Fish Farm Effluents Research Articles

Artificial aeration to increase pollutant removal efficiency of constructed wetlands in cold climate

In horizontal subsurface flow constructed wetlands, oxygen availability, which is frequently low in summer, may be even more limiting in winter when the plants are dormant. We tested the contribution of artificial aeration on pollutant removal in summer and winter, with a combination of planted, unplanted, aerated and non-aerated mesocosms treating a reconstituted fish farm effluent. Artificial aeration slightly enhanced TSS removal in all seasons regardless of treatment, probably by maintaining empty space in the head part of the gravel bed. In winter, the reduction in COD removal in non-aerated mesocosms compared to summer was totally compensated for in aerated mesocosms, in both planted and unplanted units. Artificial aeration improved summer and winter TKN removal for unplanted units, but the additional aeration did not fully compensate for the absence of plants, which suggests that the role of macrophytes goes beyond the sole addition of oxygen in the rhizosphere. Artificial aeration also improved TKN removal in planted units, but to a lower extent than for unplanted units. Our results suggest that artificial aeration represents a promising approach to improve removal efficiency in horizontal subsurface flow constructed wetland (HSSFCW), especially for fresh water fish farms in cold climate, where artificial aeration is readily available.

Reduction of land based fish-farming impact by phytotreatment pond system in a marginal lagoon area

Intensive land-based fish farming facilities discharged wastewaters into an inlet of the Orbetello lagoon (Southern Tuscany, Italian West Coast), producing large blooms of the green macroalga Ulva rigida C. Ag., accompanied by dystrophic crises. Enclosing a part of this critical inlet resulted in a new pond for phytotreatment of the fish-farming effluents. Between June 1998 and May 1999 a trial study was carried out, during the daylight, in order to: a) define the changes in the wastewater physico-chemical parameters after discharge through the phytotreatment, b) quantify the removal of nitrogen and phosphorus, c) estimate the efficiency of U. rigida biomass produced within the pond in reducing wastewater nutrient loads. During the test period the water physico-chemical parameters were recorded; water dissolved nitrogen (N) and phosphorus (P), Ulva thalli total N and P (UTN, UTP) and macroalga biomass were determined. The results indicated that: 1) the phytotreatment system affected greatly pH and DO values and it reduced thermal impact on the lagoon; 2) the wastewater inorganic nutrient loads were reduced significantly while dissolved organic components (DON and DOP) were not meaningfully lowered; 3) the diurnal nutrients removal was not directly due to U. rigida uptake because the biomass produced annually stored only 1.63% and 0.91% of the dissolved inorganic N (DIN) and P (SRP) inflow values, respectively. Inefficiency of phytotreatment action was probably due to the drop of U. rigida nutrient uptake owing to high macroalgae density inside the pond. Therefore, other processes must be taken into account to explain N and P abatements, such as ammonia volatilization and nitrification/denitrification for N, and the precipitation of insoluble compounds for P.

The effects of light and temperature on the photosynthesis of the Asparagopsis armata tetrasporophyte ( Falkenbergia rufolanosa), cultivated in tanks

The integrated aquaculture of the tetrasporophyte of Asparagopsis armata Harvey ( Falkenbergia rufolanosa) using fish farm effluents may be viable due to the species high capacity of removing nutrients and its content of halogenated organic compounds with applications on the pharmaceutical and chemical industries. In order to optimize the integrated aquaculture of F. rufolanosa, we followed the daily variation of the potential quantum yield ( F v/ F m) of PSII on plants cultivated at different biomass densities and different total ammonia nitrogen (TAN) fluxes to check if they are photoinhibited at any time of the day. Moreover, the photoinhibition under continuous exposure to highly saturating irradiance and its potential for subsequent recovery in the shade was assessed. The potential for year round cultivation was evaluated by measuring rates of O 2 evolution of plants acclimated at temperatures ranging from 15 to 29 °C, the temperature range of a fish farm effluent in southern Portugal where an integrated aquaculture system of F. rufolanosa was constructed. Photoinhibition does not seem to be a major constrain for the integrated aquaculture of F. rufolanosa. Only when cultivated at a very low density of 1.5 g fresh weight (FW) l − 1 that there was a midday decrease in maximal quantum yield ( F v/ F m). At densities higher than 4 g FW l − 1 , no photoinhibition was observed. When exposed to full solar irradiance for 1 h, F. rufolanosa showed a 33% decrease in F v/ F m, recovering to 86% of the initial value after 2 h in the shade. A midday decline of the F. rufolanosa F v/ F m was also observed under the lowest TAN flux tested (∼6 μM h − 1 ), suggesting that this fast and easy measurement of fluorescence may be used as a convenient diagnostic tool to detect nutrient-starved unbalance conditions of the cultures. Maximum net photosynthesis peaked at 15 °C with 9.7 mg O 2 g dry weight (DW) − 1 h − 1 and remained high until 24 °C. At 29 °C, the net oxygen production was significantly reduced due to a dramatic increase of respiration, suggesting this to be the species' lethal temperature threshold. Results indicate that F. rufolanosa has a considerable photosynthetic plasticity and confirm it as a good candidate for integrated aquaculture at temperatures up to 24° C and cultivation densities of at least 5 g FW l − 1 . When cultivated at these densities, light does not penetrate below the first few centimetres of the surface zone. Plants circulate within the tanks, spending around 10% of the time in the first few centimetres where they are able to use efficiently the saturating light levels without damaging their photosynthetic apparatus.

