Water Quality Maintenance and Mineral Assimilation by Plants Influence Growth of Hybrid Tilapia in Culture with Vegetable Crops

TL:DR;  The study investigates the integration of fish and vegetable production in a recirculating aquaculture system, focusing on the effects of varying tank-to-biofilter volume ratios on fish growth, plant yield, and water quality. Key findings include:

• System Design: Hybrid tilapia were grown in tanks, and tomatoes and cucumbers were cultivated in biofilters using sand as a substrate. The system employed biofilters that alternately flooded and drained, enhancing nutrient distribution and aeration.
• Biofilter Volume Impact: Increasing biofilter volume generally improved fish growth rates and biomass production, reduced feed conversion ratios, and enhanced water quality by decreasing nitrogenous compounds like TAN and NO₂⁻. Larger biofilters also supported better nutrient uptake by plants, contributing to stable water pH and improved fish health.
• Water Quality and Nutrient Management: Larger biofilters increased dissolved oxygen levels and reduced harmful nitrogen concentrations, although pH management required amendments in some experiments. The integration of plants helped assimilate nutrients, reducing the need for chemical adjustments.
• Efficiency and Sustainability: The study highlights the potential of integrated aquaculture systems to produce high yields of both fish and vegetables efficiently, particularly in resource-limited settings. Optimizing the tank-to-biofilter ratio is crucial for maximizing productivity and sustainability.

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1M.R. McMurtry2, R.G. Hodson3 and D.C. Sanders4,5
University of North Carolina Sea Grant College Program and Department of Horticultural Science, North Carolina State University, Raleigh, NC 27695

Additional index words. biofiltration, Cucumis sativus , hydroponics, integrated aquaculture, Lycopersicon esculentum, olericulture, Oreochromis mossambicus, Oreochromis niloticus, sand culture.

Partial funding for this research is from the United States Department of Agriculture Special Grant P.L. 89-106: “Agricultural Adjustment in Southeast Through Alternative Cropping Systems.” Additional funding was from a grant by the “Orange Presbytery” of the Presbyterian Church of North Carolina. 2 Research Associate, University of North Carolina Sea Grant Program, and The Office of International Programs, North Carolina State University. 3 Associate Director, University of North Carolina Sea Grant Program and Associate Professor, Department of Zoology, North Carolina State University. 4 Professor, Department of Horticultural Science, North Carolina State University. 5 The authors gratefully acknowledge the assistance of L. Barrons, M. Buchanan, P. David, DeRuiter Seeds Inc., B. Haning, R. Jones, P. Lineberger, N. Mingis, P. Nelson, R. Patterson, M. Pridgen, C. Prince, C. Spivey, J. Stoop, R. Tucker, Rex Plastics and the University of Hawaii for their help on the project.

Submitted for publication to The Journal of the World Aquaculture Society, 1994, Paper No. of the Journal Series of the North Carolina Agricultural Research Service, Raleigh, NC 27695-7643. Mention of a trademark, proprietary product, or vendor does not imply its approval to the exclusion of other products or vendors that may also be suitable.

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ABSTRACT

Fish and vegetable production were linked in a recirculating water system. Hybrid tilapia (Oreochromis mossambicus (Peters) x O. niloticus (L.)) was grown in tanks and fed a 32% protein feed. Tomato (Lycopersicon esculentum Mill. ‘Laura’) was grown in summer 1988, cucumber (Cucumis sativus L. ‘Fidello’) in fall 1988, and tomato ‘Kewalo’ in spring 1989 in a Raleigh, N.C., greenhouse. Four tank to biofilter volume ratios were studied.Plants were grown in biofilters at 4 plants m-2 and irrigated 8 times daily with water from the associated fish tank. Biofilter drainage was returned to the associated tank. Each system received identical nutrient inputs and each plant received equal water. Biological filtration, aeration, and plant assimilation of minerals and N-compounds maintained water quality suitable for tilapia production.Dissolved oxygen levels, make-up water inputs, fish biomass and fish growth rates increased with biofilter volume. Total ammoniacal-N, NO2-, and NO3- concentrations decreased with increasing biofilter volume. Water pH declined rapidly when the systems were operated without plants. When horticulture was included, water pH remained stable at approximately pH 6.0.Fruit yields per unit increase in fish biomass and per biofilter increased with increasing biofilter volume. Fruit yields and fish biomass increase per plant declined with increasing biofilter volume. Fish growth associated with the largest biofilter was 120% of that associated with the smallest biofilter.

INTRODUCTION

Recirculating aquaculture water has been used for hydroponic cultivation of higher plants (Lewis et al. 1978a, 1978b, 1981; McMurtry 1990; McMurtry et al. 1990, 1993a, 1993b, 1994; Näegal. 1977; Nair et al. 1985; Rakocy 1989a, 1989b; Watten and Busch 1984).

All previous plant-integrated aquaculture systems, other than those reported by this author and Rakocy, have specified the removal by sedimentation of more than 95% of the suspended-solid fraction of the waste products from the culture water prior to plant applications (Rakocy 1989b).

Hydroponic vegetable production has been demonstrated to reduce NO3- concentrations in recirculating aquaculture water (Lewis et al. 1978a, 1978b, 1981; McMurtry et al. 1990, 1993a, 1993b, 1994; Nair et al. 1985; Kane 1987; Rakocy 1989a, 1989b), and eliminated the need for subsequent microbial denitrification.

