Michele Astrid Burford

Nutrient and microbial dynamics in high-intensity, zero-exchange shrimp ponds in Belize

Microbial and phytoplankton processes, and their effect on water quality were examined over a 3-week period in five high-intensity (120 animals m−2) shrimp (Litopenaeus vannamei) ponds of varying crop ages at Belize Aquaculture Ltd., (BAL) in Central America. These ponds were characterized by zero water exchange throughout the crop, plastic lining and high aeration rates. Nitrogen (N) and organic carbon (C) inputs, in the form of fishmeal-based feed, grain-based feed and molasses, resulted in high concentrations of dissolved organic and inorganic N (2.29–5.56 and 0.17–10.66 mg l−1, respectively) and dissolved organic C (14.20–48.10 mg l−1). Phosphate levels were also high, ranging from 0.07 to 1.17 mg l−1. The high nutrient concentrations promoted the growth of bacteria, phytoplankton (mostly autotrophic flagellates) and protozoa. Up to 40% of the bacteria were associated with flocculated matter. However, bacterial numbers and oxygen (O2) consumption in the water column did not appear to increase with crop age. This may be due to a reduction in the C/N ratio below the optimum for bacterial growth. Up to 22% of the O2 consumption was due to nitrification and there was some indication of lowering of total ammoniacal N (TAN) concentrations and an increase in nitrite and nitrate levels in older crops. Both phytoplankton and bacteria were responsible for high rates of ammonium uptake. In ponds with high nitrate concentrations, nitrate uptake rates were also high. Phytoplankton productivity remained high irrespective of crop age and ponds fluctuated between net O2 production (autotrophy) and net O2 consumption (heterotrophy) irrespective of crop age. This reflected the highly dynamic nature of the bacterial and phytoplankton populations with frequent blooms and crashes of individual phytoplankton species. The high mixing rates resulted in phytoplankton and other detritus remaining suspended in the water column. However, a small area of sludge (<2% of pond area) did accumulate containing a high N and C content, and high pore water TAN. This study showed that despite what is generally considered as poor water quality in the ponds, i.e. high nutrient concentrations, high and unstable phytoplankton numbers, and high bacterial numbers, shrimp production was high relative to conventional ponds. There appeared to be scope for increasing bacterial production in these systems by increasing the C/N ratio, and hence C availability for bacterial growth. However, it remains to be established which microbial processes are likely to be promoted, and if the benefits of this outweigh the costs.

The contribution of flocculated material to shrimp (Litopenaeus vannamei) nutrition in a high-intensity, zero-exchange system

Abstract High-intensity, zero-exchange shrimp ponds contain a high density of flocculated particles, rich in bacteria and phytoplankton, compared with flow-through systems. The flocculated particles provide a potential food source for shrimp. Short-term tank experiments were conducted to determine the retention of nitrogen (N) from natural biota, dominated by flocculated particles, in white shrimp ( Litopenaeus vannamei ) at a high-intensity, zero-exchange shrimp farm in Central America (Belize Aquaculture (BAL)). There were two treatments: ‘floc’ and ‘floc+20%’ (3×1000-l replicate tanks each) based on two densities of flocculated particles. The floc density in the ‘floc’ treatment was typical of shrimp growout ponds at BAL, whereas the ‘floc+20%’ treatment had a 20% higher density of flocculated particles. Three consecutive experiments were conducted with 1, 5 and 9 g shrimp, respectively. At the start of the experiment, 15 N-ammonium was added to the tanks and assimilated by the natural biota. Shrimp were maintained in these tanks for 48 h after the 15 N-nitrogen enrichment. After this time, shrimp were found to be enriched with 15 N-nitrogen. It was calculated that between 1% and 3% of the particulate nitrogen in the tanks, principally from the flocculated particles, was retained by the shrimp. The proportion of estimated daily nitrogen retention of the shrimp contributed by the natural biota was calculated to be 18% to 29% for 1 to 9 g animals in the floc treatment. There was a tendency for greater retention in the floc+20% treatments, but this trend was not consistent. This study suggests that natural biota, which in this system was largely flocculated particles, can contribute substantially to the nutrition of L. vannamei . There are, therefore, benefits for shrimp in the promotion of flocculated particles in L. vannamei ponds. Whether this translates into improvements in shrimp growth and production efficiency remains to be established.

