The food and growth of 0-group flatfish on nursery grounds in the Clyde Sea Area

In early summer plaice (Pleuronectes platessa L.) in the Clyde Sea Area eat Tellina siphons and spionid palps. As they grow, they take progressively larger siphons, fragments of spionids and some crustaceans. In late summer they eat entire spionids and other sedentary polychaetes. There is no significant predation on fish. The common dab [Limanda limanda (L.)] took a wider range of prey species than the plaice, but also depended mainly on polychaetes, crustaceans and bivalves. Depth distribution may limit both intra- and interspecific competition between these fish and also influence the choice of prey. The results from the main nursery areas of Irvine and Ayr Bays were similar to those from the other bays throughout the Clyde Sea Area and support earlier work at Firemore Bay in north-west Scotland and in the Dutch Wadden Sea.The growth rate of the fish increases during the early summer and may be related to increasing food availability, feeding experience of the fish and longer day length. Growth rate between areas appeared to be correlated with benthic productivity. 0-group plaice in Irvine, Ayr and Firemore Bays grew to different mean lengths by the end of the summer growth season, being 82 mm, 73 mm and 61 mm, respectively, for the period 1972–1974. This agrees with the known differences in standing stocks of intertidal macrobenthos in these bays (17·0, 11·1 and 1·3 g m^?2 dry weight, respectively). The significance of this in relation to survival is discussed. Offshore migration of the larger fish in the autumn may result in apparent slowing of growth rates prematurely in inshore areas. The year-class strength depends more on the food availability in the nursery areas than on the number of plaice that settle on the bottom.     

Anthropogenic disturbance on nursery function of estuarine areas for marine species

Estuaries serve as nursery grounds for many marine fish species. However increasing human activities within estuaries and surrounding areas lead to significant habitat loss for the juveniles and decrease the quality of the remaining habitats. This study is based on the data of 470 beam trawls from surveys that were conducted in 13 French estuaries for the purpose of the European Water Framework Directive. It aimed at testing the effects of anthropogenic disturbances on the nursery function of estuaries. With a multispecific approach based on ecological guilds, two fish metrics, abundance and species richness of Marine Juvenile migrant fishes, were used as proxies for the estuarine nursery function. Indices of heavy metal and organic contaminations were used to estimate anthropogenic disturbances impacting these estuaries. Fish metrics were described with statistical models that took into account: (a) sampling protocol, (b) estuarine features and (c) contamination. The results of these models showed that the fish metrics highly depend on the sampling protocol, and especially type of gear, depth and salinity, which highlights the necessity of considering such metrics at the sampling (trawl haul) scale. Densities and species richness of Marine Juvenile fishes appeared to be strongly and negatively correlated to contamination indices. These results are consistent with the hypothesis that human disturbances impact the nursery function of estuaries. Finally, the densities of Marine Juvenile migrant species appeared as a potential robust and useful fish indicator for the assessment of the ecological status of estuaries within the Water Framework Directive.     

Recruitment patterns of young-of-the-year mugilid fishes in a West African estuary impacted by climate change

With the persistence of the sub-Saharan drought since the 1970s, the Sine Saloum estuary (Senegal) – the second largest coastal Biosphere Reserve of West-Africa – has become an “inverse estuary” and hypersaline (salinity > 60) in its upstream part. A one-year survey was conducted from April 2007 to March 2008 at eight sites distributed along the salinity gradient, to investigate the recruitment patterns of young-of-the-year mugilids in such an impacted ecosystem. Fishes were sampled monthly with a conical net and a beach seine in salinities ranging from 31 to 104. Samples were identified to the species level. For the smallest individuals (<20 mm SL) a PCR–RFLP technique, developed on the mitochondrial 16S ribosomal RNA region, was used for identification. A total of 8438 juveniles belonging to six of the eight species of mugilids known for the tropical Eastern Atlantic were collected: Mugil bananensis, Mugil cephalus, Mugil curema, Liza dumerili, Liza falcipinnis and Liza grandisquamis. One species, L. dumerili, represented 89% of the total catch. Length–frequency distributions revealed that M. cephalus and L. dumerili preferentially recruited during the dry season whereas the recruitment of M. curema, M. bananensis and L. falcipinnis generally occurred during the wet season. Minimal size at recruitment ranged from 9 to 19 mm SL depending on the species, the smallest size being that of L. dumerili. Despite the general salinity increase in the estuary, most parts of the Sine Saloum were suitable for the juveniles. Only the hypersaline area in the uppermost part of the estuary presented very low fish abundance for all species. According to the species, small recruits (12–20 mm SL) were collected at salinities up to 47–78, suggesting that osmoregulatory capacities had been gained early during ontogenesis, possibly resulting from an adaptation of these populations to changing environmental conditions.