Biofilters that are alternately flooded and drained were first proposed by Lewis et al. (1978) and are referred to as reciprocating biofilters (RBF). Advantages of a RBF are:

1. Uniform distribution of nutrient-laden water within the filtration medium during the flood cycle
2. Improved aeration of the biofilter from atmosphere exchange with each dewatering (Lewis et al. 1978; McMurtry et al. 1990; Paller and Lewis 1982; Nair et al. 1985; Rakocy 1989a)

These advantages benefit both the nitrifying bacteria and plant roots (Hopkins et al., 1950; Lewis et al. 1978; Paller and Lewis 1982; Rakocy 1989a, 1989b). Nitrification is limited by oxygen concentrations lower than 2 mg L-1 (Nair et al. 1985) and complete oxidation of 1 mg of NH3-N requires 4.6 mg of oxygen (Kaiser and Wheaton 1982).Benefits of integrating aquaculture and vegetable horticulture (olericulture) are:

1. Conservation of water resources and plant nutrients (McMurtry et al. 1990, 1993a)
2. Intensive production of fish protein
3. Reduced operating costs relative to either system in isolation (McMurtry et al. 1994)

The constraints of water supply, soil type and land availability do not limit the use of recirculating systems as occurs in pond or cage aquaculture (Rakocy 1989a). Water consumption in integrated systems including tilapia production is less than 1% of that required in pond culture to produce equivalent yields (McMurtry 1990; McMurtry et al. 1990, 1994; Rakocy 1989b).

Such a symbiotic system is applicable to the needs and requirements of arid or semi-arid regions where fish and fresh vegetables are in high demand (Nair et al. 1985; Rakocy 1989b; McMurtry et al. 1990). Organic vine-ripened, pesticide-free produce and ‘fresh-daily’ fish can bring premium prices, particularly during winter months in urban areas. Markets for fresh fish abound in landlocked regions and overfished coastal areas throughout the world (Nair et al. 1985; Rakocy 1989b).

Proper management of integrated systems requires the maintenance of a nutrient balance to maximize both fish and plant yields (McMurtry et al. 1993a, 1993b; Rakocy 1989b).

The objective of these studies was to evaluate the influence of fish tank to biofilter volume (BFV) ratio on fish growth rate and water quality. Plant assimilation of nutrient residual from fish production (biofilter plant population proportional to BFV) on water quality was evaluated.

MATERIALS AND METHODS

Fish Cultivation

All male (sex-reversed) hybrid tilapia (Oreochromis mossambicus (Peters) x O. niloticus (L.), Cichlidaceae) were cultivated in 500 liter in-ground tanks. Aeration was provided by regenerative blowers at a flow rate of 0.7 L.s-1 through two (3.8 x 3.8 x 15 cm) airstones per tank.

Water temperatures were kept above 25°C by two Visitherm™ 250W thermostatic aquaria heaters per tank.The rectangular tanks were formed with plywood, the bottom sloped to 45° and lined with 0.50 mm (2 @ 10 mil.) black polyethylene (Fig. 1). Each tank was coupled to a biofilter employing a builder’s grade sand as substrate. Tank water level at capacity was 10 cm below the bottom of the biofilter.

Biofilter Design

Biofilters were 1.2 m wide, 0.33 m deep and of variable length to achieve 4 ratios by volume to the fish tank (Table 1). They were lined with 0.45 mm (three @ 6 mil.) polyethylene plastic and the bottom sloped 1:200 along the length to direct drainage for return to the associated tank.

Media composition was:

• 99.25% quartz sand
• 0.75% clay
• 0.0% silt

Sand fractionation:

• Very fine sand: 1.1%
• Fine sand: 5.2%
• Medium sand: 21.0%
• Coarse sand: 38.8%
• Very coarse sand: 33.3%

Four tank-to-BFV ratios, bracketing that used in preliminary studies, were selected as treatments (McMurtry et al. 1990). Each tank-to-biofilter ratio was replicated with four independent systems per ratio.

Experimental Setup

Experiments were conducted in a polyethylene-covered greenhouse in Raleigh, N.C. Infection with the soil-borne bacterial pathogen Pseudomonas solanacearum (Smith) Smith was anticipated from experience in preliminary studies.

Preplant fumigation of the sand with methyl bromide-chloropicrin (98-2) was made at 250 kg ha-1.Each biofilter was inoculated with 1.0 liter of Fritz-zyme #7 (a suspension of Nitrosomonas Winogradsky sp. and Nitrobacter Winogradsky sp.), and irrigated with aquaculture effluents for nine days prior to planting the first vegetable crop. Tomato (Lycopersicon esculentum Mill.) or cucumber (Cucumis sativus L.) seedlings were transplanted into each biofilter at four plants m-2 in each study. Plant populations of 4, 6, 9, or 14 plants per biofilter were directly proportionate to the respective BFV.

Fish Feed and Feeding

The fish were fed a diet of modified Purina Fish Chow 5140, with a minimum analysis of:

• 32% crude protein
• 3.5% crude fat
• Not more than 7.0% crude fiber

The feed was not fortified with vitamins or trace elements (Table 2). The daily feed input rate was based on a percentage of standing fish biomass as influenced by age and mean individual weight (Pullen and Lowe~McConnell 1982). The daily ration was divided equally into two feedings administered at 0800 and 1300 hours.

The fish also grazed algae (Oscillatoria Vaucher spp., Cyanophyta and Ulothrix Kützing spp., Chlorophyta) which grew in the water and on the tank sides.

Feed Analysis

Fish food was analyzed using:

• Atomic absorption spectrophotometry for K, Ca, Mg, Fe, Mn, Zn, and Cu.
• Vanadomolybdophosphoric yellow procedure (Jackson 1958) for P.
• Kjeldahl procedure (Black et al. 1965) using a salicylic acid modification for N.
• Curcumin method (Grinstead and Snider 1967) for B.
• Turbidimetric procedure (Hunter 1979) for S.

Analyses are reported on a dry weight (DW) basis.

Irrigation System

Irrigation water was pumped from the bottom of the fish tanks eight times daily and delivered to the biofilter surfaces at a rate of 500 L m-2 of biofilter surface per day. The water flooded the biofilter surfaces, percolated through the medium, and drained back to the fish tank. The tank water level dropped approximately 25 cm during each irrigation event. Therefore, the returning water provided additional aeration resulting from the effect of the cascade. Biofilters drained intensively (rapidly) for approximately 15 minutes following cessation of irrigation and at a diminished rate for one hour.

