The Potential Use of Mangrove Forests as Nitrogen Sinks of Shrimp Aquaculture Pond Effluents: The Role of Denitrification

A generalized nitrogen budget was constructed to evaluate the potential role of mangrove sediments as a sink for dissolved inorganic nitrogen (DIN) in shrimp pond effluents. DIN concentrations were measured in pond effluents from three semi-intensive shrimp ponds along the Caribbean coast of Colombia between 1994–1995. Mean NH₄⁺ concentrations in the discharge water for all farms were significantly higher (67 × 12 μg/L) than in the adjacent estuaries (33 × 8 μg/L). Average NH₄⁺ concentrations in the pond discharge over all growout cycles were similar, representing an approximate doubling in relation to estuarine water concentrations. In contrast, NO₂^-+ NO₃^- concentrations were similar in both pond effluent and estuarine waters. Dissolved inorganic nitrogen loading of the ponds was similar. The estimated reduction of DIN in pond effluents by preliminary diversion of outflow to mangrove wetlands rather than directly to estuarine waters would be × 190 mg N/m² per d. Based on this nitrogen loss and depending upon the enrichment rate, between 0.04 to 0.12 ha of mangrove forest is required to completely remove the DIN load from effluents produced by a 1-ha pond.     

Water column production and nutrient characteristics in mangrove creeks receiving shrimp farm effluent

Water column processes in undisturbed mangrove creeks and in creeks receiving effluent from shrimp farms in North Queensland, Australia, were studied. Small scale discharges into tidal creeks did not elevate dissolved nutrient concentrations compared with non‐impacted creeks, but did elevate concentrations of particulate nutrients, chlorophyll and suspended solids proximal to the site of the effluent discharge. Rates of primary and bacterial production downstream from the discharge exceeded rates in the shrimp ponds because of the synergistic effects of turbulent mixing and eutrophication. In the lower reaches of the mangrove creeks and immediately offshore, standing stocks of particulate material and rates of primary and bacterial production were within the range of values found in non‐discharge areas. Our results suggest that water column production by phytoplankton and bacteria strips nutrients from shrimp farm effluent and repackages them in more bioavailable forms.     

Sydney rock oyster, Saccostrea commercialis (Iredale & Roughley), filtration of shrimp farm effluent: the effects on water quality

Shrimp pond effluent water can contain higher concentrations of dissolved nutrients and suspended particulates than the influent water. Consequently, there are concerns about adverse environmental impacts on coastal waters caused by eutrophication and increased turbidity. One potential method of improving effluent water quality prior to discharge or recirculation is to use bivalves to filter the effluent. In this study, we examined the effects of the Sydney rock oyster, Saccostrea commercialis (Iredale & Roughley), on the water quality of shrimp pond effluent. Effluent from a shrimp farm stocked with Penaeus japonicus (Bate) was pumped directly into 34-L tanks stocked with different densities of oysters. Combinations of live and dead oysters were used to test the effects of three different densities of live oysters (24, 16 and 8 live oysters per tank). The concentrations of total suspended solids, the proportion of organic and inorganic matter, total nitrogen, total phosphorous, chlorophyll a and the total number of bacteria in the pond effluent water were determined before and after filtration by oysters. The oysters significantly reduced the concentration of all the parameters examined, with the highest oyster density having the greatest effect. Shrimp pond effluent contained a higher proportion of inorganic matter (72%) than organic matter (28%). The organic component appeared to be mainly detritus, with chlorophyll a comprising only a minor proportion. Filtration by the high density of oysters reduced the effluent total suspended solids to 49% of the initial level, the bacterial numbers to 58%, total nitrogen to 80% and total phosphorous to 67%. The combined effects of settlement and oyster filtration reduced the concentration of chlorophyll a to 8% of the initial effluent value.     

