Mediterranean Fish Farms Research Articles

Culturing the sea cucumber Holothuria poli in open-water integrated multi-trophic aquaculture at a coastal Mediterranean fish farm

The survival and growth of the sea cucumber Holothuria poli were assessed during a 12-month field study when cultured at a commercial fish farm in Malta as part of an integrated multi-trophic aquaculture (IMTA) system. Sea cucumbers were cultured directly below a fish cage at 0 m, E0, at 10 m (E10) and 25 m (E25) distances from the cage and at two reference sites (R1 and R2) located over 800 m away from the fish farm. Mass mortalities were recorded at E0 within the first month of the study due to smothering by settled wastes. All individuals died at one of the reference sites, R1, by the end of the study. After deducting missing sea cucumbers, survival rates at E10 (23%) and E25 (33%) from the fish cage were similar to the remaining reference site (R2) (27%). Stocking density and physical disturbances to the sea cucumber cage setup were also probable cause for the low survival rates. The relative weight gain (RWG) and specific growth rates (SGR) of H. poli varied significantly between sites close to the fish farm and the reference site. The SGR of H. poli at E10 (0.18 ± 0.02% day−1) and E25 (0.20 ± 0.01% day−1) was positive over the whole study period while no average growth was recorded at the reference site (−0.04 ± 0.07% day−1) over the same period. Differences in RWG and SGR were recorded throughout the study. The overall growth observed in H. poli by January was followed by a drop in growth rate across all sites and an increase in SGR at E25 in July. Slower growth rates were observed as water temperature approached 15 °C. The results indicated that the sediments near the commercial fish cage provided an enriched source of food that supported significantly better growth in H. poli. This suggests that H. poli in IMTA might have the potential to uptake organic farm waste and increase aquaculture production, albeit with important considerations for setup design and stocking density.

Ecological risk analysis in marine fish farming: a case study of a seabass (Dicentrarchus labrax) farm located in Moroccan Mediterranean coast

The present study aimed at studying ecological interaction between a fish farm cages located in M’diq bay, in western Moroccan Mediterranean coast, and its local environment. It was carried out in 2013 and 2016 through seabed analysis in terms of organic matter and some nutrients parameters such as Nitrogen and Phosphorus, as well as through monitoring macro-fauna assemblages. The results showed recorded values relatively stable in three years’ interval and have a well-integrated fish farming activity within its ecosystem. Sediment was sandy to sandy shelly types and macro-benthic fauna were composed of six groups, Gastropods, Bivalves, Scaphopoda, Amphipods, Nematoda and Polychaeta. Organic matter and Total Organic Carbon were respectively less than 3.2% and 1.87% and very low comparing with others Mediterranean fish farms sites. Estimation of Nitrogen, Phosphorus and Carbon potential releases of M’diq fish farm gave low data related to its production capacity not exceeding 200 tons by year. Influenced with open sea location and high current state, dispersion pattern of these fish farm discharges seemed to play a key role in keeping lower values. ANOVA analysis showed no significant difference between studied parameters in the five sampling stations comparing with the reference station both on geographical and temporal levels (seasons and years).

Changes of the Mediterranean fish farm sector towards a more sustainable approach: A closer look at temporal, spatial and technical shifts