Evapotranspiration losses were replaced weekly with city water (McMurtry et al. 1994). Input water composition and pH were reported by McMurtry (1990). Culture water pH and temperature measurements were made in situ at random times daily with an Orion SA250 ATC pH meter using a Fisher double-junction pH electrode and Orion ATC probe.

Water Quality Monitoring

Diurnal modulation of pH, temperature, total ammoniacal-N (TAN), NO2^-, and NO3^- levels were assayed weekly. In the diurnal assay, the culture water of each tank was sampled prior to each filtration event, the irrigate sampled during each filtration event, and drainage from each biofilter was sampled prior to tank return. Values obtained from the random assays were compared with those taken at the same hour in the diurnal sampling of the same week.

Water samples of 120 ml were drawn at the time of each water pH assay from the top of each tank, titrated to pH 2.0, sealed, and stored at 5°C for up to two weeks prior to assays for nitrogenous compounds. Aqueous TAN and NO2^- concentrations were assayed on an Orion SA270 Ion Specific Electrode (ISE) meter using Fisher NH(3+4) and NO2^-, ISE electrodes. Aqueous NO3^- concentrations were assayed on an Orion Research Ionalyzer model 407A meter with a Fisher NO3^- ISE electrode and were verified using a modified salicylic acid and NaOH colorimetric procedure (Cataldo et al. 1975) with a Beckman model DB-G grating spectrophotometer. Culture tank dissolved oxygen (DO) measurements were made at 0730 and 1300 hours in situ with an Otterbine Barebo 111 DO meter at least weekly. Methyl orange alkalinity was determined by titration.

Fish Biomass Measurement

Fish biomass was determined after removal of all fish from the tank. The fish were sedated with Quinadine, blotted dry, and weighed individually. Fish biomass increase per time interval was calculated by subtraction of the respective stocked biomass. Fish were returned to the same tank with adjustments made (fish added or removed) to maintain a uniform (±2.5%) biomass between tanks.

The following metrics were calculated:

• Feed conversion ratio (FCR)
• Monthly production rate (MP)
• Monthly specific growth rate (MSG)
• Daily rate of increase in biomass (DRIB)

Experimental Design

The experiments were conducted as a randomized complete-block design with four replicates. Analyses of variance were made for factorial experiments with Statview™ 512+ on a personal computer. When F-test(s) warranted, LSDs were calculated.

Experiment 1

Fish were stocked on 5 May 1988 at a uniform stocking density, mean individual weight (Pmi), and initial biomass (Bi) as seen in Table 3a. An initial feeding rate of 4.3% of Bi d-1 was increased when inputs were consumed within 15 minutes. Daily feed input increased with fish biomass and was 2.2% of final biomass (Bf) per day at harvest 99 days from stocking.

Tomato ‘Laura’ was transplanted into the biofilters on 13 May 1988. This indeterminate greenhouse variety was grown as a single-stem. Fruit were harvested at the incipient color stage (McMurtry 1993b). The crop was terminated after harvest at four trusses.

 

Experiment 2

Fish were restocked on 25 August 1988 so that expected Bf during the succeeding interval would be lower than the 17 kg m-3 occurring in Experiment 1. Stocking densities, Pmi, and Bi are given in Table 3b. A feed rate of 5.0% of Bi d-1 was maintained until the fish were harvested after 42 days.

The system was operated for 42 days without plants grown in the biofilters to assess whether or not olericulture was contributing to pH buffering of the water. Incremental additions of CaMg(CO3)2 were made to each biofilter after water pH fell below pH 4.0 in order to raise water pH and reestablish nitrification.

Fish biomass per tank was equalized across treatments by removing the largest individuals in appropriate tanks prior to replanting the biofilters. Feed input rate was adjusted to 3.4% of Bi d-1 and maintained until fish were harvested at 85 days from restocking. Cucumber ‘Fidello’ was transplanted into the biofilters on 22 September 1988 and pruned to a single-stem.

Following CaMg(CO3)2 inputs, water pH in most tanks remained below pH 6.0, which was deemed too low for balanced nutrient assimilation by cucumber. Therefore, CaO was added to the tank water approximately twice weekly in quantities sufficient to raise water pH in each tank to above 6.5 following each application.

Experiment 3

Fish were stocked on 5 January 1989 at a uniform stocking density, Pmi, and Bi as seen in Table 3c. An initial feed rate of 1.8% of Bi d-1 was reduced gradually when feed remained uneaten for more than 15 minutes. Fish were harvested 132 days from stocking. The semi-determinate, bacterial wilt-resistant tomato ‘Kewalo’ was planted on 5 January 1989 and grown as a single-stem (McMurtry 1993b). Fruit were harvested at the incipient color stage.

RESULTS

Experiment 1

Mean fish growth rate (G) and the increase in total biomass increased with increasing BFV while the MSG and DRIB were not significantly different but tended to increase with BFV (Table 3a). Mean FCR tended to decrease as BFV increased. Mean individual size at harvest (Pmf) was not different among treatments while Bf and MP differed among treatments.

Diurnal mean DO concentration increased as water temperature decreased with increasing BFV (Table 4a). Water DO concentrations ranged from 4.8 and 7.8 mg L-1 with minimal day to day variation (data not shown). Total alkalinity increased from 40 mg L-1 in week one to 180 mg L-1 by week five, but remained stable through week eight and was not assayed thereafter (data not shown).

The TAN and NO2- concentrations decreased with increasing BFV (Table 4a). Initial TAN concentrations increased from 0.0 mg L-1 over the first seven weeks to mean high levels ranging from 10.8 to 30.2 mg L-1 with decreasing BFV (data not shown). Initial NO2- concentrations increased from 0.0 mg L-1 over the first four weeks to mean high levels ranging from 3.0 to 8.1 mg L-1 with decreasing BFV (data not shown).

