Search Results (2)

Anguillid eels are the iconic example of catadromous fishes, because of their long-distance offshore spawning migrations. They are also a good model for research on the onset mechanisms of migrations to breeding areas, because the migrations begin in inland waters. When eels transform from yellow eels to silver eels, it is called silvering. Silver eels show various synchronous external and internal changes during silvering, that include coloration changes, eye-size increases, and gonadal development, which appear to be pre-adaptations to the oceanic environment and for reproductive maturation. A strong gonadotropic axis activation occurs during silvering, whereas somatotropic and thyrotropic axes are not activated. Among various hormones, 11-ketotestosterone (11-KT) drastically increases during spawning migration onset. Gradual water temperature decreases simulating the autumn migratory season, inducing 11-KT increases. Administration of 11-KT appeared to cause changes related to silvering, such as early-stage oocyte growth and eye enlargement. Moreover, 11-KT may be an endogenous factor that elevates the migratory drive needed for the spawning migration onset. These findings suggested that water temperature decreases cause 11-KT to increase in autumn and this induces silvering and increases migratory drive. In addition, we newly report that 11-KT is associated with a corticotropin-releasing hormone that influences migratory behavior of salmonids. This evidence that 11-KT might be among the most important factors in the spawning migration onset of anguillid eels can help provide useful knowledge for understanding endocrinological mechanisms of the initiation of spawning migrations. Full article

The Allis shad, Alosa alosa (Linnaeus, 1758) and the Twaite shad, Alosa fallax (Lacépède, 1803) are two anadromous and congeneric Clupeidae species, i.e., closely related, even hybridising with each other, which makes it very difficult to differentiate between them. The marine phase represents the major part of the life cycle of these species, yet there are still large gaps in the knowledge of the biology, ecology and distribution during this phase. To overcome this lack of data, this study analysed the biometric characteristics, as well as different tissues and organs, of specimens of the genus Alosa, caught as bycatch by inshore fisheries on the continental shelf of the coastal region of Galicia, NW Iberian Peninsula. Specimens of the genus Alosa (N = 345) were acquired on daily first sale notes between January and March 2021 at the fish markets of A Guarda, Malpica and A Coruña, three of the most important landing sites for bycatches of these species in Galicia, whose fleet operates mainly on the continental shelf. Analysis of gill rakers revealed a slightly higher percentage of A. fallax than A. alosa, as well as the presence of probable F1 hybrids. At the demographic level, although there were significant differences, with A. alosa having the highest mean age, the results showed a typical age class distribution of the adult contingent, at the age of sexual maturity, for the three taxa. Condition index results clearly reflected the size differences between the parental species and the intermediate character of the hybrids, with A. alosa having the highest value for this index, the hybrids intermediate values and A. fallax the lowest values. Trophic spectrum was common to all three taxa, including fish, plankton, benthos and cephalopods; however, A. fallax showed more pronounced ichthyophagous behaviour than A. alosa and the hybrids. Both species and hybrids share habitat, simultaneously occupying both shallow coastal areas and deeper, more distant areas. Part of the occupied habitat is in Natura 2000 areas. Two new areas are proposed to increase the protection of these species, while fulfilling the EU’s obligations towards them. Full article