Effect of growth substrates on water quality, catfish (Clarias gariepinus) culture and spinach (Spinacia oleracea) propagation under aquaponic system

B O Maranga^1,3, R N Kagali¹, K M Omolo² and P O Sagwe⁴

¹ Department of Zoology, Jomo Kenyatta University of Agriculture and Technology, P O Box 62000-00200, Juja, Nairobi, Kenya
marangabenard@gmail.com
² Department of Biochemistry, Jomo Kenyatta University of Agriculture and Technology, P O Box 62000-00200, Juja, Nairobi, Kenya
³ Sagana Aquaculture Centre, Kenya Marine and Fisheries Research Institute, P O Box 451- 10230, Sagana, Kenya
⁴ Kegati Aquaculture Centre, Kenya Marine and Fisheries Research Institute, P O Box 3259-40200, Kisii, Kenya

Abstract

Modern technologies such as aquaponic system should be embraced to capitalize on fish and crop production. pumice, charcoal and a homogeneous mixture of (pumice and charcoal) were used as substrates and assessed on a control (aeroponic) for their suitability in nutrients removal from water forC. gariepinus culture and S. oleracea propagation under aquaponic system. Specified water parameters in fish tanks; the inlets and outlets of the hydroponic units, were determined in-situ and in the laboratory. C. gariepinus post fingerlings weighing 14.97±0.5g and length 14.05±0.5cm were stocked at 50 fish/tank in 12, 1000L tanks under aquaponic system. Experimental fish were subjected to the same diet that was analysed for its proximate composition and found to contain 35% crude proteins content (CP). Fish were sampled after every three weeks while mortality was recorded on occurrence. The outcome of the trials on water quality, C. gariepinus growth performance and survival rates revealed statistical variation (p<0.05) for the treatments used. The pumice substrate had better performance in percentage nutrients reduction for the nutrients analysed, followed by the mixture of pumice charcoal substate. Charcoal substrate outperformed the other treatments in percentage reduction efficiency for phosphates. The control treatment was the least in percentage reduction efficiency. Mean weight gained by fish for the treatments differed statistically at (p<0.05) with weight gains of 93.81g, 77.57g, 69.79g and 55.37g for pumice, pumice charcoal, charcoal and control treatments respectively. The survival rates ranged between 92-98%. S. oleracea performed better under a mixture of pumice and charcoal treatment in comparison to the other treatments under the present study. Results from this study suggest that nutrient reduction efficiency can be increased by replicating the hydroponic units with appropriate substrate to increase retention time of water in the hydroponic for increased fish and crops production.

Key words: aquaculture, growth performance, reduction efficiency, growth media, water management

Introduction

The changing climate caused by global warming has resulted to unpredictable precipitation pattern that has negatively impacted on capture fisheries globally leading to downwards trajectory of fish caught from the wild (Seneviratne et al 2012). The declining trend of capture fisheries is a clear indication that aquaculture will be a major source of fish proteins in the near future to meet the demand of fish worldwide (FAO 2018). Presently, aquaculture accounts for close to half of fish consumed globally (FAO 2016). Aquaculture enterprise contribution to global fish production increased significantly from 25.7% in the year 2000 to 46.8% in 2016 (Gichana et al 2019). The growth has been made possible by the leading countries in aquaculture production adopting to new technologies involving intensive fish culture. In Sub-Saharan Africa, aquaculture in most countries including Kenya is practiced using outdated extensive and semi-intensive production systems (Opiyo et al 2018). The pond system mostly used in many countries in Africa require large tracts of land and plenty of water for fish production per unit area. Adoption of modern technologies such as aquaponics utilizes limited space and water for mixed production of both fish and vegetables for household consumption.

The reliance of olden type of culture systems has contributed to Africa aquaculture production to be less than 3% globally (FAO 2014). With the rapid population growth that have exerted pressure on available land coupled with unpredictable precipitation pattern, the status of aquaculture productivity has worsened in Africa. Therefore, shifting from extensive and semi-intensive systems and to intensive systems will provide sustainable solution to enhance aquaculture productivity in the continent. To tackle the issues of limited land and water scarcity, adoption of modern culture technologies such as aquaponic system should be embraced, (Barbu et al 2016) to capitalize on fish production. The integration of recirculating aquaculture and hydroponics referred to aquaponics is a promising solution for enhancing intensive fish and crops production (Yesiltas et al 2020) in areas constrained with land and water resources. Under aquaponic system, waste produced in the aquaculture system is directed to the hydroponic units for nutrients removal by the activities of micro-organisms attached to substrates (Boxman et al 2017) and uptake by plants. To maintain conducive water environment for fish growth under aquaponic system, effective plant growth substrates should be used for efficient restoration of water for fish culture (Maucieri et al 2018).