At termination of the tomato crop, TAN and NO2- concentrations ranged from 0.7 to 1.1 mg L-1 and 0.02 to 0.07 mg L-1, respectively (data not shown).Mean water pH generally decreased with increasing BFV (Table 4a). Water pH increased from pH 6.5 to 7.4 in each treatment over the first 2 weeks as bacterial and plant populations became established (data not shown).

Water pH declined to approximately pH 6.0 in all treatments by week 5 and remained stable through termination of the tomato crop (data not shown). Total make-up water increased with BFV and water consumption per unit biofilter area declined with increasing BFV (Table 4a). No amendments were made to adjust water pH.

Experiment 2

Water pH declined rapidly from approximately pH 6.0 in all treatments to pH 4.3 or less during the interval with no crop in the biofilters (data not shown). Subsequent CaMg(CO3)2 amendment, given in Table 4b, raised the mean pH to 5.5 or greater (data not shown).

The mean fish biomass increase ranged from 1.88 to 3.04 kg m-3 and G ranged 1.85 to 2.74 g fish-1 d-1 at 42 days from stocking. The FCR ranged from 1.43 to 3.50, but there was no consistent trend with BFV (data not shown). The ending feed input rate was 3.1% of Bf d-1.Water pH at termination of the cucumber crop was pH 6.0, 5.5, 5.8, and 6.4 with increasing BFV, respectively (data not shown).

Cucumber yield per biofilter was 11.18, 10.04, 11.41, and 33.32 kg and yield per plant was 2.80, 1.67, 1.27, and 2.38 kg with increasing BFV, respectively (data not shown). Correlation of diurnal mean pH and fruit yield per biofilter within treatments were 0.992, 0.901, 0.968, and 0.928 with increasing BFV, respectively (r2= 0.984, 0.812, 0.937, and 0.861 with P= 0.008, 0.099, 0.032 and 0.072, respectively).

Feed input rate at day 85 from transplant of cucumber was 1.0% of Bf d-1.Composite 127 day fish growth rates (G, MSG and DRIB) and fish biomass increase tended to increase with BFV (Table 3b). Composite 127 day FCR tended to decrease as BFV increased. Mean Pmf did not differ between any treatment combination. The Bf and the MP rate increased with BFV through the 1:.1.50 v/v ratioMean water temperature generally declined with increasing BFV (Table 4b). Differences in water pH were not correlated to BFV.

Total make-up water increased with BFV and water consumption per unit area declined with increasing BFV. Lime amendment was identical across treatments while CaO amendments were inversely proportional and negatively correlated to mean water pH over time (CV= -4.84, CR= -0.82, r2= 0.673, P= .0001).

Experiment 3

The G, MSG, and DRIB rates did not differ significantly but tended to increase with BFV (Table 3c). The FCR in response to BFV was inconsistent. The fish biomass increase, Bf and MP did not differ among treatments. The feed input rate at day 77 was 0.9% of Bi d-1 and was 0.6% of Bf d-1 by the end of the 132 day feeding regime (data not shown).The DO levels increased with BFV (Table 4c). Water DO concentrations ranged from 5.6 and 6.1 mg L-1 with minimal day to day variation (SD=0.31, data not shown).

Water temperature decreased with increasing BFV (Table 4c).The TAN, NO2- and NO3- concentrations decreased with increasing BFV (Table 4c). Mean NO3- concentrations differed between the 1: 2.25 v/v treatment ratio and each other ratio. The TAN and NO2- concentrations initially ranged from 0.03 to 0.20 mg L-1 and 0.05 to 0.10 mg L-1, respectively, and increased over 2 and 10 weeks to mean high levels ranging 1.18 to 1.49 mg L-1 and 0.06 to 0.35 mg L-1, respectively, with decreasing BFV (data not shown).

At peak tomato harvest the TAN and NO2- concentrations ranged from 0.29 to 0.32 mg L-1 and 0.06 to 0.09 mg L-1, respectively, with decreasing BFV (data not shown). The NO3- concentrations increased with BFV, initially ranged 88 to 230 mg L-1, increased for 2 weeks to a range of 99 to 246 mg L-1, and at peak tomato harvest had declined to 30 to 241 mg L-1 (data not shown).Mean water pH tended to increase with BFV but differences were not significant because CaO inputs were made to maintain levels above pH 6.0 (Table 4c).

Total makeup water increased with BFV and water consumption per unit area declined with increasing BFV. Water pH had remained low following Experiment 2 and weekly additions of CaO were made until pH remained above pH 6.0 in all treatments (data not shown). Total CaO input to each tank was negatively correlated to mean pH (CV = -13.04, CR.= -0.86, r2 =0.732, P= .0001) (Table 4c). Water pH remained stable through termination of the tomato crop following the CaO inputs (data not shown).

 

Total Fish Growth and Mean Water Quality in Experiments 1, 2 and 3

The fish biomass increase in all experiments and the G, MSG, and DRIB rates increased or tended to increase with BFV (Table 5). Total fish biomass increase per plant decreased with increasing BFV while cumulative fruit yields per kg fish biomass increase increased with BFV.

The average water DO concentration increased and temperature, TAN, NO2-, and NO3- decreased with increasing BFV (Table 6). Mean water pH over time was not related to BFV. Total make-up water increased and water consumption per unit area declined with increasing BFV. Inputs of CaO were negatively correlated to diurnal mean water pH (CV.= -15.14, CR = -0.75, r2= 0.554; P= 0.0009).

Water drawn from fish tanks for irrigation had TAN and NO2- concentrations approximately twice that of the water returning after biofiltration (Fig. 2). The percentage reduction in TAN and NO2- concentrations with each filtration event decreased with increasing BFV (data not shown). Percent reduction in NO3- concentration with each filtration event was much less than TAN or NO2- (data not shown).Fish growth rates from other recirculatory systems that included olericulture were compared, contrasting similar Pmi, Pmf and culture intervals (Table 7). Growth rate (G) was negatively correlated to stocking density, regardless of culture system (Fig. 3).

