Françoise Lagardère

Influence of ontogenetic and environmental changes in the otolith microchemistry of juvenile sole (Solea solea)

We analysed otolith composition, specifically Sr/Ca ratios, with the aim of determining whether metamorphosis and the transition to benthic life of Bay of Biscay sole occur in marine or estuarine conditions. Otoliths of wild juveniles (0-group) collected in two estuarine nurseries showed characteristic Sr/Ca chronologies, with a significant decrease of the Sr/Ca ratio from the core to the accessory growth centres. As the otolith Sr/Ca ratio decreases in respect of the ambient salinity, this could suggest a relative synchronism in the timing of estuarine nursery entrance. In order to validate this hypothesis, hatchery-produced larvae were reared in a tidal marsh mesocosm until they completed metamorphosis (around 1 month post-hatching) and settled on the bottom. The otoliths from these juveniles exhibited a similar Sr/Ca pattern, which could not be explained as a result of habitat changes, and especially salinity variations, but rather reflected an ontogenetic signal. Since both wild and reared soles achieved high growth rates during metamorphosis, this suggests a link between a high metabolic rate and the observed Sr/Ca drop. In addition, ontogenetic changes during this transitional phase could involve the setting of functional mechanisms, responsible for the regulation of free Sr 2+ in either plasma or endolymph or both. Nevertheless, this common trend appeared to be modulated by environmental cues: Sr/Ca ratios were significantly higher for reared fish compared to wild fish during the larval period. A possible explanation is that experienced temperatures were much higher in the incubator than in the field. Furthermore, Sr/Ca values obtained in the otolith juvenile area, with the lowest ratios for soles from the Loire nursery, suggest that the salinity regime of estuarine nurseries could exert an environmental control on otolith Sr/Ca ratios, thus providing ecological records of up-estuary migration after metamorphosis.

Seasonal variation in growth of larval sole Solea solea (L.) and consequences on the success of larval immigration

Abstract This study investigates whether seasonal differences in growth rates during the pelagic larval phase are likely to effect the success of larval immigration. Daily increments in otoliths of larvae sampled in the northern Bay of Biscay, from February to May 1992, were used to estimate growth rates and spawning dates. Significant seasonal variations in growth occurred which were related to water temperature. Growth rates remained constant between late December and March, but showed a rapid increase from April onwards. Spawning dates, back-calculated from the larvae at the onset of metamorphosis (stage 4b), occurred over an 18-week period from December to early April, with peaks in late January and early February. 0-group sole were collected in the coastal nursery ground of the Bay of Vilaine at the beginning (April) and the end of the settlement period (June). Otolith analysis showed that the first and largest batch of immigrants caught in April were spawned between late December and February (mostly in early February). At the end of the settlement period 95% of the 0-group sampled were spawned between January and February, with a median spawning date in late January (week 4). These spawning date distributions were compared with literature data on spawning in the Bay of Biscay. Most of the surviving 0-group appeared to originate from the main spawning period, although during spawning early in the season they had experienced slow growth rate. Though still preliminary, the results suggest that seasonal differences in growth rate do not affect the success of larval immigration.

Vertical distribution and feeding activity of metamorphosing sole, Solea solea, before immigration to the Bay of Vilaine nursery (northern Bay of Biscay, France)

To evaluate the impact of metamorphosis on the vertical distribution and feeding activity of sole, Solea solea, larvae passing from offshore spawning grounds to the Bay of Vilaine, sampling series at fixed stations were carried out in April 1991 and April 1993 at depths from 50 to 30 m. Comparisons between plankton and bottom samplin series indicated differences in vertical distribution of larvae in pre-metamorphic and metamorphic steps. Metamorphosing larvae displayed a tendency to concentrate in the lower part of the water column, mainly during the day. Gut contents, analysed for prey identification, fullness index and carbon content, indicated that metamorphosing larvae fed mostly on plankton. Variations in fullness index were observed not only during the day, but also depended on tide and wind-induced mixing conditions. Larvae sampled in mixed spring-tide waters had highly variable carbon estimates, resulting in unclear diel activity. More larvae fed actively at neap-tide, which allowed the observation of a diurnal feeding activity through hourly changes in carbon estimates. It is concluded that immigrating sole were not yet able to settle but prepared themselves for demersal life (i) without undergoing starvation and (ii) by modifying the patterns of vertical distributions. The presence of a larval swimbladder suggests they can adjust their vertical movements, depending on tidal cycles, which could in turn favour coastal accumulation of metamorphosing larvae and pulses of new settlers entering the nursery grounds.

