Many salmonids, including brook trout (Salvelinus fontinalis), contain both anadromous (migrant) and nonanadromous (resident) forms within a population (partial migration). Although partial migration is commonly observed, the mechanisms governing the adoption of migration or residency are poorly understood. We used field estimates of fish growth coupled with in situ estimates of food consumption rates to demonstrate that a trade-off exists between the ability to efficiently exploit local environments (resident approach) and the capacity to capitalize from large-scale environmental heterogeneity (migrant approach). We demonstrate that in the year before migration, migrant brook trout have consumption rates 1.4 times higher than those of resident brook trout. However, migrants have lower growth efficiencies (ratio of growth to consumption) than residents, indicating that migrants have higher metabolic costs. Residents and migrants also differed in their stable carbon isotope signatures (δ13C), a time-integrated measure that has been linked to habitat use. Fish muscle δ13C of migrants was depleted by 1 ± 0.1‰ compared with that of residents, and this could not be explained by any biases introduced by the time of sampling or the size of fish sampled. Our findings thus agree with the notion that a link exists between metabolic costs (efficiency) and the adopted life-history strategy.

Summary 1.Isotopic signatures of consumers provide a time integration of their feeding history, and as a result of movements, are often out of line with signatures of their local resources. Such disequilibrium can be useful for inferring the spatial scale of consumer movement. δ13 C signatures of dissolved inorganic carbon as well as periphyton and invertebrates, exhibit pronounced gradients along rivers. We outline an analytical framework to estimate the spatial scale of movement of riverine fish by comparing the slopes of their δ 13 C signature gradients to that of the stream invertebrates they consume. For free-ranging juvenile Atlantic salmon ( Salmo salar L.), unconstrained by barriers, δ 13 C signatures departed considerably from invertebrate signatures, and along-stream slopes were as little as half those recorded for stream invertebrates. Movement estimates for these fish, based on their signature slopes, are ~20 km. 3. By contrast, stream resident salmonids (whose movements are constrained by physical barriers) and sedentary taxa such as sculpins and sticklebacks, have carbon signatures much closer to invertebrate signatures where they were collected. For these groups, our method yields negligible estimates of movement, similar to those of invertebrates. 4. Although this method cannot provide precise estimates of how much individual organisms move, or reveal details of movement history, it may provide an effective complement to telemetric and other methods of studying movement.

Summary1. Migratory and resident forms of salmonids coexist in many river systems. Although such coexistence is widespread, little is known about its ecological basis and no studies have compared the habitat use of premigratory juveniles and residents. 2. We employed a comparative approach to explore the differential habitat use of juvenile anadromous and resident brook trout. This required the investigation of habitat use in streams closed to anadromy, containing only resident brook trout Salvelinus fontinalis ('resident-only' streams) and streams open to anadromy, containing coexisting Atlantic salmon Salmo salar and anadromous and resident brook trout ('migrant-resident' streams). 3. We demonstrate that fast habitats (riffles) are occupied more frequently in streams with migratory brook trout relative to riffle habitats of streams with only resident brook trout. In contrast, occupation of slow current velocities (pools) was observed in both migrant-resident and resident-only streams as both stream types contain resident brook trout. The net effect is a wider distribution of occupied habitats (pool and riffles) in migrant-resident streams relative to resident-only streams, resulting in few, if any, unused habitats. 4. These results are consistent with previously reported bioenergetic, morphological and stable isotope differences observed between anadromous and resident brook trout. 5. Our findings suggest that a link exists between juvenile habitat use, metabolic costs and life-history strategies.

This study describes the ontogenetic and seasonal feeding patterns of anadromous brook trout (Salvelinus fontinalis, also known as sea trout) inhabiting the estuarine Saguenay River (Quebec, Canada) using both stomach content and stable isotope analyses. Sea trout of the Ste. Marguerite River (Quebec, Canada) entered the saline waters of the Ste. Marguerite Bay in early May before venturing into the Saguenay River fjord for the remainder of the summer period. Upon their arrival, first-year migrants stayed relatively close to river mouths and initially fed on freshwater aquatic invertebrates. However, they quickly shifted their diet to marine prey items such as amphipods and mysids for the rest of their first summer at sea. These prey items were generally larger than freshwater prey; the prey spectrum at sea was both larger and wider than that found in freshwater and, as such, likely contributed to the trout's rapid growth rates at sea. The diet of migrants in subsequent years at sea (second-year migrants) consisted primarily of marine crustaceans and fish, the latter being most important when feeding in the upper Saguenay River. Trout shifted to piscivory at all marine sites at a size of 25 cm, regardless of time spent at sea, although the importance of piscivory varied with season and site.Résumé : Notre étude décrit les patrons ontogéniques et saisonniers d'alimentation de l'omble de fontaine anadrome (« truite de mer »; Salvelinus fontinalis) qui habite l'estuaire du Saguenay (Québec, Canada) à l'aide à fois de l'étude des contenus stomacaux et de l'analyse des isotopes stables. La truite de mer de la rivière Sainte-Marguerite (Québec, Canada) pénètre dans les eaux salines de la baie de Sainte-Marguerite au début de mai avant de s'aventurer dans le fjord du Saguenay pour y passer le reste de l'été. À leur arrivée, les migrateurs d'un an restent relativement près des embouchures des rivières et se nourrissent au départ d'invertébrés aquatiques d'eau douce. Ils changent cependant rapidement de régime et utilisent des proies marines telles que des amphipodes et des mysidacés pour le reste de leur premier été en mer. Ces proies sont généralement de plus grande taille que les proies d'eau douce; la gamme de proies en mer est aussi plus importante et plus étendue que celle d'eau douce et elle contribue ainsi aux taux élevés de croissance de la truite en mer. Le régime alimentaire des migrateurs des années subséquentes en mer (les migrateurs de seconde année) comprend surtout des crustacés et des poissons marins, ces derniers étant particulièrement importants lorsque la truite de mer se nourrit dans le Saguenay supérieur. Dans tous les sites marins, la truite de mer devient ichtyophage à une taille de 25 cm, quel que soit le temps passé en mer, bien que l'importance de l'ichtyophagie varie en fonction de la saison et du site.[Traduit par la Rédaction] Morinville and Rasmussen 2027

Phenotypic variation linked to habitat use has been observed in fish, both between and within species. In many river systems, migratory and resident forms of salmonids coexist, including anadromous (migrant) and resident brook trout, Salvelinus fontinalis. In such populations, juvenile anadromous (migrant) brook trout, prior to migration, inhabit regions of higher current velocity than residents. Because it is more costly to occupy fast currents than slow currents, differences in morphology minimizing the effects of drag were expected between the two forms. As predicted, migrant brook trout were found to be more streamlined (narrower and shallower bodies) than resident brook trout, and these differences persisted into the marine life of the fish. Migrants also exhibited shorter pectoral fins, which facilitate pelagic swimming, indicating that migrants, prior to their migration to the sea, possess the appropriate morphology for swimming in open water habitats. The reported differences between migrants and residents were powerful enough to derive discriminant functions, using only five of the seven measured traits, allowing for accurate classification of brook trout as either migrants or residents with an overall correct classification rate of 87%. Importantly, this study contributes to the notion that a link exists between morphology, habitat use, metabolic costs and life-history strategies.