Niels Jepsen

Survival of radio-tagged Atlantic salmon (Salmo salar L.) and trout (Salmo trutta L.) smolts passing a reservoir during seaward migration

High mortality-rates of seaward migrating salmonid smolts when passing reservoirs and lakes have earlier been found in the Danish River Gudena watershed. To reveal the causes of mortality of migrating smolts in Lake Tange, a 12 km long, shallow reservoir, 50 salmon smolts and 24 trout smolts were tagged with internal miniature radio-transmitters, and released in the river just upstream the reservoir on May 1, 1996. The salmon smolts were hatchery-reared, while the trout smolts were wild fish, caught in a smolt trap. The tagged smolts were tracked daily for 3 weeks, and when possible the cause of death was determined. During the 3-week period, 90% of the tagged smolts died. The main cause of death for both trout and salmon was predation from fish and birds. The most important predator was pike (Esox lucius L.), being responsible for 56% of the observed mortality. Avian predators were assumed to be responsible for 31% of the observed mortality. No trout smolts left the reservoir, but 5 salmon-smolts got out through the turbines. Others did traverse the reservoir, but were unable to enter the river downstream, and were later eaten. The present results suggest that mortalities for migrating smolts through Lake Tange are of such a magnitude, that stocking of juveniles in the river upstream is futile, and further, that the establishment of a natural population of salmon or sea-trout in river Gudena, upstream Tange, is unrealistic under present conditions.

The Physiological Response of Chinook Salmon Smolts to Two Methods of Radio-Tagging

Abstract Smolts of hatchery-reared chinook salmon Oncorhynchus tshawytscha were radio-tagged by gastric insertion or surgical implant, and their physiological response was measured and compared to that of control fish. Plasma levels of cortisol, glucose, and lactate were measured before tagging and at 3 h, 24 h, 7 d, and 14 d after tagging. Significant increases in concentrations of cortisol, glucose, and lactate occurred at 3 h after tagging in both treatment groups. After 24 h cortisol levels were still elevated in both groups of tagged fish, whereas the levels of glucose and lactate had returned to normal for the surgically implanted fish but still remained higher than those of the controls for gastrically implanted fish. After 7 and 14 d, differences between treatment and control fish were not significant. Body size and physiological response to tagging among fish 14–26 cm showed no correlation. The results show that radio-tagging is indeed stressful for chinook salmon smolts but that the presence of ...

Behavioural interactions between prey (trout smolts) and predators (pike and pikeperch) in an impounded river

Movements of radio-tagged pike Esox lucius (L.), pikeperch Stizostedion lucioperca (L.) and outward migrating sea trout smolts Salmo trutta (L.) were studied in a shallow Danish reservoir to obtain information of predator–prey interactions between these species. Twenty pikeperch (55–74 cm) and 19 pike (52–72 cm) were tagged. Female pikeperch spent more time near the outlet sluice during the smolt run (May) than at other times of the year, apparently actively hunting the smolts delayed in this area. In contrast, male pikeperch did not seem to participate in the smolt predation but remained stationary during the smolt run, presumably guarding their nests. Most tagged pike were present at the spawning grounds during the peak of the smolt run, where they had little chance of smolt encounter. Twenty migrating trout smolts were radio-tagged in the river upstream of the reservoir. Ten of these were located in the vicinity of the outlet sluice at least once, but were unwilling or unable to find and enter the sub-surface outlet sluice. Only one tagged smolt left the reservoir. After 1–12 days in the reservoir, the remaining smolts were eaten by pikeperch or pike and the results indicate that female pikeperch and few female pike have adjusted their behaviour to predation on smolts during the smolt run. The smolt predation in this man-made reservoir is higher than in natural lakes, probably due to the changed physical environment and introduced predators, such as pikeperch. The outlet sluice practice and the temporal overlap between smolt run and predator-spawning may be key factors in smolt survival. Copyright

The use of external electronic tags on fish: an evaluation of tag retention and tagging effects

External tagging of fish with electronic tags has been used for decades for a wide range of marine and freshwater species. In the early years of fish telemetry research, it was the most commonly used attachment method, but later internal implants became preferred. Recently, the number of telemetry studies using external tagging has increased, especially with the development of archival tags (data storage tags, DSTs), pop-up satellite archival tags (PSATs) and other environment-sensing tags. Scientific evaluations of the tagging method are rather scarce for most species. We identified 89 publications, reporting effects of external tagging for 80 different fish species, which constitute the main basis for this review. External attachment holds certain benefits compared to other tagging methods, for example, speed of application, and it may be the only option for fishes with a body shape unsuitable for surgical implantation, or when using tags with sensors recording the external environment. The most commonly reported problems with external tags are tissue damage, premature tag loss, and decreased swimming capacity, but the effects are highly context dependent and species specific. Reduced growth and survival have also been recorded, but direct mortality caused by external tagging seems rare. Most of the studies reviewed evaluate tag retention, survival, and tissue reactions. There is a general need for more research on the effects of external tagging of fish with electronic tags, but particularly there are few studies on predation risk, social interactions, and studies distinguishing capture and handling effects from tagging effects. For PSATs, especially those that are large relative to fish size, there are particular problems with a high proportion of premature tag losses, reduced swimming capacity, and likely increased predation, but there remains a paucity of tag effect studies related to the use of PSATs. Before embarking on a field study employing external tagging with electronic tags, we recommend the use of appropriate pilot studies, controlled where possible, to quantify potential impacts of tagging.

