a b s t r a c tSince 2006, three invasive Gobiids from the Ponto-Caspian area established in the River Rhine and their abundances nowadays regularly exceed 80% of the fish community. Between 2009 and 2011, densities of gobies in the Rhine increased while their condition decreased, assuming that the populations are approaching or even reached the capacity of the ecosystem. Consequently, we hypothesized a high level of competition on food resources within this group of invasive gobies that all exhibit the same sedentary life style, which might strengthen the differentiation of the ecological niche on a spatial and temporal axis. Invasive gobies were caught with electro fishing and beach seining in different types of habitats over a period of two years in the Lower Rhine, analyzing the food of more than 1500 gobies of the three species Neogobius fluviatilis (NF), Neogobius melanostomus (NM) and Ponticola kessleri (PK). All species showed an opportunistic feeding strategy. In NF and PK, a clear shift in preferred food resources was observed between individuals smaller and larger 50 mm that occurred in parallel with a habitat shift from sandy areas to riprap structures in PK, but not in NF that were only found on gravel and sand. In contrast, there were no distinct changes in food and habitat preference in NM. Small NM were found from spring to autumn on the sandy nearshore areas where they competed on food resources with juvenile PK in spring, and with NF in late summer, respectively. Abundance of juvenile NF and NM increased during the night in sandy nearshore areas in October. This behavior is assumed as predator avoidance against large piscivorous NF as well as native pikeperch, because active feeding occurred mainly during the day. The results on the three invasive Gobiids in the Lower Rhine give important hints how fine-tuned spatial and temporal characteristics in intra-and inter-specific competition shape the ecological niche of these invaders in their new environment.

Angling catch records are frequently used to reveal fish population developments. It is therefore important to understand the determinants of angling catches. This study focused on angler‐related, biotic and abiotic factors influencing catchability of Eurasian perch, Perca fluviatilis L. A multi‐lake (21 lakes) study based on angling diaries collected in Mecklenburg‐Vorpommern, Germany (2006/2007), found that angler‐related factors such as fishing experience, species preference and bait/lure type had a large impact on perch catch rates. Additionally, environmental conditions (nutritional status and water transparency) affected either the size or the number of perch caught by anglers. Catch rates varied seasonally, which was confirmed by an experimental fishery on a gravel pit (2008). This portion of the study showed that altered food availabilities in the course of the year caused food limitation in perch, which in turn facilitated high catch rates and female‐biased exploitation in autumn. It is concluded that both angler‐related and abiotic factors interact affecting perch catch rates and size of perch captured in recreational angling.

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Background: The 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.Results: Overall, 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. Conclusions:Fish 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.

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