Marine fish larvae experience high and variable mortality during their early life stage as a part of the plankton ecosystem of the open sea. This thesis work focuses on larvae of the European eel, Anguilla anguilla, in the eel family Anguillidae. In the Order Anguilliformes, there are roughly 800 eel species – roughly five percent of all oceanic fish species, and while most eels are predominantly oceanic, the 19 Anguillidae species are catadromous – they spawn and grow as larvae in marine environments, but as juveniles and adults they inhabit brackish and freshwater environments. Eel larvae are distinct from most other fish larvae by being long-lived, transparent, highly migratory and having a complex biology and ecology. This, together with their former economic importance on the European continent, has made them the subject of research for over a century.
The main aim of this PhD thesis was to gain understanding of their early life biology and ecology as a component of the Sargasso Sea’s planktonic ecosystem, and specifically focus on their feeding and growth in this area. To explore this, the studies of the PhD developed via the use of three primary, and partially overlapping methods: morphologic, microscopic and genetic investigations.
Morphological and individual DNA-barcoding identification methods were combined to study larval fish diversity and distributions in the Sargasso Sea. The examination of more than 3,500 individual specimens from the Sargasso Sea showed noticeable high diversity of the ichthyoplankton community, with these oligotrophic waters harboring larvae of more than 150 different species. The European eel was 12th in order of abundance, and we saw high spatial variability in larval distributions over the region. Abundances of most species were enhanced in the vicinity of marine fronts. These results show that open-ocean environments, far from eutrophic coastal areas, can function as spawning and rearing areas for many species of fish.
Temporal and spatial variability in growth rates of eel larvae, were studied by scanning electron microscopy (SEM) examination of micro-growth rings imprinted in the ear bones (otoliths). Anguilla anguilla proved to have the lowest growth rates of all the 19 known species in the genus. We saw no spatial tends in growth rates estimates over the over 1800 km oceanic region, but laterspawned larvae showed lower growth rates than the earlier-spawned larvae. High variability in growth rates was seen, whether specimens were from a single station or assembled for the whole Sargasso Sea region. Calculated spawning frequencies peaked at new-moon, suggesting that lunar light intensity has some influence on the spawning intensity of A. anguilla.
Finally, the diet of eel larvae presently is unknown, as their stomach contents rarely contain visually identifiable material, so we used “metabarcoding”, next-generation genetic sequencing, to identify composition of the eel stomach contents. The stomach contents of the eel larvae proved to be mostly made up of Cnidarian plankton, and this was distinct from the Crustacean and protist taxa that predominated in the analyzed marine snow particles. Results thus suggest that consumption of marine snow is not an important component of their diet. Further, heightened abundances of Cnidaria plankton in the immediate environment gave further indication of these as potential food items.
Hence, the present multidisciplinary examination of the ecology and biology of larval European eels, reveals their overall place in the diverse ichthyolarval environment of the Sargasso Sea, and important findings about potential prey items, as well as high growth rate variations points to a complex early life which is strongly linked to the bio-physical conditions of the region. The gained information contributes to our understanding of the oceanic, early life phase, of the eel life-cycle, and can, hopefully lead to management plans which can facilitate the recovery and welfare of these important fishes.