Sébastien Lefebvre

Pacific oyster (Crassostrea gigas) feeding responses to a fish-farm effluent

Abstract Bivalves have often been used in integrated fish-farming to enhance the economical value of by-products and/or to improve water quality. However, no physiological studies have dealt with the contribution of the two main sources of organic matter potentially present in a fish-farm effluent: living cells of phytoplankton and detritical matter (fish-faeces and uneaten feed). This study evaluated feeding responses of the Pacific oyster ( Crassostrea gigas ) to a land-based fish-farm effluent comprised mainly of fish-faeces ( Dicentrarchus labrax ) and compared them with those obtained with a diatom ( Skeletonema costatum ). A particular distinction of the main sources of organic matter was made in the experiments, the two diets being evaluated separately and mixed. Feeding responses were evaluated using the biodeposit method with special attention being paid to pre-ingestive processes and absorption efficiency (AE). Experiments were carried out between May and July 1998 in the laboratory at constant water temperature (20°C). Results showed that food quality (nature of particulate organic matter, POM, organic content of the total suspended matter, TSM) had no influence on clearance rates. However, clearance rates were significantly reduced for oysters in advanced stage of gametogenesis. S. costatum was preferentially ingested compared to the fish-faeces when offered in a mixed diet. Absorption efficiency and energy content (56% and 15 J mg −1 , respectively) of the fish-faeces were lower than those estimated for S. costatum (66–70%, 20 J mg −1 ). Nevertheless, these values were high for such a detritic type of food. This study confirms that both concepts of biomechanical filter and economical value improvement of fish-farming waste using oysters are of interest.

Open-air treatment of wastewater from land-based marine fish farms in extensive and intensive systems: current technology and future perspectives

Marine land-based fish farms located in coastal wetlands (salt-pond zones, lagoon banks, etc.), whether extensive or intensive, send farm effluents directly to the sea or after short periods of stocking in retention reservoirs. The aims of our investigation have been to compare the efficiency of current and potential water treatment procedures in open-air. Wastewater retention ponds in commercial farms (Atlantic coasts of France) are efficient in removing up to 1 metric ton of particulate material (dry weight) per hectare and per day (faeces and unconsumed feed), but are inefficient in reducing dissolved wastes, both organic (urea, amino acids, protein) and inorganic (total ammonia nitrogen, phosphates). Forthcoming outdoor technology to treat these forms of waste were examined by trials at different sites: treatment by foam fractionation in extensive systems (Italian fish pond culture), treatment by microalgae production (Skeletonema costatum) and oyster filtration (Crassostrea gigas) in intensive systems (sea bass farm, Dicentrarchus labrax). It can be concluded that foam fractionation coupled with aeration and water circulation is a good way to treat and recirculate wastewaters in extensive systems, but that a multiple treatment combining a retention pond, foam fractionation and microalgae-bivalve filtration, is the best solution to treat all these forms of wastes from intensive systems.

Water treatment of land-based fish farm effluents by outdoor culture of marine diatoms

The feasibility of using fish farm effluents was evaluated as a source of inorganic nutrients for mass production of marine diatoms. Batch cultures were conducted from May to July 1995 in 16-L outdoor rectangular tanks, homogenized by gentle aeration (0.2 Lair L−1 h−1). The effluents from the two fish farms studied were both characterized by high concentrations of inorganic materials (NH4-N, PO4-;P, Si(OH)4-Si) and were shown to support production of marine diatoms. Moreover, periodic measurements of inorganic matter levels in the cultures showed that clearance was efficacious (90% in 3–5 days). Water purification efficiency and culture productivity were further increased through appropriate nutrient balancing. When effluents were limited in silicate, addition of Na2SiO3 induced a significant increase in both diatom biomass and nutrient removal efficiency. In this case, up to 720 000 cell mL−1 were produced dominated bySkeletonema costatum. By contrast, in effluents loaded with silicate, adjustment of the N:P:Si ratio by NH4-N and PO4-P supplementation then gave increased biomass production. In this case, the maximum cell density found was 450 000 cell mL−1, dominated byChaetoceros spp.

Modeling approach of nitrogen and phosphorus exchanges at the sediment–water interface of an intensive fishpond system

Abstract The sediment–water interface in aquaculture ponds is both a sink and a source of various substances that are potentially toxic for cultured species. The sediment to water nitrogen and phosphorus exchanges were studied at the sediment–water interface in an intensive earthpond fish-farm on the French Atlantic coast. This was to define the contribution of diffusive fluxes to the total dissolved nitrogen and phosphorus produced during the 1997–1998 rearing period. Fluxes of particulate organic nitrogen (PON) and particulate phosphorus (PP) were modeled and validated using a set of observations. Diffusive fluxes were modeled using an empirical function of temperature based on in situ sediment porewater concentration profiles. An average of 15% of PON and 10% of PP, produced by fish food waste and fish faeces were sedimented in fishponds. Ammonia and phosphate diffusive fluxes (μmol m−2 h−1) were expressed as a function of temperature (T). J mod NH 4 + =(0.144T+3.49)×10 −6 exp (0.11T+16.81) J mod PO 4 3− =(0.086T+1.8)×10 −6 exp (0.09T+15.76) PON and PP stocks in the sediment decreased during the summer and increased during the winter. However, sedimentation and mineralization–diffusion processes were approximately balanced over the 2-year period. Ammonia and phosphate diffusive fluxes accounted for only 1–4% and 4–15%, respectively, of the total dissolved nitrogen and phosphorus components produced during the rearing period.

