Lindsay A. Trott

The impact of shrimp pond effluent on water quality and phytoplankton biomass in a tropical mangrove estuary

Abstract Water quality and phytoplankton biomass were examined over a three-year period in a mangrove estuary receiving periodic inputs of effluent from adjacent shrimp ponds, and in two adjacent, non-impacted estuaries, in north Queensland, Australia. Chlorophyll a, dissolved oxygen (DO), biological oxygen demand (BOD), pH, and salinity at the discharge site in the receiving estuary were significantly higher than in the two control estuaries. There were no significant differences between the impacted and control estuaries in total suspended solids (TSS) and dissolved nutrient concentrations. Water quality and phytoplankton biomass were within ambient levels within 1 km downstream of the discharge site, based on a comparison with long-term, pre-impact data for the estuary. Within 1–2 months after pond discharge ceased, water quality and phytoplankton biomass at the discharge site returned to levels equivalent to those in the control estuaries. The limited spatial and temporal impact suggests that the effluent was dissipated by tides and assimilated and/or mineralized by the estuarine food web. Our results imply that tidal mangrove estuaries have some capacity, at least over short spatial and temporal scales, to process intermittent inputs of pond-derived nutrients.

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.

Rates of nitrogen fixation on coral reefs across the continental shelf of the central Great Barrier Reef

This study was undertaken in 1981 to determine whether there were major variations in potential rates of nitrogen fixation on apparently bare coralline substrate from reefs across the continental shelf of the central Great Barrier Reef. Nitrogen fixation, measured as rates of ethylene production (nmol cm-2h-1), was significantly lower on substrata from two inner-shelf reefs, (0.46 and 1.07) than on two middle-shelf reefs (2.10 and 2.97) and on two outer-shelf reefs (3.20 and 3.81). By contrast, algal biomass (mg cm-2) on experimental substrate was significantly higher on inner-shelf reefs (80.8 and 59.4) than on middleshelf (27.1 and 23.8) and outer-shelf reefs (26.4 and 22.4). The rate of nitrogen fixation was positively correlated with the proportion of “bare” substratum and significantly higher concentrations of dissolved inorganic nitrogen were found in waters over the reefs than in water flowing onto those reefs. The abundance of algal-grazing fishes was reported previously to be significantly lower on inner-shelf reefs. It is suggested that this cross-shelf variation in the activity of algal-grazing fishes may be a determinant of the observed cross-shelf variations in potential nitrogen fixation.

Estimates for the heritability of size in juvenile Penaeus monodon prawns from half-sib matings

Abstract Eighteen half-sib groups of the giant tiger prawn, Penaeus monodon , were obtained using artificial insemination of two females by each male. The total length (mm) and wet weight (mg) of the young were measured at 6 and 10 weeks after hatching. Sire component estimates were approximately 0.10 for both total length and wet weight at each age. Dam components were greater suggesting large non-additive genetic effects and/or common environment effects that could not be differentiated on the available data. A reduction in the dam component for both total length and wet weight from approximately 0.5–0.6 at 6 weeks to 0.3–0.4 at 10 weeks suggested significant maternal effects were present. The results emphasised the strong environmental controls on prawn growth, particularly potential maternal effects that are likely to play a role earlier in development. These influences resulted in highly variable data, emphasising the need for higher degrees of replication of sire and dam groups in order to detect significant sire effects in P. monodon .

Interspecific hybridization of the tiger prawns Penaeus monodon and Penaeus esculentus

Interspecies hybrids were produced by artificial insemination of Penaeus monodon females with P. esculentus males. Successful spawnings were achieved from two of the 13 matings attempted. Hatch rates were low (< 4%) but 85% of the vigorous larvae produced survived to PL1. The hybrid identity of the larvae was confirmed by allozyme analysis at five diagnostic loci. Tank-reared juveniles grew at an overall rate of approximately 0.09 g day−1 to a weight of 10 g in tanks until the prawns were stressed at 4.5 months. A number of viral diseases were expressed subsequently, including IHHNV, and the experiment was terminated. There was no indication of hybrid vigour in growth rates, and the high growth rate of P. monodon was not combined simply with the colour patterns of P. esculentus. A variety of body colour and pattern variants, ranging between those of P. monodon and P. esculentus, was observed in the hybrids.

Rates and pathways of benthic mineralization in extensive shrimp ponds of the Mekong delta, Vietnam

