Spatial strategies of wild Atlantic salmon parr: Exploration and settlement in unfamiliar areas

1. Relationships between distributions of animals and the resources they use can be expected to depend critically on the mobility of individual animals within populations. However, there is little information on the movements of individuals within populations of animals in most natural aquatic systems, so it is difficult to model accurately the processes that underlie their distributions. 2. Aspects of the processes involved in the colonization of vacant areas by stream-resident Atlantic salmon Salmo salar (69-114 mm length) were measured under near-natural conditions. In five separate trials over summer months, groups of salmon were introduced into enclosed 30-m long sections of stream, each comprising three distinct 10-m long regions of habitat. The subsequent movements of each individual fish within the enclosures were monitored remotely and continuously using a passive integrated transponder (PIT) tracking system. 3. Considerable variation was observed between the activity of different individual salmon. Some of the fish (range between trials, 3-33%) settled into localized home ranges without moving between regions, 10-38% of the fish moved within two regions, and 37-87% of the salmon moved through all three regions of their enclosure. A fraction (0-20%) of some of the populations was particularly mobile and never settled but continued to move throughout all three regions of the enclosure. Within the scale of this current study, it would appear that, for territorial animals such as salmon parr, certain members of the population will settle in new territories after very little exploration of their new environment. 4. The propensity to explore was independent of habitat type, but was directly proportional to the size of the fish. 5. The time from release until 50% of fish in populations settled (excluding the mobile fraction) ranged from 0.3 to 2.4 days. Activity levels were particularly high and initial movements by fish were directed upstream in trial 1, early in the summer, perhaps reflecting upstream migration by salmon parr within the population of the burn at this time.