Browsing by Subject "fisheries"
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Item Floodplain River Foodwebs in the Lower Mekong Basin(2013-11-15) Ou, ChoulyThe Mekong River is one of the world?s most important rivers in terms of its size, economic importance, cultural significance, productivity, and biodiversity. The Mekong River?s fisheries and biodiversity are threatened by major hydropower development and over-exploitation. Knowledge of river food web ecology is essential for management of the impacts created by anthropogenic activities on plant and animal populations and ecosystems. In the present study, I surveyed four tropical rivers in Cambodia within the Mekong River Basin. I examined the basal production sources supporting fish biomass in the four rivers during the dry and wet seasons and explored the relationship between trophic position and body size of fish at various taxonomic levels, among local species assemblages, and across trophic guilds. I used stable isotopes of carbon and nitrogen to estimate fish trophic levels and the principal primary production sources supporting fishes. My study provides evidence that food web dynamics in tropical rivers undergo significant seasonal shifts and emphasizes that river food webs are altered by dams and flow regulation. Seston and benthic algae were the most important production sources supporting fish biomass during the dry season, and riparian macrophytes appeared to be the most important production source supporting fishes during the wet season. In the river with strong flow regulation from an upstream impoundment, seston and benthic algae were even more important production sources supporting fishes during the dry season. My findings challenge the Eltonian theory of size-based trophic structure in food webs and also contradict the broadly accepted prediction of the fishing-down-the-food-web concept. Eltonian and fishing-down-the-food-web concepts propose that trophic level is strongly correlated with body size, but I found no significant correlation between body size and trophic position for fish assemblages. Results suggest that body size distributions are not useful for prediction the trophic structure of communities with diverse detritivores, omnivores and insectivores, but that it is a good predictor of trophic position among piscivorous fishes.Item Food web architecture in natural and impounded rivers of the Upper Parana drainage basin, Brazil(Texas A&M University, 2007-04-25) Hoeinghaus, David JosephFreshwater ecosystems are some of the most threatened on the planet. Efforts to conserve, restore, or otherwise manage large rivers and the services they provide are hindered by limited understanding of the functional dynamics of these systems. This shortcoming is especially evident with regard to trophic structure and energy flow. In this study I use natural abundances of carbon and nitrogen isotopes to examine patterns of energy flow and food-chain length of large-river food webs characterized by different landscape-scale hydrologic features. Ten locations along an approximately 500 km stretch of the Upper Paran???? River Basin, Brazil, provided the setting for this work. Carbon derived from C3 plants and phytoplankton were the dominant energy sources across all webs, but relative contributions differed among landscape types (low-gradient river, high-gradient river, river stretches downstream of reservoirs, and reservoirs). Increases in food chain length corresponded with higher relative importance of phytoplankton derived carbon, likely due to size-structured effects of the phytoplankton-zooplankton-secondary consumer trophic link. River impoundment corresponded with decreased ecological and economic efficiency of fisheries production, an important ecosystem service provided by many tropical rivers.Item Patterns of fish and macro-invertebrate distribution in the upper Laguna Madre: bag seines 1985-2004(2009-05-15) Larimer, Amy BethThe Laguna Madre is a hypersaline lagoon. Despite harsh conditions, the upper Laguna Madre (ULM) is a highly productive ecosystem and a popular sportfishing area, especially for spotted seatrout and red drum. It is also the most important Texas bay for commercial fishing of black drum. TPWD?s Coastal Fisheries division began conducting routine monitoring of coastal fishery resources in 1977 to guide management. The goal of the present study was to improve understanding of spatiotemporal trends in relative abundance of selected fish and macro-invertebrate species in the upper Laguna Madre. I used TPWD?s bag-seine and water-quality data from the years 1985-2004 to examine variation in species? relative abundances and relationships to several environmental factors. I hypothesized that one or more of these variables, alone or in combination, were related to spatial and temporal trends in community composition. I used detrended correspondence analysis (DCA) to measure species turnover (beta diversity) and to determine which model (linear or unimodal) of species response along a gradient to apply. I used canonical correspondence analysis to relate species abundances directly to explanatory variables. The explanatory variables were tested for significance and the variance partitioned among three groupings: temporal, spatial and environmental. DCA indicated complete species turnover along two dimensions: seasonal and spatial. It also indicated that a unimodal method such as CCA was appropriate for further analysis. The CCA model included 39 variables. The included variables explained 14% of the variation in species abundance in the data set. Since the first four axes explained 67% of the variation contained in the first two DCA axes, the chosen explanatory variables were sufficient to explain the majority of the tractable variation in species abundance. The variance partitioning procedure indicated that temporal effects were the most important in explaining species variation in the Upper Laguna, followed by the spatial component. The pure environmental component explained the least amount of variation. In this study, much of the variability in species abundance was due to the spawning patterns of estuary-dependent species, most of which spawn in the spring and summer months, leading to higher abundances from spring through fall.