102 research outputs found

    Competition, predation, and density-dependent mortality in demersal marine fishes

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    The relative roles of competition and predation in demographic density dependence are poorly known. A tractable experimental design to determine such effects and their interactions for demersal (seafloor oriented) fishes and similar sedentary species is cross-factoring multiple densities of new recruits with the presence and absence of predators. This design allows one to distinguish between density-dependent mortality due to competition alone, predation alone, or an interaction between the two, especially when supplemental field observations are available. To date, 14 species of marine fish have been examined with some variant of this design, and for 12 species predation was demonstrated to be the sole or major cause of density dependence. However, as competition may be slow acting relative to predation, the importance of competition can be underestimated in short-term experiments. On the Great Barrier Reef, we conducted a long-term field experiment in which multiple densities of new recruits of a planktivorous damselfish were cross-factored with the presence or absence of resident piscivorous fish on patch reefs. During the first 10 months, no density-dependent mortality was detected, regardless of whether resident predators were present or absent. By the end of the experiment at 17 months, per capita mortality was strongly density dependent and highly compensatory in both predator treatments; all reefs ultimately supported nearly the same adult density regardless of experimental treatment. Examination of treatment effect sizes suggested that competition was the main source of density-dependent mortality, with predation being merely a proximate agent of death. We hypothesize that predators were ineffective in this system compared with similar studies elsewhere because prey density was low relative to ample prey refuges provided by highly complex corals. Combined with previous studies, these findings indicate that density-dependent mortality in demersal marine fishes is often caused by interplay of predation and competition, whose roles may be altered by variation in habitat complexity and larval supply. These conclusions are relevant to marine fisheries models, which typically assume that density dependence is due solely to intraspecific competition

    Current velocity and catch efficiency in sampling settlement-stage larvae of coral-reef fishes

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    Light traps and channel nets are fixed-position devices that involve active and passive sampling, respectively, in the collection of settlement-stage larvae of coral-reef fishes. We compared the abundance, taxonomic composition, and size of such larvae caught by each device deployed simultaneously near two sites that differed substantially in current velocity. Light traps were more selective taxonomically, and the two sampling devices differed significantly in the abundance but not size of taxa caught. Most importantly, light traps and channel nets differed greatly in their catch efficiency between sites: light traps were ineffective in collecting larvae at the relatively high-current site, and channel nets were less efficient in collecting larvae at the low-current site. Use of only one of these sampling methods would clearly result in biased and inaccurate estimates of the spatial variation in larval abundance among locations that differ in current velocity. When selecting a larval sampling device, one must consider not only how well a particular taxon may be represented, but also the environmental conditions under which the device will be deployed

    Integration of submersible transect data and high-resolution multibeam sonar imagery for a habitat-based groundfish assessment of Heceta Bank, Oregon

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    In the face of dramatic declines in groundfish populations and a lack of sufficient stock assessment information, a need has arisen for new methods of assessing groundfish populations. We describe the integration of seafloor transect data gathered by a manned submersible with high-resolution sonar imagery to produce a habitat-based stock assessment system for groundfish. The data sets used in this study were collected from Heceta Bank, Oregon, and were derived from 42 submersible dives (1988–90) and a multibeam sonar survey (1998). The submersible habitat survey investigated seafloor topography and groundfish abundance along 30-minute transects over six predetermined stations and found a statistical relationship between habitat variability and groundfish distribution and abundance. These transects were analyzed in a geographic information system (GIS) by using dynamic segmentation to display changes in habitat along the transects. We used the submersible data to extrapolate fish abundance within uniform habitat patches over broad areas of the bank by means of a habitat classification based on the sonar imagery. After applying a navigation correction to the submersible-based habitat segments, a good correlation with major boundaries on the backscatter and topographic boundaries on the imagery were apparent. Extrapolation of the extent of uniform habitats was made in the vicinity of the dive stations and a preliminary stock assessment of several species of demersal fish was calculated. Such a habitat-based approach will allow researchers to characterize marine communities over large areas of the seafloor

    Comparison of the Effects of Hexavalent Chromium in the Alimentary Canal of F344 Rats and B6C3F1 Mice Following Exposure in Drinking Water: Implications for Carcinogenic Modes of Action

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    Exposure to high concentrations of hexavalent chromium (Cr[VI]) in drinking water is reported to induce oral mucosa tumors in F344 rats and intestinal tumors in B6C3F1 mice. To investigate the modes of action underlying these tumors, 90-day drinking water studies (with interim necropsy at day 8) were conducted with concentrations of 0.1–182 mg/l Cr(VI), administered as 0.3–520 mg/l sodium dichromate dihydrate. Blood and tissue samples were analyzed for chromium content, oxidative stress, iron levels, and gross and microscopic lesions. Results for the F344 rats are described herein and compared with results from B6C3F1 mice published previously. After 90 days of exposure, total chromium concentrations in the rat and mouse oral mucosae were comparable, yet significant dose-dependent decreases in the reduced-to-oxidized glutathione ratio (GSH/GSSG) were observed only in rats. In the duodenum, changes in GSH/GSSG were only observed in mice. Levels of 8-hydroxydeoxyguanosine were not increased in the oral or duodenal mucosae of either species. Glutathione levels were increased in the duodenum but decreased in the jejunum of both species, indicating potential differential responses in the intestinal segments. Histiocytic infiltration was observed in the duodenum of both species, yet duodenal cytokines were repressed in mice but increased in rats. Serum and bone marrow iron levels were more decreased in rats than mice. Collectively, these data suggest that Cr(VI)-induced carcinogenesis in the rodent alimentary canal involves oxidative stress; however, differences in histopathology, cytokines, and iron status suggest potential contributions from other factors as well
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