Yves Desaunay

Quantitative description of habitat suitability for the juvenile common sole (Solea solea, L.) in the Bay of Biscay (France) and the contribution of different habitats to the adult population

Abstract This study describes the spatial distribution of young-of-the-year sole based on autumnal beam trawl surveys conducted in the Bay of Biscay (France) during a 15-y period. Previous studies showed that habitat suitability for juvenile sole varies according to physical factors such as bathymetry, sediment structure and river plume influence. These factors, which are known exhaustively for the entire Bay of Biscay from static maps (bathymetry and granulometry) or temporal maps based on a hydrodynamic model (the river plume), were used as descriptors in a generalised linear model of habitat suitability in order to characterise the distribution of juvenile 0-group sole according to delta distribution. This model was used to identify the habitats in which juvenile 0-group sole are concentrated. The respective areas of these habitats were determined from a Geographic Information System (GIS), and their respective contribution to the sole population in the Bay of Biscay was calculated in terms of the estimated number of young fish (GIS area×density derived from the model). Despite the great variability of survey data, this quantitative approach emphasises the highly important role of restricted shallow, muddy estuarine areas as nursery grounds of sole in the Bay of Biscay and demonstrates the relation between interannual variations of nursery habitat capacity (with respect to estuarine extent) and sole recruitment.

Relationship between interannual variations of the river plume and the extent of nursery grounds for the common sole (Solea solea, L.) in Vilaine Bay. Effects on recruitment variability

Abstract This study describes the spatial distribution of juvenile sole on the basis of a 10-year beam trawl survey of Vilaine Bay, an estuarine nursery ground within the Bay of Biscay (France). A significant relationship between fluvial discharges in winter-spring and the area covered by high densities of 0-group juveniles was indicative of the favourable effects of freshwater supply on nursery size. The extent of the river plume influences both the larval supply and the size and biotic capacity of habitats in estuarine nursery grounds and determines the number of juveniles produced. The correlation between river flow and sole recruitment for Vilaine Bay was confirmed for the Bay of Biscay stock. As the recruitment of the Bay of Biscay sole stock depends partly on the influence of river plumes on nursery grounds, it appears to be less variable than for other sole stocks affected essentially by larval supply. The balance between the different factors influencing recruitment differs according to the area concerned, and recruitment variability depends on the respective roles of these factors.

Seasonal distribution of young sole (Solea solea (L.)) in the nursery ground of the Bay of Vilaine (northern Bay of Biscay)

Abstract The distribution of young sole was studied in the bay of Vilaine nursery ground as a function of age and season for the 0, 1 and 2-groups. During 1988 and 1989, bimonthly trawl surveys were carried out from the estuary to the 50-m-isobath along a 65-km transect. A regular seasonal pattern demonstrated two combined trends. An offshore movement is observed with increasing age; this general trend is marked by an inshore migration in spring and an offshore one in winter. The fluctuations of the distribution pattern are discussed in relation to sedimentary characteristics, to trophic requirements and to hydroclimatic variations. The main conclusion is that young soles are concentrated inside a closed nursery area. In their third winter some participate in spawning migration and they get mixed, to a certain extent, with newly recruted soles from nearby nursery grounds.

Quality of coastal and estuarine essential fish habitats: estimations based on the size of juvenile common sole (Solea solea L.)

Abstract Survival and growth of early fish stages are maximal in coastal and estuarine habitats where natural shallow areas serve as nurseries for a variety of widely distributed species on the continental shelf. Processes occurring in these nursery grounds during the juvenile stage affect growth and may be important in regulating the year-class strength of fishes and population size. The need, therefore, exists to protect these essential fish habitats hence to develop indicators to estimate their quality. The purpose of the present study was to use the growth of juvenile sole as a means of comparing the quality of coastal and estuarine nursery habitats in the Bay of Biscay (France). These sole nurseries were clearly identified from studies based on trawl surveys carried out during the last two decades. The size of 1-group juveniles at the end of their second summer, as estimated from these surveys, is an indicator of growth in these habitats during the juvenile phase and can be used to compare habitat quality. A model taking into account the role of seawater temperature in spatial and interannual variations of juvenile size was developed to compare growth performance in the different nursery sectors. This study shows that the size of juvenile sole after two summers of life is not density-dependent, probably because the size of the population adapts to habitat capacity after high mortality during early-juvenile stages. Size is on one hand positively related to temperature and on the other hand higher in estuarine than in non-estuarine habitats. This high growth potential of juvenile fish in estuarine areas confirms the very important role played by estuaries as nursery grounds and the essential ecological interest of these limited areas in spite of their low water quality. If a general conclusion on habitat quality is to be reached about studies based on the growth of juvenile fish, it is necessary to use not only an integrative indicator of growth, like size, representative of the intrinsic habitat quality, but also more sensitive and less integrative means, such as otolith increments or caging experiments, which better respond to anthropogenic disturbance. Moreover, it is necessary to take juvenile densities into account.

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.

Movement of sole (Solea solea) in the Bay of Biscay : coastal environment and spawning migration

During the studies on sole recruitment in the Bay of Biscay, transport process attracted considerable attention and led to important research on migration and dispersal. The last step to complete the study of the spatial component of the life cycle of sole in the northern part of the Bay of Biscay is the offshore migration of young adults towards the spawning grounds. the present study deals with this pre-spawning migration. the main question concerns the kind of linkage between nurseries and spawning grounds. The principal results are based on extensive tagging experiments carried out in autumn 1992 in two distinct nursery grounds, Bay of Vilaine and Pertuis (4620 soles of 2-group and older). One year later (January 1994) catches of tagged individuals reached 18.1% and 11.9% of the initial samples, respectively. From September to February a 50-km net displacement of the centre of the population was observed towards the offshore deeper area. A small trend to move onshore appeared in spring. In parallel with these net movements individuals were dispersed around the centre of gravity of the population. This dispersion was higher in a direction parallel to the coast. Individuals from different nurseries were mixed over the same spawning areas of the Bay of Biscay, and some individuals were also captured in and close to the English Channel. Changes in distribution were in phase with environmental changes and some elements concerning topographic preferences also appeared. Some evidence for orientation was observed but random dispersal seemed more important. No particular link between a given nursery and a spawning ground in the vicinity appeared.

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.

Mean age and migration patterns of Anguilla anguilla (L.) glass eels from three French estuaries (Somme, Vilaine and Adour Rivers)

Abstract Glass eels were sampled, according to the 1991 EIFAC Working Party recommendation (Anon., 1991), in three estuaries, about 800 km apart along the French coast. Only preliminary data are presented here for the peak of immigration in February - March 1992. Biometrics, otolith structure and estimates of larval duration lead to several assumptions on the early life dynamics. Results show that: i) the body size, the otolith development and the mean age are not correlated with the latitude; ii) the migration over the continental shelf is not reflected in the otolith proportionally to the shorter distance from the shelf edge to the estuary; iii) marginal structures of the otolith could be related to the delay before entering freshwater.