Nitrogen budget and effluent nitrogen components at an intensive shrimp farm

This study examined the operation of an intensive tropical shrimp farm in relation to the nitrogen (N) budget and the N components of the discharged effluent. Weekly samples were taken for total N (TN) concentration from the farm intake and discharge water over a 10-month period. A N budget was calculated based on TN data, continuous datalogger records of water exchange volumes, and farm records for feed addition, harvest and sediment removal. TN levels in the intake water were low throughout the 10 months and only contributed 5% of the N input to the farm. Most of the N (90%) entered the farm ponds as formulated shrimp food. Within the ponds, 22% of the input N was converted to harvested shrimp, 14% remained in the sediment, while most of the remainder (57%) was discharged to the environment. Only 3% of input N was unaccounted for, and assumed to be lost to the atmosphere via denitrification or volatilization of ammonia. More intensive sampling of effluent (three times a day) was done over 7-day periods in February (late summer) and July-August (winter) to provide detailed information on the N composition. All parameters varied substantially both within and between days. Forty-two to forty-five percent of the discharged N was in particulate form, mostly phytoplankton. The dissolved N fraction had two main components: dissolved organic N (DON), which comprised 37-43% of TN, and total ammonia N (TAN), which comprised 12-21% of TN. Comparison between the results of this and previous studies suggests that little progress has been made in improving nitrogen utilization efficiency of intensive Penaeus monodon shrimp farming over the past decade. Thus a major challenge facing this industry is to improve both environmental and economic performance by developing and implementing an integrated approach to reducing nitrogen waste.

The fate of nitrogenous waste from shrimp feeding

This study characterized and quantified the dissolved nitrogen (N) waste from shrimp (Penaeus monodon) feeding. The subsequent utilization of the dissolved N (DN) compounds by the microbial community in shrimp pond water was also examined. There were three main sources of soluble N from feeding; gill excretion, leaching from formulated feed, and leaching from shrimp faeces. The main source of DN was ammonia excreted from shrimp gills. However, there was also a significant amount of DN leached from feed and faeces over the course of a few hours. Most of this was in the form of dissolved organic N (DON) compounds. In the case of feed, a significant proportion of this was dissolved primary amines (DPA, 23%) whilst in faeces, it was urea (26%). Urea leached from shrimp faeces was rapidly utilized by the microbial community in pond water. However, other DON compounds appeared to be less bioavailable. Dissolved organic N leached from formulated feed appeared to be less effectively utilized by the microbial community and is likely to accumulate in pond water. Dissolved organic N leachates from formulated feed and faeces are, therefore, likely to have a significant impact on water quality in shrimp ponds, both by the accumulation of DON, and stimulation of the growth of the microbial community. There is, therefore, considerable scope to improve water quality, and hence reduce nutrient discharges from shrimp farming, by reducing overfeeding, and improving feed retention by shrimp.

Increased incidence of Cylindrospermopsis raciborskii in temperate zones--is climate change responsible?

The bloom-forming, toxic cyanobacterium, Cylindrospermopsis raciborskii exhibits global distribution. In recent years both the occurrence and dominance of this species, particularly in temperate regions, has increased. Whilst this may be due to increased sensitivity of analytical detection methods or more rigorous sampling routines, it is possible that this expansion has been assisted by a number of changing conditions in these environments. The geographical expansion of both the organism and toxin production can be attributed to phenomena such as eutrophication and climate change. In this review, we discuss the occurrence of C. raciborskii with respect to current literature against the backdrop of increasing global temperatures. Critically, we identify a concerning trend between the geographical spread of this organism and global climate change.

A synthesis of dominant ecological processes in intensive shrimp ponds and adjacent coastal environments in NE Australia

One of the key environmental concerns about shrimp farming is the discharge of waters with high levels of nutrients and suspended solids into adjacent waterways. In this paper we synthesize the results of our multidisciplinary research linking ecological processes in intensive shrimp ponds with their downstream impacts in tidal, mangrove-lined creeks. The incorporation of process measurements and bioindicators, in addition to water quality measurements, improved our understanding of the effect of shrimp farm discharges on the ecological health of the receiving water bodies. Changes in water quality parameters were an oversimplification of the ecological effects of water discharges, and use of key measures including primary production rates, phytoplankton responses to nutrients, community shifts in zooplankton and delta15N ratios in marine plants have the potential to provide more integrated and robust measures. Ultimately, reduction in nutrient discharges is most likely to ensure the future sustainability of the industry.

The effect of feeding frequency on water quality and growth of the black tiger shrimp (Penaeus monodon)