Environmental Assessment of Channel Catfish Ictalurus punctatus Farming in Alabama

An environmental assessment was made of Alabama channel catfish Ictalurus punctatus farming which is concentrated in the west‐central region of the state. There are about 10,000 ha of production ponds with 10.7% of the area for fry and fingerlings and 89.3% for food fish. Food fish production was about 40,800 tons in 1997. Watershed ponds filled by rainfall and runoff make up 76% of total pond area. Water levels in many of these ponds are maintained in dry weather with well water. The other ponds are embankment ponds supplied by well water. Harvest is primarily by seine‐through procedures and ponds are not drained frequently. The main points related to Alabama catfish farming and environment issues are as follows: 1) catfish farming in Alabama is conservative of water, and excluding storm overflow, about two pond volumes are intentionally discharged from each pond in 15 yr; 2) overflow from ponds following rains occurs mostly in winter and early spring when pond water quality is good and stream discharge volume is high; 3) total suspended solids concentrations in pond effluents were high, and the main sources of total suspended solids were erosion of embankments, pond bottoms, and discharge ditches; 4) concentrations of nitrogen and phosphorus in effluents were not high, but annual effluent loads of these two nutrients were greater than for typical row crops in Alabama; 5) ground water use by the industry is about 86,000 m³/d, but seepage from ponds returns water to aquifers; 6) there is little use of medicated feeds; 7) copper sulfate is used to control blue‐green algae and off‐flavor in ponds, but copper is rapidly lost from pond water; 8) although sodium chloride is applied to ponds to control nitrite toxicity, stream or ground water salinization has not resulted from this practice; 9) fertilizers are applied two or three times annually to fry and fingerling ponds and occasionally to grow‐out ponds; 10) hydrated lime is applied occasionally at 50 to 100 kg/ha but this does not cause high pH in pond waters or effluents; 11) accumulated sediment removed from pond bottoms is used to repair embankments and not discarded outside ponds; 12) sampling above and below catfish pond outfalls on eight streams revealed few differences in stream water quality; 13) electricity used for pumping water and mechanical aeration is only 0.90 kW h/kg of production; 14) each metric ton of fish meal used in feeds yields about 10 tons of dressed catfish. Reduction in effluent volume through water reuse and effluent treatment in settling basins or wetlands does not appear feasible on most farms. However, some management practices are recommended for reducing the volume and improving the quality of channel catfish pond effluents.     

Characteristics of Effluents from Central Arkansas Baitfish Ponds

Scientific information on baitfish effluents is important to provide a basis for the development of appropriate and cost-effective management practices that minimize environmental impacts. Effluents from 10 commercial golden shiner Notemigonus crysoleucas ponds in central Arkansas were sampled December 2000 through June 2001. Grab samples of the first and last 10% of pond volume were collected during intentional draining events. Effluents were sampled as they exited pond drainpipes and at the ends of drainage ditches just prior to stream discharge. Concurrent receiving stream samples were collected upstream and downstream of the discharge point. Total nitrogen (TN), total phosphorus (TP), 5-d biochemical oxygen demand (BOD_S), and total suspended solids (TSS) of each sample were measured. Mean whole effluent concentrations for the first 10% were 36 mg TSS/L, 9 mg/L BOD₅, 2 mg TN/L, and 0.5 mg TP/L. The water quality of the first and last 10% of pond effluent were not significantly different ( P < 0.05). Filtering effluents through a 5-pm mesh screen did not significantly reduce nutrient concentrations. Serial fractionation of effluents resulted in small but significant decreases in TSS concentrations in samples filtered through the 10, 8, and 5-μm meshes ( P < 0.05). Effluent discharge through farm ditches generally did not improve effluent water quality. Effluents collected at ditch ends were significantly less than drainpipe samples in BOD, concentrations only ( P < 0.05). Limited data on receiving stream water quality indicated that only TP concentrations were greater in pond effluents than in receiving streams. Overall, baitfish pond effluents are similar in composition to effluents of other phytoplankton-based pond production systems.     

The efficiency and condition of oysters and macroalgae used as biological filters of shrimp pond effluent

