In northern freshwater lakes, several fish species have populations composed of discrete morphs, usually involving a divergence between benthic and limnetic morphs. Although it has been suggested that swimming demand plays an important role in morphological differentiation, thus influencing habitat selection, it is unclear how it affects reaction norms, patterns in character correlation, and levels of morphological integration. We examined whether swimming demand could induce morphological plasticity in the directions expected under divergent habitat selection, and evaluated its influence on the morphological integration in Arctic charr ( Salvelinus alpinus) and brook charr ( S. fontinalis), two congeneric species exhibiting conspicuous and subtle resource polymorphism, respectively. We found that changes in morphology were induced by differential swimming demands in both species. The length of the pectoral fin was the character that responded most strongly according to the predicted morphological expectations under divergent habitat selection. High levels of morphological plasticity, relatively low levels of integration, and differences found in the morphological correlation structure among water velocity treatments suggest that constraints on morphological change are unlikely in either species, thus allowing great potential for phenotypic flexibility in both species. The magnitude of character integration, however, was larger for Arctic charr than for brook charr. This latter result is discussed in the light of the differences in the level of polymorphism between the two species in the wild. The results of the present study indicate that swimming demand alone may not be sufficient to generate the polymorphism encountered in nature. Given that both diet and swimming demands can induce morphological changes, it would be important to conduct experiments targeting the interaction between the morphological modules related to trophic and swimming demands.

The understanding of the relationships between environmental factors and evolutionary forces is of importance to preserve opportunities for the continuation of dynamic evolutionary and ecological processes. This involves the identification and quantification of the relative importance of environmental factors that may influence these processes. Nevertheless, environmental factors are generally interpreted in terms of hypothetical inferences as relationships between environmental and genetic variables are often difficult to quantify. In this study, we used a statistical framework based on canonical correspondence analysis in order to determine the relative contribution of drainage pattern and environmental factors in structuring inter‐ and intrapopulational genetic diversity among brook charr populations as depicted by microsatellite analysis. These procedures simultaneously analyse several sets of variables and determine their relative contribution. The results revealed the influence of drainage pattern, altitude and human‐induced factors on the pattern of genetic diversity and, particularly, the important role of historical events in explaining patterns of contemporary genetic diversity among populations. The statistical framework used in this study provides an efficient way to empirically relate variations of genetic diversity and descriptive variables in general.

A quantitative survey of 26 oligotrophic Quebec lakes indicated that the presence of creek chub (Semotilus atromaculatus) or white sucker (Catostomus commersoni) had an impact on brook charr (Salvelinus fontinalis) populations. Evidence for competitive interactions among these species was provided by the observation that (1) the mean annual yield (kilograms per hectare) of charr was significantly reduced when they lived sympatrically with these species, (2) charr shifted their food habits from benthic organisms to Zooplankton in the presence of creek chub and (or) white sucker, and (3) the length of the gill rakers of charr was significantly higher in lakes containing chubs and suckers. As the length of the gill rakers is reported to be correlated with the extent of plankton feeding, this latter result was interpreted as a morphological change associated with increased exploitation of Zooplankton in sympatry. When occurring with creek chub, the niche shift of brook charr resulted in a significant change in Zooplankton community structure. The proportion of large Zooplankton organisms (>1.1 mm) declined whereas that of the smaller ones (<0.6 mm) increased. Also, the mean length of Holopedium, the larger cladocerans, was significantly reduced in sympatric creek chub–charr lakes compared with lakes where charr lived allopatrically. A similar impact on Zooplankton was not as clear when charr occurred with white sucker. When living with white sucker, brook charr populations appeared to be sufficiently reduced as to have little or no effect on Zooplankton communities.

The specific objectives of this study were to determine if there is individual specialization in habitat use by lacustrine brook charr Salvelinus fontinalis and if so, if specialization is related to fish morphology. Localizations of 28 brook charr equipped with thermosensitive radiotransmitters were recorded during three summers (1991, 1992, and 1993) in two lakes of the Mastigouche Reserve (Québec, Canada). Fifty per cent of the fish were found mainly in the benthic zone (hereafter benthic individuals), 18% in the pelagic zone (pelagic individuals), and 32% travelled regularly between the two zones (generalist individuals). The observed interindividual differences in habitat preference were related to differences in body morphology and coloration: (i) the pectoral fins of benthic and generalist individuals were significantly longer than those of pelagic ones; and (ii) the coloration of the lower flank of benthic and generalist individuals was silver-grey while that of pelagic individuals was red. The results of this study suggest that brook charr inhabiting oligotrophic lakes of the Canadian Shield exhibit trophic polymorphisms, where some individuals are specialists better adapted to feeding in the littoral zone whereas others are specialists better adapted to feeding in the pelagic zone. The potential for reproductive isolation between the two morphs is discussed. 1997 The Fisheries Society of the British Isles 1997 The Fisheries Society of the British Isles

River discharge and water temperature are frequently cited as controlling the upstream migration of adult salmonids to their spawning areas. The results of earlier studies on the effect of these environmental factors were examined. The statistical methods employed in some of these studies failed to consider the serial correlation often found in migration time series. To assess the effects of discharge and temperature on the migratory activity of the landlocked Atlantic salmon (ouananiche, Salmo salar), 12 years of data on spawning migrations in the Mistassini R., Quebec, Canada, were analysed and the results obtained by ordinary least squares regression and time series regression were compared. In six of the 12 years, upstream migratory movement was related negatively to changes in river flow, suggesting that fish favour falling water phases for ascent. Water temperature appeared to have little effect on migratory movement. The mean body size of migrating fish decreased significantly throughout the summer; early migrants were on average 11·4% larger (mean fork length 522 mm) than late migrants (469 mm). Larger, 3-lake-year salmon migrated 7·2 days earlier than 2-lake-year salmon. Because the residuals from ordinary regression exhibited strong autocorrelation, time series regression was more appropriate than ordinary regression for the analysis of migration time series. 1996 The Fisheries Society of the British Isles