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Despite the common view that conditions in winter strongly influence survival and population size of fish, the ecology of salmonids has not been as extensively studied in winter as in other seasons. In this paper, we review the latest studies on salmonid winter survival, habitat use, movement and biotic interactions as they relate to the prevailing physical and habitat conditions in rivers and streams. The majority of research conducted on the winter ecology of salmonids has been carried out in small rivers and streams, where temperatures are above zero and where there is no ice. Investigations in large rivers, regulated and dredged rivers, and under conditions of different ice formations are almost totally lacking, presumably related to sampling difficulties with these systems. The studies-at-hand indicate that a multitude of physical and biological factors affect the survival, behavior, and habitat use of salmonids in winter. The general concept that winter functions as a critical period for the survival of young salmonids is not well supported by the literature. Instead, overwinter survival of juvenile fish appears to be context-dependent, related to specific habitat characteristics and ice regimes of streams. In general, over wintering salmonids prefer sheltered, low velocity microhabitats, are mainly nocturnal, and interact relatively little with conspecifics or interspecifics. Specific descriptions of microhabitat preferences of salmonids are difficult to make due to highly disparate results from the literature. We suggest that future research should be directed towards (1) being able to predict the dynamics of freezing and ice processes at different scales, especially at the local scale, (2) studying fish behavior, habitat use and preference under partial and full ice cover, (3) evaluating the impacts of man-induced environmental modifications (e. g. flow regulation, land-use activities) on the ecology of salmonids in winter, and (4) identifying methods to model and assess winter habitat conditions for salmonids.

The decision to migrate or not is regarded as genetically controlled for many invertebrate and vertebrate taxa. Here, we show that the environment influences this decision. By reciprocally transplanting brown trout (Salmo trutta L.) between two sections in a river, we show that both migratory and non-migratory behaviour can be environmentally induced; migratory behaviour developed in a river section with high brown trout densities and low specific growth rates, whereas non-migratory behaviour developed in a section with low brown trout densities and high specific growth rates. In a laboratory experiment, we tested the effect of food availability on the development of migratory and non-migratory body morphologies and found that most brown trout became migrants when food levels were low but fewer did so at high food levels. Thus, the decision to migrate seems to be a plastic response, influenced by growth opportunities.

1. The European eel population has decreased drastically during recent decades, and new EU-legislation calls for measures to change this negative trend. This decline has been attributed to a number of factors, including habitat fragmentation by structural barriers that prevent eels moving between freshwater and the sea. The success of downstream migrating adult silver eels migrating past a hydroelectric plant (HEP) in Sweden was examined by radio-telemetry, and the results were considered in a historical context by analysing catch data from the river for 1957-2006. 2. The choice of routes and passage success were quantified for three treatment groups and one control group of silver eels. The first treatment, the reservoir group (n = 50), was released into the reservoir upstream of the HEP, and these fish could proceed downstream by passing through the HEP (20 mm rack and turbines) or by entering the spill gates into the former channel, bypassing the HEP. The second treatment group (inside rack, n = 15) was released downstream of the 20-mm rack and had to pass through the turbines to continue migration to the sea. The third treatment group consisted of dead radio-tagged eels (n = 6) that were released into the turbines to study the extent of drifting by dead individuals. Finally, the control group (n = 50) was released downstream of the HEP to test for effects of confounding factors. 3. Most live individuals displayed migratory behaviour and continued to proceed downstream after release. Only 8% of the fish released in the reservoir or downstream of the HEP (control) did not migrate. The probability of reaching the next HEP, 24 km further downstream, was high for the control group (96%) and the reservoir-released individuals that passed the HEP via the spill gates and the former channel (83%). Survival was low and size-dependent for the individuals that passed the turbines (40%) and even lower for the individuals that had to pass through the rack and the turbines (26%). The overall passage success for eels released in the reservoir was 30%, including both routes. 4. Annual catch data from 1957 to 2006 showed that the number of eels in the River Ä tran has decreased. Despite this decrease, escapement biomass has remained unchanged, because of the fact that the mean size of eels has doubled. Passage data from 2007 show that changes in size and abundance have resulted in a reduction of relative escapement to the sea to values that are 21-24% of what they were in 1957-66. However, this low level of 2167 escapement could potentially be rectified if appropriate measures facilitating HEP passage are successfully implemented, since the potential escapement biomass in the river, owing to the large size of the eels, has changed little since the 1950s.

The role of the environment in partial migration: food availability affects the adoption of a migratory tactic in brown trout Salmo trutta.Abstract -Alternative reproductive tactics are commonly reported for salmonids (Pisces) and typically involve large migratory and small resident individuals. Variation in migratory tendency should reflect the different benefits and costs that the two different phenotypes face with regard to fitness. Therefore, the effect of food availability on the adoption of a migratory tactic in brown trout was investigated. Fifty trout were placed in each of 12 tanks and fed at three different levels. Growth-related variables were measured regularly, and at the end of the experiment, the proportion of migrants and residents was recorded. Low food availability led to increased numbers of migratory fish. The expected sex-bias was also present, with a lower percentage of resident females than resident males. As all fish originated from the same gene pool, the changing proportions of the migratory tactics can be classified as phenotypic plasticity. The study provides evidence that the different phenotypes reflect alternative tactics within a conditional strategy. Some differences in growth-related variables were present between the sexes, and a very pronounced difference in condition factor was found between resident and migratory males, but not in females. Thus, the results provide evidence that different selective forces may be acting on the sexes.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecology.Abstract. Along a gradient of increasing productivity in lakes, the abundance of perch (Perca fluviatilis) first increases and then decreases, whereas the abundance of ruffe (Gymnocephalus cernuus) and that of roach (Rutilus rutilus) continue to increase. Perch, unlike roach and ruffe, undergoes dramatic ontogenetic diet shifts. The decrease in the abundance of perch along the productivity gradient may be due to competition from roach at its plankton-feeding stage and from ruffe at its benthic-macroinvertebrate-feeding stage. Previous studies of these three species have focused on the effects of roach on the other species. To further examine the competitive relationships among these species, we studied the effect of increasing the density of ruffe, while maintaining a relatively high, constant density of roach, on the diet and growth of perch. Food resources were also monitored during this 2-mo experiment, which was conducted in 10 enclosures in a pond in southern Sweden. Based on prior experiments we expected roach, an efficient planktivore, to reduce the biomass of zooplankton in all enclosures. Moreover, we expected that the increased density of benthic-feeding ruffe would force perch to increase its consumption of zooplankton, resulting in reduced growth.We found, as predicted, that the biomass of cladocerans and copepods decreased over time but was not affected by ruffe density. The biomass of the commonly consumed macroinvertebrates Sialis (a megalopteran), trichopterans, and ephemeropterans was lower in enclosures with ruffe than in enclosures without ruffe. Moreover, the biomass of Sialis decreased and that of chironomids and the clam Pisidium increased over time. High ruffe densities were related to an increased proportion of zooplankton in the diet of perch in August, but not in September. The diet of ruffe changed little along the ruffe gradient. Only the proportion of trichopterans (in August) and Pisidiumn (in September) in the diet of ruffe increased along the ruffe gradient. The growth of both perch and ruffe decreased with increasing ruffe density, whereas the growth of roach was constant. These data suggest that one reason perch decrease in abundance along a productivity gradient may be because they are competitively sandwiched between planktivore and benthivore specialists.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org..Wiley-Blackwell and Nordic Society Oikos are collaborating with JSTOR to digitize, preserve and extend access to Oikos.

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