The evolving story of catadromy in the European eel (Anguilla anguilla)

Anguillid eels were once considered to be the classic example of catadromy. However, alternative life cycles have been reported, including skipping the freshwater phase and habitat shifting between fresh, brackish, and saltwater throughout the growth phase. There is a lack of knowledge regarding these alternate life strategies, for example, the proportion of individuals in the population that adopt them compared to classic catadromy. We provide a description of these alternate life cycle strategies in temperate anguillids, their possible drivers, and the methods available to investigate them. These methods (lethal and non-lethal), include otolith microchemistry, fatty acid and stable isotope analyses, parasite identification, blood transcriptomics, and electronic tags. We argue that since the current management framework for the European eel and other temperate eels is based mainly on the freshwater component of the population, it ignores eels growing in saline waters. Many of the factors that are thought to be responsible for the precipitous decline of the eel population are more prevalent in freshwater systems. Therefore, the contribution of saline eels may be more important than currently estimated. The habitat-shifting ability of eels may be all the more crucial for the persistence and recovery of those species that are endangered

Do sea trout Salmo trutta parr surveys monitor the densities of anadromous or resident maternal origin parr, or both?

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Diel changes in the fish assemblage in a coastal surf-zone area in the eastern Baltic Sea

The composition of the fish assemblage in the surf zone in Eru Bay, Gulf of Finland, was highly variable in relation to the time of day (dawn, noon, dusk and midnight) during the ice-free season in 2008. The diel variation in the surf-zone fish assemblage composition was also associated with seasonal changes. Species that exhibited the most variation during the diel cycle were the European smelt (Osmerus eperlanus), gudgeon (Gobio gobio), bleak (Alburnus alburnus), three-spined stickleback (Gasterosteus aculeatus), ninespine stickleback (Pungitius pungitius), small sandeel (Ammodytes tobianus), and gobies (Pomatoschistus spp.). To our knowledge, this is the first study that describes diel variations in the fish assemblage composition of a non-tidal brackish surf-zone environment using an annual data set. The results imply that time of day effects species abundances, and this should be taken into account in future studies where the aim is to evaluate littoral fish assemblages of the Baltic Sea