A combination of radio and acoustic telemetry was used to monitor the out-migration of hatcheryreared Atlantic salmon, Salmo salar L., in the River Testebo, its estuary and coastal system. As with many other Baltic rivers, the hydropower regulated River Testebo once had a self-sustaining salmon population that is now extinct. Substantial losses of smolts in the river (48-69%) and inner part of the estuary (43-47%) were found, but after leaving the estuary, the success of post-smolts moving out of the Bay was sufficiently high (83-89%) to conclude that habitat within the bay is not a factor limiting initial marine survival. The results suggest that hatchery-based recovery of a wild salmon population in the river will not be successful unless other actions, such as habitat improvement, are included. K E Y W O R D S :Atlantic salmon, migration, smolt, stocking, tag.

No abstract

The present research aimed to investigate the existence of clock gene expression rhythms in tilapia, their endogenous origin, and how light and feeding cycles synchronize these rhythms. In the first experiment, two groups of fish were kept under an LD cycle and fed at two different time points: in the middle of the light (ML) or in the middle of the dark (MD) phase. In the second experiment, fish fed at ML was fasted and kept under constant lighting (LL) conditions for 1 day. In both experiments, the samples from central (optic tectum and hypothalamus) and peripheral (liver) tissues were collected every 3 h throughout a 24 h cycle. The expression levels of clock genes bmal1a, clock1, per1b, cry2a, and cry5 were analyzed by quantitative PCR. All the clock genes analyzed in brain regions showed daily rhythms: clock1, bmal1a, and cry2a showed the acrophase approximately at the end of the light phase (ZT 8:43-11:22 h), whereas per1b and cry5 did so between the end of the dark phase and the beginning of the light phase, respectively (ZT 21:16-4:00 h). These rhythms persisted under constant conditions. No effect of the feeding time was observed in the brain. In the liver, however, the rhythms of clock1 and cry5 were influenced by feeding, and a shift was observed in the MD fish group (ZT 3:58 h for clock1 and 11:20 h for cry5). This study provides the first insights into the molecular clock of tilapia, a very important fish species for aquaculture. It also reveals the endogenous origin of clock gene rhythms and the ability of feeding time to shift the phase in some clock genes in the peripheral, but not the central, oscillator.

The survival of brown trout Salmo trutta embryos and first-year juveniles was studied using in situ bioassays during the snowmelt-driven spring flood in 12 streams in northern Sweden. Unlike in most previous studies on the impact of acidity on brown trout, the streams in this study were high in dissolved organic carbon (DOC), and as a result organic acids play a primary role in controlling pH. During the spring flood period DOC concentrations increased strongly in most streams and, in combination with dilution of acid-neutralizing capacity, resulted in a decrease in pH and an increase in total and inorganic monomeric aluminum. High mortality of brown trout juveniles occurred during the spring flood and was best correlated to the high H þ concentration. No toxic effect could be directly attributed to measured inorganic aluminum concentrations. An empirical model to predict juvenile brown trout mortality in DOC-rich streams was developed, and based on these predictions a critical chemical threshold of pH 4.8-5.4 is proposed for firstyear juveniles. There was high embryo and yolk sac fry survival during the spring flood, even at sites with pH as low as 4.0, suggesting that the pH threshold in DOC-rich waters is lower than the previously established threshold for low-DOC systems. We discuss the complex role that DOC has in humic-rich surface waters, where it both causes a pH decrease and protects against aluminum toxicity in fish. The results suggest that first-year juveniles are likely to be the stage most vulnerable to the effects of episodic pH depression associated with the snowmelt period in northern boreal systems. This results from asynchrony in the seasonality of the spring flood acid pulse and the seasonality of trout embryo development, which is slow in cold northern waters.

In general, hatchery salmonid smolts experience higher mortality during migration than wild smolts, which is suggested to be due to domestication effects and that hatchery fish lack experience of the natural environment. However, possible differences in feeding during smolt migration between hatchery and wild smolts have rarely been addressed. We compared the number of feeding smolts and stomach fullness among wild Atlantic salmon smolts, hatchery-reared smolts released as 1-year-old parr, and hatcheryreared smolts released as 2-year-old smolts during their descent to sea in River Tornionjoki. In addition, estimations of prey selection among the smolt groups were conducted. A high proportion of wild smolts and smolts stocked as parr actively fed during the smolt migration. A lower proportion of smolts stocked as smolts was feeding and their stomach fullness were much reduced in comparison with the two other groups. The study also indicated that the feeding of migrating smolts is selective rather than opportunistic.In conclusion, this study suggests that stocked 2-yearold smolts may enter sea with an inferior foraging behaviour and it is a possibility that this may contribute to the observed low post-smolt survival in the Baltic Sea.