Substrates in aquaponic systems influence water status by providing attachment media for nitrogen converting bacteria and acting as particle filter medium to improve on water quality. Substrates also provide attachment for plants that are used for nutrients uptake within the aquaponic system. However, clogging, formation of death zones and difficulty in cleaning are challenges experienced after using substrates for a long duration without cleaning or replacement (Yesiltas et al 2020). Best management practices should be observed in the fish rearing units to ensure the amount of organic load draining to the hydroponic units is minimized to prevent clogging. As such, substrates will enhance development of bacteria responsible for breakdown of ammonia to absorbable form by plants. Effectiveness of substrates is essential in holding water and air for maintenance of optimal condition for roots and consequently growth of organisms (Maucieri et al 2018) within the aquaponic unit. Variation in growth of organisms has been observed in aquaponic systems when different substrates are subjected to the organisms (Geisenhoff et al 2016). Therefore, it is important to identify right substrates to maximize growth of the cultured organisms in aquaponic system.

Water is required in appropriate quality and quantity to enable sustainable production for intensive aquaculture to succeed (FAO 2018). Close monitoring of water quality under intensive aquaculture system is essential to avoid stunted growth and mortality due to high stocking densities of fish placed in limited water recirculating within the system. Water quality parameters such as temperature, pH, dissolved oxygen and nutrients must be maintained at optimal levels to enhance growth of all microorganisms under intensive aquaculture. Excess feeds and poor feed utilization efficiency by fish affects water quality as a result of nutrients enrichment of water within the culture systems (Filbrum et al 2013). As such, fish productivity and yield are reduced by hypoxia and blooming of aquatic plants such as phytoplankton and algae that compete for oxygen with fish during the process of respiration. For optimal water environment for fish production to be achieved, excess feeding should be avoided so as to prevent water quality deterioration and unnecessary wastage of feeds (Kosemani et al 2017). Plants grown in the hydroponic unit should be able to uptake nutrients from culture significantly to maintain the integrity of water within the system. The effective nutrients reduction may also be achieved by the use of appropriate substrates that will enable biological processes of nutrients conversion by microorganism to take place (Espinosa-moya et al 2018) in the hydroponic units. This study was aimed at assessing the following materials; pumice, charcoal and a mixture of (pumice and charcoal) mixed homogenously at 50 % ratio for their use as substrates and tested on a control (aeroponic) for their suitability in nutrients reduction from water for growth of C. gariepinus culture and S. oleracea under aquaponic system.

Materials and methods

Experimental design

The 16 weeks study was conducted from June to September 2021 at a private fish farm in Mogotio, Kenya. Solar powered aquaponic system comprising of twelve fish rearing tanks of 1000 litres capacity were arranged in four sets of three tanks each in a randomized complete block design. The four treatments comprising of three tanks were connected using 2-inch polyvinyl plastic pipes to the hydroponic units measuring (2X1X0.5) m which were filled with different substrate types. The substrates were placed in three hydroponic units that included, pumice, charcoal and a mixture of (pumice and charcoal) mixed homogenously at 50 % ratio, while the fourth hydroponic unit had no substrate (aeroponic) and was used as a control for the study. Each treatment had 1500 litres sump in which a solar Maji Pump (24VDC) was placed to facilitate the recirculation of water within the systems. Proximate analysis of the commercial diet was conducted at food science laboratory of Jomo Kenyatta University of Agriculture and Technology (JKUAT) while water nutrients were analyzed at Kenya Marine and Fisheries Research Institute KMFRI laboratory, Baringo station, Kenya.

Photo 1. Setting up of the aquaponics experimental system in Mogotio, Baringo county in Kenya

Sourcing of fish and diet

C. gariepinus post fingerling of average weight 14.97±0.5g and length 14.05±0.5cm were sourced from a private fish hatchery in Nairobi, Kenya and transported in polythene bags aerated with oxygen to the study site while the experimental diet of crude protein (CP) 35% and 2mm pellets was sourced from Unga Farm Care, animal feed manufacturing company within Nakuru city, Kenya.

