Urbanization impacts on production and recruitment of Fundulus heteroclitus in salt marsh creeks

It is unclear how urbanization affects secondary biological production in estuaries in the southeastern USA. We estimated production of larval/juvenile Fundulus heteroclitus in salt marsh areas of North Carolina tidal creeks and tested for factors influencing production. F. heteroclitus were collected with a throw trap in salt marshes of 5 creeks subjected to a range of urbanization intensities. Multiple factor analysis (MFA) was used to reduce dimensionality of habitat and urbanization effects in the creeks and their watersheds. Production was then related to the first 2 dimensions of the MFA, month, and year. Lastly, we determined the relationship between creek-wide larval/juvenile production and abundance from spring and abundance of adults from autumn of the same year. Production in marsh (g m-2 d-1) varied between years and was negatively related to the MFA dimension that indexed salt marsh; higher rates of production were related to creeks with higher percentages of marsh. An asymptotic relationship was found between abundance of adults and creek-wide production of larvae/juveniles and an even stronger density-dependent relationship was found between abundance of adults and creek-wide larval/juvenile abundance. Results demonstrate (1) the ability of F. heteroclitus to maintain production within salt marsh in creeks with a lesser percentage of marsh as long as this habitat is not removed altogether and (2) a density-dependent link between age-0 production/abundance and subsequent adult recruitment. Given the relationship between production and marsh area, natural resource agencies should consider impacts of development on production when permitting construction in the southeastern USA.

Population Structure and Habitat Use of the Endangered Watercress Darter

Despite Alabama's exceptionally diverse freshwater fish fauna, many of its fish species face extinction. Some of the state's most imperiled species dwell within coldwater springs, but a deficit of knowledge about their ecology hampers protection efforts. The watercress darter Etheostoma nuchale is a species confined to five springs in the greater Birmingham metropolitan area. Roebuck Spring likely harbors the largest population. Its pool has been surveyed occasionally, but there had been no survey of its run, a shallow stream flowing from the pool. We investigated the darter's use of the run, its habitat preferences, and characteristics of the habitats where it is most abundant. We quantified the abundance of stream microhabitats, then estimated darter density in the stream's five most common habitats using a throw trap, a high-walled metal frame dropped in the habitat. We found darters at densities vastly exceeding typical estimates derived using seine nets. We estimated the run harbored 116,932 (79,358–155,965) darters, two-thirds of which were juveniles. The most preferred habitat was coontail Ceratophyllum demersum, a submergent plant not previously known to provide darter habitat. Coontail grew prolifically in swift currents, which was surprising given that darter habitats described previously had little to no current. Coontail provided a more structurally complex habitat than the plants of the other microhabitats studied. Our results suggest that spring runs can support substantial densities of the watercress darter if dense submergent vegetation is present.

Catchability model selection for estimating the composition of fishes and invertebrates within dynamic aquatic ecosystems

We evaluated alternative catchability models for estimating the composition of fishes and decapods in two coastal rivers sampled via capture–recapture boat electrofishing, seine removal sampling, and throw trap removal sampling. Information criteria were used to select between linear and nonlinear catchability models and assess heterogeneity in catchability coefficients across populations, sampling reaches, and sampling events. The selected electrofishing catchability model assumed linearity between catch per unit effort and population density, with heterogeneity in catchability coefficients among taxa and between rivers. Linear models were selected for seine and throw trap samples, with heterogeneity in catchability coefficients among taxa, sampling reaches, and sampling events. We emphasize the importance of estimating population density, rather than abundance exclusively, when accurate community composition estimates are needed. The analytical techniques employed have broad-scale application in quantifying key components of ecosystems by integrating information from multiple sampling gears targeting different taxa or trophic guilds and in accounting for variable sampling intensity and catchability heterogeneity in the estimation of community composition.

Simple relationships to predict attributes of fish assemblages in patches of submerged macrophytes

Submerged macrophytes play an important role in structuring habitats and, therefore, in determining patterns of aquatic biodiversity. Because these plants are widespread in shallow areas of many Neotropical reservoirs, the present work investigated if variables related to habitat structure, measured in patches of submerged macrophytes (Egeria densa and E. najas), can be used to predict fish assemblage attributes (fish density and species richness). Based on patch characteristics at fine spatial extents (macrophyte patches within reservoir arms), we considered plant biomass, volume and proportional volume (i.e. percentage of macrophyte volume in the water column) as potential predictors. Fish and macrophytes were sampled with a 1-m² throw trap in littoral habitats of Rosana Reservoir, Paranapanema River, and simple correlation analyses were performed. Fish richness and abundance were highly correlated with all variables (R = 0.53 to 0.90), a relationship consistently observed in all sites. When compared to biomass, plant volume and proportional volume did not yield stronger correlations. We observed stronger correlations when E. densa and E. najas patches were analyzed separately (mono-specificity), probably because particular effects of each macrophyte on habitat structuring were removed (e.g. unnoticed morphological differences or unknown effects on habitat quality). The high R values observed in all pairwise relationships are uncommon in ecological studies, highlighting the predictive potential of variables related to habitat structure. These results suggest that, at small spatial extents, macrophyte biomass may represent an interesting predictor of fish density and richness in reservoirs with extensive colonization of submerged plants.