Current shrimp pond management practices generally result in elevated concentrations of nutrients, suspended solids, bacteria and phytoplankton compared with the influent water. Concerns about adverse environmental impacts caused by discharging pond effluent directly into adjacent waterways have prompted the search for cost‐effective methods of effluent treatment. One potential method of effluent treatment is the use of ponds or raceways stocked with plants or animals that act as natural biofilters by removing waste nutrients. In addition to improving effluent water quality prior to discharge, the use of natural biofilters provides a method for capturing otherwise wasted nutrients. This study examined the potential of the native oyster, Saccostrea commercialis (Iredale and Roughley) and macroalgae, Gracilaria edulis (Gmelin) Silva to improve effluent water quality from a commercial Penaeus japonicus (Bate) shrimp farm. A system of raceways was constructed to permit recirculation of the effluent through the oysters to maximize the filtration of bacteria, phytoplankton and total suspended solids. A series of experiments was conducted to test the ability of oysters and macroalgae to improve effluent water quality in a flow‐through system compared with a recirculating system. In the flow‐through system, oysters reduced the concentration of bacteria to 35% of the initial concentration, chlorophyll a to 39%, total particulates (2.28–35.2 µm) to 29%, total nitrogen to 66% and total phosphorus to 56%. Under the recirculating flow regime, the ability of the oysters to improve water quality was significantly enhanced. After four circuits, total bacterial numbers were reduced to 12%, chlorophyll a to 4%, and total suspended solids to 16%. Efforts to increase biofiltration by adding additional layers of oyster trays and macroalgae‐filled mesh bags resulted in fouling of the lower layers causing the death of oysters and senescence of macroalgae. Supplementary laboratory experiments were designed to examine the effects of high effluent concentrations of suspended particulates on the growth and condition of oysters and macroalgae. The results demonstrated that high concentrations of particulates inhibited growth and reduced the condition of oysters and macroalgae. Allowing the effluent to settle before biofiltration improved growth and reduced signs of stress in the oysters and macroalgae. A settling time of 6 h reduced particulates to a level that prevented fouling of the oysters and macroalgae.     

Effects of shrimp pond water on phytoplankton: importance of salinity and trophic status of the receiving environment

Aquaculture generates a large load of effluents rich in organic matter and nutrients that may be introduced into the environment. This study aimed to assess in a microcosm experiment, the effect of shrimp pond water mixed with Patos Lagoon estuary water on phytoplankton chlorophyll a and primary production, simulating two salinities. Chlorophyll a, dissolved inorganic nutrients and primary production were measured in two experiments. In Harvest I, salinity of shrimp pond and environment water was similar, and chlorophyll a showed different trends over time, according to the amount of nitrogen available. In Harvest II, with different salinities and high nutrient concentrations in environment water, chlorophyll a levels showed a similar increasing trend over time in all mixtures. Net primary production showed differences among treatments in the first sampling in Harvest I, but not in the second, whereas no differences were observed among treatments in Harvest II. We conclude that shrimp pond effluent can lead to short‐term variations in chlorophyll a and primary production levels, with similar salinities. Salinity differences result in lower chlorophyll a and primary production values than expected according to the nutrient input. Differences in salinity can be an important management strategy to choose the best harvest period.     

The use of high rate algal ponds for the treatment of marine effluent from a recirculating fish rearing system

A high rate algal pond (HRAP) system was used to treat effluent from a recirculating sea water aquaculture system in southern France. Dicentrarchus labrax L. were farmed at a high density, with effluents containing an average of 10 mg L^?1 dissolved inorganic nitrogen (DIN) and 1.3 mg L^?1 reactive phosphorus (RP). On a yearly basis, the algal pond removed 59% of the dissolved nitrogen and 56% of the phosphorus input, which was converted into 3.3 kg DW m^?2 algae. Green macroalgae were dominant throughout the year and the algal biomass mirrored the seasonal changes in daily irradiance and temperature. This first year study supports the possibility of treating marine aquaculture wastes using HPAPs, although conditions will have to be found to mitigate the strong influence of climate on the algal community during winter. During the more temperate season, only 150 m² of treatment ponds would be necessary to remove the nutrients produced by 1 ton of fish. Treated water was characterized by a high pH, elevated levels of dissolved oxygen (midday value) and low concentrations of nutrients and suspended solids. The absence of toxic phytoplankton meant that the water could be recycled through the farm tanks.     

Suspended Solids from Baitfish Pond Effluents in Drainage Ditches

Effluents from aquaculture facilities vary between species and among production systems. Drainage ditches commonly convey effluents from central Arkansas baitfish ponds. Ditches could potentially reduce suspended solids prior to effluent release into receiving streams through settling. We characterized suspended solids in effluents from baitfish ponds and evaluated changes in suspended solids in drainage ditches. We also characterized drainage ditches based on width, depth, slope, and percent vegetation cover. Average (± SD) total suspended solids (TSS) at the point of discharge was 52 (± 41) mg/L, while volatile suspended solids (VSS) averaged 22 (± 23) mg/L. Screening effluents did little to alter their composition. Approximately 76% of TSS were less than 5 μm. There were no significant changes in effluent solids along drainage ditches 100 m from the point of discharge and no significant correlations between ditch characteristics and changes in either TSS or VSS. Existing ditches are quite variable and are not necessarily effective in removing solids present in baitfish effluents. Screening and use of ditches as settling basins seem impractical for effluent treatment given the characteristics of solids in baitfish effluents.     

