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 Ltd (BAL)). There were two treatments: 'floc' and 'floc + 20%' (3 x 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.

Microbial and phytoplankton processes, and their effect on water quality were examined over a 3 week period in 5 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 (O 2 ) 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 O 2 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 O 2 production (autotrophy) and net O 2 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 porewater 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.

Shrimp domestication and genetic improvement programmes began in late 1980s, in the United States of America, under the United States Marine Shrimp Farming Program (USMSFP), using the Pacific whiteleg shrimp Penaeus vannamei. The USMSFP was based on proven concepts from the livestock and poultry industries and began with establishing a specific pathogen‐free (SPF) shrimp stock. The original shrimp stock was obtained using rigorous screening of captured wild shrimp for selection of individuals naturally free of major shrimp pathogens. Although the concept of SPF animals was well defined for terrestrial animals, it was relatively new for aquaculture, and it took some time to be adopted by the aquaculture community. In the early 1990s, parallel to USMSFP, several other programmes on genetic improvement of shrimp were also initiated in Latin America. Subsequently, several new terminologies and products, such as specific pathogen resistant (SPR) shrimp, specific pathogen tolerant (SPT) shrimp and even ‘all pathogen exposed’ (APE) shrimp, entered the shrimp industry vocabulary and became commercial. This led to confusion in the shrimp industry about the meaning, relationship and significance of these new terms with respect to SPF. This position paper attempts to clarify these concepts, provide science‐based definitions, reconfirms the importance of developing, maintaining and using domesticated, specific pathogen‐free (SPF) shrimp stocks (which may also achieve SPR and/or SPT status) to reduce the risk of disease outbreaks and increase production and profit. The same principles would apply to development of domesticated SPF stocks for other species used in aquaculture. The paper also discusses the difficulties of confirming and certifying SPF status due to the presence of endogenous viral elements (EVEs) and calls for internationally agreed science and evidence‐based technical guidelines for producing healthy shrimp.

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