Abstract
BackgroundThe aim of telemetry studies is often to determine the fate and mortality rates of fish. A moving fish is usually regarded as alive and a long-term stationary fish as dead—and the site where it became stationary as the site where it died. Downstream transport of dead fish in rivers can lead investigators to mistake dead fish for live fish. We examined downstream movements of 60 dead Atlantic salmon smolts and 55 dead European silver eels, equipped with radio transmitters and released at hydropower stations in three German rivers.ResultsOverall, dead smolts drifted up to 2.4 km downstream and dead eels up to at least 30.1 km downstream. Smolts released in an Archimedes screw turbine drifted up to 1.1 km and eels up to at least 5.1 km downstream. Most smolts stopped moving further downstream within 1 week after release, whereas the eels (or their transmitters) moved downstream over several weeks, or even months, after release. However, the distance moved varied considerably also within species and among release sites. Eighteen (30%) dead smolts and nine (16%) dead eels moved upstream or disappeared from the rivers, indicating that they had been taken by scavengers. Some of these (four smolts and one eel) had recordings, indicating that they had been taken out of the river by birds.ConclusionsFish can drift considerable distances downstream after they have died in a river. This can make it difficult to identify dead fish, and the exact site and time of death from telemetry studies. Furthermore, dead fish can be moved within the river, or taken out of the river, by scavengers. These results are relevant for studies of mortality at power stations, but also for studies of tagged fish in rivers in general. When designing telemetry studies and interpreting telemetry data, investigators should keep in mind that dead fish (or their transmitters) may drift downstream. Investigators may also consider if it is useful to release dead fish as a control groups when planning and performing fish mortality studies.
Highlights
The aim of telemetry studies is often to determine the fate and mortality rates of fish
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
The present results indicate that assessment of European silver eel mortality requires a larger study area than assessment of Atlantic salmon smolt mortality
Summary
The aim of telemetry studies is often to determine the fate and mortality rates of fish marked with electronic tags [e.g., 1, 2]. A moving fish is usually regarded as alive, while a long-term stationary fish is regarded as dead— and the site where it became stationary is regarded as the site where it died [e.g., 3, 4]. Based on release of six dead European eels Anguilla anguilla into hydropower turbines, drift of dead fish up to 4.2 km downstream was demonstrated. Such downstream movements of dead fish have implications for calculating mortality rates and identifying site of mortality in telemetry studies. There is little available information on potential downstream movement of dead fish in rivers
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