The MP per unit volume, regressed on stocking density, was found to be greater in this system (study) than in all other previous systems (studies) that had removed the suspended solid waste fraction prior to olericulture application of effluent. Mean MP from the three other culture techniques used in this comparison, adjusted to a uniform stocking density of 100 m-3, would be 3.0 kg m-3 as compared to the treatment mean MP of 5.8 kg m-3 resulting from this study.

DISCUSSION

A rapid decline in FCR was observed in the first experiment when standing fish biomass exceeded 12 kg m-3 regardless of BFV. Fish were stocked in Experiments 2 and 3 so that expected Bf would not exceed 10 kg m-3 in order to minimize the quantity of non-ingested feed. The differential in fish weight gain between experiments is attributed to the differences in Pmi and stocking density.

Fish production in Experiments 2 and 3 was limited by a reduction in number of individuals cultured and by their relatively large Pmf. Growth rate (G) was similar between experiments. Because FCR declines with increasing fish size and/or age (Pullen and Lowe-McConnell, 1982), the feed input per mean standing fish biomass and per fish biomass increase was greater in Experiments 2 and 3 than in Experiment 1.Biofiltration maintained water quality at acceptable levels for tilapia.

Nitrogenous compounds, which frequently limit production in recirculatory aquaculture (Lewis et al. 1978), never reached toxic levels and were extracted by the plants (McMurtry 1990, McMurtry et al. 1993a). Yield of both fish and fruit per biofilter increased with BFV in both studies. Mean fruit yield per biofilter ranged 13.66 to 31.65 kg in Experiment 1 and ranged 19.88 to 33.11 kg with increasing BFV (McMurtry 1993b). Increased nutrient uptake by the plants with increasing yield resulted in improved water quality, and therefore, increased fish growth rates with increasing BFV (McMurtry 1993a, 1993b).

The rate of thermal energy transfer between the water and filter substrate increased with biofilter mass resulting in lower diurnal-mean water temperatures with increasing BFV. Microbial conversions and plant assimilation maintained sub-lethal concentrations of aqueous N-compounds although the assayed levels were in excess of reported toxicities of 48 h LD50 = 2.4 mg NH3-N L-1 (Redner and Stickney 1979) and 0.45 mg NO2–N L-1 (Balarin and Haller 1982) for tilapia.

No clinical signs of nitrite toxicity were detected and the fish grew well.

Traditional recirculatory aquaculture has relied on carbonate inputs to neutralize the acidification resultant in nitrification (Rakocy 1989b). Alkaline amendment was not necessary when N input rate approximated N assimilation rates, as in Experiments 1 and 3. This was believed to be due to:

1. Nitrification occurring in the biofilters where organic matter accumulated to provide buffering capacity
2. Both ammoniacal-N and NO3–N was available to plants
3. Plant N uptake was mainly NO3- which increased alkalinity of the medium

Availability of both NH4+ and NO3- ions buffers nutrient solution pH during plant nutrient assimilation (Haynes and Goh 1978; Noggle and Fritz 1983) and NO3- uptake was in exchange for OH- ions or bicarbonate ions produced during respiration (Kirkby and Hughes 1970; Riley and Barber 1971). The need for CaO amendments in Experiment 3 were considered to be due to residual acidity from Experiment 2. Once water pH was reestablished within an acceptable range for plant growth (pH 6.0-6.5), it remained stable through the conclusion of Experiment 3.

Buffering of water pH also may be attributed to NH4+ reacting with OH- ions released during plant anion adsorption to form NH4OH (Noggle and Fritz 1983) or to carbonate and/or bicarbonate ions formed in the reaction of ammonia gas, CO2 and H2O (Berber 1968).

Comparison of growth and production levels between culture systems is complicated by Pmi and Pmf, stocking density, and feed quality. Good tilapia growth rates were attributed partially to water pH remaining below pH 7.0. The greatest percentage of ammoniacal-N generated in fish metabolism remains non-toxic to fish at pH levels <7.0. Fish would have reduced their feeding activity if pH had increased above pH 7.0 (Rakocy 1989a).

Ammoniacal-N concentrations can be regulated by adjusting feed input rate (Rakocy 1989a). Optimum ratios between feed input rate, standing fish biomass, system water volume, and biofilter volume needs to be established for various combinations of fish and vegetable species (Rakocy 1989b).Uniform crop development and satisfactory performance of this system can be partially attributed to the reciprocating water movement. Muir (1982) found that high oxygen availability in the biofilter favored nitrifying bacteria over heterotrophic aerobes and starch hydrolyzers that compete for attachment sites.

This integrated food-production technique produces good yields of both fish and vegetables and reduces total production costs relative to separately operated culture systems (Rakocy 1989b; McMurtry 1990; McMurtry et al. 1994).