Preliminary investigation on the enrichment of heavy metals in marine sediments originated from intensive aquaculture effluents

In the last decades, marine aquaculture has experienced an important development around the world. One of the main environmental impacts of these activities is the high amount of particulate matter discharged, which comes from uneaten food and faeces. Although the concentration of heavy metals in these particles is relatively low, the total amount of solids is so high that the accumulation of metals in the sediments may become an important problem. This fact has been mentioned in previous works, but it was not studied in depth. In this work, we demonstrate the accumulation of selected heavy metals (Cu, Zn, and Pb) and organic matter (estimated as Loss on Ignition, LOI) in marine sediments as a consequence of intensive marine aquaculture. The study was carried out in an arm-of-the-sea of the Bay of Cádiz (SW Spain) where several intensive marine aquaculture facilities are located. The data obtained were statistically treated to determine whether the spatial and temporal gradients in heavy metals and organic matter in sediments were related to aquaculture. Although the area is relatively pristine in regards to heavy metals, aquaculture activities have increased metals (Zn, Cu, Pb) and organic matter concentrations significantly. The average enrichments were: 140%, 362%, 97% and 445%, respectively. The results suggested that trace metals' enrichment in the sediments may be attributed to the fish farm effluents, although metal concentrations are not likely to cause harmful effects in the marine ecosystem.

δ15N as a natural tracer of particulate nitrogen effluents released from marine aquaculture

The flow of particulate nitrogen from marine net pen fish farm effluents to the surrounding biofouling community was quantified by means of stable isotopes of nitrogen. Plastic mesh substrates were deployed at 8 m depth near a sea bream fish farm and at a nearby reference site in the northern Gulf of Aqaba (Red Sea) to assess whether natural fouling organisms could sequester substantial quantities of farm-derived particulate nitrogen waste. A mixing equation, incorporating differences in nitrogen stable isotope composition, δ15N, between particulate organic matter (“source”) and fouling organisms (“sink”) at the fish farm and reference site, was used to estimate the amount of farm-derived nitrogen that was incorporated by the fouling community. Among the conspicuous fouling organisms examined, sponges, tunicates and polychaetes showed greatest uptake of fish farm N, where the mean fractions of farm-derived N estimated over the 2-year period of observation were 19±7, 22±6 and 31±8% of total organisms’ N content, respectively, with maximal recorded seasonal values of 68, 85 and 57%, respectively. Mean N uptake by mixed fouling communities (conspicuous + cryptic organisms) was as much as fivefold higher than that calculated for the sum of conspicuous taxa, suggesting that the retention efficiency is greater in mixed than in mono-specific biofouling communities.