The Mediterranean is a miniature of the world with 22 countries sharing the its coastline. In the past few decades aquaculture production in Mediterranean showed an increasing trend but brought with it several environmental and socio-economic issues. In order to establish sustainable development, the Mediterranean aquaculture sector has adopted integrated management plans. Within this framework, our aim was to evaluate the state of the Mediterranean aquaculture industry with respect to sustainable growth, focusing mostly on the spatial scale. We used the evaluation framework for ICZM proposed by Billé (2007) that takes into consideration how multiple policies can be accountable for sustainable management practices. In order to identify the different management regimes and trends in the area, satellite images were processed as a diagnostic tool to locate and analyse the characteristics of the Mediterranean fish farms in cages over time. Google Earth historical images were used to map the characteristics of the Mediterranean fish farms in three different periods between 2006 and 2017. The average coastal surface occupied by the fish cages in the Mediterranean was 0.01%; only two countries currently exceed this percentage due to having a large number of cages (Turkey and Greece) and five others due to their limited coastline. In total, 15,499 cages were counted in recent years in the Mediterranean area, 46.5% of them belonging to larger cluster aggregations. Over time, square steel cages have been gradually replaced by larger and technically more advanced circular cages that significantly increased the finfish production rates. To understand the level of integration done by Mediterranean aquaculture we examined four dimensions of aquaculture issues: (i) policy and management; (ii) environment; (iii) economy - industry and (iv) social. State-of-the-art aquaculture policy and management systems were integrated in national legislations, boosting the production, ensuring at the same time better environmental quality from and for the aquaculture. Despite these positive results for the Mediterranean aquaculture industry, other factors, such as social issues related with the general bad image of aquaculture and the conflicts with local stakeholders, seem to inhibit the further development of aquaculture production. Thus, studies like this can identify factors that have favoured the expansion of aquaculture but also the evolving needs of the stakeholders in the context of changing market and social conditions.

Different survival of three populations of European sea bass (Dicentrarchus labrax) following challenge with two variants of nervous necrosis virus (NNV)

Viral Nervous Necrosis (VNN, also called viral encephalo- and retinopathy (VER)), is a widespread disease of marine aquaculture caused by betanodavirus (or nervous necrosis virus - NNV), a segmented positive sense RNA virus, member of the nodaviridae family. VNN affects predominantly marine fish and cause significant losses to the Mediterranean fish farming industry, including the production of European sea bass (Dicentrarchus labrax). Of the four circulating genotypes of betanodavirus, red-spotted grouper NNV (RGNNV) and the reassortant genotype red-spotted grouper/striped jack NNV (RG/SJNNV) are most prevalent in the Mediterranean. Inheritable resistance against VNN has been detected in sea bass, and selective breeding could be a mean to limit this untreatable disease. In the current study, we compare resistance to disease among three populations from the Atlantic Ocean (AT), Eastern Mediterranean (EM) and Western Mediterranean (WM), by challenge trials using both a highly pathogenic isolate of RGNNV and a lower pathogenic reassortant isolate of RG/SJNNV. The survival of the three populations were modelled with a logistic regression, and the odds ratio (OR) of surviving was calculated. The challenge with RG/SJNNV reduced the odds of surviving three-fold (OR = 0.29 [0.07-0.87]), whereas the challenge with RGNNV reduced the odds of surviving 100-fold (OR = 0.01 [0.00-0.03]). Overall, the EM population had 3.32 (1.92–5.86) times higher odds of surviving the challenge than the AT and WM stocks. All survivors were harboring viral RNA in the brain, as demonstrated by RT-qPCR. However, viral RNA levels were in average lower in survivors from the EM population in both challenges, though only significantly lower in the challenge with RG/SJNNV (p < 0.01). The survival results combined with the RT-qPCR results indicate that the EM sea bass population has a natural resistance to disease caused by RGNNV, possibly associated with limited viral entry into and/or replication in the brain.

Mycobacterium pseudoshottsii in Mediterranean Fish Farms: New Trouble for European Aquaculture?

Mycobacterium pseudoshottsii, a slow-growing mycobacterium closely related to M. marinum, has been isolated only in wild fish in the United States and in Japanese fish farms to date. Here, we report cases of mortality in three farmed fish species (Dicentrarchus labrax, Sparus aurata, and Sciaenops ocellatus) caused by M. pseudoshottsii in Italy. Samples underwent necropsy, histology, and culture with pathogen identification based on PCR and sequencing of housekeeping genes (16S rRNA, hsp65, rpoB). Multifocal to coalescing granulomatous and necrotizing inflammation with acid-fast bacilli were observed in the parenchymatous organs, from which M. pseudoshottsii was isolated and identified. Phylogenetic analysis confirmed the results of gene sequencing and allowed subdivision of the isolates into three distinct groups. M. pseudoshottsii poses a potential threat for Mediterranean aquaculture. Its origin in the area under study needs to be clarified, as well as the threat to the farmed fish species.

