We synthesized evidence for unintended consequences and trade‐offs associated with the passage of fishes. Provisioning of fish passageways at dams and dam removals are being carried out increasingly as resource managers seek ways to reduce fragmentation of migratory fish populations and restore biodiversity and nature‐like ecosystem services in tributaries altered by dams. The benefits of provisioning upstream passage are highlighted widely. Possible unwanted consequences and trade‐offs of upstream passage are coming to light, but remain poorly examined and underappreciated. Unintended consequences arise when passage of native and desirable introduced fishes is delayed, undone (fallback), results in patterns of movement and habitat use that reduce Darwinian fitness (e.g. ecological traps), or is highly selective taxonomically and numerically. Trade‐offs arise when passage decisions intended to benefit native species interfere with management decisions intended to control the unwanted spread of non‐native fishes and aquatic invertebrates, or genes, diseases and contaminants carried by hatchery and wild fishes. These consequences and trade‐offs will vary in importance from system to system and can result in large economic and environmental costs. For some river systems, decisions about how to manage fish passage involve substantial risks and could benefit from use of a formal, structured process that allows transparent, objective and, where possible, quantitative evaluation of these risks. Such a process can also facilitate the design of an adaptive framework that provides valuable insights into future decisions.

Aquatic resource managers are continually faced with construction or site development proposals which, if allowed to proceed, would ultimately alter the physical structure and cover of fish habitat. In the absence of clear quantitative guidelines linking the change in habitat to fish, resource managers often use the change in habitat area as a basis for decisions. To assess the weight of scientific evidence in support of management decisions, we summarized both the observational and experimental freshwater fish-habitat literature. We then extracted data from experimental studies (where possible) for inclusion in a meta-analysis, to provide a more rigorous assessment of the published results of experimental habitat manipulations. We found relatively strong and consistent correlational evidence linking fish and physical habitat features, yet inconsistent evidence when narratively reviewing the experimental literature. On the whole, decreases in structural habitat complexity are detrimental to fish diversity and can change species composition. Increases in structural complexity showed increases, decreases, or no measurable changes in species and (or) communities. The majority of our meta-analyses resulted in supporting a direct link between habitat and fish abundance or biomass, with fish biomass responding most strongly to habitat change. Habitat alterations are most likely to affect individual species or community structure, and thus evaluating the extent of the effect on a biological basis depends on management objectives.

The study of cisco diversity in inland lakes of North America has been plagued by taxonomic uncertainty linked to high phenotypic plasticity and an ongoing reliance on morphology to differentiate species. More recently, this uncertainty has hindered the development of conservation plans and status assessments of ciscoes. This study presents the first range-wide comparison of morphological and genetic variation between Cisco (Coregonus artedi) and Shortjaw Cisco (C. zenithicus). Using morphological and genetic data from 17 lakes, three sets of analyses were undertaken to evaluate alternate hypotheses explaining the pattern of cisco phenotypic diversity in inland lakes. Morphotypes (MTs) representing the two taxa were phenotypically distinct (largely reflective of differences in gill raker number and jaw morphology) within lakes but highly variable across lakes. Shortjaw Cisco was only recognizable when compared to sympatric Cisco and some populations were morphologically similar to Cisco from other lakes. Analysis of AFLP data revealed two genetic clusters that conformed to differences in geography (eastern and western groups), rather than hypothesized taxonomic boundaries. Genetic variation strongly suggests that each of these unique sympatric pairs of MTs originated recently and locally, in parallel, from the ancestral Cisco. Phenotypic and genetic distinctiveness between MTs were not related. MTs were sometimes clearly recognizable despite a lack of genetic differentiation, suggesting that the canalization of phenotypic plasticity is unevenly completed across lakes. These results provide evidence that the taxonbased approach is clearly inadequate for the protection of Shortjaw Cisco. In Canada, status assessment should aim to identify lake-specific designatable units (DU). Given the idiosyncratic nature of each instance of Shortjaw Cisco, it is expected that the strength of morphological, biological, ecological and genetic evidence for individual DUs will vary among lakes.