Integration of consumer-targeted microalgal production with marine fish effluent biofiltration – a strategy for mariculture sustainability

EU regulations recommend effluent treatment and nutrient recycling for aquaculture sustainability, so a study was undertaken to provide base-line data for the integration of commercial fish-farm effluents with the production of microalgae. The project relates to a specific bivalve consumer (Tapes decussatus) and biofiltration. Effluent inorganic nutrient composition was assessed and evaluated as culture media for Phaeodactylum tricornutum, Tetraselmis suecica and Tetraselmis sp. Optimization of the microalgal compartment included studies on preparation of a simple medium, nutrient or dilution rate manipulation and nutrient removal. Cell harvest was increased chiefly by N correction (6-fold for Tetraselmis sp.) and semi-continuous or continuous operation (by a factor of 3 to 11). Nutrient removal efficiency was high for ammonium and nitrite-nitrogen (80–100%), depending on species, nutrient ratio (Si correction for P. tricornutum) and culture regime for nitrate (41–100%) or phosphorus (21–99%). Data obtained under cyclostat cultivation (yields of 1.38 and 0.50×106 P. tricornutum or Tetraselmis sp. cells mL−1 d−1 and nutrient uptake rates of 2.32 mg N L−1 d−1 and 0.96 mg P L−1 d−1) were used to show clam production and simultaneous wastewater treatment possibilities through the proposed fish-microalgae-clam integrated aquaculture system.

Influence of sewage inputs and fish farm effluents on dissolved nitrogen species in a chalk river

Influence of sewage inputs and fish farm effluents on dissolved nitrogen species in a chalk river

Survivorship of coral juveniles in a fish farm environment

Intensive fish farming is an emerging coastal activity that can potentially enhance sedimentation and promote eutrophication in fringing coral reefs. Here, we investigate the effect of fish farm effluent on the juvenile survivorship of the reef-building coral Seriatopora caliendrum. One-month old juvenile corals (on terracotta tiles) were deployed in fish farm and reference (reef) sites in Bolinao, the Philippines at a depth of 2 m. After forty days, no survivor was recovered in the fish farm, while survivorship was low (11%) in the reference site, with the survivors’ growth rate at 3.3 polyps mo −1 or 1.3 mm 2 mo −1. The fish farm deployed tiles were covered with muddy sediment and were colonized by barnacles, whereas those in the reference site were overgrown by a short stand of filamentous macroalgae. Environmental monitoring revealed higher nutrient levels (ammonia and phosphate), sedimentation rate, and organic matter flux, as well as diminished water transparency and dissolved oxygen levels in the fish farm compared to the reference site. Hence, intensive fish farming offers a suite of physical, chemical and biological modifications of the coastal marine environment which have a detrimental effect on the survivorship of coral juveniles.

Influence of Sewage Inputs and Fish Farm Effluents on Dissolved Nitrogen Species in a Chalk River

Nitrate, ammonium and dissolved organic nitrogen (DON) concentrations were measured in the lower reaches of the River Test in Hampshire to assess the impact of two effluent point sources. An 18-month survey from July 2001 to December 2002 was carried out at locations above and below fish farm inputs and a sewage effluent outfall. The concentration of nitrate exceeded 400 µM at the four sites on all sample dates. A consistent increase in ammonium concentration was measured downstream of the fish farm, but not the sewage effluent input. DON samples collected downstream of the fish farm did not show a consistent increase in concentration, compared with the upstream sampling location, whereas the sewage effluent input at Romsey was a point source of DON to the River Test with a mean increase of 15 µM. Nitrate was the major component of dissolved nitrogen in this aquifer-fed chalk-bed river system, with up to 10% being DON and ammonium comprising <1%.

Outdoor phytoplankton continuous culture in a marine fish–phytoplankton–bivalve integrated system: combined effects of dilution rate and ambient conditions on growth rate, biomass and nutrient cycling

Natural phytoplankton populations were cultured in outdoor continuous cultures using fish-farm effluents as the source of nutrients. The dilution rate was assumed to be the integrating factor of phytoplankton growth and biomass development (flux and stock). In this context, the combined effects of (i) dilution rates of the outdoor culture and (ii) ambient conditions were tested on phytoplankton growth, biomass and cycling of the major nutrient elements (C, N and P). Experiments were carried out in outdoor polyester tanks (0.7 m deep), homogenised by gentle aeration. Si/P ratio was balanced at around 5 in the inflow in order to induce diatom domination while maintaining high N and P assimilation by phytoplankton. Nutrient cycling was assessed through analyses of the different forms of particulate and dissolved nutrients in the inflow and the outflow. Culture dilution rates determined the longevity of the culture and the assimilation efficiency of nutrients. Dissolved phosphorus was the most limiting nutrient. The optimal dilution rate was approximately 0.5 day −1 at 10 °C and 1.5 day −1 at 20 °C with a mean diatom biomass of 9 μM P. Under these conditions, 80% of the dissolved nutrients provided to the tanks were transformed, a production of 8 g C m −2 day −1 and an assimilation rate of 0.3 g P m −2 day −1 were recorded. Assimilation by diatoms was the major pathway of nutrient cycling. During the experiment, a bottom sediment developed progressively and this also played an important role in denitrifying the excess dissolved nitrogen in the fish-farm effluent. However, the results showed that diatom biomass can collapse and we hypothesize that this was the consequence of an increase in cellular sinking rates due to cell aggregation under nutrient or light stress. Modelling approaches are needed in future research in order to determine optimal dilution rates taking into account phytoplankton growth rates, nutrient inputs and ambient conditions (e.g. light and temperature).

