Jocelyne Marchand

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.

The influence of environmental conditions on settlement, distribution and growth of 0-group sole (Solea solea (L.)) in a macrotidal estuary (Vilaine, France)

Abstract In the Vilaine estuary, the environmental conditions at the onset of the settlement of metamorphosing sole have been surveyed for 5 years. The inshore migration occurs either in early or in late April depending on the hydroclimatic conditions: sole are observed in the estuary when bottom water salinity varies between 25 and 30 S and water temperature is up to 11°C. During spring, their distribution pattern is similar in all years: first, accumulation at the entrance of the estuary, then concentration in the upper parts and finally, colonization of the whole estuarine area by juveniles. Interannual variations in growth rates were observed during their first estuarine phase, in particular when the estuary is transformed into a ‘ria’ with high water temperature and salinity. These biological features are discussed according to ontogenic changes occurring during these early life stages. From field and experimental data and from literature information on sole and other pleuronectiform species, a conceptual model on relationships between environmental parameters and metamorphosis processes in estuarine areas is proposed.

The influence of seasonal salinity and turbidity maximum variations on the nursery function of the Loire estuary (France)

In the Loire estuary which is a macrotidal estuary, the river flow and tides induce large seasonal variations in salinity gradients and in location and density of the turbidity maximum. These characteristics influence colonization patterns developed by juveniles of euryhaline fish populations (structure, distribution, density) and the dynamics of benthic communities which constitute their food resources. In some hydrological conditions, the upstream limit of the nursery function may be controlled by the turbidity maximum which induces anoxic conditions: reduction in densities or mass mortalities may be observed.

Uniform variation in genetic traits of a marine bivalve related to starvation, pollution and geographic clines

Consistent patterns of genetic variation in the marine bivalve Macoma balthica (L.) were found after exposure to low levels of copper, starvation, and along geographic clines. The geographic clines were related to temperature and salinity. Genetic differences were primarily found in the LAP (Leucine aminopeptidase) locus; under stress the frequency of one specific allele, and thereby the heterozygosity, decreased strongly. The degree of changes depended on age and condition of the animals; adult animals showing stronger changes than juveniles. Low concentrations of copper yielded stronger genetic changes than high concentrations. It is concluded that genetic diversity and stress-sensitivity of species change predictably along a gradient in response to environmental harshness. Animals near the southern limit of their range were more sensitive to additional stress.

A comparative study on the relation between copper and condition in marine bivalves and the relation with copper in the sediment

The relation between condition and copper concentration was assessed in three bivalve species (the Baltic clam Macoma balthica, the cockle Cerastoderma edule and the mussel Mytilus edulis) from several Dutch and French estuaries. In general, the copper concentration in the bivalves was negatively related to condition. The slope of this relation was strongest in the Baltic clam M. balthica, with an almost inversely proportional change of Cu concentration at a change of condition, indicating an almost constant Cu content (body burden) per individual, i.e. no elimination or accumulation of Cu. In the mussel M. edulis, the slope of this relation was absent or the weakest, with an almost uniform Cu concentration at a change of condition, indicating an almost proportional elimination (or accumulation) of Cu at a decrease (or increase) of the condition. Thus in Baltic clams the copper body burden is kept at an almost constant level, whereas in mussels the copper concentration is almost constant. In Baltic clams, but not in mussels and cockles, spatial differences were related to copper concentrations in the sediment. It is discussed that mussels maintain a homeostatic Cu concentration within a short time-period (days; copper highly exchangeable by complexation to metallothioneins), whereas the Cu concentration in Baltic clams is more slowly regulated (several weeks; fossilization of copper in mineral granules) and remains on an average in (partitioning) equilibrium with the Cu concentration in the sediment. Therefore, the Baltic clam is a better indicator of sediment copper pollution than the commonly used mussel. In addition, the observed differences between species were compared with differences in their life-strategies. The Cu concentration in the sediment is strongly related to the silt fraction (< 16 μm) of the sediment.

Sensitivity to stress of the estuarine bivalve Macoma balthica from areas between the Netherlands and its southern limits (Gironde)

Abstract Variation in the sensitivity to stress of Macoma balthica was measured in several French and Dutch estuaries. For adult and juvenile Macoma balthica exposed to copper under conditions of starvation, differences in mortality rate, condition, glycogen, burrowing rate and copper content were assessed. No significant differences were observed between adults and juveniles; the influence of treatment and origin was always evident. Animals from the most southern estuaries, Loire and Gironde, near to the speciess southern limit of distribution, showed, in the field, the strongest deviations for the ecophysiological traits measured, and were in the experiments the most sensitive to stress.