Effects of different feed management treatments on water quality for Pacific white shrimp Litopenaeus vannamei

Increasing feeding rates may provide an increase in production, thus nutrients such as nitrogen, phosphorus and organic matter will also increase. These nutrients promote a greater oxygen demand and concentrations of toxic metabolites which can lead to frequent problems with low dissolved oxygen and an abundance of blue‐green algae. Four feed management practices were evaluated among sixteen 0.1 ha ponds culturing Pacific white shrimp (Litopenaeus vannamei). Feeding treatments included hand feeding using the Standard Feeding Protocol (SFP), SFP plus 15% from 8 to 16 weeks, an automatic‐solar timer which fed SFP+15%, and an AQ1 acoustic demand feeder allowing up to 12 kg/day·pond based on shrimps feeding response. Samples were analysed at weeks 0, 4 and 8–16 for the following parameters: chlorophyll a, total ammonia nitrogen, nitrite–nitrogen, nitrate–nitrogen, total nitrogen, total phosphorus, soluble reactive phosphorus, total suspended solids, total suspended volatile solids, turbidity, conductivity, salinity and biological oxygen demand. Samples were collected and shipped overnight to Auburn, Alabama for off‐site analysis. On‐site water quality was also obtained at the farm. The AQ1 acoustic demand feeder produced the most shrimp with a yield of 4,568 kg/ha; however, the AQ1 also had the highest total ammonia nitrogen and nitrite–nitrogen levels late in the growing season. The AQ1 feeder may be a viable, reduced labour and cost alternative for the shrimp commercial industry; however, such technologies must also be matched to the ability of the production system to process nutrients.     

A Survey of Water Quality Characteristics of Effluent from Hawaiian Aquaculture Facilities

Ten water quality parameters were measured in influent and effluent water at 11 aquaculture facilities in Hawaii. The data were grouped into four categories based on the types of organisms cultured: freshwater fish, freshwater prawn, marine fish, and marine shrimp. Within each category, concentrations of most parameters were lognormally distributed and spanned one to two orders of magnitude. Geometric mean concentrations of suspended materials, total nitrogen, total phosphorus, and pigments were highest in effluent from freshwater prawn ponds and lowest in marine fish pond effluent. Nitrate/Nitrite and total ammonia concentrations were higher in fish pond effluent than in crustacean pond effluent. Parameter concentrations were generally higher in effluent than in influent water, with freshwater fish and prawn ponds exhibiting the greatest increases in suspended materials and pigments. In contrast, nitrate/nitrite concentrations were lower in effluent than in influent waters. These data provide a basis for analyzing the environmental impacts of warm-water aquaculture effluent discharges.     

Experimental Evaluation of the Halophyte,Salicornia virginica,for Biomitigation of Dissolved Nutrients in Effluent from a Recirculating Aquaculture System for Marine Finfish

The ability of the halophyte, Salicornia virginica, planted in drainage lysimeters to biomitigate dissolved nutrients in effluent from a recirculating aquaculture system (RAS) for marine finfish was evaluated. Seawater effluent from a RAS producing black sea bass, Centropristis striata (filtered to reduce total suspended solids), was used as irrigant. Plant growth and dissolved N and P removal were determined as a function of leachate fraction (LF%) – that is, proportion of irrigant that leaches from the plant‐substrate lysimeter. Lysimeters were irrigated weekly to produce 30, 40, and 50% LF. A control (unplanted) lysimeter was included at the 30% LF. Plant growth was excellent in all LF% treatments until Day 141 when salt buildup in the lysimeter substrate inhibited nutrient uptake. Salt accumulation was mitigated at higher LF%, so that plant biomass and net removal (μg) of dissolved N and P by the p‐s lysimeter remained higher (P < 0.05) at the 40 and 50% than at the 30% LF. On Day 141, percent removal efficiency at the 50% LF was 79.2% for inorganic N and 73.9% for total phosphorus. Through Day 355, substrate salinity was minimized and plant biomass and nutrient removal were maximized at the 50% LF. S. virginica is an effective biofilter for dissolved nutrients in effluent from an RAS for marine finfish.     

