In freshwater fish, the transition from dependence on maternal yolk reserves to independent foraging can be an early critical period, with survival during this stage having a strong influence on population abundance and cohort strength. Information concerning Atlantic salmon Salmo salar and brown trout Salmo trutta as model species is reviewed to show how population dynamics are influenced by habitat use during the transitional stage and illustrate the role of maternal provisioning along with density-dependent and -independent factors. The allocation of resources in yolk and timing and position of spawning strongly influence the biotic and abiotic environments of juveniles and their subsequent performance. Vulnerability to predators, adverse environmental conditions and restricted conditions over which they can successfully forage result in specific habitat requirements for newly independent juveniles. The availability of slow-flowing habitats at stream margins during the first month of independence is crucial. Alteration of natural flow regimes and physical habitat structure, associated with a wide range of anthropogenic influences, can have significant deleterious effects on the availability of critical juvenile habitat. A model combining habitat structure and the relationship between density-dependent and -independent mortality is presented to explore the range of conditions under which the transitional period would have a strong influence on population abundance. This model provides a framework for establishing thresholds or optima for habitat availability that will favour sufficient recruitment out of the transitional stage. Using the modelling framework, managers can make informed decisions on the utility and cost effectiveness of fisheries and habitat management activities designed to increase juvenile survival during the transition to independence. A range of management options is outlined for improving habitat quality and increasing juvenile survival during the transitional period, including restoration of structural complexity, provision of suitable flow regimes, and tailoring stocking and reintroduction strategies to mimic natural dynamics.
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