Fish assemblages in small lakes (£50 ha) in central Ontario were characterized to determine the impact of smallmouth bass (Micropterus dolomieu) predation and habitat complexity on the structure of littoral zone fish assemblages. Data were collected employing minnow traps and visual assessment. Although species richness did not differ between lakes with and without smallmouth bass, species composition and relative abundance did differ. We identified two distinct fish assemblage types: one characterized by small-bodied species, mainly cyprinids, and a second by largebodied centrarchid species, e.g., smallmouth bass. Smallmouth bass appear to reduce abundance, alter habitat use, and extirpate many small-bodied species such as brook stickleback (Culaea inconstans), fathead minnow (Pimephales promelas), pearl dace (Margariscus margarita), and Phoxinus spp.Résumé : La caractérisation des peuplements de poissons du littoral de petits lacs (£50 ha) du centre de l'Ontario a permis d'évaluer l'impact de la prédation de l'Achigan à petite bouche (Micropterus dolomieu) et de la complexité de l'habitat sur la structure des communautés. Les données consistaient en des récoltes faites à la nasse et des estimations visuelles. La composition spécifique des peuplements de poissons et l'abondance relative des espèces différaient entre les lacs où l'achigan était présent et ceux où il était absent, mais pas la richesse en espèces. Il a été possible de distinguer deux types de peuplements, l'un composé de poissons à corps élancé, en majorité des cyprinidés, et l'autre dominé par des poissons centrarchidés à corps massif, e.g., l'achigan. La présence de l'achigan semble donc réduire l'abondance des poissons, modifier l'utilisation de l'habitat et éliminer plusieurs des poissons à corps élancé, tels que l'Épinoche à cinq épines (Culaea inconstans), le Tête-de-boule (Pimephales promelas), le Mulet perlé (Margariscus margarita) et les Phoxinus spp.[Traduit par la Rédaction] MacRae and Jackson 351

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Omnivory is ubiquitous in aquatic and terrestrial ecosystems and may increase stability of food webs. Turtles are longlived and among the most abundant organisms in lentic aquatic habitats. Many species are omnivores and thus may be especially important in regulating ecosystem dynamics in these systems. Turtles are also one of the most threatened vertebrate groups in the world; if they indeed play pivotal roles in lentic ecosystems, their declines may disrupt or even destabilize those ecosystems. In order to assess the role of turtles in an aquatic ecosystem, we examined sources of primary production, trophic structure, and ontogenetic diet shifts of five species of turtles, other herpetofauna, and fish in a large shallow lake in northern Florida. Stable isotope analyses (d 13 C and d 15 N) of 42 species of animals and five sources of primary productivity revealed that macroalgae and/or particulate organic matter were the foundation of the food web. Trophic structure of the herpetofauna and fish indicated a high degree of omnivory (TPs of 3-4), few specialists at high trophic levels (TP $ 4.0), and few strict primary consumers (TP = 2.0). Several predatory fish (Micropterus salmoides, Lepomis gulosus, and Pomoxis nigromaculatus) and herpetological predators (e.g., Alligator, Nerodia) were functionally trophic omnivores with trophic positions lowered by predation on primary and low trophic position secondary consumers. We found two major divisions in the trophic structure of turtles-an herbivore (Pseudemys floridana) and four omnivores (Apalone ferox, Chelydra serpentina, Sternotherus odoratus, and Trachemys scripta). There were at least three types of omnivores, species that are generalist feeders throughout their life (e.g., C. serpentina, S. odoratus), species that change their trophic position during ontogeny (e.g., A. ferox), and species with the same trophic position through ontogeny but resulting from different diet composition as juveniles and adults (e.g., T. scripta). Trophic position of some species indicated that even when turtles are primarily predatory, they also function as vegetative consumers and facultative scavengers (e.g., T. scripta, S. odoratus, A. ferox). Given that omnivory is a critical process that can stabilize food webs, there will certainly be dramatic shifts in how energy and nutrients flow through these lentic ecosystems if turtle populations decline.