Description, systematics and ecology of a new tanaidacean (Crustacea, Peracarida) species from mediterranean fish farms

An undescribed species of tanaidacean belonging to the genus Hexapleomera, tribe Pancolini, Hexapleomera bultidactyla sp. nov. was found in fouling community samples from off-coast fish farms cages in the western Mediterranean Sea. The species can be distinguished from other Hexapleomera species by the presence of a ventral apophysis on the dactylus of the chela in males. Other diagnostic characters (in combination) include a male antennule with five aesthetascs, the female with three, the maxillule palp with four terminal setae and maxilliped basis and coxa each with two setae; the male fixed finger with four ventral setae and proximal apophysis, the female chela fixed finger with a proximal triangular apophysis, an apophysis on the coxa of pereopod 1, a pleopod 3 basis with three outer setae, and an uropod of four segments. Although several substrata were investigated, the species was most abundant where the turf formed by Ceramiaceae algae and the hydroid Aglaophenia sp. was dominant. An updated identification key to all the species of Hexapleomera is provided.

Laboratory development of &lt;em&gt;Capitella&lt;/em&gt; sp. A (Annelida: Capitellidae) from a NW Mediterranean fish farm reared under different organic enrichment conditions

The polychaete Capitella sp. A, collected in a NW Mediterranean fish farm (Les Cases d’Alcanar, Tarragona, Spain), was cultured for the first time under experimental conditions with different organically enriched sediments to study the differences in development and growth. The species proved to be dioecious and had lecithotrophic development. Sizes of individuals and duration of the developmental stages varied widely, as in most known species of Capitella. In organically enriched sediments, the juveniles were seen one day after hatching and immature females (i.e. with yellow ovaries) after 52 days. Females may reach maturity (i.e. show white intra-coelomic oocytes) at about 64 days old, and the species had a life span of 167 days. According to its development, Capitella sp. A differs from all known lecithotrophic species of the genus. The results also proved that organically enrichment enhanced growth and survival, whereas lowering food can cause morphological alterations such as reduced size in male genital spines.

Rapid dispersion of escaped meagre (Argyrosomus regius ) from a coastal Mediterranean fish farm

Meagre (Argyrosomus regius) is an emerging species in aquaculture considered a locally absent species in the Western Mediterranean. Little is known about meagre escape incidents from farms. To evaluate escape impacts and develop mitigation measures, knowledge about the behaviour of escaped meagre is necessary. In this study, a rapid dispersion of escaped meagre was observed following a simulated escape incident in a coastal Mediterranean farm, using acoustic telemetry (acoustic tag: AT) and mark-and-recapture techniques (external tag: ET). A small proportion of AT meagre (22.7%) remained within the farm boundaries 24 h after the simulated escape. A total of 84 meagre (9.2%) were captured by local trammel netters nearby the farm facility within the first 48 h after escape. Only two AT individuals (15.3%) were detected by acoustic receivers located in coastal areas within 2 days after dispersal from the farm and eight ET meagre (0.8%) were recaptured along the shoreline during the first week. Two AT individuals (15.3%) were considered to be dead by predation few hours after escape. The rest of the AT individuals (N = 6, 46.2%) left the facility alive, and they were never detected by receivers nor reported back by fishermen. Thus, our results highlight the necessity of establishing escape mitigation strategies at Mediterranean fish farms, i.e. recapture programmes 24–48 h after the escape incident in collaboration with local fishermen, to diminish potential impacts related to meagre escapees.

