Characterization of water quality factors during intensive raceway production of juvenile Litopenaeus vannamei using limited discharge and biosecure management tools

Abstract

Disease epizootics have negatively affected production and expansion of the shrimp culture industry. This, along with environmental concerns regarding limited water resources and contamination of receiving streams, has caused the industry to investigate more sustainable and biosecure management practices. A study was conducted to evaluate the effect of limited water exchange on water quality, growth and survival of the Pacific white shrimp Litopenaeus vannamei postlarvae (PL) in greenhouse-enclosed raceways. Concentrations of NH₄-N did not exceed 2.0 mg l⁻¹ during this period; whereas, NO₂-N exceeded 26.4 mg l⁻¹, indicating assimilation of primary amines by primary productivity. Periodic removal of suspended solids by a common pressurized sand filter and injection of oxygen into culture water resulted in high-survival rates for both raceways (97.5 and 106.0%) with an average biomass yield of 4.29 ± 0.06 kg m⁻³. Shrimp samples collected during the nursery trial and at harvest showed no signs of bacterial or viral pathogen infections.

Introduction

Rapid expansion of shrimp aquaculture can be attributed to technological advances, high-market demand and reduced supplies of wild stocks (Barg et al., 1999). Despite this growth, the shrimp farming industry has experienced frequent setbacks, largely as a result of catastrophic viral epizootics (Lightner et al., 1997). This problem has persisted into the present and is evidenced by the recent outbreak of Taura Syndrome Virus Disease in South Texas. To reduce potential for infection of commercial shrimp production facilities with catastrophic disease-related viruses, an appropriate and cost-effective strategy towards disease prevention via improved biosecurity is required.

Probably the most common pathway for pathogen introduction into an aquaculture facility is via introduction of water (Lotz and Lightner, 1999). In traditional intensive shrimp culture, a substantial portion of pond water is discharged daily into receiving streams, necessitating similar replacement with source water. According to Timmons and Losordo (1994), the production of 1.0 kg of penaeid shrimp using traditional, flow-through criteria requires approximately 20,000 l of water, which is ultimately released into receiving streams. In the past, acceptable water quality for pond production was maintained largely by release of waste pollutants via continuous large-volume water exchange (Hopkins et al., 1993, Moss et al., 1999). In many cases, no attempt was made to reduce these loads prior to discharge. Discharge of nutrient-rich or high-oxygen demand pond effluent into low recharge receiving streams has precipitated conflict with other potential users (Boyd and Yoo, 1994). Effluent from production systems is typically high in waste nutrients (e.g., nitrogen and phosphorus), total suspended solids (TSS), volatile suspended solids (VSS), biochemical oxygen demand (BOD) and chemical oxygen demand (COD). Because of substantial temporal and chemical variation in discharged pond water (Preston et al., 2000) as well as receiving streams (Trott and Alongi, 2000), the assessment of impact remains complicated.

This discharge issue has generated substantial interest in the shrimp farming industry to achieve zero or limited water exchange in production systems. For these systems to be effective, though, proper feed management, adequate aeration and circulation, natural productivity and nitrogen cycling processes must be carefully managed. The typical operational criteria impacting recirculating aquaculture systems (RAS) as well as their basic design components have been described extensively by Timmons et al. (2001). Several major operational issues impact these types of production systems: management of solids, availability of nutrients to shrimp and other system biota, nutrient ratios and processing of nitrogenous waste.

Solid waste is generated either directly or indirectly from feed added to the system and is present as uneaten feed residuals, metabolites, manures and/or microorganisms growing in the system (Viadero and Noblett, 2002). For traditional open shrimp production systems, Funge-Smith and Briggs (1998) identified feed as a significant source of organic matter (31–50%) but contributing few solids (4–8%) to the system (Funge-Smith and Briggs, 1998). Erosion of pond soil was the major source of both solids (88–93%) and organic matter (40–60%). Because many limited water exchange production ponds and raceways are plastic-lined and have a reduced requirement for input water, it stands to reason that most solids and organic matter are ultimately derived from feed. According to Thakur and Lin (2003), the major water quality-related problem associated with closed system production of shrimp is the potential for rapid eutrophication, resulting from increasing concentration of nutrients and organic matter over the culture period. Ultimately, this buildup impacts carrying capacity of the culture environment (Lin, 1995) to the point that commercial success depends upon striking a balance between waste production and assimilation capacity (Thakur and Lin, 2003). A study conducted in zero-exchange shrimp ponds in Belize (Burford et al., 2003) showed that despite experiencing “poor” water quality in ponds (i.e., high-nutrient concentrations, high and unstable phytoplankton and bacterial numbers), shrimp yields surpassed those achieved in conventional (i.e., traditional) ponds.

