INTRODUCTION:

Pond management strategies to produce food-size catfish are varied and have evolved over time toward increased intensification¹. Pond catfish culture is intensified further using the split-pond system², which evolved from the partitioned aquaculture system³. Photosynthesis supplies dissolved oxygen during the day, and flow rate between the two sections of the split pond is adjusted to ensure the minimum required oxygen concentration for fish growth². Paddlewheel aeration is used in the fish-production section at night.

The eutrophicated pond system could represent a further intensification of catfish culture. High yields of fish or shrimp are produced in the new pond system because two important production-limiting factors, dissolved oxygen and total ammonia-nitrogen (TAN) concentrations, are maintained at near-optimal levels despite high stocking and feeding rates^4,25. In an outdoor pond production system, a complex of living organisms is closely associated with particulate organic matter and is maintained in suspension by continuous aeration. TAN excreted by the culture animal is utilized by the phytoplankton and bacteria that are part of this complex of living organisms. The catfish have been grown successfully in an outdoor pond production system⁵ with maintaining ammonia and oxygen level in pond ecosystem. In traditional pond culture, mechanical aeration is used at night to maintain dissolved oxygen concentration above critical levels, and algal photosynthesis is the primary source of dissolved oxygen and the principal sink for excreted TAN. The objective of this study was thus to evaluate variation in water quality and fish growth for catfish stocked in new and old pond production system.

METHODS AND MATERIALS:

Two ponds in which one is 5-year and another is 10-year-old equipped with an aerator to maintain minimum dissolved oxygen concentration above 40% of saturation in this study. Water only was added to replace losses to evaporation and seepage, but the volume added was not quantified. Salt was added to all culture units to ensure that chloride concentration exceeded 100mg/L. The ponds were filled on with water contained total alkalinity as 226.4 mg/L CaCO₃ and after one month fertilized with a total of 0.5kg (N-P-K), 4.5kg dried molasses. Thereafter, catfish (Clarias batrachus) as 47g/fish weight were stocked into ponds as 5.6fish/m² and 12.6fish/m² density. The fish population in each pond was sampled periodically using a seine net to monitor fish growth. On each sample date at least three samples per pond unit of 25fish/sample were weighed and the fish returned to the culture unit. No mortalities were recorded during sampling.

All culture units were harvested completely one year after stocking. At harvest, a minimum of 75 fish per culture unit were weighed individually and remaining fish were weighed in bulk. Fish number was calculated by dividing the bulk weight by the mean individual weight. Individually weighed fish were assigned to size classes (kg/fish/size range): sub-marketable (<0.34kg/fish), out-of-size (0.34-0.45kg/fish), 0.45(0.45-0.57kg/fish), 0.57(0.57-0.68 kg/fish), 0.68(0.68-0.79kg/fish), 0.79(0.79-0.91kg/fish), 0.91(0.91-1.02kg/fish) and 1.02(≥1.02kg/fish). Fish were fed a 32% protein commercially extruded feed daily to apparent satiation and quantities recorded. Feed conversion ratio was calculated for each pond as the total feed (dry-weight basis) fed divided by the net fish yield.

Water samples were collected weekly from each culture unit at about 0800 h: A 90-cm column sample was collected monthly from ponds for water quality parameters during the study period. Flow injection analysis also was used to quantify total ammonia-nitrogen (TAN) in filtered samples using the o-phthaldialdehyde method⁶. Water samples were filtered through a 0.45-μm pore size glass fiber filter for chlorophyll a analysis. Chlorophyll a was extracted in 2:1 chloroform: methanol from the phytoplankton retained on the filter, and the chlorophyll a concentration in the extract was determined by spectroscopy⁷. Dissolved oxygen (DO) and temperature in each pond were monitored continuously by a galvanic oxygen sensor and a thermister connected to a data logger. Mean (±SD) DO was 6.4±1.7 and 7.2±1.3mg/L in ponds and temperature as 25.0±5.0 and 24.6±5.2ºC in ponds during this investigation. The care was taken to ensured fish well-being, fed fish, and maintained water levels as needed. All other activities were suspended.

