Etude des nourriceries de la baie de Seine orientale et de l'estuaire de la Seine. Synthese des connaissances. Identification d'une nourricerie en estuaire de Seine. Analyse de la fonctionnalite de l'estuaire comme nourricerie

The study carried out from 1995 to 1997 within the framework of "Le Havre" harbour extension project, concerned the description of halieutic populations and the characterization of a coastal nursery in the oriental part of the Seine bay and in the Seine estuary. The main objective was to describe the specific and demographic composition of marine populations as well as their spatio-temporal distribution, and then to assess the biologic and halieutic role of this area. The study also included a research about the dietary needs of fish juveniles living in the estuary. Methodology Five series of surveys were carried out in subtidal and intertidal zones. They were realised in autumn and winter, from September 1995 to March 1997, in an area extending from the coast to a distance of 20 meters depth, between the "Pont de Normandie", Ouistreham and Antifer. Autumn is the most favourable season to assess juveniles abundance in nurseries areas. Nevertheless winter surveys were planed to determine seaward extension of the nursery area at this time of the year. In subtidal zones, two beamtrawls were used (2 m and 3 m wide) with a mesh size of 20 mm. Intertidal zones were sampled with a one meter beamtrawl of 8 mm mesh size. The study area was divided into 13 strata which were considered homogeneous for bathymetry and environment. Between 3 and 6 trawling were carried out in each stratum. At each station all the species were identified, and for each species, animals were counted and weighed. Commercially important fishes were measured. For six species, the age was determined from otoliths (all fishes except for bass) or scales (only bass). Halieutic study Species richness During the whole study, a total number of 110 species (fishes: 50, molluscs: 34, crustaceans: 16) was identified. At each survey more than 70 species, on average, were identified . In autumn, the Seine estuary and its mouth appeared to be the richest area in terms of number of species. In winter the species richness was at its highest in the estuary mouth. The average number of species found in this area during all the surveys varied between 35 and 38. In the other zones the average number varied between 12 and 30. Areas directly influenced by the estuary are characterized by a larger average number of fish species. More than 20 species were identified in this latter area whereas less than 20 species were identified elsewhere. Dominant species and abundance Seastars (Asterias rubens) and Ophiuridea are the most abundant in the whole prospected zone, except in the estuary. The average density of all the different species indicates that species other than fishes are numerically dominant. Their density reaches 180 ind/1000 m2, when fish density varies between 30 and 60. When the data are analysed without taking into account seastars and ophiuroidea, brown shrimp Crangon crangon is numerically dominant. Among the fishes, Trisopterus luscus, Dicentrarchus labrax, Sprattus sprattus and Solea solea are the numerically dominant species. The estuary area differs by the numerical dominance of species like Clupea harengus, D. labrax, P. flesus, S. solea and C. crangon. Other areas, outside the estuary, are characterized by numerically dominant species like Callionymus lyra and Buglossidium luteum which don't live in brackish waters. In autumn, as well as in winter, the estuary and its mouth play a major role for fishes : more than 90 ind./1000 m2 in the mouth and less than 40 ind./1000m2 more seaward (average numbers in autumn). Age and length Most of the fishes captured in the whole area subject to the study are less than 2 years old. The Length structure shows that the majority of fishes are shorter than the legal minimal length. Spatial distributions The analysis of spatial distributions shows that juvenile fishes are preferentially present in under 10 m depths . The distributions of the nine species of juvenile fishes previously mentioned, cover each other. The capacities of the coastal zone - situated between the Orne and Seine estuaries - to be a nursery area - are thus confirmed. Nevertheless, in this area, each species has a distribution influenced by its own biologic requirements. D. labrax and P. flesus are more often observed in brackish and muddy waters, with concentration areas near the « Pont de Normandie ». C harengus is abundant inside the estuary, even if it is also present outside. S. solea is essentially captured on fine and muddy sediments. This species seems to be quite subject to the Seine estuary influence and, to a lesser extent, to the Orne influence. On the contrary, P. platessa and L. limanda hardly enter into the estuary. Brown shrimp, C. crangon, a commercially important species, is also essentially catched at less than 10 m depths, with high concentrations in estuary area. The Observations made in 1981 regarding the same