No abstract

The distribution of fishes is influenced by a host of physico-chemical and biological variables, including temperature and oxygen, prey abundance, feeding or assimilation rates, and predation risk. We used hydroacoustics and midwater trawls to measure the vertical distribution of pelagic fishes during a series of research cruises on Lake Superior's western arm in 2001 and 2004. Our objective was to assess vertical structuring in the fish assemblage over varying light levels. We observed variability in vertical structuring of both ciscoes (Coregonus spp.) and their primary predator, the siscowet (Salvelinus namaycush siscowet). Our results indicate that deepwater predators and prey migrate extensively over a diel cycle. This migration pattern is most consistent with changes in the distribution of prey resources for siscowet and diel variability in predation risk controlled by changing light levels for ciscoes. The magnitude of vertical migration in ciscoes increased with higher abundance of siscowets, supporting predation risk as a driver of cisco distribution. This study describes the extent of vertical migration in each group of fish, provides a statistical description of the pattern, and discusses the implications for trophic interactions in the Lake Superior food web.Résumé : La répartition des poissons est affectée par une quantité de variables physico-chimiques et biologiques, dont la température et l'oxygène, l'abondance des proies, les taux d'alimentation et d'assimilation et le risque de prédation. Nous avons utilisé des méthodes hydroacoustiques et des chaluts de mi-profondeur pour mesurer la répartition verticale des poissons pélagiques durant une série de croisières de recherche dans le bras occidental du lac Supérieur en 2001 et en 2004. Notre objectif était de déterminer la structuration verticale du peuplement de poissons en fonction de la variation des intensités lumineuses. Nous avons observé de la variabilité dans la structuration verticale tant chez les ciscos (Coregonus spp.) que chez leur prédateur principal, le touladi siscowet (Salvelinus namaycush siscowet). Nos résultats indiquent que les prédateurs et les proies d'eau profonde font d'importantes migrations sur un cycle journalier. Ce patron de migration s'accorde le mieux avec les changements dans la répartition des ressources des proies chez le siscowet et avec la variabilité journalière du risque de prédation contrôlée par les changements de l'intensité lumineuse chez les ciscos. L'étendue de la migration verticale des corégones augmente en fonction de l'abondance des siscowets, ce qui vient appuyer le risque de prédation comme facteur explicatif de la répartition des ciscos. Notre étude décrit l'étendue de la migration verticale chez chaque groupe de poissons et fournit une description statistique du patron de migration; les conséquences sur les interactions trophiques dans le réseau alimentaire du lac Supérieur font l'objet d'une discussion.[Traduit par la Rédaction] Hrabik et al. 22951991Eshenroder and Burnham-Curtis 1999;Knudsen and...

Integrated, quantitative expressions of anthropogenic stress over large geographic regions can be valuable tools in environmental research and management. Despite the fundamental appeal of a regional approach, development of regional stress measures remains one of the most important current challenges in environmental science. Using publicly available, pre-existing spatial datasets, we developed a geographic information system database of 86 variables related to five classes of anthropogenic stress in the U.S. Great Lakes basin: agriculture, atmospheric deposition, human population, land cover, and point source pollution. The original variables were quantified by a variety of data types over a broad range of spatial and classification resolutions. We summarized the original data for 762 watershed-based units that comprise the U.S. portion of the basin and then used principal components analysis to develop overall stress measures within each stress category. We developed a cumulative stress index by combining the first principal component from each of the five stress categories. Maps of the stress measures illustrate strong spatial patterns across the basin, with the greatest amount of stress occurring on the western shore of Lake Michigan, southwest Lake Erie, and southeastern Lake Ontario. We found strong relationships between the stress measures and characteristics of bird communities, fish communities, and water chemistry measurements from the coastal region. The stress measures are taken to represent the major threats to coastal ecosystems in the U.S. Great Lakes. Such regional-scale efforts are critical for understanding relationships between human disturbance and ecosystem response, and can be used to guide environmental decision-making at both regional and local scales.

Several hypotheses have been proposed to explain diel vertical migration (DVM); however, they have generally been applied to DVM behavior of a single trophic level. We evaluate the costs (predation risk) and benefits (foraging rate and growth rate potential) of different hypothetical and observed DVM trajectories for a three-level pelagic food chain in Lake Superior containing opossum shrimp (Mysis relicta), deepwater ciscoes (Coregonus spp.), and lake trout (Salvelinus namaycush). Lake trout appear to be maximizing foraging and growth rates by tracking vertically migrating ciscoes, while the DVM trajectories of ciscoes suggests a trade-off between predation risk and growth. For ciscoes, two alternative DVM trajectories both minimize the ratio of risk to growth: a shallow trajectory that follows low light levels down to 80 m during the day and a deep trajectory (below 150 m) that tracks highest Mysis densities. Observed cisco DVM trajectories appear to follow the shallow high risk – high growth trajectory in 2001, but switch to the deep, low risk – low growth trajectory in 2004 when lake trout density was higher and the density of ciscoes was lower.

Populations of cisco Coregonus artedi in the Laurentian Great Lakes supported large‐scale commercial fisheries and were the primary forage of piscivores during the first half of the 20th century. However, by 1970 populations had collapsed in all of the lakes. Since then, ciscoes have staged a recovery in Lake Superior. In this synthesis, we describe the status of ciscoes in Lake Superior during 1970–2006 and provide a comprehensive review of their ecology. Better understanding of age estimation techniques, application of hydroacoustic and midwater trawl sampling, and compilation of long‐term data sets have advanced our understanding of the species. Management agencies contemplating rehabilitation of cisco populations should recognize that (1) knowledge of cisco ecology and population dynamics is increasing; (2) ciscoes are long‐lived; (3) Great Lakes populations are probably composed of both shallow‐water and deepwater spawning forms; (4) large year‐classes can be produced from small adult stocks; (5) large variation in year‐class strength is probably intrinsic to Great Lakes populations; (6) despite the longevity and early maturity of ciscoes, stocks can be overfished because large year‐classes are produced infrequently; (7) regional environmental factors appear to play a large role in reproductive success; and (8) rainbow smelt Osmerus mordax are likely to have a negative effect on cisco recruitment under certain conditions. A top‐down approach for rehabilitating lake trout Salvelinus namaycush in Lake Superior probably benefited cisco recovery through lake trout predation on invasive rainbow smelt populations. We argue that managing for populations of exotic alewives Alosa pseudoharengus to support popular recreational fisheries of exotic Pacific salmonids in the other Great Lakes conflicts with stocking efforts to rehabilitate native lake trout in those lakes. If native fish rehabilitation is a serious and primary goal for management agencies in the Great Lakes basin, we propose that an ecosystem‐based approach to modifying the environment for the benefit of native fish species (i.e., decimation or eradication of invasive species) is required.