Husbandry and environmental conditions explain temporal variability of wild fish assemblages aggregated around a Mediterranean fish farm

The aggregation of wild fish around fish farms is spatially and temporally variable in terms of species composition and structure. In general, temporal variability has been less well studied than the spatial aspect of these aggregations. We analyzed temporal variability at differ- ent scales following a hierarchical sampling design, with year, season and time of day as the main factors, with the levels of the latter factor corresponding with feeding intensity at the farm. Envi- ronmental and production variables were considered, such as water temperature, photoperiod, feed supplied, number of stocked fish and stocked biomass. Dominant fish species were analyzed separately to determine possible segregation processes. The most relevant sources of variation in abundance and assemblage structure were seasonal changes among years and hourly changes with different feeding intensities between seasons. Feed supplied was the production variable that best explained the temporal pattern with respect to abundance, and this variable, combined with hours of daylight, determined the assemblage structure. Liza ramada and Sarpa salpa domi- nated the assemblage in winter and were always aggregated around the farm. Boops boops, Ob - lada melanura and Sardinella aurita dominated the assemblage in spring, summer and autumn, showing a marked seasonal pattern. These 3 species, although present around the farm most of the year, displayed periods of distinct maximum abundance corresponding to their reproductive period. This study emphasizes the importance of determining the temporal patterns in abundance of dominant species in order to better understand the interaction between fish farming and wild fish assemblages, and to ascertain the contribution of wild fish to the recycling of aquaculture- derived wastes.

Metals in tissues of seabass and seabream reared in sites with oxic and anoxic substrata and risk assessment for consumers

Twenty-eight metals and elements were measured in the muscle, liver, gills, bone and intestine of farmed seabass and gilthead seabream from four Mediterranean fish farms. The influence of fish species and the effect of environmental conditions on the metal accumulation in fish tissues was investigated. Most concentrations were lower in muscle and higher in liver and bone than in other body tissues. Seabass accumulates more elements in its tissues than seabream. Fish reared in coarse, oxic sites accumulate more elements with higher concentrations in muscle, bone and intestine and with lower concentrations in liver and gills than fish reared in silty, anoxic sites. This may be attributed to feed type and sediment properties. According to the metal pollution index, hazard quotient, selenium health benefit values, carcinogenic risk of arsenic, maximum safe consumption and the permitted limits, the consumption of both farmed species should be considered as safe for human health.

Benthic community response to sediment organic enrichment by Mediterranean fish farms: Case studies

There is a great deal of scientific information about the biogeochemical processes in sediments near fish farms and how those processes may be driven under nutrient enrichment to environmental negative effects, as biodiversity decrease is observed in benthic assemblages. Within an adaptive management framework, a good monitoring program can be used to adjust farm management to avert serious and persistent impacts to the sediment. Macrobenthic community represents a good indicator able to detect environmental effects resulting from organic enrichment and, based on this, was selected to monitor Italian marine fish farms. In order to assess the environmental sediment disturbance resulting from marine aquaculture along the Italian coasts three marine fish farms were considered based on their different environmental features: two located in the Tyrrhenian Sea and one in the Adriatic Sea. The selected cage fish farms produce European sea bass Dicentrarchus labrax (L.), gilthead sea-bream Sparus aurata (L.) and meagre Argyrosomus regius (A.) The study ranges from 2001 to 2012, at different times in each fish farm, and was planned according to the fattening cycles. The samples for benthic community analyses were collected by means of a van Veen grab in selected stations located from 0 to 1000m from the sea cages, the farthest as control stations. In total 134 stations were sampled for more of 20,000 individuals belonging to more than 450 species. Data analyses were performed with statistical methods and biotic indices were calculated. The results showed changes in benthic communities at the stations located just beneath the cages and connected with high cultured biomass. The impacted sites showed loss of biodiversity and increase of opportunistic species such as Capitella Sp. I and Nebalia strausi. Changes in the benthic communities were more or less evident in relation to substrata composition, hydrodynamic conditions and cultured biomass. The observed impact was limited to a few tens of meters around the sea cages of all the three studied fish farms.