zone, within the framework of « SAUM baie de Seine » are confirmed, though differences of abundance of some species could be high between 1981 and 1995-97. Studies carried out in the Somme bay lead to the same kind of observations : juveniles are predominant in coastal and estuary zones and juveniles are present at less than 10 m depths . The role of the intertidal areas Surveys conducted in the intertidal zone provide essentially qualitative information which are complementary to the ones obtained in the subtidal zone. These two kinds of information cannot be compared because of a difference in methodology. C. crangon, Gobiidae, Cerastoderma edule and some fishes like P. platessa and D. labrax are numerically dominant in the intertidal zone. All fishes (species whose age has been determined) are under one year old and abundant. Though their densities cannot be directly compared to the ones obtained in the subtidal zone, their abundance indicates that the intertidal zone plays an essential role for the juveniles of many species. In this area, juveniles are shorter than the ones of the same group of age, which have been catched in the subtidal zone. That is the confirmation of the major role of the intertidal zone, especially the muddy grounds of the estuary, for the youngest fishes of many species. Interannual variability of abundance An important result from the surveys made during two years is the high interannual variability of abundance of most species, including juveniles of many species like S. solea and D. labrax. This observation has also been made in other sites, particularly in the bay of Somme. This might be considered as a characteristic of coastal nurseries Trophic study Nurseries areas are high productive zones, of primordial importance for biological cycles of many fish species, particularly for diet. In addition to the demographic analysis, the other objective of this study was to identify the diets of some fish. Methodology The stomach contents study concerned 6 species well captured in the Seine estuary, each of them having a behaviour different from the others. We studied 3 flatfishes (S. solea, P. platessa, P. flesus) and 3 round fishes (D. labrax, T. luscus, M. merlangus). To assess the season's influence, samples were collected in autumn and winter. The predator's length influence has been studied by using 3 length classes (<16 cm, from 16 to 21 cm, >=21 cm) for each species. For each predator species, a global study of stomach contents was first carried out. After that, analyses were made for each predator length class and for each season. These analyses concerned prey species numbers, frequencies and biomass. The study was also carried out using a global diet index, MFI "main food item" (Zander 1982, in Rosecchi and al., 1987) which gathers in one value the three parameters, number, frequency and biomass. Primary and secundary preys of each predator could thus be better determined. Results The preys identified belong to various ecological environments. If we consider preys frequencies in stomach contents and global amounts of preys, it appears that a group of preys has a particular importance. It concerns crustaceans (Mysidacea, Corophium volutator, Crangon crangori), Annelida {Pectinaria koreni, Nereis diversicolor) and one Bivalvia (Abra alba). Corophium and Nereis belong to muddy intertidal benthic environment. They are eaten by very young bass, flounders and plaices. Nereis forms a part of the young sole's diet, and, in winter, of flounder and bass (medium and large size sole) diet. These fishes feed on muddy bottoms invertebrates. Pectinaria and Abra alba are characteristic of muddy fine sands of subtidal environment. They are respectively eaten by sole (medium and large size sole), flounder (for Pectinaria) and by flounder and plaice (for Abra). Mysidacea and C. crangon belong to suprabenthic and pelagic environment. They are eaten by bass and pout. Fish , belonging to pelagic environment, are principally eaten by whiting.S. solea, P. platessa and P. flesus have a great relationship with benthic fauna. The smaller bass have also a great affinity for benthic preys of intertidal environment (Corophium). Unlike these, hardly any link has been observed between subtidal round fish and the Seine estuary benthos. M. merlangus feeds on pelagic preys (essentially fish). D. labrax and T. luscus prefer crustaceans, most often C. crangon. For each species, different kinds of diet behaviour are observed according to fish length. There is a particular prey typology to which the diversity of the estuary environments can answer. Subtidal zones contain bentho-demersal resources with low densities (<100 ind./m2 in average) but with great biomass (big animals), whereas in coastal intertidal zones there are small animals with large quantities (several thousands of CorophiumlmT) that will be eaten by the youngest fishes which are abundant in intertidal area. C. crangon is abundant in the estuary. Diet needs of fishes through their different phases of growth can be satisfied by the diversity of brown shrimp l