Combining Radio and PIT-Telemetry to Study the Large and Fine-Scale Movements of Stocked and Wild Brown Trout (Salmo trutta L.) in a Northeastern Stream, Portugal

Abstract

Stream-resident salmonid movements have been the subject of numerous studies and their behaviour is relatively well-known (Harcup et al., 1984; Heggenes, 1988). For example, brown trout (Salmo trutta) is described as a sedentary species based on the behaviour displayed, often associated to the strong site attachment to a territory or home range (Bridcut & Giller, 1993; Armstrong & Herbert, 1997). Other salmonids like brook (Salvelinus fontinalis) (Roghair & Dolloff, 2005) and cutthroat trout (Oncorhynchus clarki) (Hegennes et al., 1991) showed similar behaviour. However, there are studies reporting a wide range of movements for brown (Meyers et al., 1992; Young, 1994), cutthroat (Hilderbrand & Kershner, 2000) and brook (Gowan & Fausch, 1996) trout populations. Trout behaviour can be modified by natural (e.g. fish density, food availability) and especially by man induced factors (e.g. environmental degradation, harvest and stocking) responsible for major threats of wild populations (Laikre et al., 2000). Indeed, stocking of hatchery-reared brown trout is a management tool commonly used to improve the recreational fishing (Cowx, 1999). This activity is responsible for a sudden artificial increase of fish density in a particular area. Negative impacts on wild populations, such as genetic contamination, competition, predator attraction and disease transmission were often referred (White et al., 1995; Einum & Fleming, 2001; Weber & Fausch, 2003) and are potentially amplified with the dispersal failure, since many hatchery-reared trout tend to remain near of the stocking site (Cresswell, 1981; Aarestrup et al., 2005). There are also contradictory results, as reported by Bettinger & Bettoli (2002) where stocked trout dispersal reached over 12 km in the downstream direction, just 24 hours after their release. Cortes et al. (1996) found for Portuguese salmonid streams that, during three successive years (2000 to 2003), less than 20% of stocked brown trout remained in the stream segment, one month after the release. However, in this study a mark-recapture method was used that did not allow to assess the main causes of the fish depletion and was not appropriate for the observation of fish behaviour. In fact, a wide variety of techniques, grouped as capture dependent (e.g. mark-recapture, telemetry) and independent (e.g. visual observation) methods, were used for the investigation of the spatio-