After confirming homogeneity of variance and normality, data were analyzed using the two-tailed t-test, linear regression, and frequency procedures of SAS version 9.4. The repeated measures mixed-models procedure was used to compare slopes of growth curves. Percent data were arcsin transformed. Feed conversion ratio (FCR) data could not be normalized and were analyzed by non-parametric one-way analysis of variance.

RESULTS AND OBSERVATIONS:

Water-quality dynamics varied between treatments. Feed input and culture environment were the main factors that affected water quality. The daily feed rate showed higher in pond 1 in comparison of pond 2 in initial period of stocking because stocking rate was 2.1 times greater. During the first 60 d, fish in both ponds were fed a mean of 15.6 g/m³ feed daily, 8.0g/m³ in 60-120 days and thereafter 27g/m³ up to harvest day of catfishes in the experimental ponds.

Chlorophyll a concentration (Figure 1), an indicator of phytoplankton biomass, was similar in ponds in initial period (averaging 264.0mg/m³) and corresponded to a moderate phytoplankton bloom. During the first 60 days after fish were stocked, chlorophyll a averaged 222 mg/m³ in pond 1 compared to 267mg/m³ in pond 2 observed in this study. Subsequently, chlorophyll a concentration in both treatments converged to a mean of 387 mg/m³ that persisted throughout the remainder of the experiment. Chlorophyll a concentration did not differ significantly between treatments after 210 days as 534 mg/m3 in pond 1 and 582 mg/m3 in pond 2 after 210 days (P = 0.878) or on last day of study (P = 0.175).

Figure 1: Mean chlorophyll a in ponds stocked with catfish during the 210-day study.

Increased phytoplankton biomass can be important in sustaining dissolved oxygen concentration in aquaculture systems, particularly when the algal community is dominated by eukaryotes such as chlorophytes and diatoms as in the case of pond 2, which are better oxygenators of the water compared to bloom-forming cyanobacterians persisted in pond 1during the study period.

Dissolved inorganic nitrogen (DIN) dynamics differed between ponds (Figure 3) as TAN and NO₂-N in pond 2 was higher and more variable, and NO₃-N was lower.

Figure 2: Mean Ammonia in ponds stocked with catfish during the 210-day study.

Excretion of feed nitrogen by fish in pond 2 exceeded phytoplankton TAN uptake and resulted in a TAN spike of 5.8mg/L after 60 days followed by a secondary TAN spike of 6.3mg/L after 90 days of catfish stocking in this study. Phytoplankton productivity in pond 2 likely was nitrogen- limited prior to the first TAN spike and phytoplankton TAN uptake in pond 1 during the remainder of the study kept TAN concentration low.

Figure 3: Mean pH in ponds stocked with catfish during the 210-day study.

Pond 2 pH oscillated around pH 8.1 throughout the study, whereas in the pond 1 pH declined after 1.5 months because of nitrification but was maintained around pH 7.4 by periodic addition of sodium bicarbonate (Figure 3). Pond 1 pH was significantly lower (P < 0.001) than pond 2 pH after completion of 8 months in this study, but not different (P = 0.060) on last day of the study.

Figure 4: Mean Phosphate in ponds stocked with catfish during the 210-day study.

Soluble reactive phosphorus (PO₄-P) concentration (Figure 4) was similar in pond 1 and pond 2 at the start of the experiment. Pond 1 PO₄-P concentrations remained low throughout the culture cycle.

Figure 5: Size distribution of catfish harvested after 210 days from ponds

Fish in both treatments grew rapidly throughout the study (Figure 5). Fish were sampled periodically for growth, and growth curve analysis showed that intercepts and slopes did not differ significantly (P = 0.243) between treatments. However, at harvest, fish from pond 2 were significantly larger than those from the pond 1 in this study (P = 0.035; Table 1).