Water quality analysis

Physical water quality parameters (dissolved oxygen, temperature, pH, conductivity and total dissolved substance) were monitored on weekly basis at 08:30hrs using a multiparameter water quality meter (YSI) while water samples were collected in 350ml sample collection bottles and taken to the laboratory for ammonia, phosphorous, nitrites and nitrates analysis using UV Shimadzu spectrometer (UV-1800ENG240V, SOFT).

The nutrient reduction efficiencies of different substrates in the hydroponic units were calculated using the formula, (Oladimeji 2018).

% Reduction Efficiency = [(a-b)/a] X 100

where,

a= concentration in the inlet water

b= concentration of the outlet water

Proximate composition of experimental diet

Proximate composition for specified parameters of the experimental diet was determined according to AOAC method specifications 950.46 (AOAC, 1995) and results presented as shown in table 1.

Stocking and sampling of fish

A total of 600 C. gariepinus post fingerlings were stocked at a rate of 50 fish per 1000 litre rearing tank and acclimatized for a duration of one week before the start of the trial. Mortality experienced during the first week was removed from the rearing tanks, counted and the number recorded. Fish were hand fed to apparent satiation twice daily at 10:00hrs and 16:00hrs during the entire experimental period except the days when sampling was done. The feeding rate was adjusted according to the change in body weight of experimental fish immediately after sampling was done. Fish sampling was conducted after every three weeks during the entire study period. Fishing net of 0.5 mm mesh size was used to catch fish from the rearing tanks. Buckets were half-way filled with water and 50 fish were placed per bucket per tank for every treatment. A mitre rule mounted on a board was used to determine the length of fish while weighing balance, WTC 2000, was used to determine the weight of fish. The biometrics were recorded and fish were taken back to their respective rearing tanks which were covered with nets to prevent predation from predatory birds.

Determination of growth performance and survival

At the conclusion of the experiment, fish were harvested using a fishing net, counted, weight and length documented. Survival, Growth and feed efficiency were evaluated by the following standard formula.

Daily growth (DG) (g) = Final weight/Time (Exp days)

Body weight gain (BWG) (g) = Final weight – Initial weight

Specific Growth Rate (SGR) (%) = 100% X [ (In Final weight (g) – In Initial weight (g)) / Time (Exp days) ]

Fish Food Conversion Ratio (FCR) = Feed provided (g) / Weight gain (g)

Survival rate (SR) (%) = 100 X (Final number of fish) / (Initial number of fish)

Determination of plant growth parameters

Fifteen Spinacia oleracea seedlings sourced from a Nakuru approved vegetable nursery were planted in each of the hydroponic unit at a spacing of 30cm by 45cm. The plants were allowed to grow for one and a half months and were harvested continuously on monthly basis, for three months. Sterilised pair of scissors was used to cut the leaves from the plants. The length of the leaves was measured from the tip of the leafy part to the tip of the stalk while the breadth was measured at the broader region of the leaf from the end of one side of the leaf margin to the end of the other side of the leaf margin and values recorded. The number of leaves per plant were counted during harvesting and summed up for every treatment. The number of plants that the actual harvesting was done were also counted during every harvesting session for all the treatment on the same day.

Statistical analysis

Statistical analysis was performed using SPSS version 23 for windows. Means for; fish weight and length, spinach length and breadth as well as water quality parameters for the treatments were compared using Analysis of Variance (ANOVA). Statistical variation for the inference tests were performed using the Tukey-HSD post hoc, at 95 significance level. The results were presented using tables and a graph plotted using Microsoft excel spreadsheet for windows 2010.