Water quality and plankton densities in mixed shrimp-mangrove forestry farming systems in Vietnam

Water quality and plankton densities were monitored in shrimp ponds at 12 mixed shrimp‐mangrove forestry farms in Ca Mau province, southern Vietnam, to detail basic water chemistry and assess whether conditions are suitable for shrimp culture. In general, water quality was not optimal for shrimp culture. In particular, ponds were shallow (mean ± 1SE, 50.5 ± 2.8 cm), acidic (pH < 6.5), had high suspended solids (0.3 ± 0.03 g l^?1), low chlorophyll a/phytoplankton concentrations (0.2 ± 0.05 µg l^?1 and 8600 ± 800 cells l^?1 respectively) and low dissolved oxygen (DO) levels (3.7 ± 0.15 mg l^?1). Eight out of the 12 farms sampled had potentially acid sulphate soils (pH < 4.2). Salinity, DO and pH were highly variable over short time‐periods (hours); DO in particular was reduced to potentially lethal levels (1–2 mg l^?1). Seasonal variations in water chemistry and plankton communities (i.e. salinity, DO, phosphate, temperature, phytoplankton and zooplankton densities) appear to be driven by differences in rainfall patterns. The presence or absence of mangroves on internal pond levees (‘mixed’ versus ‘separate’ farms) and the source of pond water (rivers versus canals) were of lesser importance in determining water quality patterns and plankton biomass. Zooplankton and macrobenthos densities were sufficient to support the current (low) stocking densities of shrimp. However, natural food sources are not adequate to support increases in production by stocking hatchery reared post larvae. Increasing productivity by fertilization and/or supplemental feeding has the potential for adverse water quality and would require improvements to water management practices. Some practical strategies for improving water quality and plankton densities are outlined.     

Bioremediation and reuse of shrimp aquaculture effluents to farm whiteleg shrimp,Litopenaeus vannamei : a first approach

Shrimp aquaculture effluents were bioremediated in a two‐phase system (System A) using the black clam Chione fluctifraga and the benthic microalgae Navicula sp., and then reused to farm whiteleg shrimp Litopenaeus vannamei. In the experimental design, Systems B and C had an identical structure as System A, but no clams or microalgae were added. System B received the same shrimp effluents while System C received only estuarine water. Shrimp raw effluents had a poor water quality. System A improved the water quality by decreasing the concentrations of total nitrogen, total ammonia nitrogen (TAN), nitrites, nitrates, phosphates, total suspended solids (TSS) and organic suspended solids (OSS). System B also decreased the concentration of TAN, TSS and OSS via sedimentation, but the effect was less pronounced than that observed in System A. Shrimp reared in the bioremediated effluents (System A) had better production (3166 kg ha^?1) and higher survival (89.2%) than those reared in effluents from Systems B (2610 kg ha^?1, 75.1%) and C (2874 kg ha^?1, 82.1%). It is concluded that the bioremediation system was moderately efficient and the bioremediated effluents were suitable to farm L. vannamei.     

Estimates of Bottom Soil and Effluent Load of Phosphorus at a Semi-intensive Marine Shrimp Farm

A phosphorus budget for a single crop was prepared for a 685‐ha semi‐intensive shrimp farm that consistently has produced about 3000 tonnes/yr of black tiger prawn, Penaeus monodon. Phosphorus inputs were shrimp stock, 0.31 kg/ha; triple superphosphate, 1.38 kg/ha; incoming water, 25.8 kg/ha; and feed, 65.3 kg/ha. Phosphorus outputs were harvested shrimp, 5.43 kg/ha, and outflow for water exchange and draining, 42.7 kg/ha. The high clay‐content soil in pond bottoms adsorbed 45.2 kg/ha of phosphorus. Water was taken from and released back into the same estuary and bay. The phosphorus contribution of shrimp farming to the receiving water body was the difference between the amount of phosphorus in effluent and that in the incoming water, which was 16.9 kg/ha. Bottom soil accumulated 67.8% of phosphorus added to the ponds. Another estimate of soil phosphorus uptake based on the relationship between cumulative phosphorus applied to ponds as fertilizer and feed and soil phosphorus concentration suggested that 63.2% of fertilizer and feed phosphorus had accumulated in pond bottoms. The farm effluent phosphorus load was 23.5 tonnes/yr. The estuary and bay system has an estimated volume of 4.8 × 10⁹ m³, and the annual phosphorus input from the farm had a concentration equivalent of 0.005 mg/L, and there were no other major inputs of phosphorus. The estuary and bay are flushed by freshwater inflow and tidal action, and the farm input is not likely to cause eutrophication.     