Table 1: Mean (±SE) gross and net fish yields, individual fish weight, survival and feed conversion ratio for Clarias batrachus harvested in ponds after a 210-day study.

Treatment	Gross Yield (Kg/m³)	Net Yield (Kg/m³)	Fish Weight (g/fish)	Survival (%)	FCR
Pond I	2.5±0.0	2.3±	430±18	92.2±0.4	1.6±0.0
Pond II	4.3±0.5	4.1±0.5	542±22	84.7±1.0	2.0±0.1
Pr˃t	<0.001	<0.001	0.035	0.442	0.036

Significant (P < 0.001) treatment differences were detected for size classes of harvested fish (Figure 6). Results of frequency analysis showed that the number of sub-marketable fish approximated the expected number for each treatment. There were fewer than expected out-of-size and 0.45kg/fish size class fish harvested from pond 1, whereas for the pond 2 there were greater than expected fish catches in numbers. The fish catches as of 0.57, 0.68, and 1.02kg/fish size classes in each treatment approximated expected quantity, while there was greater than expected 0.79 and 0.91kg/fish size-class fish harvested from pond 2, whereas for the pond 1 there were fewer than expected fish catches in this study. Density-related interactions appear to have affected fish growth more in the pond 1 during the study period.

DISCUSSIONS:

Mean daily feed ration and total feed consumed by fish in ponds were consistent with results from previous catfish studies in variable aged ponds^5,8,23.

Low nutrient input likely contributed to the initial decline in pond chlorophyll a concentration. In the pond 1, high feed nutrient input beginning at stocking resulted in rapid phytoplankton growth, and mean chlorophyll a concentration during the first 4–8 weeks was consistent with concentrations reported previously for catfish culture^{5, 8}. The filamentous cyanobacteria blooms generally dominate during late summer in catfish production ponds (9), whereas the consistently high nutrient loading rates in the pond 1 contributes to phytoplankton communities dominated by chlorophytes and diatoms⁸.

Chlorophyll a concentration increased in pond 2 (R² = 0.651, P < 0.001) but decreased in pond 1 system (R² = 0.671, P < 0.001) as the weekly mean of daily feed input increased during the one year study. Increasing chlorophyll a concentration in response to increasing feed nutrient input is common in ponds used for the intensive culture of catfish^10,11. However, the negative chlorophyll a-feed input relationship in pond 1 is in contrast to other published studies on catfish culture where chlorophyll a concentration was independent of feed input or increased with increased feed input^5,8. Variation among studies in fish stocking rate and initial size and the rapid increase to sustained high chlorophyll a concentrations likely contributed to the different observed chlorophyll a-feed input relationships.

Algal uptake of TAN is the primary mechanism that controls TAN concentration in catfish ponds^12,13. A NO₂-N spike (days 110–138) that followed the TAN spike (day 96) may indicate that some nitrification occurred in ponds. The nitrification is more rapid in pond 2 in comparison of pond 1 due to more phytoplankton growth in old ponds. The observed DIN dynamics in ponds in the present study, including the minor role of nitrification, were consistent with those observed in commercial catfish ponds¹⁴. Nitrification typically begins 4–6 weeks into the culture period^{5, 15} because nitrifying bacteria grow slowly⁴. Chronic or acute exposure to elevated un-ionized ammonia concentration can decrease fish growth or be toxic¹³.

The proportion of unionized ammonia was higher in pond 2 rather than pond 1 due to high temperature, more alkalinity and photosynthesis in the old ponds used in catfish culture in tropic regions. These concentrations are less than the LC₅₀ and growth-limiting unionized ammonia concentrations reported for channel catfish¹³. In addition to feed input, two other factors contributed to the differences in water quality observed in present research as interaction and nutrient transformations from water column to soil sediments. It includes nitrification and de-nitrification^12,17, accumulation of organic nitrogen¹⁷, and Phosphate adsorption in sediment pore water¹⁸.