Vibrio renipiscarius sp. nov., isolated from cultured gilthead sea bream (Sparus aurata).

Two strains of Gram-negative, facultatively anaerobic, slightly halophilic bacteria, isolated from healthy gilthead sea bream (Sparus aurata) cultured in Spanish Mediterranean fish farms, were different from their closest relatives, Vibrio scophthalmi and V. ichthyoenteri, by phenotypic, phylogenetic and genomic standards. The strains were negative for decarboxylase tests and lacked extracellular hydrolytic activities, but were able to ferment d-mannitol, sucrose, cellobiose and d-gluconate, among other carbohydrates. The major cellular fatty acids were C16:1 and C16:0, in agreement with other species of the genus Vibrio. Their 16S rRNA gene sequences were 98.4 and 97.2 % similar to those of the type strains of V. scophthalmi and V. ichthyoenteri, and the similarities using other housekeeping genes (ftsZ, rpoD, recA, mreB and gyrB) and indices of genomic resemblance (average nucleotide identity and estimated DNA-DNA hybridization) between the isolates and those type strains were clearly below intraspecific levels, supporting the recognition of the strains as members of a separate novel species. Thus, we propose the name Vibrio renipiscarius sp. nov., with DCR 1-4-2T ( = CECT 8603T = KCTC 42287T) as the type strain.

Draft Genome Sequences of Vibrio renopiscarius Strains CECT 8603T and CECT 8604, Two Marine Gammaproteobacteria Isolated from Cultured Gilthead Sea Bream (Sparus aurata).

Vibrio renopiscarius DCR 1-4-2T (CECT 8603T) and DCR 1-4-12 (CECT 8604) were isolated from healthy gilthead sea bream (Sparus aurata) from Mediterranean fish farms (Castellón, Spain). Their draft genome sequences (30 and 44 contigs, respectively) have 4.3 Mbp and a G+C content of 45.2 mol% and contain almost 3,700 protein-encoding genes.

Fish farming and anti-fouling paints: a potential source of Cu and Zn in farmed fish

AEI Aquaculture Environment Interactions Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections AEI 5:163-171 (2014) - DOI: https://doi.org/10.3354/aei00101 Fish farming and anti-fouling paints: a potential source of Cu and Zn in farmed fish Marina Nikolaou*, Nikos Neofitou, Konstantinos Skordas, Ioanna Castritsi-Catharios, Lamprini Tziantziou School of Agriculture, Department of Ichthyology and Aquatic Environment, University of Thessaly, Fytokou St., N. Ionia, 38446 Volos, Greece *Corresponding author: maze55@live.com ABSTRACT: The accumulation of copper and zinc, the basic components of anti-fouling paints, was examined in cultured sea bass Dicentrarchus labrax and sea bream Sparus aurata. Samplings were carried out at 4 eastern Mediterranean fish farms. Two of the farms used nets treated with anti-fouling paints, and 2 used untreated nets. At each farm samples of sediment and fish tissue (muscle, liver and gills) were analysed for heavy metal concentrations. The results showed that while total copper and zinc concentrations in sediments were quite similar in samples collected from the 4 farms, the extractable copper concentration in sediment from farms using anti-fouling treatment was 2 to 3 times higher than from those using untreated nets. Statistical analysis revealed no significant differences for either metal between sediment samples from farms using anti-fouling treatment and those which did not, except for extractable copper concentrations. However, analysis of the biological samples showed that copper concentrations in muscle tissue were lower in samples from farms using untreated nets, with the highest copper concentrations being observed in fish livers from farms using anti-fouling paints. General linear model results indicated significant differences between heavy metal concentrations in fish samples from farms using anti-fouling paints and those which did not. Furthermore, 1-way ANOVA indicated that these differences were confined to liver tissue for zinc, whereas significant differences for copper were seen in all tissues (except for gills of sea bream) for both species studied. These results indicate that the use of anti-fouling paints may be a potential source of metal accumulation in cultured fish. KEY WORDS: Aquaculture · Anti-fouling · Copper · Zinc · Fish · Eastern Mediterranean Full text in pdf format PreviousNextCite this article as: Nikolaou M, Neofitou N, Skordas K, Castritsi-Catharios I, Tziantziou L (2014) Fish farming and anti-fouling paints: a potential source of Cu and Zn in farmed fish. Aquacult Environ Interact 5:163-171. https://doi.org/10.3354/aei00101 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AEI Vol. 5, No. 2. Online publication date: June 18, 2014 Print ISSN: 1869-215X; Online ISSN: 1869-7534 Copyright © 2014 Inter-Research.

