Fish assemblage structure, and factors and mechanisms appearing important in the ecological maintence of these structures, were examined for 18 small lakes in northern Wisconsin during summer and winter. The study was focused around the following questions. Are there discrete, repeatable groups of fish assemblages? If so, are they temporally stable? What are the relations between fish assemblage structure and habitat complexity, physical disturbance, biotic interactions and the insular nature of small lakes? A comparative approach was used to generate hypotheses and propose explanations concerning the roles of these factors in structuring the assemblages. Multivariate classification, ordination, and discriminant analyses helped discern two assemblage types: Umbra—cyprinid and centrarchid—Esox. Each had a distinctive species composition and seasonal change in composition. Environmental characteristic of the lakes occupied by each assemblage type also differed consistently. The type of assemblage present in a lake appeared related to oxygen concentrations in winter, interacting with the availability of refuges from either a severe physical environment (low oxygen during winter) or from large pisvivores. Centrarchid—Esox assemblages occurred in lakes with high winter oxygen levels, and also in lakes with low oxygen if a stream or connecting lake could provide a refuge from these conditions in winter. When no refuge was present, low winter oxygen lakes lacked piscivorous fishes, but contained Umbra—cyprinid assemblages. The relationships between species richness in summer and environmental factors were generally similar for the two assemblage types, but the relative importance of individual factors differed. In winter, richness relationships in centrarchid—Esox assemblages for most environmental factors were reversed from those of summer. No significant seasonal change occurred in the Umbra—cyprinid assemblages. Habitat complexity factors, particulary vegetation diversity, were significantly related to summer species richness in both assemblage types. Lake area was also related to summer richness for both types, but the slope of the species—area regression was much steeper for Umbra—cypinid assemblages than for those in centrarchid—Esox lakes. Species richness relationships with winter oxygen concentration were negative in both seasons in Umbra—cyprinid lakes, but the relationship was positive for centrarchid—Esox assemblages in winter. A measure of lake connectedness was related to summer richness in centrarchid—Esox lakes. These patterns suggest that centrarchid—Esox assemblages are in ecological equilibrium but that a disturbance—induced disequilibrium occurs in Umbra—cyprinid assemblages.

No abstract

Effects of lipid extraction on stable carbon and nitrogen isotope analyses of fish tissues: potential consequences for food web studies Un resumen en espanol se incluye detrás del texto principal de este artículo.Abstract -We examined whether solvent-based lipid extractions, commonly used for stable isotope analysis (SIA) of biota, alters d 15 N or d 13 C values of fish muscle tissue or whole juvenile fish. Lipid extraction from muscle tissue led to only small (<1&) isotope shifts in d 13 C and d 15 N values. By contrast, ecologically significant shifts (+3.4& for d 13 C and +2.8& for d 15 N) were observed for whole juvenile fish. Sample variance was not affected by lipid extraction. For tissue-specific SIA, two sample aliquots may be required: a lipid-extracted aliquot for stable carbon isotope analysis when differing lipid content among tissues is a concern, and a nonextracted aliquot for d 15 N determination. Whole organism SIA is not recommended because of the mix of tissues having different turnover times; for very small fish, we recommend that fish be eviscerated, decapitated, and skinned to minimise differences with samples of muscle tissue.

Many autecological effects of temperature on fish are known, and fishery biologists have begun to incorporate this knowledge into population-level relations that can be used to assess possible effects of climatic warming on fishes and their habitats. However, the problem of extrapolating these or other relations to multispecies assemblages is not straightforward, given the complexity of community-level phenomena. I present a conceptual framework that views fish assemblages as products of a series of filters, operating at different spatial and temporal scales, through which an assemblage's component species must pass. This framework can facilitate an understanding of the processes that organize fish assemblages and suggest ways in which the complex problem can be divided into manageable pieces. I apply this framework in an examination of small-lake fish assemblages in three regions on two continents. The procedure reveals local and regional relations of richness and composition and highlights the importance of isolation, extinction, and colonization, as well as temperature, that must be considered in climate change assessments. This community-level framework can organize accumulated knowledge of fish assemblages, identify causal processes behind community-level patterns, and focus research needed for the management of fish assemblages in the face of major anticipated changes in climate.

Northern pike (Esox lucius) are often considered to be specialist piscivores, but under some circumstances will continue to eat invertebrates as adults. To examine effects of fish assemblage composition on the trophic ecology of pike, we combined stable isotope analysis (SIA) of carbon and nitrogen and stomach content analysis (SCA) on pike from five lakes in northern Alberta, three of which contain only pike ("pike-only") and two that also contain yellow perch (Perca flavescens) or white sucker (Catostomus commersoni) ("pike-other"). Fish were more important as prey and empty stomachs, which often characterize piscivores, were significantly more frequent in pike-other than in pike-only lakes. However, even though invertebrates were more important for pike in pike-only lakes, SIA and SCA indicated that invertebrates were also an important component of pike diets in pike-other lakes. SIA and SCA also revealed considerable intrapopulation variation in trophic ecology, with individuals in some populations differing by as much as two trophic levels. Comparisons of stomach contents and isotope signatures of the same fish suggested that within these variable populations, specialization on invertebrates or fish was a long-term trait of some individuals. SIA indicated that trophic position increased and diets shifted to a greater importance of littoral prey as pike grew in pike-only lakes, but not in lakes with other fish present. Trophic adaptability in northern pike is expressed at both the population level, where the trophic ecology is sensitive to differences in prey regimes, and at the organismal level, in the form of intrapopulation variation and individual specialization.