Mesophilic anaerobic treatment of sludge from saline fish farm effluents with biogas production

The mesophilic anaerobic treatment of sludge from saline fish farm effluents (total solids (TS): 8.2–10.2 wt%, chemical oxygen demand (COD): 60–74 g/l, sodium (Na): 10–10.5 g/l) was carried out in continuously stirred tank reactors (CSTRs) at 35 °C. COD stabilization between 36% and 55% and methane yields between 0.114 and 0.184 l/g COD added were achieved. However, the process was strongly inhibited, presumably by sodium, and unstable, with propionic acid being the main compound of the volatile fatty acids (VFA). When diluting the sludge 1:1 with tap water (Na: 5.3 g/l), the inhibition could be overcome and a stable process with low VFA concentrations was achieved. The results of the study are used to make recommendations for the configuration of full-scale treatment plants for the collected sludge from one salmon farming licence and to estimate the energy production from these plants.

Rapid assessment of rivers using macroinvertebrates: the role of experience, and comparisons with quantitative methods

We assessed rapid biological assessment (RBA) of macroinvertebrate communities in comparison with quantitative sampling at 18 pairs of river sites in south-eastern Australia. One member of each pair served as a reference site and the other was affected by mild to moderate human disturbance from a point or diffuse source (fish farm effluents, small municipal sewage discharges, a dam, agriculture and grazing). Samples were taken from riffles, stream edges and rocks in pools, mostly using hand nets (RBA) and Hess samplers (quantitative). Macroinvertebrates in RBA samples were always subsampled by live-picking by eye on site for 30 min. Comparisons were made between novice operators (university undergraduates) and experienced river biologists in the application of RBA, including sample identification. Quantitative samples were collected only by experienced river biologists, preserved and picked under stereomicroscopes in the laboratory, either in their entirety or after mechanical subsampling. Specimens were identified to family level for both methods. Novices recorded slightly fewer families than experts and misidentified some specimens, and expert data for the same site and habitat were on average more consistent than novice data. Nevertheless, Procrustes analysis of ordinations showed that differences between RBA and quantitative data did not depend on the type of RBA operator. And regardless of the operator, RBA data were better than quantitative data at discriminating between reference and disturbed sites for all three habitat types. We conclude that this type of RBA is sensitive and cost-effective, and we recommend improvements to inter-operator consistency.

Assessment of the removal efficiency of fish farm effluents by grey mullets: a nutritional approach

A ¢eld study was carried out to assess the use of the bottom-feeding grey mullet (Mugil cephalus) within benthic enclosures as a means to reduce the benthic impacts of a net cage ¢sh farm in the Gulf of Aqaba, Red Sea. Five experimental 1-m 3 net-cage enclosures were stocked with 12 grey mullets each (¢sh weights 50, 70 or 144 g). The enclosures had no bottom and the mullets had access to the enclosed organically enriched sediments and to particulate matter (PM) falling from the overlying cages. Sediment traps were used to quantify and qualify the PM falling from the ¢sh farm to the sea £oor. Simultaneously, a feeding trial was performed with mullets (50 and 70 g) in experimental tanks. In order to estimate the potential growth rate and to quantify energy and protein requirements, the ¢sh were fed a formulated diet with known composition. After approximately 70 days at sea, mullets in the enclosures had used up all the available food in the sediment and gained up to 0.78 g day � 1 ¢sh � 1 . Applying the values for energy and protein requirements for maintenance and growth derived from the experimental trial, estimates indicated that the grey mullets eiectively removed 4.2 g organic carbon, 0.70 g nitrogen and 7.5 mg phosphorus kg � 1 mullet m � 2 day � 1 from the organically enriched sediment. Thus deployment of grey mullets may be an e⁄cient means to improve the quality of sediments below intensive net-cage ¢sh farms.

