Salt Marsh Ecosystem Support of Marine Transient Species

Abstract

One of the most important reasons stated in legislation for protecting salt marshes is their support of commercially and recreationally important nekton (fish and crustaceans). Yet, there is a surprising level of uncertainty among scientists regarding the role of salt marshes in supporting secondary production. The emphasis has been on "marine transient" species (in earlier literature often referred to as "estuarine dependent") because they have life histories that seem designed to place young-of-the-year or juveniles in marsh habitats and because these species are often of commercial or recreational value. Salt marshes are believed to provide: 1) trophic support resulting in high growth rates, 2) increased survivorship due to lowered mortality, and 3) a suitable physico-chemical environment for development of young fishes. In this paper, we consider the evidence for each of these, with an emphasis on the trophic and survivorship aspects. The seasonally warmer temperatures of estuaries and salt marsh creeks apparently provide a metabolic advantage that supports high growth rates. The influence of marsh-derived organic matter in estuarine food webs is apparent, and its importance to marine transient fishes is supported by dietary, behavioral, and isotopic evidence. The major pathways by which marsh organic matter is transferred to fish are largely indirect, through microbial and invertebrate intermediaries. Invertebrates are the primary link to fish consumers of marsh-associated production, transforming microphytes, organic detritus, and microbial detrital heterotrophs into available biomass. Although most detrital organic carbon entering salt marsh systems, mainly from emergent grasses, is apparently respired by heterotrophs, the support of consumers by marsh plant detritus and microalgae can be equally important. The use of salt marsh detritus in food webs usually occurs in close proximity to the salt marsh indicating that outwelling of salt marsh organic matter offshore is not the dominant way that salt marshes support offshore fisheries. Salt marsh support of offshore fisheries is more probably by direct export of juvenile fish biomass and a trophic relay involving ontogenetic and cyclic migrations of nekton species, rather than export of organic detritus. Understanding the controls on marine transient fish mortality is probably the most problematic and least studied aspect of their ecology. The few estimates of mortality rates of fishes in estuaries are as high as, or higher than, mortality rates of fishes in other marine and freshwater ecosystems. However, because of faster growth rates, fish spend less time in the small stages with the higher mortality rates. Within estuaries, mortality rates for some species, but not all, are lower in marsh creeks compared to more open areas. The value of marshes as refuge habitat is probably due to the interaction of temperature, turbidity, and vegetative structure in restricting the foraging of piscine predators.