Results

Physical and chemical water parameters

Physical water parameters and nutrients are presented in Tables 2 and 3. Water quality analysis indicated statistical variation (p≤0.05) observed for dissolved oxygen (DO), temperature, ammonia, nitrates and nitrites between treatments. Dissolved oxygen ranged from 2.2- 4.2 and the water temperature which was affected by the prevailing weather condition of the study area was maintained between 19.4 ^oC and 23.5 ^oC in the fish rearing tanks (Table 2). The other parameters including pH, conductivity, total dissolved substance and phosphates didn’t differ statistically (p>0.05) between fish rearing tanks. The water pH varied between 6.81 and 8.18, while conductivity and total dissolved substance ranged between 50-68 μScm ^-1 and 25-34 μScm^-1 respectively. The maximum value for ammonia, nitrates, nitrites and phosphates analyzed were 3.32m/L, 5.22m/L, 0.99m/L and 0.94m/L respectively. Low ammonia levels were observed in fish tanks that had a mixture of pumice and charcoal and high level of ammonia was observed in the control fish tanks that had no substrates. The nutrients varied statistically (p<0.05) except for nitrates which were statistically indifferent (p>0.05) between the treatment of fish tanks.

Descriptive statistics of nutrients in the hydroponic unit is presented in Table 3. The mean statistical levels for ammonia, nitrates, nitrites and phosphates were similar (p>0.05). The nutrients, ammonia, nitrate, nitrates and phosphates ranged between 0.66 mg/L-2.91 mg/L, 0.09 mg/L-4.11 mg/L, 0.03 mg/L-0.92 mg/L and 0.02 mg/L-0.61 mg/L respectively.

Figure 1. A bar graph of percentage reduction efficiency (%) of nutrients (mg/L) in the aquaponic system under different substrates

Growth performance of C. gariepinus under different substrates treatments

The proximate analysis for the trial diet (Table 1) shows proximate composition of nutritive values of different parameters which indicated the suitability of the diet in C. gariepinus culture at post fingerling stage of growth. The diet was subjected to fish under three different substrates treatments tested on aeroponic (control) with the expectation of fish growing at the same rate to achieve final uniform weights for the four treatments. However, results on growth performance of C. gariepinus as presented in (Table 4) indicated a variation of final average weights of experimental fish after the four-month trial. The treatment with pumice substrate attained the highest final weight of 108.78g, followed by pumice charcoal substrate with 92.54g, then charcoal substrate with 84.76g while the control treatment was least with 70.34g.

The survival rates of fish under the set growth conditions subject to different substrates was between 92-98% with most of fish mortality experienced in the second day after stocking. Most mortality was recorded under the control treatment with 8 deaths, followed by the charcoal treatment with 6 deaths, pumice treatment with 4 deaths while the pumice charcoal had the least mortalities with 2 as shown on (table 4). There was no significant difference (p>0.05) observed in survival rate (SR) among all the treatment under the present study.

Growth performance of Spinacia oleraceaunder different substrates treatments

Table 5 presents the growth parameters of S. oleracea grown in different substrate treatments under aquaponic system. Statistical variation (p<0.005) was observed for breadth, length and number of leaves harvested per plant with p values of 0.023, 0.045 and 0.003 respectively while there was statistical insignificance (p>0.005) in the number of plants harvested during the study period with p value of 0.087. The range of the number of plants harvested during the study period was between 6 and 8 for charcoal treatment while the pumice and the mixture of pumice and charcoal treatments had similar number of plants harvested of between 8 and 11 plants. The number of leaves harvested from the plants during the study duration ranged between (24-32) leaves for charcoal treatment, (40-48) leaves for pumice treatment and (41-48) leaves for a mixture of pumice and charcoal treatment. The range in breadth of harvested leaves were between (6-12.2) cm for charcoal substrate, (6.2-13.2) cm for pumice substrate and (7.2-15) cm for the mixture of pumice and charcoal substrate. The range for length of leaves were between (12-24) cm, (12-24) cm and (14-30) cm for charcoal, pumice and a mixture of pumice and charcoal substrates respectively.

