Summary Lipids have more negative δ 13C values relative to other major biochemical compounds in plant and animal tissues. Although variable lipid content in biological tissues alters results and conclusions of δ 13 C analyses in aquatic food web and migration studies, no standard correction protocol exists. 2. We compared chemical extraction and mathematical correction methods for freshwater and marine fishes and aquatic invertebrates to better understand impacts of correction approaches on carbon ( δ 13 C) and nitrogen ( δ 15 N) stable isotope data. Fish and aquatic invertebrate tissue δ 13C values increased significantly following extraction for almost all species and tissue types relative to nonextracted samples. In contrast, δ 15 N was affected for muscle and whole body samples from only a few freshwater and marine species and had a limited effect for the entire data set. 4. Lipid normalization models, using C : N as a proxy for lipid content, predicted lipid-corrected δ 13 C for paired data sets more closely with parameters specific to the tissue type and species to which they were applied. 5. We present species-and tissue-specific models based on bulk C : N as a reliable alternative to chemical extraction corrections. By analysing a subset of samples before and after lipid extraction, models can be applied to the species and tissues of interest that will improve estimates of dietary sources using stable isotopes.

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IVaterEoo, V%'aberkns, Onb. N2E 361 Cunjak, R. A., and G. Power. 7986. Winter habitat utilization by stream resident brook trout (Salvelinus fsntinalis) and brown trout (Salrno erutta). Can. 8 . Fish. Aquat. Sci. 113: 1970 Habitat utilization by brook trout (Salvelinus fontina%is) and brown trout dSalrno erutta) is described from three winters of u~derwater observations in a southern Ontario river. Older trout (>age 1) generally occupied positions in deeper and faster water thaw age 0+ trsut. In winter, at sites of syrnpatry, brown trout occupied greater focal point water depths than brook trsut; both species had similar focal point water velocities. At all sites, and for both age groups and species, there was a strong preference for positions beneath cover. Relative to summer, trout positions in winter were characterized by slower water velocities and greater overhead cover. In winter, most trout were in aggregations, usually in pools beneath cover and close to point sources of groundwater discharge. Gregarious behaviour appeared to increase as water temperatures decreased; no such relationship was evident in the summer. Specific strategies for overwintering varied between sites and age groups but generally conformed to the theory sf energetic cost minimization for position choice. These variable patterns appear to be adaptive.Les auteurs traitent de ~'utilisation de Ifhabitat par I'srnble de fontaine (Salvelinus fontinalis) et la truite brune (Salms frutta) suite 3 trois hivers d'observations sous-marines effectuees daws un cours d'eau du sud de ('Ontario.bes truites plus ag6es (Sge 1 +) se trouvaient g6neralement dans des eaux plus profondes et A ecaulernent plus rapide que celles d15ge 8-k. En hiver et dans Ies lieux de sympatrie, les truites brunes occugaient des prsfondeurs plus focalis6es que les ornbles de fontaine; la focalisation par rapport aux vitesses d'ecoulernent etait semblable chez les deux especes. Les deux grouges d'sge des deux esp&ces prkentaient une forte prefkrence pour les Bieux sous couvert, ceci A tsus les sites. bes psissons pr6ft5raient des vitesses df@coulernent msins importantes et un couvert plus important en hiver, comparativement 2 116t6. En hiver, la plupart des poissons se regroupaient, gkn6ralement dans des fosses sous couvert, 2 proximite des sources ponctuelles d'eau souterraine. Ce comporterrsent grbgaire semblait s'accroitre 2 mesure que la temperature diminuait; une relation de ce type n'a pas 6t6 notee en 6t6. Les stratkgies adoptees au csurs de I'hiver variaient selon les sites et les groupes d'ige, mais elks se conforrnaient gen6ralernent 2 la thkorie de la minimisation des coQts 6nergetiques lors du choix d'un emplacement. 91 semble que ces cornportements variables ssient de nature adaptative.

Abstract– Habitat is important in determining stream carrying capacity and population density in young Atlantic salmon and brown trout. We review stream habitat selection studies and relate results to variable and interacting abiotic and biotic factors. The importance of spatial and temporal scales are often overlooked. Different physical variables may influence fish position choice at different spatial scales. Temporally variable water flows and temperatures are pervasive environmental factors in streams that affect behavior and habitat selection. The more frequently measured abiotic variables are water depth, water velocity (or stream gradient), substrate particle size, and cover. Summer daytime, feeding habitats of Atlantic salmon are size structured. Larger parr (>7 cm) have a wider spatial niche than small parr. Selected snout water velocities are consistently low (3–25 cm. s−1). Mean (or surface) water velocities are in the preferred range of 30–50 cm. s−1, and usually in combination with coarse substratum (16–256 mm). However, salmon parr demonstrate flexibility with respect to preferred water velocity, depending on fish size, intra‐ and interspecific competition, and predation risk. Water depth is less important, except in small streams. In large rivers and lakes a variety of water depths are used by salmon parr. Summer daytime, feeding habitat of brown trout is also characterized by a narrow selection of low snout water velocities. Habitat use is size‐structured, which appears to be mainly a result of intraspecific competition. The small trout parr (<7 cm) are abundant in the shallow swift stream areas (<20–30 cm depths, 10–50 cm. s−1 water velocities) with cobble substrates. The larger trout have increasingly strong preferences for deep‐slow stream areas, in particular pools. Water depth is considered the most important habitat variable for brown trout. Spatial niche overlap is considerable where the two species are sympatric, although young Atlantic salmon tend to be distributed more in the faster flowing and shallow habitats compared with trout. Habitat use by salmon is restricted through interspecific competition with the more aggressive brown trout (interactive segregation). However, subtle innate differences in behavior at an early stage also indicate selective segregation. Seasonal changes in habitat use related to water temperatures occur in both species. In winter, they have a stronger preference for cover and shelter, and may seek shelter in the streambed and/or deeper water. At low temperatures (higher latitudes), there are also marked shifts in habitat use during day and night as the fish become nocturnal. Passive sheltering in the substrate or aggregating in deep‐slow stream areas is the typical daytime behavior. While active at night, the fish move to more exposed holding positions primarily on but also above the substrate. Diurnal changes in habitat use take place also in summer; brown trout may utilize a wider spatial niche at night with more fish occupying the shallow‐slow stream areas. ...

Underwater observations at two sites along a small Nova Scotian river were carried out between December and April (water temperature range = 0.5–7.0 °C) to describe the winter microhabitat of young Atlantic salmon (Salmo salar). Salmon (5–15 cm fork length) were consistently found hiding beneath rocks (mean diameter = 16.8–23.0 cm) in riffle-run habitats where mean water depths were 40.9–48.9 cm and mean water velocities were 38.7–45.7 cm∙s−1. Many of the salmon were found overwintering within redd excavations. "Home stones" were distributed closer to midstream than to river banks and where sediment compaction was minimal. Monthly collections of fish (ages 1 and 2) indicated that feeding continued over winter. The data suggest a nocturnal activity pattern and photonegative response by young salmon during winter.