Aggregations of bluefish Pomatomus saltatrix (L.) at Mediterranean coastal fish farms: seasonal presence, daily patterns and influence of farming activity

Bluefish, Pomatomus saltatrix (Linnaeus, 1766), is commonly observed close to Mediterranean open-sea fish farms. It usually preys on wild fish that are attracted to farms, but also on farmed fish by biting holes in sea cages net walls. In the current telemetry study, we found that the tagged bluefish stayed close to fish farms during spring and early summer. However, most of the tagged fish disappeared from the farms during autumn, when the sea water temperature dropped. When aggregating at farms, bluefish were present at deeper waters during day time, but moved closer to the surface during night time. In addition, periods when there was operational activity at the farms might influence on daily swimming depth of bluefish. Therefore, bluefish individuals seem to find an optimal foraging habitat around Mediterranean fish farms.

Risk assessment for parasites in cultures of Diplodus puntazzo (Sparidae) in the Western Mediterranean: Prospects of cross infection with Sparus aurata

The sharpsnout seabream Diplodus puntazzo is of interest in Mediterranean fish farming. Disease is an important problem because parasites can spread quickly in culture conditions and fish often develop high parasite burdens. Here we assess the risk that documented parasites pose to the sustainability of D. puntazzo farming. This study specifically considers metazoan and protist parasites recorded from wild and farmed D. puntazzo in scientific literature. Risk assessment studies involve the identification, characterization and qualitative quantification of the risk in question (parasitoses in this case) and the probability of establishment. We considered the parasite species which may be difficult to manage as a priority for research into potential management strategies. Those parasites which could be transmitted from cultures of gilthead seabream (Sparus aurata) were also included in this study. Four groups of parasites represented a risk to D. puntazzo farming, ranging from moderate to high: Ciliophora, Myxozoa, Monogenea and Copepoda. Three parasite species were considered high risk to D. puntazzo cultures: Amyloodinium sp., Cryptocaryon sp. and Enteromyxum leei. These species were responsible for high mortalities in cultures of these and other fish species. In addition Sparicotyle chrysophrii, Caligus ligusticus and Gnathia vorax entail a moderate risk to D. puntazzo Mediterranean farms. No important episodes have been related to caligids and isopods in Mediterranean sparids, nevertheless they should be properly managed to prevent future problems.

Modelling biogeochemical fluxes across a Mediterranean fish cage farm

An integrated approach is described for modelling interactions between off-shore fish cages and biogeochemical fluxes of carbon (C), nitrogen (N) and phosphorus (P). Two individ- ual-based population dynamic models for European seabass Dicentrarchus labrax and gilthead seabream Sparus aurata were coupled with a Lagrangian deposition and a benthic degradation model. The individual models explicitly take into account the effects of water temperature and feed availability on fish growth. The integrated model was tested at a Mediterranean fish farm where a comprehensive set of in situ environmental and husbandry data was available. Tests were performed to compare the predicted and observed total organic carbon (TOC) concentrations in surface sediment under and near fish cages. At a local scale, the model output simulated the spa- tial distribution of 4 biogeochemical indicators, namely: TOC concentrations, C fluxes towards the seabed and C:N and C:P ratios. These allowed the most impacted areas and more extended areas of intermediate organic enrichment to be identified. The model was also used for estimating the mass balance of C, N and P, in order to determine the potential cumulative effects of multiple fish farms in the same area. The C, N and P fluxes among feed, fish and environment were calculated for each fish species over 24 mo of farm activity. The results showed that the amount of dissolved N directly released into the water column in inorganic form (ammonia/urea) was comparable to that deposited on the seafloor in particulate form as uneaten feed and faeces. A larger fraction of P (about 65%) was released as faeces. Results from the integrated model yielded useful informa- tion for assessing the sustainability of an area for aquaculture activities that could be used to pro- vide a scientific rationale for fish farm development in new areas.

