Shallow Soft Bottom Areas Research Articles

A quantitative estimate of the function of soft-bottom sheltered coastal areas as essential flatfish nursery habitat

Essential fish habitat suitability (EFHS) models and geographic information system (GIS) were combined to describe nursery habitats for three flatfish species (Solea solea, Pleuronectes platessa, Dicologlossa cuneata) in the Bay of Biscay (Western Europe), using physical parameters known or suspected to influence juvenile flatfish spatial distribution and density (i.e. bathymetry, sediment, estuarine influence and wave exposure). The effects of habitat features on juvenile distribution were first calculated from EFHS models, used to identify the habitats in which juvenile are concentrated. The EFHS model for S. solea confirmed previous findings regarding its preference for shallow soft bottom areas and provided new insights relating to the significant effect of wave exposure on nursery habitat suitability. The two other models extended these conclusions with some discrepancies among species related to their respective niches. Using a GIS, quantitative density maps were produced from EFHS models predictions. The respective areas of the different habitats were determined and their relative contributions (density × area) to the total amount of juveniles were calculated at the scale of stock management, in the Bay of Biscay. Shallow and muddy areas contributed to 70% of total juvenile relative abundance whereas only representing 16% of the coastal area, suggesting that they should be considered as essential habitats for these three flatfish species. For S. solea and P. platessa, wave exposure explained the propensity for sheltered areas, where concentration of juveniles was higher. Distribution maps of P. platessa and D. cuneata juveniles also revealed opposite spatial and temporal trends which were explained by the respective biogeographical distributions of these two species, close to their southern and northern limit respectively, and by their responses to hydroclimatic trends.

Distribution and quality of plaice ( Pleuronectes platessa) nursery grounds on the Swedish west coast

Plaice ( Pleuronectes platessa) has spatially restricted nursery grounds located in shallow soft bottom areas, where the nursery areas only make up a small fraction of the species distribution range. The importance of different coastal areas for recruitment is expected to depend on the quality and size of the nursery grounds. This paper describes the geographical extension of plaice nursery grounds at a regional level along the Swedish west coast. Densities of juvenile plaice were used as the response variable of habitat quality and were compiled based on records from autumn sampling in the various regions. Different aspects of nursery ground quality were related to the density of 0-group plaice, to evaluate the contribution of different factors to the geographical pattern observed. Larval supply was found to be the most important component of nursery quality showing a close relationship with the density of juvenile plaice on a regional scale. The relative contribution of plaice recruits from Swedish nursery grounds to the Skagerrak/Kattegat stock was evaluated using historical data. Swedish nursery grounds were estimated to contribute 77% of the recruits to the adult stock in the area.

Disentangling the effects of El Niño on a population of the polychaete <i>Sigambra bassi</i> in the Bay of Ancón, Peru

Abstract. The macrobenthic community in shallow soft-bottom areas in the Bay of Ancón, Peru, is characterized by low biodiversity due to low oxygen concentrations. During El Niño (EN) events, higher water temperature and higher concentrations of dissolved oxygen induce a temporary increase in biodiversity. However, the structure and dynamics of the emerging macrobenthic community and populations, especially the polychaete Sigambra bassi, vary strongly among events. The reasons for this variation are poorly understood, in particular the relative influence of abiotic versus biotic factors. To disentangle how abiotic and biotic factors influence the different responses of the population of S. bassi, population models were developed based on detailed long-term monitoring data, which include four El Niño events. The results show that S. bassi abundances are favored by abiotic environmental conditions during EN, namely high temperature and dissolved oxygen concentration, but these abiotic effects are modulated by different biotic processes. In two EN events with relatively similarly high temperature anomalies (EN 1982-1983 and 1997-1998) different biological interactions (competition, predation, facilitation) and different species composition of the community resulted in different responses of the population of S. bassi.

Seasonal and inter-annual variation in occurrence and biomass of rooted macrophytes and drift algae in shallow bays

Submerged rooted macrophytes and drift algae were studied in shallow (0–1 m) brackish soft-bottom bays in the Åland Islands, N Baltic Sea, in 1997–2000. The study was performed by aerial photography and ground-truth sampling and the compatibility of the methods was evaluated. The study provided quantitative results on seasonal and inter-annual variation in growth, distribution and biomass of submerged macrophytes and drift algae. On an average, 18 submerged macrophyte species occurred in the studied bays. The most common species, by weight and occurrence, were Chara aspera, Cladophora glomerata, Pilayella littoralis and Potamogeton pectinatus. Filamentous green algae constituted 45–70% of the biomass, charophytes 25–40% and vascular plants 3–18%. A seasonal pattern with a peak in biomass in July–August was found and the mean biomass was negatively correlated with exposure. There were statistically significant differences in coverage among years, and among levels of exposure. The coverage was highest when exposure was low. Both sheltered and exposed bays were influenced by drift algae (30 and 60% occurrence in July–August) and there was a positive correlation between exposure and occurrence of algal accumulations. At exposed sites, most of the algae had drifted in from other areas, while at sheltered ones they were mainly of local origin. Data obtained by aerial photography and ground-truth sampling showed a high concordance, but aerial photography gave a 9% higher estimate than the ground-truth samples. The results can be applied in planning of monitoring and management strategies for shallow soft-bottom areas under potential threat of drift algae.