Underestimation of chemical contamination in marine fish muscle tissue can be reduced by considering variable wet:dry weight ratios

Whether considered as a risk for human health or as ecological tracers, contaminants concentrations measured in fish muscles are commonly expressed relative to wet or dry mass. Comparison of results required conversion factors (CF) but accurate values are scarce and case-specific. The present paper is aimed at investigating errors linked with the use of the theoretical value. Muscles dry and wet masses were measured in 15 fish species to determine the actual CF. Most CF were lower than the theoretical wet:dry ratio of 5 classically used, with variations at individual and species level. Muscle lipid content (inferred by C/N ratios) was a crucial factor explaining discrepancies, claiming for caution when working with lipid-rich species. The observed variability demonstrated that using the theoretical CF may be inaccurate, when actual CF largely differs from the theoretical value. Dedicated measurement is the better approach when accuracy is required.

Linking life-history traits, spatial distribution and abundance of two species of lugworms to bait collection: A case study for sustainable management plan

Arenicola spp. are marine benthic polychaetes dug for bait by anglers. Without regulation, this activity can lead to the decrease of lugworms population meanwhile affecting the physical characteristics of the beach and the biodiversity. Here, we identified through morphology and genetics two species of lugworms, Arenicola marina and A. defodiens, within a Marine Protected Area of the Eastern English Channel (France). For each species, abundance and spatial distribution were assessed using a stratified random sampling and interpolation at four studied sites, as well as some life-history traits. These data were compared to lugworms collection data to estimate its sustainability and to provide potential management measures. At one site, A. marina was present in large numbers on the higher and middle shore, whereas A. defodiens occupied the lower shore. At the other sites, both species co-occurred on the lower shore, and A. marina individuals were less numerous and lacking recruits. Spawning periods for A. marina occurred in early autumn and in late autumn for A. defodiens. The size at first maturity of A. marina was at 3.8u202fcm of trunk length (between 1.5 and 2.5 years old). One site (Au) appeared in need for management when linking abundance data with bait collection, where harvest of both species represented ∼14% of the total amount of lugworms and was above the carrying capacity of the beach for A. marina. The retail value associated to lugworm harvesting within the MPA was estimated at the same level as the shrimp retail value. Our results highlight the need for some fishery regulations.

Realized niche analysis of phytoplankton communities involving HAB: Phaeocystis spp. as a case study

The link between harmful algal blooms, phytoplankton community dynamics and global environmental change is not well understood. To tackle this challenging question, a new method was used to reveal how phytoplankton communities responded to environmental change with the occurrence of an harmful algae, using the coastal waters of the eastern English Channel as a case study. The great interannual variability in the magnitude and intensity of Phaeocystis spp. blooms, along with diatoms, compared to the ongoing gradual decrease in anthropogenic nutrient concentration and rebalancing of nutrient ratios; suggests that other factors, such as competition for resources, may also play an important role. A realized niche approach was used with the Outlying Mean Index analysis and the dynamics of the species realized subniches were estimated using the Within Outlying Mean Indexes calculations under low (L) and high (H) contrasting Phaeocystis spp. abundance. The Within Outlying Mean Indexes allows the decomposition of the realized niche into realized subniches, found within the subset of habitat conditions and constrained by a subset of a biotic factor. The two contrasting scenarios were characterized by significantly different subsets of environmental conditions and diatom species (BV-step analysis), and different seasonality in salinity, turbidity, and nutrients. The subset L environmental conditions were potentially favorable for Phaeocystis spp. but it suffered from competitive exclusion by key diatom species such as Skeletonema spp., Thalassiosira gravida, Thalassionema nitzschioides and the Pseudo-nitzchia seriata complex. Accordingly, these diatoms species occupied 81% of Phaeocystis spp.s existing fundamental subniche. In contrast, the greater number of diatoms, correlated with the community trend, within subset H exerted a weaker biological constraint and favored Phaeocystis spp. realized subniche expansion. In conclusion, the results strongly suggest that both abiotic and biotic interactions should be considered to understand Phaeocystis spp. blooms with greater consideration of the preceeding diatoms. HABs needs must therefore be studied as part of the total phytoplankton community.