Metals in benthic macrofauna and biogeochemical factors affecting their trophic transfer to wild fish around fish farm cages

Benthic macroinvertebrates and wild fish aggregating in the vicinity of four Mediterranean fish farms were sampled. Concentrations of metals and other elements were measured in macrofaunal taxa and in fish tissues (muscle, liver, gills, bone, gonad, stomach, intestine, and stomach content). Biological and geochemical characteristics play an important role in metal accumulation in benthic invertebrates, and consequently in metal transfer to higher trophic levels. Macroinvertebrates accumulated lower concentrations of most metals and elements than their respective sediment, except As, P, Na, Zn and Cd. Elemental concentrations of benthic organisms increased with increasing sediment metal content, except Cd, and with % silt, refractory organic matter and chlorophyll-a of sediment due to the influence of sediment geochemistry on metal bioavailability. Tolerant species were found to accumulate higher concentrations of most metals and elements, except for Cd, than equilibrium species. The ecological and morphological characteristics of the benthic invertebrates can affect the bioaccumulation of metals and elements in macrobenthos. Hg and P were found to increase their concentrations from zoobenthos to wild fish aggregating around fish cages feeding on macrofauna.

Heavy metals, trace elements and sediment geochemistry at four Mediterranean fish farms

Trace element concentrations in sediment were investigated at four fish farms in the Eastern Mediterranean Sea. Fish farms effects were negligible beyond 25–50m from the edge of the cages. Based on elemental distribution, sediments from the farms were separated into coarse oxidized and silty reduced ones. Fish feed is richer in P, Zn and Cd than reference and impacted stations. Comparison among impacted stations and the respective reference stations shows that, in anoxic sediments, all elements had higher concentrations at the impacted stations than at reference stations while in oxic sediments, many elemental concentrations were lower at impacted stations than at reference stations. The behavior of elements and therefore their distribution is affected by changes in sediment grain size, organic content and redox regime. Elements in sediments around fish farms can be clustered into five groups according to these environmental variables. In silty and anoxic sediments, element concentrations were higher than in coarse and oxic ones. Several approaches were used to assess potential sediment toxicity (enrichment factors, geoaccumulation indices, contamination factors) as well as to assess the potential danger to aquatic life (Sediment Quality Guidelines, SQG). Cu, Zn and Fe can cause from threshold to extreme effects on aquatic life in anoxic, fine-grained sediments and As can cause threshold effects in all types of sediment around fish farms. Other elements (Cr, Pb, Mn) can also cause unwanted effects when compounded with elevated background levels.

Trophic structure and energy fluxes around a Mediterranean fish farm

A fish farm in Southeastern Spain was described using an Ecopath mass-balanced model, aimed at characterising its structure, the interactions among ecological groups and the impact of fish farms and fisheries. The model comprised 41 functional groups (including the artificial food input). Comparing consumption and respiration to total system throughput suggests lower energy use in the fish farm, resulting in an accumulation of detritus. The production to total system throughput ratio was low due to the low efficiency of the modelled ecosystem. The connectance and system omnivory indexes were low, typical of a simple or immature food web in terms of structure and dynamics. Artificial food pellets provided energy and nutrients to sustain system function and generate a considerable reserve from which it can draw to meet unexpected perturbations. The study shows the substantial effect the artificial food pellets have on the wild aggregated fishes, which could act to buffer the ecosystem and hence prevent environmental degradation.