David J. Yozzo, Virginia Institute of Marine Science, School of Marine Science, College of William and Mary, Gloucester Point, VA 23062
This ongoing project examines the distribution and comparative life history attributes of resident fishes in high marsh and marsh/upland ecotone habitats at the Virginia Coast Reserve. Permanent and ephemeral aquatic habitats (ponds, ditches, sloughs) are common landscape features of the infrequently flooded intertidal zone and the interface between tidal wetlands and adjacent terrestrial habitats. These habitats support a distinct resident nekton community and may provide abundant forage for wading birds and other upper level consumers. Fishes and other resident nekton (primarily decapod crustaceans) have been collected monthly since June 1994 from five sampling locations within the Phillips Creek/Brownsville component of the VCR-LTER site. Both permanent and seasonally flooded habitats are represented within the study locations, which were selected to best represent variation in physico-chemical conditions and geomorphology of the intertidal high marsh environment.
Physico-chemical conditions varied widely among the study sites. Salinity ranged from 0 ppt at a relict agricultural pond to > 40 ppt at a high-salinity pocket marsh adjacent to Phillips Creek. Dissolved oxygen levels within habitats varied widely throughout the study period, and hypoxic conditions (< 3 mg/l) were regularly encountered during the summer in conjunction with high temperatures and reduced water levels. Other habitat characteristics, including substrate type, availability of invertebrate food items, and presence/absence of submerged aquatic vegetation (SAV) are being monitored at all sites, with particular regard to the role of these variables in determining the nursery function of a particular habitat.
The fish community of high salt marsh habitats is comprised of several widely distributed, environmentally-tolerant species. A total of 1032 individuals representing 7 species were collected during June - November 1994 (Table 1). The mosquitofish (Gambusia affinis holbrooki) was abundant in permanent, brackish and freshwater habitats. Mosquitofish were either absent or rare in high salinity or ephemeral marsh habitats. Mummichogs (Fundulus heteroclitus) were most abundant in ephemeral habitats, particularly in late fall and winter when water levels were highest. The spotfin killifish (Fundulus luciae) is a resident high marsh species and was most abundant in high salinity habitats, particularly in mid-summer. This species was present at both permanent and ephemeral brackish and polyhaline sites. Sheepshead minnows (Cyprinodon variegatus) were most abundant at a polyhaline marsh site, and were occasionally present in brackish habitats. The rainwater killifish (Lucania parva) was regularly collected at the polyhaline marsh site. Additional species collected infrequently from high marsh habitats include the inland silverside (Menidia beryllina) and juvenile American eels (Anguilla rostrata). In addition to fishes, decapod crustaceans were seasonally abundant in intertidal high marsh habitats. Grass shrimp (Palaemonetes pugio) were abundant in collections from the polyhaline site from August through November. This species was either rare or absent from other high marsh locations. Juvenile blue crabs (Callinectes sapidus) were occasionally collected at the polyhaline marsh site. Freshwater crayfish (Cambarus robustus) were occasionally collected from a low salinity ditch adjacent to the headwaters of Phillips Creek. The important nursery function of high marsh habitats is exemplified by the large percentage of juveniles collected during the study period. 42% of all fishes collected during June - November 1994 were young-of-the-year.
During Spring-Summer 1995, intensive sampling of the primary study sites will be conducted in order to determine life history attributes of selected high marsh species. Primary emphasis will be placed on the comparative life history of mosquitofish from freshwater and saline habitats. This live-bearing species is capable of rapid evolution of life history traits in response to environmental factors. The population ecology of mosquitofish in saline environments, particularly within a salt marsh ecosystem, is not well-documented. Preliminary data collection has revealed significant differences in length distribution and sex ratios between the two populations examined at the VCR. It is possible to visually discern individuals from the different populations due to variation in pigmentation, a trait which may evolve in response to environmental parameters, such a substrate type. Fecundity, age and size class distribution, feeding habits and relative abundance patterns of mosquitofish and other resident species will be compared between disparate marsh habitats. The role of environmental vs. biotic factors (predation, competition, etc.) in determining species distribution and seasonal/spatial patterns of habitat use will be evaluated. It is anticipated that additional collection sites, including high marsh environments located on the Virginia barrier islands, and marshes contiguous with the nearby Machipongo River, will be surveyed in late spring/summer 1995.
North Phillips Creek TNC* Phillips Creek*
Duck Pond Brownsville Pocket Marsh Boardwalk High Marsh
Gambusia affinis 434 (39%) 199 (27%) 1 (0%) 0 1 (0%)
Fundulus heteroclitus 0 15 (7%) 56 (79%) 7 (0%) 55 (27%)
Fundulus luciae 0 7 (71%) 144 (51%) 13 (100%) 0
Cyprinodon variegatus 0 0 55 (51%) 0 2 (0%)
Lucania parva 0 0 36 (78%) 0 0
Anguilla rostrata 1 (100%) 0 4 (100%) 0 0
Menidia beryllina 0 0 2 (0%) 0 0
* ephemeral sites