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McMurtry, M.R., D.C. Sanders, R.P. Patterson, and A. Nash. 1993b. Yield of tomato irrigated with recirculating aquaculture water. J. Prod. Agric. 6(3):331-332, 428-432.
McMurtry, M.R., D.C. Sanders, R.G. Hodson, and B.C. Haning. 1994. Food value, water use efficiency, and economic productivity of an integrated aquaculture-olericulture system as influenced by component ratio. HortTechnology (accepted for publication).
Muir, J.F. 1982. Recirculated water systems in aquaculture. Pages 357-447 in Muir, J.F. and R.J. Roberts, editors. Recent Advances in Aquaculture. Croom Helm Limited, London.
Nägel, Ludwig C.A. 1977. Combined production of fish and plants in recirculating water. Aquaculture 10:17-24.
Nair, A., J.E. Rakocy, and J.A. Hargreaves. 1985. Water quality characteristics of a closed recirculating system for tilapia culture and tomato hydroponics. Pages 223-254 in Proceedings, Second International Conference on Warmwater Aquaculture Finfish. Division of Continuing Education, Brigham Young University, Laie, Hawaii.
Noggle, G.R. and G.J. Fritz. 1983. Introductory Plant Physiology, 2nd edition. Prentice-Hall, Inc., Englewood Cliffs, NJ. 627 p.
Paller, M.H. and W.M. Lewis. 1982. Reciprocating biofilter for water reuse in aquaculture. Aquacultural Engineering 1:139-151.
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Rakocy, J.E. 1989a. A recirculating system for tilapia culture and vegetable hydroponics in the Caribbean. 24 p. in Proceedings, Auburn Symposium on Fisheries and Aquacultures, Sept. 20-22, 1984. Brown Printing Co., Montgomery, Alabama (in press).
Rakocy, J.E. 1989b. Vegetable hydroponics and fish culture, a productive interface. World Aquaculture 20:42-47.
Redner, B.D. and R.R. Stickney. 1979. Acclimation of ammonia by Tilapia aurea. Transactions of the American Fisheries Society 108:383-388.
Riley, D. and S.A. Barber. 1971. Effect of ammonium and nitrate fertilization on phosphorus uptake as related to root-induced pH changes at the root-soil interface. Soil Science Society of America Proceedings 35:301-306.
Watten, B.J. and R.L. Busch. 1984. Tropical production of tilapia (Sarotherodon aurea) and tomatoes (Lycopersicon esculentum) in a small-scale recirculating water system. Aquaculture 41:271-283.

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Table 1 presents the physical parameters of different tank-to-biofilter ratios used in an integrated aquaculture system. This system combines fish culture with vegetable production, using the biofilter as a medium for plant growth and water filtration. Here is an explanation of the table’s columns and their significance:

• Biofilter Ratio (v/v): This column shows the volume ratio of the biofilter to the fish tank. The ratios range from 1:0.67 to 1:2.25, indicating increasing biofilter volumes relative to the tank volume.
• Water : Biofilter (v/v): This column lists the volume ratio of water to the biofilter, which corresponds to the biofilter ratio. For example, a 1:0.67 ratio means the biofilter volume is 67% of the water volume.
• No. Plants (a/a): This column indicates the ratio of plants to the biofilter volume. As the biofilter volume increases, the number of plants also increases, reflecting a proportional relationship.
• Irrigation (m-2) (plot-1) (liter m-2 d-1): This column provides three pieces of information:
  • (m-2): The density of plants per square meter of biofilter, which is consistently 4.0 plants/m² across all treatments.
  • (plot-1): The total number of plants per plot, which increases with larger biofilters (4, 6, 9, and 14 plants respectively).
  • (liter m-2 d-1): The amount of water used for irrigation per square meter per day, which remains constant at 500 liters/m²/day for all treatments.

Table 1 highlights how increasing the biofilter volume allows for more plants to be grown, which can enhance the system’s capacity to filter water and assimilate nutrients, thus maintaining water quality for fish culture.

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Table 3 examines the influence of different tank to biofilter volume ratios on fish stocking, growth, and harvest variables across three experiments. Each experiment involved different crops in the biofilters: Laura tomato, Fidello cucumber, and Kewalo tomato.

Experiment 1: Laura Tomato in Biofilters

• Biofilter Ratios: 1:0.67, 1:1.00, 1:1.50, 1:2.25
• Growth Metrics: Mean individual weight at stocking (Pmi) and mean biomass at stocking (Bi) were similar across treatments. The average growth rate (G) and monthly specific growth rate (MSG) increased with biofilter volume, although not significantly. The feed conversion ratio (FCR) tended to decrease with larger biofilters, indicating more efficient feed use.
• Harvest Metrics: Mean weight at harvest (Pmf) did not vary significantly, but mean biomass at harvest (Bf) and monthly production (MP) increased with larger biofilters.

Experiment 2: No Crop Interval and Fidello Cucumber

• Biofilter Ratios: 1:0.67, 1:1.00, 1:1.50, 1:2.25
• Growth Metrics: Similar trends were observed with growth rates increasing and FCR decreasing as biofilter volume increased. The daily rate of biomass increase (DRIB) was higher with larger biofilters.
• Harvest Metrics: Mean biomass at harvest (Bf) and monthly production (MP) increased with biofilter volume, similar to Experiment 1.

Experiment 3: Kewalo Tomato in Biofilters

• Biofilter Ratios: 1:0.67, 1:1.00, 1:1.50, 1:2.25
• Growth Metrics: Growth rates (G, MSG, DRIB) showed a tendency to increase with biofilter volume, though not significantly. FCR was inconsistent across treatments.
• Harvest Metrics: Mean biomass at harvest (Bf) and monthly production (MP) did not show significant differences among treatments.

General Observations

• Biofilter Volume Impact: Across all experiments, increasing the biofilter volume generally improved fish growth rates and biomass production, while reducing the feed conversion ratio, indicating more efficient feed utilization.
• Water Quality: Larger biofilters contributed to better water quality, with decreases in nitrogenous compounds like TAN and NO₂⁻, which are crucial for maintaining healthy fish growth environments.
• Nutrient Uptake: The integration of vegetable crops in biofilters helped assimilate nutrients, improving water quality and supporting fish growth.

These results suggest that optimizing the tank to biofilter volume ratio is crucial for maximizing both fish and plant production in integrated aquaculture systems. The experiments demonstrate that larger biofilters enhance growth rates and production efficiency, likely due to improved water quality and nutrient availability.