Treatment of freshwater fish farm effluent using constructed wetlands: the role of plants and substrate

Freshwater fish farm effluents have low nutrient concentrations but high flow rates, resulting in pollutant load, especially phosphorus (P), causing eutrophication. The feasibility was tested of a treatment combining, within a single constructed wetland, the contribution of macrophytes for reducing organic matter and nitrogen (N), with the high efficiency of steel slag and limestone for P removal. Twenty subsurface flow (SSF) basins of 280 L with different combinations of plants (Phragmites communis or Typha latifolia) and substrates (steel slag, limestone, gravel, peat) were fed with a reconstituted fish farm effluent in a greenhouse experiment. Pollutant removal was generally very good under all treatments. N and organic matter removal were correlated with plant biomass while P removal was better in substrates with steel slag and limestone. However, the high pH of the P-adsorbing substrate was detrimental to plant growth so that no combination of plants and substrates could maximise in one step the simultaneous removal of all evaluated pollutants. Therefore, the use of two sequential units is recommended, a first one consisting of a macrophyte planted basin using a neutral substrate to remove organic matter and N, followed by a second unplanted basin containing only a P-adsorbing substrate.

Emerging effluent management strategies in marine fish-culture farms located in European coastal wetlands

Coastal wetlands are suitable sites for land-based fish culture in ponds and tanks, but environmental constraints on effluent discharges are stringent for these areas. In order to limit effluent loading, different techniques have been proposed and are beginning to be implemented by aquaculturists. On the Atlantic coast of Europe (France, Portugal, Spain, etc.), growout farms for sea bass ( Dicentrarchus labrax), sea bream ( Sparus aurata) or turbot ( Scophthalmus maximus) are often located in wetlands where salt ponds were previously built. Downstream from the rearing ponds, sedimentation ponds are used to reduce particulate matter exportation. Using fish farm effluents, the continuous mass culture of microalgae has been the subject of experiments converting ammonia and phosphates into diatoms, with the systematic addition of required amounts of limiting nutrients (silicon as sodium silicate, or phosphorus as phosphoric acid). New physical treatments may be added if partial recirculation systems are employed, such as immersed foam fractionators, specifically developed for aquaculture ponds. Integrated systems may be emergent practices for reducing the effluent pollutant discharge without additional cost, in addition to producing a complementary income to that resulting from the production of the main culture species.

Reduction of aquaculture wastewater eutrophication by phytotreatment ponds system: II. Nitrogen and phosphorus content in macroalgae and sediment

The presence of intensive fish farms in the Orbetello Lagoon (Italian West Coast) has produced a heavy impact on the environment and has led to a large-scale cyclic development of opportunistic macroalgae. The farms are equipped with phytotreatment basins to treat the effluent. Although it is possible to find reports in the literature of phytotreatment techniques suitable for fish-farming effluents, they refer solely to pilot scale. The flooded areas used for phytotreatment permit the growth of huge quantities of macroalgae, which remove nitrogen (N) and phosphorus (P) from the effluent. These ponds are unmanaged, and N and P are released when this vegetation dies during the Summer. Between June 1999 and May 2000 research was carried out on an intensive fish-farming facility that produces about 100 tonnes per year of European sea bass ( Dicentrarchus labrax L.) and gilt head sea bream ( Sparus aurata L.), occupying a volume of 9000 m 3. The wastewater from the plant is discharged to four phytotreatment ponds, arranged in series, and, ultimately, released into the nearby sea-lagoon canal. The research aims were: (a) to quantify the N and P in the effluent from an intensive fish farm facility which are retained by the macroalgae and sediments as the effluent passes through the phytotreatment system; (b) to detect any seasonal variations in nutrient distribution; (c) to determine spatial distribution of the nutrients between input and output of the phytotreatment system. During the test period the physico-chemical parameters of the water were recorded; sediment total organic carbon (STOC), sediment total nitrogen (STN), sediment total phosphorus (STP), macroalgal total carbon (MTC), macroalgal total nitrogen (MTN) and macroalgal total phosphorus (MTP) were determined. Also the micro- and macroalgal biomasses were determined and a floristic list was made. The data collected over the test period were subjected to discriminant type multivariate analysis. The results indicate that: (1) the system size and water residence time are not sufficient to produce adequate abatement; (2) the quantity of nutrients involved in this system is too high for the system itself to be controlled by macroalgal growth alone; (3) the Ulva rigida biomass develops over too short a period of the year (February–June) and is accompanied by rapid decomposition that produces extreme environmental conditions.