Discussion

Water quality and fish growth

Water quality monitoring is essential in achieving optimum growth for all organisms propagated under aquaponic system (Somerville et al 2014). For effective and efficient aquaponic system, maintaining water quality parameters within tolerable limits ensures optimal growth of fish, plants and bacteria (Goddek et al 2016). The physical water parameters tested under the present study were within the acceptable ranges for Clarias gariepinus growth and development. However, dissolved oxygen (DO) levels were lower for all the treatments than the recommended range of 5-6 mgL^-1 required for optimum growth of plants and most warm water fish in aquaponic systems (Wongkiew et al 2017). Studies carried out on channel catfish by (Boyd and Hanson 2010) showed better growth performance and feed conversion ratio (FCR) in culture systems where mean average DO concentration did not fall below 3.5 mgL^-1. The slow growth rate and higher FCR observed under the current study for control treatment may be attributed to low DO levels observed of 2.63±0.42 mgL^-1 in comparison to the rest of the treatments that had a mean average DO concentration of above 3.5 mgL^-1. Low DO levels in aquaponic systems are attributed to several biological processes that include plant roots and fish respiration, oxidation of ammonia by nitrifying bacteria and organic load production within the fish rearing tanks (Espinosa-moya et al 2018). Also, accumulation of nutrients and toxic gasses developed within fish tanks reduces affinity for DO in water as was observed in the control treatment of this study. Temperature and pH observed under the current study were maintained within the average range of (20.30±0.79 – 21.39±0.78) ⁰C and 7.19±0.17 to 7.40±0.51 respectively, creating a conducive environment for growth to all the organisms within the aquaponic system.

At temperature and pH ranges stated above, ammonia concentration ranging between (1.88±0.67 - 2.64±0.42) mgL^-1 obtained for the present study might not have deleterious effect on fish under aquaponic system. In water, ammonia exist either un-ionized (NH₃) or ionized (NH ₄⁺⁾ with the relative proportion of the two forms mainly affected by pH and temperature. Ammonia toxicity increases with rising pH and temperature levels in water. At pH below 8, only 10% of ammonia in water is considered toxic (Hargreaves and Tucker 2004) for fish. However, at any pH level, ammonia toxicity increases with rise in temperature due to reduction of dissolved oxygen in water as a result of temperature change. As such, the water quality parameters should be maintained at an equilibrium for healthy fish growth under aquaponic system. Nitrifiers are responsible for nutrients management in water under aquaponic system (Hu et al 2015), which explains the variation of ammonia, nitrites and nitrates observed under the current study. The action of Nitrosomonas and Nitrobacter majorly contribute to the oxidation of ammonium and nitrites ions in water creating a conducive environment for fish growth (Trang and Brix 2014). Other mechanisms that are associated with ammonia removal include; uptake by plant roots and assimilation by microorganisms that convert nutrients back to organic matter (Hu et al 2015). Although nitrites and nitrates are considerably less toxic than ammonia, it is important for the nutrients to be removed to prevent accumulation so as to enhance fish growth and development under favorable environment. The levels observed for the nutrients concentrations under the present study were within manageable ranges that would have not affected catfish growth negatively.

Effect of substrates in nutrients reduction efficiency

The essence of substrates under aquaponic system is to hold water, air and maintain optimal conditions for bacteria and plant development (Lennard and Goddek et al 2019). Substrates have an influence on growth performance of fish and plant development as shown under the current study (table 4 and 5) paralleling results obtained by (Roosta and Afsharipoor 2012), for straw berry growth and culture of grass carp and silver carp using perlite, cocopeat and a mixture of perlite and cocopeat mixed at different ratios. The nutrients reduction efficiency of the system was probably influenced by the substrate suitability in providing attachment, for microorganism that solubilizes nutrients for uptake by plant and conversion of toxic ammonia to nontoxic form, (Wongkiew et al 2017). The assessment on the suitability of the various substrates that were used for the present study indicated pumice substrate contributed significantly in nutrients reduction efficiency within the aquaponic system compared to other substrates used for the trial. Charcoal substrate contributed significantly in restoration of water clarity through the filtration of suspended particles but did not match pumice and the interaction between charcoal and pumice substrates in nutrient removal.