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Table 4 provides a detailed analysis of water quality and amendments, specifically focusing on different tank to biofilter volume ratios across three experiments. Here’s a breakdown of the table and its implications:

Experiment 1: Laura Tomato in the Biofilters

• Biofilter Ratios: Four different ratios were tested (1:0.67, 1:1.00, 1:1.50, 1:2.25).
• Water Quality: As the biofilter volume increased, dissolved oxygen (DO) levels increased, while temperature slightly decreased. Total ammoniacal nitrogen (TAN) and nitrite (NO2-) concentrations decreased significantly with larger biofilters, indicating better water quality.
• Water Amendments: No pH adjustments or lime/CaO were needed, suggesting that the system maintained a stable pH without external amendments.

Experiment 2: No Crop Interval and Fidello Cucumber in the Biofilters

• Biofilter Ratios: Similar ratios were tested.
• Water Quality: Data on DO, TAN, and NO2- were not provided, but pH levels varied, with the highest ratio showing a pH of 6.00.
• Water Amendments: Lime and CaO were added to adjust pH, indicating a need for external amendments to maintain suitable water conditions.

Experiment 3: Kewalo Tomato in the Biofilters

• Biofilter Ratios: The same ratios were used.
• Water Quality: DO levels increased with larger biofilters, while temperature decreased. TAN, NO2-, and nitrate (NO3-) concentrations decreased with increasing biofilter volume, suggesting improved water quality.
• Water Amendments: CaO was added to maintain pH levels, indicating some need for pH stabilization.

General Observations

• Dissolved Oxygen (DO): Increased DO levels with larger biofilters suggest better aeration and water quality, which is beneficial for fish health.
• Nitrogen Compounds: Decreased concentrations of TAN and NO2- with larger biofilters indicate effective biofiltration and nutrient uptake by plants, reducing potential toxicity.
• pH Stability: Experiment 1 maintained stable pH without amendments, while Experiments 2 and 3 required lime and CaO to adjust pH, highlighting the influence of plant presence on pH stability.
• Water Usage: Larger biofilters required more makeup water but showed reduced water consumption per unit area, indicating efficient water use.

Overall, the results demonstrate that increasing the biofilter volume improves water quality by enhancing oxygen levels and reducing harmful nitrogen compounds, although pH management may require amendments depending on the presence of crops in the biofilters

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Table 5 presents the results of fish growth over a 362-day period, focusing on how different tank-to-biofilter ratios affect fish growth and fruit yield. Here’s an explanation of the table and its implications:

Explanation of the Table

• Biofilter Ratio (v/v): This column represents the volume ratio of the biofilter to the fish tank. The ratios tested were 1:0.67, 1:1.00, 1:1.50, and 1:2.25.
• G (g d-1): This is the average growth rate of individual fish during the culture period, measured in grams per day. It increases with the biofilter ratio, indicating that larger biofilters support better fish growth.
• MSG (%): The average monthly specific growth rate, which also increases with the biofilter ratio, suggesting improved growth efficiency with larger biofilters.
• DRIB (%): The daily rate of increase of biomass, calculated using a specific formula. This metric shows a similar trend of increase with larger biofilters.
• Increase (kg m-3): This indicates the increase in fish biomass per cubic meter of water. It shows a steady increase with larger biofilters, peaking at 28.41 kg m-3 for the 1:2.25 ratio.
• Fish Increase (kg plant-1): This metric shows the increase in fish biomass per plant. It decreases with larger biofilters, suggesting that while overall biomass increases, the efficiency per plant decreases.
• Fruit Yield / Fish Increase (kg kg-1 fish increase): This ratio indicates how much fruit yield is produced per kilogram of fish biomass increase. It increases significantly with larger biofilters, reaching 6.92 kg kg-1 for the 1:2.25 ratio, indicating a more efficient system for producing fruit relative to fish biomass increase.

Results and Implications

The results from Table 5 suggest that increasing the biofilter volume relative to the fish tank volume enhances fish growth rates and biomass production. Larger biofilters improve water quality by facilitating better mineral assimilation and maintaining stable pH levels, which are crucial for both fish and plant growth. However, while the overall fish biomass increases, the efficiency per plant decreases, suggesting a trade-off between total production and per-plant efficiency.

The increase in fruit yield per unit of fish biomass increase with larger biofilters highlights the system’s potential for efficient integrated aquaculture and horticulture production. This efficiency is particularly beneficial in settings where maximizing output from limited resources is crucial, such as in arid or semi-arid regions.

Overall, the study demonstrates that optimizing the tank-to-biofilter ratio is key to enhancing the productivity and sustainability of integrated aquaculture systems, balancing fish and plant yields while maintaining water quality.

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Table 6 presents data on water quality and amendments in a study examining the impact of different tank to biofilter ratios on the growth of ‘Laura’ tomato, ‘Fidello’ cucumber, and ‘Kewalo’ tomato over a 362-day period. The table includes various parameters such as dissolved oxygen (DO), temperature, concentrations of nitrogen compounds (NH3, NO2-, NO3-), pH levels, and the amounts of water and lime amendments made.

Key Observations from the Table:

1. Dissolved Oxygen (DO): The DO levels increased with the biofilter ratio, from 5.58 mg/L at a 1:0.67 ratio to 6.26 mg/L at a 1:2.25 ratio. This indicates improved oxygenation in the system with larger biofilters.
2. Temperature: The water temperature slightly decreased with increasing biofilter volume, from 28.7°C to 27.7°C, suggesting that larger biofilters may help in stabilizing or slightly reducing water temperature.
3. Nitrogen Compounds:
   • Ammonia (NH3): The concentration of ammonia decreased significantly with higher biofilter ratios, from 4.49 mg/L to 1.87 mg/L, indicating better nitrification and ammonia removal with larger biofilters.
   • Nitrite (NO2-): Similarly, nitrite levels decreased from 0.65 mg/L to 0.32 mg/L with increasing biofilter volume.
   • Nitrate (NO3-): Nitrate concentration also decreased with larger biofilters, from 229.0 mg/L to 92.0 mg/L, showing enhanced nutrient uptake by plants.
4. pH Levels: The pH remained relatively stable across different biofilter ratios, ranging from 5.75 to 5.97, indicating effective buffering and pH management in the system.
5. Water and Lime Amendments:
   • Water Added (HOH): The amount of water added increased with larger biofilters, from 3782 liters to 7170 liters, reflecting higher water usage for maintaining system balance.
   • Lime and Calcium Oxide (CaO) Amendments: Lime usage remained constant across treatments, while CaO usage decreased with larger biofilters, from 265 g to 51 g, suggesting reduced need for pH adjustment in larger systems.

