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The endangered Rio Grande silvery minnow Hybognathus amarus was cultured in 2.44-m-diameter outdoor tanks in a 131-day yield trial to assess growth, survival, and percentage of taggable-sized fish ( total length) when stocked at 500/tank (1.07 million/ha), 1,000/tank (2.14 million/ha), and 1,500/tank (3.21 million/ha). At harvest, fish averaged 45.6 mm and 0.94 g in the 500/tank treatment, 42.6 mm and 0.74 g in the 1,000/tank treatment, and 38.4 mm and 0.55 g in the 1,500/tank treatment; the differences were significant (P = 0.05). Survival in the three treatments was 70%, 64%, and 52%, respectively, but the differences were not significant. Percent taggable-sized fish was 86%, 89%, and 65%, respectively, but the differences were not significant. Yield was 672.5 kg/ha, 1,026.6 kg/ha, and 887.8 kg/ha, respectively; yield in the 1,000/tank treatment was significantly greater than that in the 500/tank treatment, but was not significantly greater than that of the 1,500/tank treatment. This facility is a conservation facility and a major goal is to raise fish without formulated feed. Fertilization produced good growth for the first month, but little growth occurred during the second month so supplemental feed had to be used for the final 60 d of the yield trial.
Human induced hydrologic alteration is ubiquitous in North American riverscapes. These alterations have been shown to impact fishes by modifying habitats, influencing movement patterns and driving changes in community structure. Understanding these impacts is an essential first step for the conservation of fish in these systems. We use Western Silvery Minnow Hybognathus argyritis and the Milk River of southern Alberta as a model system to develop and apply an interdisciplinary approach to assess the impacts of hydrologic alteration on capture efficiency and habitat suitability of stream fishes. The capture efficiency of sampling gear is a key component of many fish research programs, and understanding the link between hydrology and capture efficiency is critical to accurately assessing the impacts of hydrologic alterations on fish. We measured seine net capture efficiency in the Milk River, and investigated the effects of flow, species, and habitat variables on capture efficiency using mixed effects models. Flow state was an important driver of capture efficiency, which increased ~5 % during augmented flow compared to natural flow. Habitat suitability assessments are commonly used to determine the impacts of hydrologic alteration on fishes, but often rely on poorly understood relationships between fish and their habitat. We used the swimming performance of Western Silvery Minnow to measure the cost of movement between habitat patches in the Milk River and incorporated this cost into a graph theoretic metric of habitat suitability (Equivalent Connected Area). Compared to augmented flow, the proportion of suitable area was ~ 475 % higher during natural flow, the mean cost of movement between habitat patches was ~ 13 % higher and Equivalent Connected Area increased ~ 0.119 (95% C.I. 0.109-0.130). By including flow as a variable in modelling capture efficiency and swimming performance as a mechanism defining habitat suitability, we show the utility and benefits of taking an interdisciplinary approach to assessing the impacts of hydrologic alteration on stream fishes.
Density-dependence is considered one of the most important regulators of population growth, and it has been documented across a wide variety of species. Typically, population growth rate and components thereof decline with increasing density (i.e., negative density-dependence); however, in species that exhibit high population densities and social behavior, positive density-dependence (i.e., Allee effect) may occur at low density. June sucker, a federally endangered lake sucker endemic to Utah Lake, Utah, USA, occurred historically at high density, and it exhibits coordinated feeding behavior. These characteristics indicate a potential for the existence of an Allee effect at current low population densities. To determine effects of density on growth, survival, and diet, I experimentally manipulated density of young June sucker in replicated enclosures in a natural environment. Larval June sucker were placed in enclosures at four different densities, and growth, survival, and diet of fish, and availability of prey (to determine selectivity) were measured at two time intervals. Both individual growth and survival were significantly lower at the lowest density compared to higher densities, indicative of a component Allee effect. Diets of individuals at low densities were more selective than diets of individuals at intermediate and high densities, suggesting a change in feeding strategy with density. Reduced growth and survival at low density suggests that corresponding, highly selective, feeding strategies may be less efficient than feeding strategies employed at higher densities. Allee effects appear to be an important consideration for recovery of this endangered species, and such effects may be common in historically abundant, but currently rare species.
4.1.1 Demographic significance Confined populations grow more rapidly than populations from which dispersal is permitted (Lidicker, 1975; Krebs, 1979; Tamarin et at., 1984), and demography in island populations where dispersal is restricted differs greatly from nearby mainland populations (Lidicker, 1973; Tamarin, 1977, 1978; Gliwicz, 1980), clearly demonstrating the demographic signi ficance of dispersal. The prevalence of dispersal in rapidly expanding populations is held to be the best evidence for presaturation dispersal. Because dispersal reduces the growth rate of source populations, it is generally believed that emigration is not balanced by immigration, and that mortality of emigrants occurs as a result of movement into a 'sink' of unfavourable habitat. If such dispersal is age- or sex-biased, the demo graphy of the population is markedly affected, as a consequence of differ ences in mortality in the dispersive sex or age class. Habitat heterogeneity consequently underlies this interpretation of dispersal and its demographic consequences, although the spatial variability of environments is rarely assessed in dispersal studies.
Fish Conservation offers, for the first time in a single volume, a readable reference with a global approach to marine and freshwater fish diversity and fishery resource issues. Gene Helfman brings together available knowledge on the decline and restoration of freshwater and marine fishes, providing ecologically sound answers to biodiversity declines as well as to fishery management problems at the subsistence, recreational, and commercial levels. Written in an engaging and accessible style, the book: considers the value of preserving aquatic biodiversity offers an overview of imperiled fishes on a taxonomic and geographic basis presents a synthesis of common characteristics of imperiled fishes and their habitats details anthropogenic causes of decline examines human exploitation issues addresses ethical questions surrounding exploitation of fishes The final chapter integrates topics and evaluates prospects for arresting declines, emphasizing the application of evolutionary and ecological principles in light of projected trends. Throughout, Helfman provides examples, explores case studies, and synthesizes available information from a broad taxonomic, habitat, and geographic range. Fish Conservation summarizes the current state of knowledge about the degradation and restoration of diversity among fishes and the productivity of fishery resources, pointing out areas where progress has been made and where more needs to be done. Solutions focus on the application of ecological knowledge to solving practical problems, recognizing that effective biodiversity conservation depends on meeting human needs through management that focuses on long term sustainability and an ecosystem perspective.