Fish-farm effluents in the River Baceiro: effects on water quality

Fish-farm effluents in the River Baceiro: effects on water quality

Application of Seaweed Cultivation to the Bioremediation of Nutrient-Rich Effluent

A seaweed biofilter/production system of being developed to reduce the environmental impact of marine fish farm effluent in coastal ecosystems as a part of an integrated aquaculture system. Several known seaweed taxa and their cultivars have been considered as candidate biofilter organisms based on their species-specific physiological properties such as nutrient uptake kinetics and their economic value. Porphyra is an excellent cadidate and shows efficient nutrient extraction properties. Rates of ammonium uptake were maintained at around 3 at 150 inorganic nitrogen at . Ulva is another possible biofilter candidate with an uptake rate of 1.9 under same conditions. A simple uptake/growth and harvest model was applied to estimate the efficiency of the biofilter/production system. The model was deterministic and used a compartment model structure based on difference equations. The efficiency of Porpyra filter was estimated over 17% of removal from the contimuous supply of 100 flow rate.

In situ biofiltration: a means to limit the dispersal of effluents from marine finfish cage aquaculture

Net pen fish farms generally enrich the surrounding waters and the underlying sediments with nutrients and organic matter, and these loadings can cause a variety of environmental problems, such as algal blooms and sediment anoxia. In this study we test the potential of biofiltration by artificial reefs for reducing the negative environmental impacts surrounding fish farms in the Gulf of Aqaba, Red Sea. Two triangular-shaped artificial reefs (reef volume 8.2 m3) constructed from porous durable polyethylene were deployed at 20 m; one below a commercial fish farm and the other 500 m west of this farm in order to monitor the colonization of these reefs by the local fauna and to determine whether the reef community can remove fish farm effluents from the water. Both reefs became rapidly colonized by a wide variety of organisms with potential for the removal of compounds released from the farms. Within the first year of this study fish abundances and the number of species reached 518–1185 individuals per reef and 25–42 species per reef. Moreover, numerous benthic algae; small sessile invertebrates (bryozoa, tunicates, bivalves, polychaetes, sponges, anemones) and large motile macrofauna (crustaceans, sea urchins, gastropods) settled on the reef surfaces. Depletion of chlorophyll a was measured in the water traversing the artificial reefs in order to assess the biofiltration capacity of the associated fauna. Chlorophyll a was significantly reduced to a level 15–35% lower than ambient concentrations. This reduction was greatest at intermediate current speeds (3–10 cm s−1), but was not influenced by current direction. The reef structures served as a successful base for colonization by natural fauna and flora, thereby boosting the local benthic biodiversity, and also served as effective biofilters of phytoplankton.

Phosphorus removal from trout farm effluents by constructed wetlands

Freshwater trout farms need a high and continuous clean water flow to keep fish exposed to a non-toxic ammonium concentration. As a result, the concentration of effluents from these farms are even below standard effluent criteria for municipal wastewater effluent for solids, nitrogen and phosphorus. Nevertheless, the mass of pollutants discharged, originating mostly from excreta and undigested fish food, must be reduced by simple and economical treatment processes. We designed and operated a three-stage system aimed at retaining solids by a 60 pm nylon rotating microscreen followed by treatment with a phosphorus-retaining constructed wetland system. Washwater from the microscreen was pumped to a series of two horizontal flow beds of 100 m3 each (0.6 m deep). Coarse (2 mm) and finer (< 2 mm) crushed limestone were used in each bed, respectively, with the first one being planted with reeds (Phragmites australis) and the second one designed to remove even more phosphorus by adsorption and precipitation. Preliminary results indicated that the microscreen captured about 60% of the suspended solids and that greater than 95% of the suspended solids and greater than 80% of the total phosphorus mass loads were retained by the beds. The potential of constructed wetlands as an ecologically attractive and economical method for treating fish farm effluents to reduce solids and phosphorus discharge appears promising.