The Atlantic sturgeon Acipenser oxyrinchus oxyrinchus has a latitudinally broad distribution along the east coast of North America, with extant populations occurring from the Saint Lawrence River to rivers in southern Georgia. This species once supported intensive caviar-based fisheries that resulted in overharvest and sharply reduced population abundances; presently, directed commercial fishing for Atlantic sturgeon is banned in U.S. waters. We sequenced a 203base-pair section of the mitochondrial DNA (mtDNA) control region of 322 Atlantic sturgeon specimens from 11 river systems across their range to elucidate their stock structure. We found a pronounced latitudinal cline in the number of composite mtDNA haplotypes and in haplotypic diversity, which increased from north to south, from previously glaciated and subsequently recolonized systems to the portion of their range unglaciated during the Pleistocene. The observed number of haplotypes per population ranged from 1 haplotype in each of the two northernmost population samples to 17 in the sample from the Savannah River. Haplotypic diversity ranged from 0.0 to 0.90. The greater genetic diversity within and among southern populations is likely a product of the persistence of these populations through the Pleistocene and to the faster mutation rates associated with their shorter generation times. Of 39 composite mtDNA haplotypes found, 64% were unique to particular populations. Monomorphism of the two Canadian populations suggested a strong founder effect. Three haplotypes unique to northern populations were probably the result of base substitutions that occurred within the past 10,000 years. In contrast with an earlier study, we found stock structure among southern populations and evidence of at least seven genetic stocks across this subspecies' range.

The fish assemblage using shallow nursery habitats in the Ogeechee River-Ossabaw Sound salt-marsh estuary, Georgia, was investigated during the winter and spring of two successive years. High river discharges during these periods produced fully freshwater conditions (all tidal stages and amplitudes) in the upper portion of the study area for up to 4 months. Abundances of Atlantic croaker Micropogonias undulatus, southern flounder Paralichthys lethostigma, silver perch Bairdiella chrysoura, and hogchoker Trinectes maculatus recruits were highest in the upper estuary. Spots Leiostomus xanthurus were more evenly distributed, but continued to use upper-estuary nursery areas during periods of high river discharge. Although the recruitment of several species likely was inhibited during discharge peaks, only striped mullet Mug//cephalus avoided freshwater conditions. Spots, southern flounder, Atlantic menhaden, and silver perch utilized shallow nursery areas on a size-specific basis. Recruitment and utilization patterns of fishes spawned in deeper areas generally were maintained throughout prolonged periods (up to 100 days) of freshwater conditions. The precise function of upper-estuary nursery areas is governed by the timing and magnitude of discharge events, but remains essentially intact through the seasonal encroachment of fresh water. Extensive and highly productive salt-marsh estuaries are a dominant feature along much of the Atlantic and Gulf of Mexico coasts of the southeastern United States. Shallow areas within estuaries provide habitat for fish assemblages that are highly dynamic both spatially and temporally. Some species are resident year-round, but an important component of these assemblages is a transient group of young-of-the-year clupeiform, perciform, and pleuronectlform fishes that are spawned in deeper estuarine, nearshore, and offshore waters (Turner and Johnson 1973). These species enter the shallow portions of estuaries primarily in the winter and early spring, occur in very high densities during their recruitment and residence periods, and leave the shallows as they grow or as the water begins to cool in late summer and early fall (Herke 1971; I Present address: University of Delaware, College of Marine Studies, Lewes, Delaware 19958. • Present address: South Carolina Wildlife and Marine Resources Department, Post Office Box 12559, Dean 1980; Weinstein2 et al. 1980; Weinstein and Brooks 1983). Comparisons of their spatial and temporal nursery use between studies are complicated by differences in habitats sampled, timing of sampling with respect to tidal-lunar cycles, and gear design and efficiency. However, it is apparent that fishes generally exhibit similar patterns throughout the region. "Waves" of recruitment and peaks of abundance unique to each species are separated through time during the late fall, winter, spring, and early summer (Weinstein2 et al. 1980). Each species associates with particular ranges of physicochemical and biotic factors, including salinity, substrate composition, veget...

No abstract

The addition of heavy, spiked, linked tickler chains (chain mat) to a 2 m beam trawl without chain mat increased the catch rates of some epibenthic species, but not others. Catch rates of the invertebrates Asten'as rubens, Ophiura ophiura, Liocarcinus holsatus and all combined flatfish species increased. In contrast, the catch rates of the epibenthic teleosts Callionymus spp. decreased, but those of Echiichthys vipera were not affected. Comparisons with data in other studies suggested that this modification increased the catch rates of epifauna to give an improved estimate of population density. Although total biomass of the catch increased with tow durations between 2.5 and 7.5 min, there was no significant effect on estimates of either standardised species abundance or biomass ha-' Although short tows reduce catch sorting time, the variation between samples was greater than for longer tows (7.5 min). Hence, it is suggested that the latter are preferable for estimating population density and community structure, although this may depend on the spatial dimensions and objectives of the study. The results are discussed in the context of the use of the gear as a sampling tool in ecological surveys of epibenthic communities In sublittoral and coastal shelf areas.