The separate and combined effects of epibenthic predation and presence of macro-infauna on the recruitment success of bivalves in shallow soft-bottom areas on the Swedish west coast

Previous studies have shown a high year-to-year variability in recruitment success of bivalves. Especially after mild winters recruitment was generally low, but the lower egg production after mild winters in Macoma balthica could only explain 7% of its recruitment variance. In the present study, I tested the hypothesis that the combined effect of a high predation pressure together with a high abundance of adult macrofauna contributes to an explanation of the low recruitment success of bivalves after mild winters. In field experiments in shallow soft-bottom bays at the Swedish west coast, adult benthic fauna (mainly consisting of lugworms Arenicola marina and cockles Cerastoderma edule) was removed and predators (mainly shrimps Crangon crangon and crabs Carcinus maenas) were excluded in some plots/cages, whereas in other plots/cages high densities of adult lugworms or cockles and predators were present. Both the absence of adult macrofauna and the absence of predators increased recruitment success, but the effect of the combined absence of adult macrofauna and predators enhanced recruitment success even more. The combined presence of high macrofauna densities and high predation pressure reduced the recruitment success for Mya arenaria by about 80% compared to the situation in which adult macrofauna and predators were absent. For C. edule, spat densities the reduction was nearly 90% and for Tellinacea spat even about 95%. Thus failure in recruitment success after a mild winter can to a large extent be explained by the presence of high densities of adult macrofauna combined with high predation pressure.

Spatial and temporal variations of fish assemblages in a shallow Mediterranean soft-bottom area (Gulf of Fos, France)

The fish assemblages of two shallow soft-bottom areas located in the Gulf of Fos (NW Mediterranean) were studied by means of 57 trawl surveys carried out on both seasonal and diel scales between 1983 and 1985. A total of 47 fish species was recorded, with a mean density and biomass of 650 individuals ha –1 and 2.67 kg ha –1, respectively. Most of the fluctuations observed were on the spatial scale. Strong differences in fish assemblages were observed between the two areas, with both higher species richness and biomass in the outer part of the gulf (La Gracieuse), while a higher density of individuals characterised its inner part (Bay of Carteau). This spatial pattern was due to a small number of species, such as the gobiids Gobius niger and Pomatoschitus minutus in Carteau, and the flatfish Arnoglossus laterna, Buglossidium luteum and Solea solea in La Gracieuse. These differences were probably induced by differences in the hydrological and benthic characteristics of the two study areas. Decreases in species richness, density and/or biomass were observed in both areas over the study period, although this trend was not significant in all cases. These decreases may have been linked to interannual variations in the mean flow rate of the Rhône River (–28.1% between 1983 and 1985), which might have induced a decrease in the density of benthic invertebrate populations. We suggest that a similar reaction occurred in fish populations, as most of them are known to feed on these invertebrates. Diel patterns did not reveal any significant trends. However, an increase in density and biomass at sunset, and a decrease before sunrise were observed in summer in both areas, and the presence of certain specific species was recorded in catches at determined hours of the diel cycle. These observations were likely due to the activity cycles (diurnal vs. nocturnal) of the fish species.

The impact of fish predation on shallow soft bottoms in brackish waters (SW Finland); an experimental study

The impact of fish predation on shallow soft bottoms was studied by field and laboratory experiments. The field manipulations (caging experiments) were carried out in 2 shallow (1.5 m) bays in the northern Baltic Sea. An a priori hypothesis that fish do not structure macrozoobenthic communities in shallow soft-bottom areas was tested. Short-term aquarium experiments clearly showed that perch ( Perca fluviatilis L.), roach ( Rutilus rutilus L.) and ruffe ( Acerina cernua (L.)) are all effective predators on benthic macroinvertebrates ( Corophium volutator) Pallas and Nereis diversicolor O.F. Müller) under laboratory conditions. Thus fish can be considered a potential structuring force on benthic communities in shallow soft-bottom areas. Caging experiments of one to two month's duration (partial and total exclosure of fish and enclosure with perch) induced only minor changes in the benthic community. These results support the a priori hypothesis. One of our main conclusions is, however, that fish predation and other biotic interactions (competition, disturbance, etc.) together with abiotic factors play some role in the structuring processes. Multilevel testing is needed in studies on biotic interactions in the marine environment. Results from a specific environment should not automatically be applied to other kinds of habitats.