Habitat for young-of-the-year rainbow trout Oncorhynchus mykiss was enhanced in a fourthorder stream during August-October 1991 by the addition of wooden structures that simulated accumulations of fine woody debris (FWD). The experiment represented a two-factorial design with the presence or absence of FWD bundles and time since debris introduction as factors. Immediately after FWD placement, fry density, individual biomass, fry condition factor, and total fry biomass were similar in treated and untreated sites. As the experiment progressed, density and total fry biomass significantly increased at treated but not at untreated sites. Individual biomass and condition factor did not differ between treated and untreated areas, and they were affected only by time since FWD placement. Because individuals at treated and untreated sites were the same size, added FWD did not affect an individual's net rate of energy gain. Rather, we hypothesize that the FWD provided structurally complex habitat that acted as a refuge from predators and as sites from which foraging forays were staged. Adding FWD to a stream can increase carrying capacity for trout fry, and adult population density may increase as a result.

We used head-capsule width, wing-pad length, larval recruitment, adult emergence, and MULTIFAN TM analysis to determine the life history of the stonefly Pteronarcys californica in the Crowsnest River, Alberta. Embryonic development, incorporating an early diapause, accounts for approximately 1 year of the life cycle. Both sexes spend a further 4 years in the larval stage. Female larvae grow faster than males and so achieve a larger size at emergence. Extended larval recruitment leads to variation in the sizes of larvae of the same year class and probably to cohort splitting. There is a high degree of synchrony in adult emergence, perhaps as a result of a summer diapause in antepenultimate-instar larvae. These phenomena (embryonic diapause, extended larval recruitment, different growth rates of the sexes, cohort splitting, and larval diapause) are possibly typical of aquatic insects with long life cycles.Résumé : Nous avons étudié le cycle biologique de Pteronarcys californica dans la rivière Crowsnest en Alberta en utilisant des mesures de la largeur de la capsule céphalique, de la longueur des fourreaux alaires, du recrutement larvaire et de l'émergence des adultes ainsi qu'une analyse MULTIFAN TM . Le développement embryonnaire, qui comporte une diapause hâtive, explique environ 1 année du cycle. Mâles et femelles passent ensuite encore 4 ans au stade larvaire. Les larves femelles croissent à un rythme plus rapide que les larves mâles et sont donc de plus grande taille à l'émergence. Le recrutement étalé des larves donne lieu à une grande variation de taille chez les larves d'une même classe d'âge et probablement aussi à la division en cohortes. L'émergence des adultes est remarquablement synchronisée, peut-être à la suite d'une diapause d'été à l'avant-dernier stade larvaire. Ces phénomènes (diapause embryonnaire, recrutement larvaire étalé, taux de croissance différents chez les mâles et les femelles, division en cohortes et diapause larvaire) sont peut-être des caractéristiques typiques des insectes aquatiques à cycles biologiques de longue durée.[Traduit par la Rédaction] Townsend and Pritchard 2280

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