Abstract The feeding strategy used in the commercial culture of shrimp can have a significant impact on pond water quality and hence growth, health and survival of the shrimp, as well as the efficiency of feed utilization. These factors contribute to the profitability of production and to the environmental impact of shrimp farming. The effect of four different feeding frequencies (3, 4, 5 and 6 feedings day −1 ) on the growth and survival of the black tiger shrimp, Penaeus monodon , and water quality was studied in an 8-week growth trial. The shrimp were held in 20×2500-l outdoor tanks containing water and sediment from a shrimp pond. The water management and aeration strategies were designed to simulate a shrimp pond system. The shrimp (initial weight of 5.6 g) were stocked at a density of 25 animals m −2 and fed a widely used, commercial pelleted feed, with all the feed being placed on feeding trays. The uneaten feed on the feeding trays was removed at specific time intervals so that in all treatments, the shrimp had access to the feed for 12 h day −1 . There were no significant ( P >0.05) differences due to feeding frequency on growth rate (1.4±0.08 g week −1 ), feed conversion ratio (FCR) (2.0±0.27) or survival (84±7.6%) of shrimp. Similarly, the water quality parameters (total N, ammonium, nitrate/nitrite, dissolved organic nitrogen, total phosphorus, phosphate, chlorophyll a , oxygen, pH, temperature, salinity, turbidity) were not different among treatments. The results suggest that there is no benefit from feeding P. monodon more frequently than 3 times day −1 when using a feed that is nutritionally adequate and has high water stability. Therefore, it may be possible to reduce feeding frequency in commercial shrimp ponds without adversely affecting water quality, shrimp growth rate and survival, thereby improving farm profitability.

Modeling nitrogen dynamics in intensive shrimp ponds: the role of sediment remineralization

Abstract A mathematical model is used to investigate the role of sedimentation and remineralization in the sediment on nitrogen (N) dynamics in intensive shrimp culture ponds. The model describes the key processes involved in N cycling that underpin the dynamics of total ammoniacal N (TAN), nitrate/nitrite (NOX) and chlorophyll a (CHL) concentrations and the sediment N pool. These parameters may, in high concentrations, impact negatively on the shrimp or the adjacent aquatic environment when water is discharged from ponds. The model was calibrated for an Australian commercial shrimp ( Penaeus monodon ) pond. Most N enters the pond system as TAN from shrimp excretion of dietary N and decomposition of wasted feed, and is subsequently taken up by phytoplankton, which, on senescence, is sedimented and remineralized. Sediment remineralization is the dominant source of TAN in the water column for all but the beginning of the production cycle. The remineralization rate of sedimented N was estimated at 6% day −1 . Nonetheless, sediment acts as a net sink of N throughout the production cycle. The effect of management strategies, including increased stocking densities, water exchange and sludge (=sedimented material) removal, on water quality was examined. Model outputs show that using current shrimp farming techniques, with water exchange rates of 7% day −1 , an increase in stocking densities above 60 animals m −2 would result in unacceptably high TAN concentrations. Both sludge removal and water exchange provide effective ways of reducing TAN and NOX concentrations and may allow substantially higher stocking densities. However, sludge removal may be the more acceptable option, given the need to meet strict regulatory requirements for discharge loads in some countries and the desire to reduce water intake to improve biosecurity.

Pulses of phosphate promote dominance of the toxic cyanophyte Cylindrospermopsis raciborskii in a subtropical water reservoir.

The role of dissolved inorganic phosphorus (DIP) in promoting dominance of the toxic nitrogen (N)‐fixing cyanobacterium Cylindrospermopsis raciborskii (Wołosz.) Seenayya et Subba Raju was examined in a subtropical water reservoir, Lake Samsonvale (=North Pine reservoir). A novel in situ bioassay approach, using dialysis tubing rather than bottles or bags, was used to determine the change in C. raciborskii dominance with daily additions of DIP. A statistically significant increase in dominance of C. raciborskii was observed when DIP was added at two concentrations (0.32 μM and 16 μM) in a daily pulse over a 4 d period in three separate experiments in the summer of 2006/2007. There was an increase in both C. raciborskii cell concentrations and biovolume in two DIP treatments, but not in the ammoniacal N + DIP treatment. In addition, overall phytoplankton cell concentrations increased with DIP addition, indicating that Lake Samsonvale was DIP limited at the time of experiments. Given the bioassay response, it is likely that dominance of C. raciborskii could increase in Lake Samsonvale with periodic injections of DIP such as inflow events.

Sources of carbon fuelling production in an arid floodplain river

Dryland rivers are characterised by highly pulsed and unpredictable flow, and support a diverse biota. The present study examined the contribution of floodplain sources to the productivity of a disconnected dryland river; that is a waterhole, after a major overland flood event. Rate measures of productivity were combined with stable isotope and biomass data on the food web in the waterhole and floodplain. The present study estimated that 50% of the fish carbon in the waterhole after flooding was derived from floodplain food sources. In the few months after retraction of the river to isolated waterholes, the large biomass of fish concentrated from the flooding decreased by 80%, most likely as a result of starvation. Based on the development of a carbon budget for the waterhole, mass mortality is hypothesised to be the cause of the high rates of heterotrophic production in the waterhole. The present study suggests that floodplain inputs are important for fuelling short-term production in waterholes, but via an unconventional pathway; that is, fish mortality. The episodic nature of flooding in dryland rivers means that changes in flow regimes, such as water regulation or abstraction, will reduce flooding and hence floodplain subsidies to the river. This is likely to have significant impacts on river productivity.