Abstract Benthic mineralization rates and pathways were measured in two extensive shrimp ponds of the Mekong delta, Vietnam. Sediments of both ponds were unconsolidated, oxic to suboxic silt-clays of neutral pH (6.8–7.3). Free sulfides and methane were not detected in the porewater, which was dominated by NH 4 + (up to 400 μM); other interstitial solutes were very variable among replicate cores, ponds, and sediment depths. Particulate C and N concentrations ranged from 2–8% and 0.15–0.36% by sediment dry weight (DW) with few depth differences between ponds. Pyrite was abundant (0.3–5.6% of sediment DW) in both ponds. Total carbon oxidation rates were not significantly different between the pond located separate from mangroves (separate pond) and the pond located within a mangrove forest (mixed pond). Fluxes of O 2 and CO 2 (=total carbon oxidation, T COX ) were highly variable, with slow rates of CO 2 release (range: 7.7–30.5 mmol m −2 day −1 ) but higher rates of O 2 consumption (range: 9.8–135.9 mmol m −2 day −1 ), especially in the separate pond. A budget of the contribution of the various diagenetic pathways to total carbon oxidation indicates that aerobic respiration accounted for 41–60% of T COX , with active manganese and iron reduction in the mixed and separate ponds, respectively. No denitrification or methane flux was detected from sediments of either pond. Rates of sulfate reduction were slow (range: 0.94–2.73 mmol S m −2 day −1 ) and highly variable, accounting for 13–26% of T COX . Rates of solute flux across the sediment–water interface were dominated by DOC, NO 2 − +NO 3 − , and Mn. There was no measurable NH 4 + flux. Most light–dark bottle O 2 fluxes indicated no net benthic primary productivity. The slow rates of benthic decomposition and the dominance of oxic and suboxic pathways reflect the slow rates of organic matter input, and phytoplankton and shrimp production in these extensive ponds.

Denitrification and Anammox in Tropical Aquaculture Settlement Ponds: An Isotope Tracer Approach for Evaluating N2 Production

Settlement ponds are used to treat aquaculture discharge water by removing nutrients through physical (settling) and biological (microbial transformation) processes. Nutrient removal through settling has been quantified, however, the occurrence of, and potential for microbial nitrogen (N) removal is largely unknown in these systems. Therefore, isotope tracer techniques were used to measure potential rates of denitrification and anaerobic ammonium oxidation (anammox) in the sediment of settlement ponds in tropical aquaculture systems. Dinitrogen gas (N2) was produced in all ponds, although potential rates were low (0–7.07 nmol N cm−3 h−1) relative to other aquatic systems. Denitrification was the main driver of N2 production, with anammox only detected in two of the four ponds. No correlations were detected between the measured sediment variables (total organic carbon, total nitrogen, iron, manganese, sulphur and phosphorous) and denitrification or anammox. Furthermore, denitrification was not carbon limited as the addition of particulate organic matter (paired t-Test; P = 0.350, n = 3) or methanol (paired t-Test; P = 0.744, n = 3) did not stimulate production of N2. A simple mass balance model showed that only 2.5% of added fixed N was removed in the studied settlement ponds through the denitrification and anammox processes. It is recommended that settlement ponds be used in conjunction with additional technologies (i.e. constructed wetlands or biological reactors) to enhance N2 production and N removal from aquaculture wastewater.

Seacage aquaculture in a World Heritage Area: the environmental footprint of a Barramundi farm in tropical Australia.

The fate of aquaculture wastes from a seacage farm within a pristine mangrove environment was studied. Seasonal and tidal differences were most important in determining water quality within receiving waters and obscured any nutrient enrichment effect by the farm. Farm wastes added significantly to the N budget status of the creek system, but overall water quality conformed to Queensland EPA Water Quality standards. Mangrove trees throughout the creek system contained (15)N signatures traceable to aquaculture feeds, but the footprint of the farm itself was best indicated by the ratio of Zn:Li in sediments. The creek became hypoxic (<2 mgl(-1)) during wet season low tides. Consequently, we recommended monitoring of water-column oxygen concentrations to warn of hypoxic conditions threatening to fish health, as well as Zn:Li ratios in sediment accumulation zones to determine the area of influence of the farm.

Utility of streamer tags to assess survival and growth of juvenile tiger prawns (Penaeus monodon) in aquaculture environments

Abstract Experiments in tank and pond environments over 5–6 months showed that growth rate of juvenile Penaeus monodon was unaffected by tagging at 15 mm carapace length (CL) or at 18 mm CL. The mortality rate of prawns tagged at 15 mm CL was consistently greater than those tagged at 18 mm CL in both environments, suggesting that the minimum size for tagging to obtain less biased survival estimates is at least 18 mm CL. Survival in the tank of prawns tagged at 18 mm CL was the same as untagged animals (40–50%), but that of prawns tagged at 18 mm CL in the pond (50%) was less than untagged prawns (70%). These results suggest an environment-specific effect on tag-induced mortality: The ability to obtain independent data from untagged animals in aquaculture ponds allowed the effect to be detected. This has been impossible in wild fisheries work and the results in this paper highlight the inadequacy of tagging methods to obtain mortality estimates from wild prawn populations. Tag colour also biased tag recovery rates, but no mechanism to explain the bias was identified. Streamer tags will prove useful in aquaculture as the tags are unlikely to affect important performance characteristics of the marked animals, such as growth rate, and reasonable relative estimates of mortality among tagged groups can be obtained. However, the use of tagged animals to gain accurate estimates of mortality of an untagged group they are meant to represent is unlikely to be successful, even in relatively well-controlled aquaculture situations, because of environment specific effects on tag mortality.