We conducted hydroacoustic, gill‐net, and push trawl surveys to quantify changes in habitat‐specific fish size and biomass in shallow (<2‐m) estuarine waters of Barataria Bay, Louisiana, in order to evaluate essential fish habitat. Surveys were conducted monthly between June 2003 and May 2004 among regions located along a north—south salinity gradient. The fish length distributions derived from the gill‐net and push trawl catches showed moderate concordance with the measured target strength distributions, indicating that our integrated approach more effectively characterized the fish community than using only a single gear type would have. Acoustic estimates showed that biomass was highest during fall (mean ± SE; 2.30 ± 0.27 g/m3) and next highest in spring (1.49 ± 0.20 g/m3), with relatively low biomass during summer (0.70 ± 0.14 g/m3) and winter (0.86 ± 0.14 g/m3); pelagic fish biomass from nets was low during winter (53.9 ± 14.9 grams per unit effort [gpue]) but relatively high in fall (846.1 ± 207.2 gpue), spring (774.3 ± 175.5 gpue), and summer (620.3 ± 140.7 gpue). Oyster habitat supported a greater biomass of pelagic fish (acoustic survey: 1.54 ± 0.15 g/m3; gill‐net survey: 467.3 ± 81.0 gpue) than soft‐bottom habitat (acoustic: 0.94 ± 0.11 g/m3; gill‐net: 315.2 ± 54.8 gpue). Among regions, the greatest biomass of pelagic fish was observed at polyhaline stations (acoustic: 1.78 ± 0.19 g/m3; gill‐net: 654.3 ± 136.5 gpue), followed by mesohaline (acoustic: 1.18 ± 0.15 g/m3; gill‐net: 378.5 ± 79.1 gpue) and oligohaline stations (acoustic: 0.82 ± 0.12 g/m3; gill‐net: 228.3 ± 50.2 gpue). Gill‐net biomass was linearly related to the acoustic biomass estimates of small pelagic fish. The complementary, multigear approach proved to be useful in evaluating habitat use and may be particularly helpful in identifying and monitoring ecosystem reference points to evaluate change and in standardizing ecosystem‐based assessment approaches.

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Louisiana wetlands are disappearing at a dramatic rate, providing an impetus for identifying the relative value to fishes of a matrix of estuarine habitat types. The distribution, relative abundance, biomass, and length of spotted seatrout, Cynoscion nebulosus (Cuvier) were examined in Barataria Bay, Louisiana, in relation to habitat type and abiotic variables. Spotted seatrout were collected from three sites located along a salinity gradient. Each site contained the three habitat types of interest: marsh edge, soft bottom and oyster shell, and were sampled monthly from May 2003 to May 2004 with gillnets. Habitat preference of spotted seatrout was not easily defined by habitat type alone, but rather their distribution, relative abundance, biomass and length distribution were influenced by a combination of habitat type and abiotic variables. These results suggest that habitat type and abiotic properties of the water act together to provide a diverse range of available habitats important to spotted seatrout. Despite the importance of incorporating habitat in fisheries management, it may be that a combination of habitats and their spatial arrangement, as well as abiotic variables, contribute to the value of estuarine habitats in support of fisheries productivity. Thus, efforts to ensure future fisheries productivity of spotted seatrout fisheries in Louisiana will likely be maximized by managing the spatial integrity of multiple habitat types as opposed to focusing on any single type.

Comparative studies documenting a relationship between male gonadal investment and the degree of sperm competition (SC) have usually considered the association between these traits to be driven by qualitative differences in the mating system, such as whether spawning occurs in pairs or groups. However, ecological and demographic differences between conspecific populations may also generate variation in the importance of SC that can drive the evolution of male gonadal investment. In this study, we examined whether variation in population density, which is predicted to influence the level of SC in many animals, is correlated with male gonadal investment among populations of the least killifish, Heterandria formosa, a species with internal fertilization in which multiple mating is common. We complemented this field study by testing whether males respond plastically to experimentally increased levels of SC by increasing investment in testis. This experiment involved two treatments. In the first, we eliminated the potential for sperm competition (NSC) by housing a single male with a single female. In the second, we created a high risk of SC by housing five males with two females. In the field survey, we found significant differences among populations in density and relative testis mass. However, there was no evidence for a correlation between population density and relative testis mass. In our lab experiment, males did not adjust their gonadal investment in response to experiencing different levels of SC for 4 weeks. Our combined results indicate that gonadal investment in male H. formosa is not related to variation in population density.

To determine which sampling method may be best employed to monitor changes in fish species composition in littoral areas of small north temperate lakes (≤50 ha), data were collected using distance sampling, based on the line transect method, and a traditional fish sampling method of baited minnow traps. These methods were independently biased in terms of their estimates of species presence and abundance. However, analyses of fish community structure indicated that minnow trap data alone provided a better measure of community structure than did distance sampling data alone. Distance sampling, although a powerful tool in estimating population density, is hindered by a multitude of environmental variables that do not preclude the use of minnow traps. Lakes with clear water containing few species with abundant individuals are best suited for distance sampling. Minnow traps proved to be a fast, efficacious sampling method capable of fairly accurately defining the littoral small-fish community structure in most lakes sampled. These traps are a reliable method for monitoring presence or absence and relative abundance of small-bodied fish species in north temperate lakes. However, the combination of minnow traps and some form of visual sampling will help to ensure that all species are detected in small north temperate lakes.