Detection and apparent survival of PIT-tagged stream fish in winter

Abstract

Environmental fluctuations exert strong control on behavior, survival, and fitness of stream biota. Technical improvements increasingly allow for tracking the response of large numbers of individuals to environmental fluctuations, for instance, by remote detection of animals equipped with PIT (passive integrated transponder) tags. PIT tags were implanted into 393 juvenile and adult brown trout Salmo trutta L. and European sculpin Cottus gobio L. in a boreal stream subjected to considerable ice formation. With weekly trackings over 6 months, we quantified apparent survival and detection probability in relation to biological, environmental, and methodological factors. Individuals with a higher physical condition in autumn showed a higher apparent survival; this pattern was consistent across all species and age classes. Detection probability decreased with increasing thickness of the surface ice layer; this effect was most pronounced for juvenile trout and benthic-living sculpin, both tagged with smaller-sized tags. Detection probability was reduced in structurally complex habitats. Our study demonstrates that apparent survival and particularly detection probability may show pronounced spatiotemporal variation. In order to compare results from different sampling occasions and sites, a good knowledge of the study site and of the regulating factors is crucial. Thanks to their small size, low prize and battery independence, PIT tags are increasingly used in the long-term monitoring of individual animals, plants or units of inorganic matter (e.g. gravel). Over the last years, reading ranges have been increased to the decimetre to metre scale, allowing for remote detection in environments where physical recapture is temporarily complicated, for instance when studying fish in ice-covered streams. Our remote tracking of PIT-tagged fish in a boreal stream subjected to pronounced ice-dynamics reveals two aspects that need consideration when analysing monitoring results: (1) Spatio-temporal variability in environmental conditions and individual habitat use affect recapture probability of tagged fish. (2) Fish with a higher physical condition in autumn show a higher apparent survival.