Phosphorus budget in shrimp aquaculture pond with mangrove enclosure and aquaculture performance

ABSTRACT:   An experiment in which water was circulated between shrimp aquaculture ponds stocked with 10 000 or 20 000 PL-15 stage Penaeus monodon , and mangrove enclosures each planted with 476 Rhizophora mucronata per enclosure, was carried out at the Samut Songkhram Coastal Aquatic Research Station, Faculty of Fisheries, Kasetsart University, Thailand. Shrimp survival rate was significantly higher ( P  < 0.001, Fisher's exact test) in ponds where 10 000 larvae was stocked and water was exchanged with the mangrove enclosure, compared with the control pond with no water exchange, over the 136 day experimental period. Phosphorus transport to the mangrove enclosure was estimated to be 0.41 kgP and 0.18 kgP over the experimental period and change in phosphorus content in mud was reduced there compared with the control pond. A load reduction effect to the environment was confirmed in this aquaculture system with mangrove enclosure compared with the phosphorus budget in the control pond, and 6.2 or 8.9 ha of mangrove area was estimated to be required by 1 ha shrimp ponds to fully process the phosphorus.     

Integrated shrimp-mangrove farming systems in the Mekong delta of Vietnam

In recent years, mangroves have been seriously threatened by shrimp culture development in the Mekong delta of Vietnam. As one way of protecting the mangroves, efforts have been made to develop shrimp-mangrove integrated farming systems through State Forestry Fisheries Enterprises (SFFEs), involving a participatory approach. The present study investigated the shrimp-mangrove farming systems in Ngoc Hien district at the southern tip of the Mekong delta through a detailed farm-level survey, including technical, environmental, social and economic factors. A total of 161 shrimp-mangrove farms on the east and west coasts of Ngoc Hien district were surveyed. The results showed that on the East coast of the delta, the pH of the bottom soil, the water alkalinity, gate width ratio (ratio of gate width to pond area), pond age and mangrove density, and flooding level of ‘flat’ (flat is where the mangroves are planted) in the dry and rainy seasons were significantly correlated with shrimp pond yields. On the west coast, the pH of the bottom soil, dissolved inorganic phosphorus, ditch area, and flooding level of ‘flat’ were found to be significantly correlated with shrimp pond yields. An economic analysis, based solely on the economic returns from shrimp culture showed that the farming systems with a mangrove coverage of 30-50% of the pond area gave the highest annual economic returns. The results demonstrate a better economic return to farmers who maintain mangroves in their farming systems.     

Characterization of intake and effluent waters from intensive and semi-intensive shrimp farms in Texas

Two commercial shrimp farms in south Texas were evaluated for influent and effluent water quality from June to October 1994. The intensive farm, Taiwan Shrimp Village Association (TSV) had an average annual yield of 4630 kg ha^?1 while the semi‐intensive farm, Harlingen Shrimp Farm (HSF), had a yield of 1777 kg ha^?1. The study had three objectives: (1) to compare influent and effluent water from the intensive and semi‐intensive shrimp farms, (2) to show which effluent water‐quality indicators exceeded allowable limits, (3) to indicate inherent problems in farms operated with water exchange and summarize how findings from this study led to changes in farms' management that limited potential negative impact on receiving streams. Water samples were collected and analysed twice a week for the TSV farm and once a week for the HSF farm. Samples were analysed for dissolved oxygen (DO), salinity, pH, ammonia‐nitrogen (NH₃‐N), nitrite‐nitrogen (NO₂‐N), nitrate‐nitrogen (NO₃‐N), total phosphorus (TP), total reactive phosphorus (TRP), five‐day carbonaceous biochemical oxygen demand (cBOD₅), total suspended solids (TSS) and settleable solids (SettSols). Most of the effluent constituents showed fluctuations throughout the sampling period often related to harvest activity. Effluent pH at TSV was lower than influent values but within the regulatory requirements set by Texas Commission of Environmental Quality (TCEQ), formerly known as Texas Natural Resource Conservation Commission (TNRCC). HSF effluent pH values were higher than its influent, but still within TCEQ limits. Effluent DO mean levels were generally below the regulatory daily mean requirement, with values at TSV often below those for influent. Effluent nutrient concentrations and net loads were generally higher at the intensive shrimp farm, with NH₃‐N mean concentrations above the daily mean set by the TCEQ on several occasions. Effluent TSS concentrations were higher than influent for both farms, with daily mean values above the TCEQ limit. The two farms presented similar TSS concentrations despite their different stocking densities. However, TSS total net load and net load per hectare were higher at the intensive farm. The semi‐intensive farm presented higher cBOD₅ concentrations and net loads despite its lower stocking density, with daily mean values above the TCEQ limit. The cBOD₅ net load at TSV presented negative values indicating higher load at the influent than at the effluent. Analyses showed no evidence of self‐pollution between influent and effluent at the two farms. The high feed conversion ratio (FCR) values (2.3 and 2.7 for the intensive and the semi‐intensive farm respectively) suggest that better feed management is needed to reduce nutrient and solid net loads release from the two farms. The data obtained from this study resulted in several modifications in design and management of the two farms that reduced the potential negative impact on receiving streams. A brief summary of the improvement in selected effluent water‐quality indicators at the intensive shrimp farm is provided.     

