Hydropower stations influence migrating fishes in many ways. Besides a delay in migration, turbines and other installations at power stations may cause injury and mortality in downstream migrating fish [e.g., 12–14]. Telemetry is a useful tool to monitor the behavior of fish during passage of hydropower stations and other sites impacted by anthropogenic activities. However, mortality is difficult to document, since fish that die during passage of a hydropower station may drift further downstream, as demonstrated in the present study, and their movements can incorrectly be regarded as movements of live fish. The results from this study are relevant for studies at power stations, but also for studies of tagged fish in rivers in general.
Dead fish (or their transmitters) moved considerable distances downstream, smolts up to 2.4 km downstream and eels up to at least 30.1 km, which are longer distances than indicated in previous studies [2, 15]. Since the fish were killed before release, these results represent downstream drift after immediate mortality. If fish are mortally wounded, but still perform some swim-like movements, it is possible that they may move even longer distances downstream from the site where they were wounded. We suggest that the downstream drift distance can be affected by many factors, such as water discharge, water currents at the site where fish died and in the areas downstream, magnitude of floods, river morphology, substrate, fish size and decay rate of the dead fish. Our study design did not allow for an in-depth analysis of the effect of different factors. Nevertheless, since releases were made of two species, at a number of different rivers, and under different conditions, the results are relevant for a wide range of conditions. Our results clearly show that there is large variation in drift distance both within and among release occasions and that information on drift distances is not necessarily transferable among sites and situations.
In general, the eels moved longer distances downstream than the smolts. Although individuals of the two species were not released at the same time, they were released at the same sites, and the difference between the species could not be explained by consistently higher water discharges during releases of eels. The transmitters used for smolts had shorter battery lifetime than those used for eels, and the smolts were therefore monitored for a shorter time period. Still, many smolts (or transmitters) were followed for 1.5 month or more, and during a similar period, most eels had already moved much longer distances than recorded for smolts. Most smolts had stopped moving long before the transmitter stopped sending signals. Hence, a shorter transmitter lifetime for smolts cannot explain the different drift distances between the species. Other factors causing different drift distances could be differences in body size and body consistency, which may influence the speed and nature of the decay process. Smolts are small compared to adult salmon and silver eels, and previous records of floating carcasses of adult salmon up to 20 km downstream [15] may suggest that the short drift distance of smolts in the present study was related to their smaller size.
Dead European silver eels drifted downstream over longer time periods than smolts, even when considering the difference in battery lifetime. Similar to the adult salmon in the study of Hewson [15], the drift of eels seemed to be facilitated by large floods in the period after release. Downstream movements of dead eels still occurred after 1.5–3.5 months, but we do not know whether the carcass still existed, or whether it was only the tag moving. The carcass of larger fish may remain for a longer time period than of smaller fish. The decay rate may also be slower at lower temperatures in the autumn, which may have contributed to a longer time period of downstream drift by eels than by smolts. Tags may drift downstream after the carcass has disappeared, and the different size and weight of the tags may also have impacted the extent to which they settled on the bottom.
A relatively large proportion of the dead fish moved upstream or disappeared from the rivers (30% of the smolts and 16% of the eels), indicating that they had been taken by scavengers. We cannot rule out that a few transmitters failed, but it is unlikely that this happened with a large proportion of the transmitters. Hence, the most likely reason for transmitters disappearing from the river is that they were taken by scavengers. This was supported by the fact that several dead fish showed movements indicating that they had been brought out of the river by birds. High predation rates in areas close to power stations are documented in several studies, and birds such as red-necked grebe Podiceps griseigena, grey heron Ardea cinerea and great cormorant Phalacrocorax carbo are known as fish predators [1, 16–18]. Great cormorants, grey heron and goosander Mergus merganser were avian predators present in our study areas (own observations). The fact that dead fish may be moved around in rivers, and even brought out of rivers by scavengers, has to be taken into account when attempting to make inferences about mortality causes and sites from telemetry data.
Information on the movements and fate of dead fish can be used to design telemetry studies and analyze telemetry data. In studies when it is not possible or desirable to include groups of dead fish, results in this and similar papers can be used to design the study to minimize problems with possible dead fish being interpreted as live fish. If the study area covers a long enough river stretch, identification of probable survivors after for instance passing a power station can be based on stationary receivers installed further downstream than dead fish likely drift. The present results indicate that assessment of European silver eel mortality requires a larger study area than assessment of Atlantic salmon smolt mortality. Large study areas may be feasible in long rivers, such as the Rhine. However, if recording mortality at a power station located close to the river mouth, where dead fish can potentially drift into the sea, it may be more difficult to obtain reliable mortality estimates. If groups of dead fish can be released as part of the study, the distribution of dead fish drift distances can be used to estimate probabilities of mortality for fish being released alive, based on their movement distance. It may also be possible in some situations to distinguish dead from survived fish based on information on the dead fish other than distribution of drift distances, like movement speed or distinctive aspects of behavior, such as movements indicating that they have been taken by scavengers. At one power station, we used recordings of dead smolts to estimate the probability of detecting mortality of live smolts potentially killed when passing an Archimedes screw and Francis turbines, based on both movement distance and the proportion of fish disappearing from the river [19]. We were able to estimate a probable upper threshold of the true mortality and the uncertainty in the estimate [19].