Free amino acids as a biochemical indicator of stress in the estuarine bivalve Macoma balthica

The use of free amino acids (FAA) as a stress indicator was assessed during a copper exposure experiment for specimens of the estuarine bivalve M. balthica from a geographical gradient extending from the Netherlands to its most southern limit of distribution in the Gironde, France. Changes in FAA were significant. A higher alanine content at the start of the experiment coincided with a lower mortality-rate during copper exposure. During copper exposure the tissue concentrations of alanine increased. It is concluded that especially the concentrations of alanine, and to a lesser degree the taurine/glycine (T/G) ratio, can be used as stress-indicators for M. balthica. Animals near the southern limit of their distribution showed the strongest deviations from average values. The age of the animals (2-7 years) had no influence on results. [KEYWORDS: free amino acids; Macoma balthica; stress; copper;bio-indicator; geographic dine Mytilus-edulis-l; pollution; proteins; cadmium]

Rheotaxis in larvae and juvenile sole (Solea solea L.): Influence of light conditions and sediment

Abstract An oval channel with an experimental section containing a sediment bed of thin sand was used to examine the effects of current on the orientation and the patterns of behaviour of larvae and juvenile sole. Behaviour of wild and laboratory-reared fish was compared. Experiments were carried out at constant temperature (12 °C) and salinity (35%.) under different light conditions and water velocities. Compared with the random distribution observed in still water, most larvae and juvenile headed into the current from low water velocities; the number of rheopositive animals increased with increasing speeds from a threshold depending on their age and whether they were in the water column or on the bottom. Larvae and small juveniles began to respond at currents of about 1 cm·s −1 ; larvae swimming in the water column began to be swept back at currents above 4 cm·s −1 , although most remained heading into the current. Near the bottom, most juveniles oriented upstream from a speed of ~ 1 cm·s −1 . Often, they swam off and just over the bottom and some buried. Juvenile sole rheotaxis persisted in the dark. Observations, confirmed by statistical analysis, showed that light conditions modify the relation between rheopositive responses and current speed. The percentage of rheopositive responses was higher in dim light or total darkness than under bright light. The implications of these observations for fish behaviour in the field are considered. The experiments indicate that the effects of current, combined with those of light and endogenous rhythmicity may control larvae and juvenile sole migrations from spawning areas to coastal areas. Nevertheless, swimming activity and orientation with respect to currents differ between day and night and are probably related to vertical, and mainly nocturnal, migrations.

Rheotaxis in juvenile sole solea solea (L.): Influence of salinity and light conditions

Abstract The effects of water currents, salinity and light regime on the orientation and swimming activity were studied in juvenile wild and hatchery-reared sole of two size ranges (10–20 mm; 21–30 mm) in an oval flume with a straight working section and a thin bed of fine sand. Short-term experiments using increasing water velocities were carried out at low salinities (S=26, 18 and 11) similar to those found in estuarine areas where recently metamorphosed juveniles settle and develop. Long-term experiments (24 h) were carried out with constant water velocities (3 cm·s −1 and 5 cm·s −1 in juveniles of 10–20 mm and 21–30 mm, respectively) and 4 water salinities (S=35, 26, 18 and 11). The behaviour of juvenile sole was dependent on current speed regardless of light conditions. In the short-term experiments, the number of rheopositive fish increased with increased current. At low salinities, regardless of the origin and the size of the animals, the relationship between water velocity and rheopositive reactions was not significantly different under different light conditions. In long-term experiments, most juveniles remained oriented against the current during the nycthemeral cycle but the percentage of rheopositive-responding fish was higher during the dark phases or in total darkness than during the illuminated phases or under constant illumination. The interrelations between current, salinity and light are discussed and a hypothesis is proposed to explain the migration of juvenile sole towards coastal areas and estuaries.

Seasonal dynamics of the zoobenthic communities in the mesohaline zone of the Loire estuary (France)

The application of correspondence factorial analysis to four types of sediment in the upper reaches of the Loire estuary enabled the significance of environmental factors in the spatial distribution, abundance and seasonal dynamics of the biotic communities to be determined. In coarse sand, the communities are stable but species are few in number and population densities are low. In muddy sediments (from muddy sand to fluid mud) population densities are high but undergo seasonal fluctuations due to changes in the hydrological regime (especially the dissolved oxygen regime). This is true for the composition of the biotic communities. The oligohaline phase, in which the fluvial element is predominant, is characterized by abundance maxima due to freshwater species. The mesohaline phase, in which the marine element is predominant, exerts a restrictive action on population densities; certain species are supplanted by others, their development being governed by the quality of the environment.