Initial mortality of radio-tagged Atlantic salmon (Salmo salar L.) smolts following release downstream of a hydropower station

The fate of radio-tagged hatchery-reared salmon smolts (Salmo salar L.) was investigated in the tailrace of a hydropower station in the Danish River Gudenaa during 2 years. Seventeen and 27 smolts were tagged and released in late May 1996 and during April 1999, respectively. Out of the total of 44 smolts, only two were recorded to leave the river and enter the estuary. In both years of study, electrofishing was used to sample tagged smolts. In 1996, these attempts were unsuccessful, while in 1999, 19 (70% of total) transmitters were retrieved in the stomachs of pikeperch (Stizostedion lucioperca (L.)) and pike (Esox lucius L.), and in grey heron (Ardea cinerea L.) nests. The present study demonstrates that the physical condition of the watershed, with respect to spawning and rearing, may not be the only crucial determining factors of the success of reintroduction of Atlantic salmon. Hydropower plants not only block the free movement of migratory fish, hence preventing the utilisation of spawning and rearing areas, but also form the basis of areas with increased mortality.

Spawning migration of sea trout (Salmo trutta (L)) in a Danish river

From September to November in 1995 a total of 49 mature sea trout were caught and radio tagged in the estuary (Randers fjord) or at the river mouth of the River Gudena in Eastern Jutland. The tagged trout were between 2 and 6 yr old with total body length of 56–85 cm. Twenty-five of the tagged trout ascended the river. They were tracked every third day, for up to six months, until death or descent. Great variation was found in migration pattern and duration of river residence. Some fish spawned and left the river, some died after spawning, while others died unspent. The sea trout preferred to stay on the southern side of the main river, and Males spent significantly more time of the freshwater stay in spawning tributaries than females. Most of the trout ascended the main spawning tributary, the River Lillea, where none passed a weir, 2 km upstream the confluence, despite the presence of a fish ladder.

Human - wildlife conflicts in Europe: Fisheries and fish-eating vertebrates as a model case

This book is about conflicts between different stakeholder groups triggered by protected species that compete with humans for natural resources. It presents key ecological features of typical conflict species and mitigation strategies including technical mitigation and the design of participatory decision strategies involving relevant stakeholders. The book provides a European perspective, but also develops a global framework for the development of action plans.

The use of coded wire tags to estimate cormorant predation on fish stocks in an estuary

One of the main obstacles to resolving the conflict between an increasing population of cormorants, Phalacrocorax carbo sinensis, and the fishing industry is the lack of documentation of the effect of the birds’ predation on fish stocks. Tagging and releasing fish with coded wire tags followed by intensive cormorant pellet sampling may be a viable method to measure the impact of cormorants on fish populations. To test this new method, we studied cormorant predation in a shallow estuary, where nearly 100 000 fish were tagged and more than 10 000 cormorant pellets were collected over a 3-year study period. A total of 112 tags were recovered from the collected pellets. Analyses of tag recovery data indicated considerable cormorant predation on tagged flounder, eel and salmon smolts, but the method did not deliver high-quality documentation, mainly because of limitations in pellet sampling. We conclude with recommendations to enhance the value of this method.

Effects of salmon lice infection and salmon lice protection on fjord migrating Atlantic salmon and brown trout post-smolts

Effects of artificial salmon lice infection and pharmaceutical salmon lice prophylaxis on survival and rate of progression of Atlantic salmon (n = 72) and brown trout post-smolts (n = 72) during their fjord migration, were studied by telemetry. The infected groups were artificially exposed to infective salmon lice larvae in the laboratory immediately before release in the inner part of the fjord to simulate a naturally high infection pressure. Groups of infected Atlantic salmon (n = 20) and brown trout (n = 12) were also retained in the hatchery to control the infection intensity and lice development during the study period. Neither salmon lice infection nor pharmaceutical prophylaxis had any effects on survival and rate of progression of fjord migrating Atlantic salmon post-smolts compared to control fish. Atlantic salmon spent on average only 151.2 h (maximum 207.3 h) in passing the 80 km fjord system and had, thus, entered the ocean when the more pathogenic pre-adult and adult lice stages developed. The brown trout, in comparison to Atlantic salmon, remained to a larger extent than Atlantic salmon in the inner part of the fjord system. No effect of salmon lice infection, or protection, was found in brown trout during the first weeks of their fjord migration. Brown trout will, to a larger extent than Atlantic salmon, stay in the fjord areas when salmon lice infections reach the more pathogenic pre-adult and adult stages. In contrast to Atlantic salmon, they will thereby possess the practical capability of returning to freshwater when encountering severe salmon lice attacks.

Phenotypic variation in metabolism and morphology correlating with animal swimming activity in the wild: relevance for the OCLTT (oxygen- and capacity-limitation of thermal tolerance), allocation and performance models

Combining physiological and morphological measures in the laboratory with registrations of detailed measures of field activity, we tested the hypothesis that individual activity patterns correlate with individual metabolism and morphology as proposed by several conceptual models. We found no evidence indicating an effect of metabolism, whereas morphology correlated with several activity measures.