Similar PO₄-P concentrations was observed in other catfish pond production trials elsewhere²³. In contrast, PO₄-P concentration increased linearly (R² = 0.915, P < 0.001) with time beginning on day 40 in the pond 1 and was a consequence of the high feed input. Mean PO₄-P concentration in the pond 1 was significantly higher (P < 0.001) than in pond 2 after 8 months of this study.

The stocking rate is known to affect catfish growth over a variety of production environments (19, 20). In this study, pond 2 have more stock in density in comparison of pond 1 which resulted large sized fish guilds in pond 1 as was reported for other fish in response to increased stocking rate (21). The fish weight CV at stock-out was 26.4% and at harvest CVs ranged from 25.2%–28.8% for fish in pond 2 and 22.3%–27.4% for fish in pond 1 and would not be expected to differ.

Since fish were fed daily to apparent satiation, competition for food should be negligible. In fact, mean feed consumption per fish was 166 and 121g/fish for the pond 1 and pond 2 respectively and did not differ significantly (P = 0.275). Differential mortality does not explain the treatment difference for mean final individual weight because mean survivals were high and not significantly different. The water-quality variables in the present experiment would not be expected to affect individual fish feed consumption or growth. The feed conversion ratio is different in ponds under study, while feed consumption did not significantly differ between treatments which may indicate that density-induced stress not affected feeding physiology in the ponds.

Gross fish yield (GFY) and net fish yield (NFY) were significantly greater (P<0.001) in the pond 2 as would be expected from the higher stocking rate. The catfish yields in the current experiment were similar to yields obtained in previous experiments in ponds (20,23). The fish survival was high in pond 2 due to macrophyte which can be used as shelter for fingerlings and also as breeding site in mature fishes (22,23). There were significant differences between treatments among size classes of harvested fish (Table 2).

Table 2: Mean (±SE) percentage of catfish population harvested after a 210-day study from ponds as sub-marketable (<0.35), out-of-size (0.35-0.45), medium (≥0.45, ≥0.57) and moderate-size (≥0.68) Kg/fish)

Fish size	<0.35	0.35-0.45	≥0.45	≥0.57	≥0.68
Pond I	2.9±0.0	14.1±0.0	83.0±0	64.0±0.0	39.4±0.3
Pond II	4.6±0.3	21.6±0.1	72.1±22	39.3±0.4	18.0±0.2
Pr˃t	0.241	0.071	0.102	0.017	0.026

The catfish size accepted by processing plants range from 0.45-0.57 to 1.81-2.26kg/fish, but the preferred size range varies from plant to plant and over time in response to market demands (24). Processors likely prefer a tighter size range (0.68-1.13kg/fish) with minimal numbers of fish 1.36kg and larger. Catfish that are too small to eat (generally <0.35kg/fish) are rejected by the plant, whereas fish that are large enough to eat but outside prevailing specifications are considered out-of-size (0.35-0.45 and >2.26kg/fish) and are discounted (24). In the present experiment, sub-marketable fish did not differ significantly (P = 0.241) between treatments and ranged from 3% – 6% of the population. There were 50% more out-of-size and more fish harvested from pond 2 were in the larger size classes.

CONCLUSIONS:

In summary, pond culture of catfish is much less intensive in new pond rather than old ponds where phytoplankton uptake is the main TAN sink in and ammonia oxidation by nitrifying bacteria are the main TAN sinks in the new pond for catfish culture. Total GFY and NFY from pond 2 are less than from the pond 1 because of lower stocking and feeding rates. Although a significantly higher percentage of fish harvested from pond 2 were of market size, the GFY and NFY of market-size fish from the pond 2 was significantly higher because of the higher stocking rate. Density-related social interactions that appear to limit fish growth in the pond 1 need to be studied and resolved.

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Received on 27.02.2023 Modified on 16.03.2023

Research J. Science and Tech. 2023; 15(2):99-104.

DOI: 10.52711/2349-2988.2023.00017