Otoliths imprinting of sole (Solea solea) from the Bay of Biscay: a tool to discriminate individuals from nursery origins?

Sole nurseries are located in the main bays and estuaries of the Bay of Biscay, where juveniles from the same stock concentrate and constitute temporarily isolated groups. This context being favourable for elemental analyses of juvenile otoliths, this study has been initiated with the aim of obtaining environmental imprints of the main nurseries of origin of juveniles recruiting to the adult stock, and of evaluating the relative contribution of these nurseries to the stock. The objectives were to compare (i) the otolith elemental imprints obtained by sampling juveniles in the Loire and the Gironde nurseries, and (ii) two multi-elemental analysis techniques: laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) and solution-based ICPMS (SB-ICPMS) adapted to small volumes. Depending on the fish origin, differences in Sr and Rb concentrations were shown using LA-ICPMS. Comparisons between the antero-dorsal and postero-ventral sites from where otolith material was ablated also suggested a spatial heterogeneity in otolith composition at least for some metals. From linear discriminant analyses, 73 % and 79 % of individuals (bootstrap estimations) were correctly classified with respect to their origin from the composition of the antero-dorsal and postero-ventral areas, respectively. The SB-ICPMS analysis was more powerful, which resulted in an 89 % rate of correct classification from a 2-variable model (Mg and Cd), whereas a 5-variable model (Li, Mg, Rb, Cd, Th) resulted in a 91 % rate of correct classification (bootstrap estimations). These results confirm that sole juveniles from the main estuaries of the French Atlantic coast could be discriminated by the elemental fingerprints of their otoliths.

Comparison between the life cycles of two Soleidae, the common sole, Solea solea, and the thickback sole, Microchirus variegatus, in the Bay of Biscay (France)

The life history of two Soleidae, the common sole, Solea solea, and the thickback sole, Microchirus variegatus, were compared in the Bay of Biscay in an attempt to set out factors which could explain settlement styles known to be different between juveniles of each species. Common sole juveniles had been shown to depend on coastal and estuarine nurseries, and the thickback sole to develop in open-sea nurseries, although the spawning grounds and spawning season of both species overlapped (offshore and at springtime, respectively). For this study, data on adult, juvenile and larva distributions were obtained from cruises carried out in the Bay of Biscay during the last decade. In addition, growth rate of larvae and planktonic interval duration, estimated by means of otolith increment analysis, were compared, as well as literature-derived information on behaviour of larvae. By comparing larval features, it appeared that thickback sole did not obtain the advantage of a slightly longer pelagic life span for an increased dispersal, due to an early shift to benthic behaviour. Other evidence was given by more specifically stated distributions that the further and deeper offshore spawning of M. variegatus, compared to S. solea, was a likely key-factor of the juvenile settlement process. This could explain why the thickback sole, contrary to the common sole, never reach coastal areas and thus settle offshore, in waters deeper than 30 m.

Anomalies in the cephalic area of laboratory-reared larvae and juveniles of the common sole, Solea solea: oral jaw apparatus, dermal papillae and pigmentation

SynopsisThe post-metamorphic common sole, Solea solea, as well as the other Soleidae and Cynoglossidae, are characterized by oral jaw elements without teeth on the ocular side. A relatively large proportion of late sole larvae and early juveniles, routinely reared in the laboratory, showed dentigerous and shape-modified premaxillae and mandibles on the ocular side. Abnormal jaws were associated with dermal papillae and a lack of pigmentation on parts of the ocular side of the head. These features are probably the result of suppressed epigenetic mechanisms normally active during ontogeny. Insights into the morphological evolution of the soleid jaw apparatus are discussed.