Elodie Réveillac

Anguilla marmorata larval migration plasticity as revealed by otolith microstructural analysis

The oceanic early-life history of Anguilla marmorata was examined in the southwestern Indian Ocean in Mayotte, Mauritius, and Reunion islands through otolith microstructural analysis. The study of the hatching dates, the first feeding check diameter (FFD), the leptocephalus (LD) and metamorphosis (MD) durations, the age at recruitment (AR), and the leptocephalus otolith growth rate (OGR) of glass eels revealed great variations in early-life traits and relationships between them. An agglomerative nesting analysis discriminated three early-life histories, differently represented according to the locality: (i) fast migrants with short LD, short MD, young AR, large FFD, and high OGR dominated in Reunion and Mayotte; (ii) midspeed migrants with intermediate LD, MD, AR, FFD, and OGR dominated in Mauritius; (iii) slow migrants with long LD, long MD, old AR, small FFD, and low OGR were recorded only in Mauritius. All possible strategies were not observed and therefore not successful at the sampling time. However, several were simultaneously expressed, which suggests larval migration plasticity at the population level. This evidence is crucial information regarding both the species dispersal capabilities and the evolution from short-migratory tropical species towards long-migratory temperate ones in the genus Anguilla.

Clues to the location of the spawning area and larval migration characteristics of Anguilla mossambica as inferred from otolith microstructural analyses

The spawning area location and the larval migration characteristics of Anguilla mossambica were studied through the examination of early life-history traits (ELHT) inferred from otolith microstructural analyses of 122 glass A. mossambica collected along the eastern coast of Madagascar, in the south-western Indian Ocean. Backcalculated hatching dates of leptocephali that recruited between November 2005 and February 2006 suggested that spawning was continuous from June to November 2005, although individual migration duration periods were variable. Larvae that exhibited the highest daily otolith growth rates had the shortest leptocephalus durations and total length (L(T)) at recruitment, possibly because they reached the competence to metamorphose more rapidly and to recruit to fresh waters. Migration durations and L(T) increased from north to south on the Malagasy eastern coast, supporting earlier suggestions that A. mossambica spawns north-east of Madagascar, west of the Mascarene Ridge in the Indian Ocean.

Influence of Green Tides in Coastal Nursery Grounds on the Habitat Selection and Individual Performance of Juvenile Fish

Coastal ecosystems, which provide numerous essential ecological functions for fish, are threatened by the proliferation of green macroalgae that significantly modify habitat conditions in intertidal areas. Understanding the influence of green tides on the nursery function of these ecosystems is essential to determine their potential effects on fish recruitment success. In this study, the influence of green tides on juvenile fish was examined in an intertidal sandy beach area, the Bay of Saint-Brieuc (Northwestern France), during two annual cycles of green tides with varying levels of intensity. The responses of three nursery-dependent fish species, the pelagic Sprattus sprattus (L.), the demersal Dicentrarchus labrax (L.) and the benthic Pleuronectes platessa L., were analysed to determine the effects of green tides according to species-specific habitat niche and behaviour. The responses to this perturbation were investigated based on habitat selection and a comparison of individual performance between a control and an impacted site. Several indices on different integrative scales were examined to evaluate these responses (antioxidant defence capacity, muscle total lipid, morphometric condition and growth). Based on these analyses, green tides affect juvenile fish differently according to macroalgal density and species-specific tolerance, which is linked to their capacity to move and to their distribution in the water column. A decreasing gradient of sensitivity was observed from benthic to demersal and pelagic fish species. At low densities of green macroalgae, the three species stayed at the impacted site and the growth of plaice was reduced. At medium macroalgal densities, plaice disappeared from the impacted site and the growth of sea bass and the muscle total lipid content of sprat were reduced. Finally, when high macroalgal densities were reached, none of the studied species were captured at the impacted site. Hence, sites affected by green tides are less favourable nursery grounds for all the studied species, with species-specific effects related to macroalgal density.

Spatial distribution and impact of the gill-parasitic Mazocraes alosae (Monogenea Polyopisthocotylea) on Alosa alosa and A. fallax (Actinopterygii, Clupeidae)

Are the distribution of Mazocraes alosae and its impact on the host similar between Alosa alosa and A. fallax according to their resemblances? Parasites were numbered on each gill of shads sampled in North-East Atlantic coastal waters and connected rivers. Their impact on host condition was measured using girth, gonado-somatic ratio, C/N ratio, and Fulton’s K. Prevalence and mean intensity of M. alosae were significantly higher for A. alosa than for A. fallax, including in sympatric conditions. The mean intensity varied among sites whatever fish species; it was higher in coastal–estuarine versus fresh waters only for A. fallax. The distribution of M. alosae was aggregated in the host population whatever species. At the host individual level, some gills (second and third for A. alosa, second for A. fallax) were significantly more inhabited than others, probably in relation with larger water volumes flowing on these gills and mazocraeid sedentary lifestyle. Despite high prevalence and intensity, no negative impact of M. alosae was demonstrated on the host condition whatever the index considered. Our study underlines the major occurrence of M. alosae on shads and the potential use of such benign parasite as biological tag to discriminate closely related host species.