Conclusion:

The results from Table 6 suggest that increasing the biofilter volume improves water quality by enhancing oxygen levels and reducing concentrations of harmful nitrogen compounds like ammonia and nitrite. This leads to a more stable and favorable environment for both fish and plant growth. Larger biofilters also contribute to better nutrient uptake by plants, reducing the need for chemical amendments like CaO.

Overall, the study indicates that optimizing the tank to biofilter ratio is crucial for maintaining water quality and maximizing the productivity of integrated aquaculture and horticulture systems.

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Table 7 presents comparative growth rates of tilapia as influenced by various recirculatory aquaculture systems integrated with plant production. It includes data on feed input, stocking, growth, and harvesting across different studies and species of tilapia.

Key Metrics in the Table

1. Feed Input and Stocking Data:
   • Species Cultured: Different species and hybrids of tilapia are used.
   • Sex Type: Indicates whether the fish were male or mixed.
   • Protein (%): The percentage of protein in the feed.
   • Days: Duration of the culture period in days.
   • FCR (Feed Conversion Ratio): Calculated as feed input divided by the difference between final and initial biomass.
2. Growth Data:
   • Pmi (Mean Individual Weight at Stocking): Initial weight of the fish.
   • Bi (Mean Biomass at Stocking): Initial biomass in kg per cubic meter.
   • G (Average Growth Rate): Growth rate of individual fish during the culture period.
   • MSG (Monthly Specific Growth Rate): Growth rate standardized to a monthly basis.
   • DRIB (Daily Rate of Increase of Biomass): Calculated using a formula involving initial biomass and growth rate.
3. Harvesting Data:
   • Pmf (Mean Individual Weight at Harvest): Weight of the fish at harvest.
   • Bf (Mean Biomass at Harvest): Final biomass in kg per cubic meter.
   • MP (Monthly Production): Production rate standardized to a monthly basis.

Results and Interpretation

• Species and System Variability: The table shows variability in growth rates and biomass production across different species and systems. For example, Oreochromis hybrid males in McMurtry’s unpublished study had a higher FCR and biomass at harvest compared to other studies.
• Feed Conversion Efficiency: The FCR values indicate the efficiency of feed conversion into biomass. Lower FCR values, such as those for Sarotherodon aureus in Watten’s study, suggest more efficient feed utilization.
• Growth Rates: The average growth rates (G) and MSG values provide insights into how quickly the fish grow, which varies with the type of system and species used. For instance, Sarotherodon aureus in Nair et al.’s study showed a higher growth rate compared to other systems.
• Biomass and Production: The final biomass (Bf) and monthly production (MP) metrics highlight the productivity of each system. McMurtry’s studies generally show higher biomass and production rates, indicating effective integration of aquaculture and plant systems.

Overall, the table demonstrates the potential for integrated aquaculture systems to enhance tilapia growth and production through efficient resource use and system design. The results suggest that system-specific factors, such as species, feed quality, and integration with plant production, significantly influence growth outcomes.

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Figure 2 illustrates the relationship between the tank to biofilter volume ratio and the concentrations of nitrogen compounds (TAN, NO₂⁻, and NO₃⁻) in the water used for irrigation and the water returned from the biofilters.

Key Components of the Figure:

1. Total Ammoniacal Nitrogen (TAN):
   • The equations $y=-0.204x+1.171,r=0.900$ and $y=-0.133x+0.709,r=0.898$ describe the linear relationships between the tank to biofilter volume ratio and TAN concentrations in the irrigation and return water, respectively.
   • Both equations have high correlation coefficients, indicating a strong linear relationship. The negative slopes suggest that as the tank to biofilter ratio increases, the TAN concentrations decrease.
2. Nitrite (NO₂⁻):
   • The equations $y=-0.049x+0.271,r=0.744$ and $y=-0.019x+0.13,r=0.561$ represent the relationship for NO₂⁻ concentrations.
   • These equations also show a decrease in NO₂⁻ concentrations with increasing tank to biofilter ratios, though the correlation is weaker compared to TAN.
3. Nitrate (NO₃⁻):
   • The quadratic equations y=145.13+108.63x−29.88×2,r=0.686y=145.13+108.63x−29.88x2,r=0.686 and y=140.31+105.79x−29.06×2,r=0.684y=140.31+105.79x−29.06x2,r=0.684 describe the relationship for NO₃⁻ concentrations.
   • These equations indicate a more complex relationship, with an initial increase in NO₃⁻ concentrations followed by a decrease as the tank to biofilter ratio increases. The correlation coefficients are moderate, reflecting a less direct relationship compared to TAN and NO₂⁻.

Interpretation:

• Tank to Biofilter Ratio Impact: The figure demonstrates that increasing the tank to biofilter volume ratio generally reduces the concentrations of TAN, NO₂⁻, and NO₃⁻ in the water, which is beneficial for maintaining water quality in aquaculture systems.
• Biofiltration Efficiency: The reduction in nitrogen compounds with higher biofilter volumes suggests improved biofiltration efficiency, likely due to increased surface area for microbial activity and plant nutrient uptake.

Overall, the figure underscores the importance of optimizing the tank to biofilter ratio to enhance water quality and support sustainable aquaculture practices.

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The regression equation for stocking density is $y=-0.015x$, indicating a negative relationship between stocking density and growth rate. This suggests that as stocking density increases, the growth rate of fish decreases.