The interaction between charcoal and pumice was expected to perform better in nutrients removal than the rest of the substrates, however it was second after pumice. The aeroponic was the least in reduction efficiency for all the nutrients analyzed under the current study because it lacked S. oleracea plants and substrates that were responsible in nutrients removal in the other treatments for the experiment. Though least in nutrient reduction efficiency, the control had the ability to reduce nutrients through other mechanisms such as nitrification and denitrification processes by microorganisms that may have developed in the aeroponic unit. The maximum reduction efficiency of ammonia for the current study was 28.33% observed under the pumice treatment which was significantly lower than 92.77% reported by (Endut et al 2016) for Clarias gariepinus culture and water spinach propagation under aquaponic system using gravel substrate. Though the reduction efficiency for ammonia was low under the current study, the ammonia levels observed were comparatively low in mean average concentration ranging between (1.88±0.67 - 2.64±0.42)mgL^-1 across all the treatments than the recommended concentration of (3.0-6.7) mgL^-1 for C. gariepinus under aquaponic system (Endut et al 2016). Phosphate reduction was highest reduced nutrient at 61% efficiency followed by nitrites at 41.38% and nitrates at 35% under pumice treatment. Higher percentage reduction values were observed for the mixed pumice and charcoal treatment compared to charcoal treatment for most of the nutrients except for phosphates as shown in Fig 1. Study by (Lin et al 2019) showed carbon source biofilter operated under anoxic period in wastewater treatment had removal efficiency of 91.15% on phosphorus thus, explaining the high reduction efficiency for charcoal substrate obtained for the present study.

Effect of substrates on fish growth

Fish under the pumice treatment achieved the highest final growth compared to fish that were subjected on the other treatments under the current study. The highest growth of fish observed under the treatment containing pumice substrate is attributed to the reduction efficiency of the substrate in reducing nutrients significantly within the system. Fish in the aeroponic unit performed the least in terms of growth compared to charcoal and the interaction between pumice and charcoal treatments. The aeroponic unit had the least reduction efficiency for all the nutrients analyzed hence may have contributed to the least growth observed for the treatment. The interaction between pumice and charcoal treatment which was second in fish weight gain, had better nutrients reduction efficiency than charcoal treatment which was third in fish growth and nutrients reduction efficiency under the present study. The statistical growth variation (p<0.05) observed for experimental fish was as a result of varying ability of substrates under different treatments in reducing nutrients levels in water within aquaponic system. Survival under the present study was over 90%, however none of the treatment attained 100% survival rate. Mortality happened within the first 3 days after stocking hence a clear indication that the deaths were not influenced by the substrates used for present study. Probably, stress acquired during packaging and transportation of experimental fish using oxygenated polythene bags contributed to the mortality observed. Therefore, alternative means of packaging e.g., aerated plastic containers should be used for transportation of fish that are beyond fingerling stage of development to avoid unnecessary mortalities as observed in the present study.

Effect of substrates on S. oleracea

There was no significant statistical variation (p<0.05) for analyzed S. oleracea parameters between treatments except for the number of leaves harvested. Better values for leave breadth, leave length, leave numbers and number of plants harvested were obtained under the treatment that had a mixture of pumice and charcoal substrate. Thus, the observation was contrary to the outcome observed for fish in comparison to the reduction efficiency of pumice and the mixture of pumice and charcoal substrates in regards to growth of S. oleracea. Unlike fish that performed better under pumice treatment, S. oleracea performed better under the mixture of pumice and charcoal treatment. The outcome may be attributed to the environment created by the substrates having varied influence in roots aerobic respiration and other metabolic processes for the plants under different treatments affecting nutrients absorption and ultimately varying S. oleracea development. Anoxic environment decreases roots permeability to water which can subsequently reduce nutrients absorption and plant growth (Estim et al 2018) in aquaponic systems. Charcoal treatment was least for all the growth parameters that were analysed for Spinacia oleracea under this study. Therefore, charcoal alone is not a good substrate for the growth of Spinacia oleracea under aquaponic systems.

Conclusion

• The variation observed in nutrients reduction efficiency under different substrate treatments influenced productivity and growth performance of C. gariepinus and S. oleracea under the solar powered aquaponics system used for this study.
  
  
• Survival was not substrate related but depended mostly on handling of fish during packing, time taken and mode of transportation from the hatchery to the study site.
  
  
• This study showed pumice is a better substrate in fish culture under aquaponic system compared to the other substrates used for the present study.

Ethical statement

The study was carried out in accordance with the international, national and institutional guidelines for the care of experimental animals.

Acknowledgement

We acknowledge the Kenya Marine and Fisheries Research Institute (KMFRI) Baringo station for providing laboratory space and equipment for analysis of water nutrients. We also acknowledge Mr David Chebon for having accepted the trials to be conducted on his farm in Mogotio, Baringo County.
This research was funded by the Government of Kenya (GoK) through the Ministry of Agriculture Livestock, Fisheries and Co-operative under the Kenya Climate Smart Agricultural Program (KCSAP).

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