A preliminary telemetry study of the migration of silver European eel (Anguilla anguilla L.) in the River Mosel, GermanyUn resumen en espan˜ol se incluye detra´s del texto principal de este artı´culo. IntroductionDeclining elver catches at the European coasts and declining commercial eel catches in many European freshwater rivers provide strong evidence for a decrease of European eel (Anguilla anguilla L.) stocks in Europe (Moriarty 1992;Dekker 2000). In addition to other possible reasons for the decline of eel populations such as the reduction of silver eel fitness by the swimbladder nematode Anguillicola crassus (Sprengel & Lu¨chtenberg 1991;Haenen et al. 1994;Kirk et al. 2000), obstacles to migration and increased hazards while crossing a rising number of dams during migration in freshwater might significantly reduce the number of spawners that return to the Sargasso Sea.Hydroelectric dams aggravate both up-and downstream migration of fish. Although fishways can facilitate upstream migration, effective downstream migration facilities are still generally lacking (for reviews see Larinier & Travade 1999;Odeh 1999). Silver eels are particularly affected by the lack of appropriate downstream migration facilities because they use the main current during their downstream passage and migrate near the riverbed (Tesch 1983(Tesch , 1994Jonsson 1991), a behaviour that automatically leads them to the water intake of hydroelectric power plants. With their elongate body shape, eels are particularly vulnerable to damage by turbine blades (Coutant & Whitney 2000), which may ultimately result in high mortality rates. Numerous previous studies Abstract -To study the behaviour of silver eels (Anguilla anguilla L.) during their downstream migration, particularly near a hydroelectric power dam, we tagged nine eels with ultrasonic transmitters and tracked their paths in the River Mosel, Germany. The onset of migration coincided with the first flood event that followed the full moon but was independent of daytime, because migration and turbine passage occurred during both day and night. During migration eels swam actively downstream with a velocity of 0.3-1.2 m Á s À1. When migrating eels arrived at the dam, they either passed through the turbines immediately or stayed upstream of the powerhouse for up to 8 days, showing a characteristic circling behaviour. Circling eels repeatedly approached the trashrack, sprinted upstream, and finally passed through the turbines with the next high water discharge. These observations are discussed with regard to the design of appropriate downstream passage facilities.
Identification of parasite species is particularly challenging in larval and juvenile hosts, and this hampers the understanding of parasite acquisition in early life. The work described here employs a new combination of methods to identify parasite species and study parasite succession in fry of perch (Perca fluviatilis) from Lake Constance, Germany. Classical morphological diagnostics are combined with sequence comparisons between parasite life-stages collected from various hosts within the same ecosystem. In perch fry at different stages of development, 13 different parasite species were found. Incomplete morphological identifications of cestodes of the order Proteocephalidea, and trematodes of the family Diplostomatidae were complemented with sequences of mitochondrial DNA (cytochrome oxidase 1) and/or nuclear (28 s rDNA) genes. Sequences were compared to published data and used to link the parasites in perch to stages from molluscs, arthropods and more easily identifiable developmental stages from other fishes collected in Lake Constance, which both aided parasite identification and clarified transmission pathways. There were distinct changes in parasite community composition and abundance associated with perch fry age and habitat shifts. Some parasites became more abundant in older fish, whereas the composition of parasite communities was more strongly affected by the ontogenetic shifts from the pelagic to the littoral zone.
Previous microsatellite analysis showed that two subpopulations of perch ( Perca fluviatilis L.) exist in Lake Constance. This raises questions of whether (i) Lake Constance was colonized by two populations that diverged in allopatry, or (ii) the two subpopulations diverged in sympatry. Sequence analysis of a 365 bp mtDNA fragment (5′ ′ ′ ′ -end of the D-loop) of perch from Lake Constance and adjacent waters revealed 10 haplotypes. We suggest colonization via the Danube river, based on the frequency and dispersion of haplotypes, and knowledge of the lake's palaeohydrological development. Pairwise F ST -values using mitochondrial DNA sequences showed no significant population subdivision. Our study provides strong evidence that subpopulations of perch in Lake Constance have diverged in sympatry.
Prior studies have shown that perch (Perca fluviatilis L.) of Lake Constance belong to two genetically different but sympatric populations, and that local aggregations of juveniles and adults contain closely related kin. In this study we analysed the genetic structure of pelagic perch larvae to investigate, if kin structured shoals already exist during early ontogenetic development or might be the result of homing to natal sites. Analysis of the gene frequencies at five microsatellite loci revealed that 3 out of 5 pelagic aggregations of larvae showed significant accumulation of kin. To investigate possible mechanisms of shoal formation, we tested if perch use olfactory cues to recognize their kin. Choice tests in a fluviarium showed preference for odours of unfamiliar kin versus unfamiliar non-kin. Additionally, we showed that perch could differentiate between the odours of the two sympatric populations and significantly preferred unfamiliar and unrelated conspecifics of their own over the foreign population. Our results present a behavioural mechanism that could lead to the observed formation of kin structured shoals in perch. We further discuss if the ability to discriminate between the own and the foreign population could result in assortative mating within populations and thus form the basis of "socially mediated speciation" in perch.
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