No abstract

Barriers to prevent spawning migrations of sea lampreys Petromyzon marinus remain an important component of an integrated sea lamprey management program in the Laurentian Great Lakes. Concerns about effects on nontarget fishes have led to the construction of specially designed vertical‐slot trap‐and‐sort fishways to mitigate potential barrier effects. To improve passage at these fishways, we used passive integrated transponder technology to assess the performance of two fishways located on low‐head sea lamprey barriers. Fishways on the Big Carp River (which flows into Lake Superior) and Cobourg Brook (which flows into Lake Ontario) were assessed for attraction efficiency, trap attraction and retention, and passage efficiency. Based on the results of these assessments, fishways were modified by increasing the trap volume and altering the funnel characteristics to reduce escapement from the trap and then reassessed. Attraction efficiency for all tagged fish was high (≥80%) at both sites in all years. Fishway modifications improved passage from 35% in 2003 to 88% and 64% in 2004 and 2005, respectively, at the Big Carp River. As expected, white suckers Catostomus commersonii, an obligate migrant, had higher attraction and passage efficiency, fewer passage attempts, and shorter migration delay than did rock bass Ambloplites rupestris, a facultative migrant. No improvements were seen at Cobourg Brook, where passage efficiency remained low (7% in 2003, 10% in 2005), probably because of the loss of attraction flow. At both fishways, individual fish averaged 3‐10 attempts to pass through the fishways and had their migrations delayed 1‐2 weeks. The observed improvements to the Big Carp River fishway, which resulted in high fishway attraction and passage rates for white suckers, suggest that vertical‐slot fishways can help mitigate the effects of low‐head barriers for some species. Our study provides a rigorous quantifiable approach to assessing fishway performance that can be employed widely and successfully to assess initial fishway design and subsequent modifications.

We removed 40%–70% of nearshore wood habitat from three lakes to test the link between habitat and productive capacity, specifically focusing on the provision of substrate for periphyton and invertebrate production by submerged wood. Our objectives were fourfold: (i) to calculate the total amount of invertebrate and periphyton biomass removed with the wood; (ii) to explore wood's value as habitat for invertebrates and periphyton; (iii) to determine the response within residual epixylic periphyton and invertebrate biomass; and (iv) to assess interactions between periphyton and invertebrates and other factors that may influence wood's productivity. Invertebrate biomass was greater on wood than in sediment, but the total available sediment area exceeded that of wood, thus a relatively small proportion of overall productivity was lost. Highly decayed wood supported greater chlorophyll a concentrations and more invertebrate biomass and diversity than fresh wood. The removal had no measurable effect on whole-lake water chemistry, nor did it result in a response in residual epixylic periphyton and invertebrate biomass. We conclude that we permanently reduced a dynamic and concentrated biomass of primary and secondary productivity in lakes by removing submerged wood habitat.

Anguillid eels are found globally in fresh, transitional and saline waters and have played an important role in human life for centuries. The population status of several species is now of significant concern. The threats to populations include direct exploitation at different life stages, blockages to migratory routes by dams and other structures, changes in river basin management that impact habitat carrying capacity and suitability, pollution, climate change, diseases and parasites. While much has been done to understand eel biology and ecology, a major challenge is to identify the key research and management questions so that effective and targeted studies can be designed to inform conservation, management and policy. We gathered 30 experts in the field of eel biology and management to review the current state of knowledge for anguillid eel species and to identify the main topics for research. The identified research topics fell into three themes: (a) Lifecycle and Biology; (b) Impacts and (c) Management. Although tropical anguillid eels are by far the least well understood, significant knowledge gaps exist for all species. Considerable progress has been made in the last 20 years, but the status of many species remains of great concern, particularly for northern temperate species. Without improved engagement and coordination at the regional, national and international level, the situation is unlikely to improve. Further, adaptive management mechanisms to respond to developments in science, policy and our knowledge of potential threats are required to ensure the future of these important and enigmatic species.

We investigated three potential mechanisms-size-dependent overwinter energy loss, size-specific predation, and size-dependent predator-induced metabolic costs-to determine the role of body size and predators on the overwinter survival and energy reserves of young-of-theyear walleyes Stizostedion vitreum. Walleyes were reared in outdoor hatchery ponds in Westport, Ontario, for three overwinter periods () in the presence or absence of predators (burbot Lota lota or adult walleyes). Other young-of-the-year fishes were provided as potential prey items. We predicted that (1) smaller individuals of a young-of-the-year walleye cohort would have lower prewinter energy stocks and would deplete them at a higher rate; (2) no size-dependent overwinter survival would occur in walleyes reared without predators; and (3) physiological stress, in combination with size-selective predation on smaller individuals, would lead to size-dependent overwinter mortality in young of the year reared with predators. Prediction 1 was not supported by lipid, protein, or specific-energy content data; indeed, smaller walleyes actually gained energy over the winter, whereas larger walleyes lost energy. Prediction 2 was supported but prediction 3 was not, as no significant survival differences were observed between four size-class quartiles in any pond or year. However, young-of-the-year walleyes reared over winter in the presence of predators lost significantly more weight and energy and had significantly lower lipid concentrations at the end of winter than did those reared in the absence of predators. Although no differences in overwinter survivorship were detected between young-ofthe-year walleyes reared in predator and predator-free ponds, the presence of significant predatorinduced energetic costs in the absence of size-selective mortality suggests that the size-selective mortality observed in some natural walleye populations is the direct effect of the presence of predators.