No abstract

Environmental DNA (eDNA) can be used to detect the presence and abundance of aquatic organisms from water samples. Before implementing this methodology as a tool for monitoring, more knowledge is needed on variation in eDNA concentrations in relation to species abundance and potential confounding factors. Shedding and decay of eDNA may vary extensively over the season and are dependent on environmental factors such as water temperature and on biological processes such as activity level and reproduction. In lotic systems, eDNA concentrations are also affected by downstream transport of eDNA. Sessile freshwater mussels provide an ideal study system for investigating the relationship between species spatial distribution and eDNA concentrations in lotic systems. We quantified freshwater pearl mussel (Margaritifera margaritifera) eDNA concentrations at four localities in a natural river with detailed knowledge of mussel spatial distribution: (a) upstream of the known species distribution, just downstream (b) a small and (c) a large aggregation and (d) 1,700 m downstream of the large aggregation. To study seasonal variation, we quantified eDNA concentrations during three periods: (a) in late spring, with cold water and relatively inactive mussels; (b) in mid-summer, with higher water temperature and active mussel filtration; and (c) in late summer, during the release of larvae.Species detection was highly reliable, with no detection of eDNA upstream of the species distribution and complete detection downstream of the large aggregation.Detection success of the small aggregation was low, with 13% of the samples testing positive. Downstream transport was efficient, with no significant decrease in eDNA concentrations over 1,700 m river distance. Seasonal variation was strong, with a 20-fold increase in eDNA concentrations from late spring to late summer, during reproduction. Our results highlight both the potential and challenges of eDNA monitoring in lotic systems. K E Y W O R D Sconservation genetics, environmental DNA, freshwater mussels, species detection, unionidae | 65 WACKER Et Al.

The behaviour of wild (n = 43, mean L T = 152 mm) and hatchery-reared (n = 71, mean L T = 198 mm) Atlantic salmon and wild anadromous brown trout (n = 34, mean L T = 171 mm) post-smolts with acoustic transmitters was compared in a Norwegian fjord system. There was no difference in survival between wild and hatchery reared salmon from release in the river mouth to passing receiver sites 9.5 km and 37.0 km from the release site. Mortality approached 65% during the first 37 km of the marine migration for both groups. There was no difference between wild and hatchery-reared salmon either in time from release to first recording at 9.5 km (mean 135 and 80 h), or in the rate of movement through the fjord (mean 0.53 and 0.56 bl s -1 ). Hatchery-reared salmon reached the 37 km site sooner after release than the wild salmon (mean 168 and 450 h), but rate of movement in terms of body lengths per second did not differ (mean 0.56 and 0.77 bl s -1 ). The brown trout remained a longer period in the inner part of the fjord system, with much slower rates of movement during the first 9.5 km (mean 0.06 bl s -1 ).

Eight hatchery-reared Atlantic salmon Salmo salar post-smolts, implanted with acoustic depth sensing transmitters and manually tracked for 5-12 h in the Hardangerfjord (Norway), spent most of their time (49-99%) at 1-3 m depth during the day, whereas four of seven fish tracked were found close (<0Á5 m) to the surface at night, with a strong negative cross-correlation between general swimming depth and surface light intensity. Hence, the actual swimming depth of post-smolts during their early marine migration may depend on the light conditions, although the individual variation in vertical movement pattern was large. No cross-correlations were found between light intensity and swimming depth during daytime periods with rapid changes in light intensity, indicating that other factors than light intensity were important in initiating the irregular dives that were recorded down to 6Á5 m depth.

The physiological consequences of “premature migratory return” to freshwater for wild sea-run brown trout (Salmo trutta) smolts infested with sea lice (Lepeophtheirus salmonis) were investigated in the laboratory. Osmoregulatory, metabolic, and stress markers were analysed in order to assess the potential consequences of transfer to freshwater, 19 days after the challenge with L. salmonis. Infestation intensity was significantly reduced following transfer to freshwater, and mortality rates were markedly higher in infested fish maintained in seawater vs. fish that were transferred to freshwater. Significant sea lice effects, consistent across a number of physiological markers, were apparent once L. salmonis developed to the mobile stages. Plasma chloride, lactate, and cortisol all were significantly higher than control values, and liver glycogen concentration was significantly reduced in infested fish in seawater. After return to freshwater, these physiological measures returned to control levels, but significant lice effects persisted for fish maintained in seawater. Premature return of infested sea-run brown trout to freshwater does, therefore, confer significant short-term physiological benefits across a range of osmoregulatory, metabolic, and stress markers.