The traditional approach to intensive culture of shrimp has used high rates of water exchange (e.g., flushing) and assumed that shrimp derive the majority of their nutrition from feed (as opposed to biotic pond resources). Past and recent researchs (Rubright et al., 1981, Hunter et al., 1987, Moriarty et al., 1987, Leber and Pruder, 1988) have indicated the importance of benthic fauna to penaeid nutrition in ponds. Litopenaeus vannamei cultured in RAS typically use microalgae and associated detritus as food sources (Moss et al., 1999). The contribution of selected fractions of shrimp pond water to growth enhancement of L. vannamei was demonstrated by Moss et al. (1992).

Another area of interest in the development of intensive minimal water exchange systems is the biogeochemical cycling of nutrients important to overall maintenance of the penaeid shrimp culture system. Several recent studies have addressed this cycling via development of either management strategies or nutrient budgets (Avnimelech, 1999, Burford and Williams, 2001, Thakur and Lin, 2003, Burford et al., 2003, Wang, 2003, Otoshi et al., 2003, Jackson et al., 2003). Thakur and Lin (2003) showed for closed culture of Penaeus monodon that despite the major source of nutrient input being feed (76–92% of nitrogen and 39–67% of phosphorus), shrimp could assimilate only 23–31% and 10–13% of these nutrients, respectively. For open intensive ponds, Funge-Smith and Briggs (1998) found only 6–11% of carbon applied was assimilated, implying it was either incorporated into plankton biomass, volatilized or trapped in sediments. The main source of dissolved nitrogen in ponds is excretion from shrimp gills as ammonia; however, a substantial amount of dissolved organic nitrogen (DON) comes from feed (dissolved primary amines) and feces, as urea (Burford and Williams, 2001). High-density, zero exchange culture of shrimp in plastic-lined ponds in Belize (Burford et al., 2003) resulted in high concentrations of dissolved inorganic nitrogen (DIN; 2.29–5.56 mg l⁻¹), DON (0.17–10.66 mg l⁻¹), DOC (14.2–48.1 mg l⁻¹) and phosphate (0.07–1.17 mg l⁻¹). With added aeration, a high level of bacterial flocculation occurs. This results in establishment of a high DOC/DN ratio in culture water, which ultimately promotes breakdown of organic matter, and reduces both DIN and feed input (Avnimelech et al., 1994, Avnimelech, 1999, Browdy et al., 2001).

There is currently strong research interest in establishing a proper ratio between carbon and nitrogen levels in shrimp culture water in order to enhance nitrogen cycling and promote growth of bacteria as a low-cost nutrient source. Avnimelech (1999) used glucose to reduce inorganic nitrogen in experimental tanks containing P. monodon. He suggested that feeds containing 30% protein could be amended by an additional 46.5% carbohydrate in order to assimilate ammonium flux into microbial protein. This called for use of a 20.48% CP feed having a C/N ratio of 15.75:1. Net uptake of DIN occurs only when C/N ratio of organic matter in coastal marine ecosystems is higher than 10 (Lancelot and Billen, 1985). However, in intensive closed shrimp pond production, Burford et al. (2003) showed that increased pond age (i.e., increased addition of carbon) had no major effect on heterotrophic status of the pond and that due to high availability of nitrogen and phosphorus to phytoplankton, ponds fluctuated between autotrophy and heterotrophy. They suggested that a heterotrophic system might only be achieved if feed carbon input exceeded that of primary production.

In order to improve production scheduling, most limited water exchange production systems require integration with a preliminary nursery phase. Use of this phase has long been recognized to have several benefits over direct stocking of PL in commercial shrimp production operations (Sandifer et al., 1991). Intermediate production of juvenile shrimp in smaller raceway impoundments at high densities has been shown to improve survival and feeding efficiency (Samocha and Benner, 2001, Samocha et al., 2000, Samocha et al., 2002, Samocha et al., 2003). From the standpoint of space utilization, it is more efficient to stock production ponds with larger, juvenile shrimp which will also reduce production cycle duration. These systems must be managed under biosecure criteria with simple strategies towards management of dissolved nitrogen and solids to avoid potentially toxic and eutrophic conditions. The present study investigated the use of phytoplankton in combination with periodic solids removal and oxygen supplementation as management tools for the production of juvenile shrimp in plastic-lined limited water exchange shrimp nurseries.

The objectives of this study were to (1) characterize change in various physiochemical water quality factors during duplicate limited water exchange raceway production trials of juvenile shrimp; (2) evaluate maintenance of biosecurity conditions; and (3) based upon results, suggest means whereby the present research system could be modified for commercial production.