Treatment of shrimp farm effluent with omnivorous finfish and artificial substrates

Long‐term environmental sustainability and community acceptance of the shrimp farming industry in Australia requires on‐going development of efficient cost‐effective effluent treatment options. In this study, we aimed to evaluate the effectiveness of a shrimp farm treatment system containing finfish and vertical artificial substrates (VAS). This was achieved by (1) quantifying the individual and collective effects of grey mullet (Mugil cephalus L.) and VASs on water and sediment quality, and (2) comparing the retention of N in treatment systems with and without the presence of finfish (M. cephalus and the siganid Siganus nebulosus (Quoy & Gaimard)), where light was selectively removed. Artificial substrates were found to significantly improve the settlement of particulate material, regardless of the presence of finfish. Mullet actively resuspended settled solids and reduced the production of nitrate when artificial substrates were absent. However, appreciable nitrification was observed when mullet were present together with artificial substrates. The total quantity of N retained by the mullet was found to be 1.8– 2.4% of the incoming pond effluent N. It was estimated that only 21% of the pond effluent N was available for mullet consumption. When S. nebulosus was added, total finfish N retention increased from 1.8% to 3.9%, N retention by mullet also improved (78±16 to 132±21‐mg N day^?1 before and after siganid addition respectively). Presence of filamentous macroalgae (Enteromorpha spp.) was found to improve the removal of N from pond effluent relative to treatments where light was excluded. Denitrification was also a significant sink for N (up to 24% N removed). Despite the absence of algal productivity and greater availability of nitrate, denitrification was not higher in treatments where light was excluded. Mullet were found to have no effect on the rates of denitrification but significantly reduced macroalgal growth on the surface of the water. When mullet were absent, excessive macroalgal growth led to reduced dissolved oxygen concentrations and nitrification. This study concludes that the culture of mullet alone in shrimp farm effluent treatment systems does not result in significant retention of N but can contribute to the control of macroalgal biomass. To improve N retention and removal, further work should focus on polyculturing a range of species and also on improving denitrification.     

Chemical and Physical Properties of Shrimp Pond Bottom Soils in Ecuador

Chemical and physical analyses were conducted on bottom soil samples from 74 brackish-water ponds representing 40 shrimp farms in Ecuador. Most ponds had soils with pH > 6 and total carbon concentrations < 2.5%. Carbon was mostly in organic form, for the average concentration of carbonate carbon was 0.06%. The C: N ratio was 8 to 10 in soils with < 2.5% carbon. In ponds built in former mangrove areas, soil carbon was > 2.5% and C: N ratios were 25 to 30. Ponds soils in former mangrove areas also tended to be high in total sulfur and low in pH. Lack of correlation between carbon and sulfur in mangrove soils suggested that most of the sulfur was inorganic and presumably in sulfides. Soils containing above 0.4% free carbonate (as equivalent CaCO₃) had pH values > 7. Although carbonate concentration was a major factor controlling soil pH, calcium hardness of pond waters was strongly influenced by salinity (and calcium) in the water supply. Total phosphorus concentrations averaged 898 mg/kg, and dilute acid extractable phosphorus usually accounted for 25–35% of the total. Concentrations of major cations and minor elements varied greatly in soils and exhibited ranges of up to three orders of magnitude. Contrary to opinions of shrimp producers, many pond soils in Ecuador are not acidic and few soils have a high organic matter content. Proper use of soil and water testing could greatly improve the efficiency of liming and other soil management practices.