Island, Georgia, USA. Three species accounted for >99% of the 41,023 individuals collected. These were the killifishesFundulus heteroclitus (57.0%) andF. luciae (4.0%), and the daggerblade grass shrimp,Palaemonetes pugio (38.4%). YoungF. heteroclitus were used in field enclosure experiments to relate abundance data to actual areal densities. Average annual estimated density of young nekton on the surface of the intertidal marsh at low tide was 7.2 individuals m⁻². Early life stages of estuarine resident species, particularly those with demersal young, are not affected by the same physical processes influencing larval supply and recruitment variability in marine-spawned species. In salt marshes, biotic factors (e.g., adult reproductive activity, predation, and food limitation) may be more important as proximate causes of variation during the early life histories of resident nekton.