Rohu - Fertilizers and fertilization

Production of natural food through fertilization is one of the important factors for better survival and growth of fish in ponds. Good phytoplankton growth is pivotal for sustained zooplankton density in a pond. This requires an adequate availability of nutrients such as phosphorous and nitrogen including certain micronutrients. Since the natural availability of these nutrients is low, ponds must be adequately fertilized to sustain good plankton growth. The efficacy of fertilizers and manures for improved natural productivity mainly depends on the N: P and C: N ratios in the pond sediment. N:P ratios of 2:1 to 4:1 and C:N ratio of 10:1 to 20:1 are desirable for sustained primary productivity of pond water. Because freshwater ponds in India usually contain adequate quantities of potassium, potassium fertilizers are generally not applied (Jena and Das, 2006).

For ponds that can be drained and dried it is advisable to dry the pond till cracks appear in the bottom soil and to plough the substratum to improve soil aeration and microbial decomposition of organic matter. Depending on the soil pH, lime is applied (Table 4). Changes in water pH exerts stress and affects the haematology of all three species of Indian major carps, although rohu is the least prone to stress caused by altered water pH (Das et al., 2006). After correction for pH by liming, organic and inorganic manures and fertilizers are applied. Organic manures are usually preferred due to the slower rate at which nutrients are released into the water column. The nutrient composition of various organic and inorganic fertilizers is presented Table 5 and 6, respectively.

Several fertilization schedules have been developed for nursery ponds. Most commonly, raw cow dung is applied at a rate of 4000-6000 kg/ha, two weeks before stocking. If poultry manure is used it is applied at a 3rd of the cow dung dose. If, mohua oil cake is used as a fish poison to kill predatory and nuisance fish in the nursery ponds, the dose of manure is reduced to 50 percent of the recommended dose. A phased manuring practice is also used whereby a mixture of groundnut oil cake at 750 kg, cow dung at 200 kg and single super phosphate at 50 kg/ha has also been shown to be effective for the production of desired plankton densities. A thick paste of half of the above quantities are prepared by addition of water and applied as an initial dose 2–3 day prior to stocking and the remainder is applied in 2–3 split doses depending on plankton density (Jena et al., 1998).

Fingerling rearing ponds are generally fertilized with cow dung at 5 000-10 000 kg/ha, depending on the nutrient status of pond. One third of the required dose is applied 8-10 days before stocking of fry and the remainder is applied in equal portions at fortnightly intervals. As mentioned earlier, when poultry manure is used the dose is reduced to 1/3rd of the cow dung application rate.

Grow-out ponds are usually fertilized with cow dung at 10 000-20 000 kg/ha/year or poultry manure at 4 000-8 000 kg/ha/year and in combination with inorganic fertilizers such as urea at 100 N kg/ha/year and super phosphate at 50 P kg/ha/year.  Based on the nutrient status of the soil, carp ponds are classified as low, medium and highly productive ponds and this determines the rate at which fertilizers are applied (Table 7). One third of the total amount is normally applied two weeks before stocking and the remainder is applied at two weekly intervals in equal proportions. Cow dung is the main organic fertilizer used by farmers. Depending on availability, other organic manures such as buffalo manure, pig manure, duck droppings and domestic sewage are also used.

Bio-processed organic manure (e.g. bio-gas slurry) has also been tested and application rates have been standardized. It has been reported that the application of bio-gas slurry at a rate of 30 000 – 45 000 kg/ha/year has the same beneficial effect as other manures, although the BOD is lower and the nutrient release rate is faster (Jena and Das, 2006). The use of Azolla as a nitrogenous biofertilizer has also been standardized. At an application rate of 40 000 kg/ha/year Azolla provides the full nutrient compliment required for intensive aquaculture (Jena and Das, 2006). Tripathi et al. (2000) using Azolla as a bio-fertilizer, have achieved a gross production of more than 15 000 kg/ha/year. Vermi-compost has also been tested and found to be a useful alternative pond fertilizer (Deolalikar and Mitra, 2004).

A modified fertilization schedule adopted by farmers in Andhra Pradesh, India
Farmers in Andhra Pradesh are of the opinion that large ponds are generally better suited for the production of carps. The average pond size in the region is about 4 ha, although some ponds are as big as 100 ha. These ponds are built such that they can be drained, dried and refilled. Lime is applied to the dry pond bottom at 250–500 kg/ha as an initial application. The ponds are then filled with water and poultry manure is applied at approximately 20 000 kg/ha. At least 30 percent of the total amount is applied as a primary dose and once the pond water turns green, the ponds are stocked with stunted carp seed as described earlier. The remainder of the fertilizer is applied at fortnightly or monthly intervals based on the colour of the pond water. Along with organic manures, inorganic fertilizers are also commonly applied. The most commonly applied inorganic fertilizers are single super phosphate, urea, di-ammonium phosphate and 28-28-0 fertilizer complex. In addition, some farmers also apply potash and zinc fertilizers. The general application rate for inorganic fertilizers is reported to be quite high, often the level reaching by some farmers at about 2 000 kg/ha (Veerina et. al., 1993). Inorganic fertilizers are also applied on a periodic basis to ensure adequate plankton production. In addition to fertilizers, lime is also applied on a regular basis to regulate pH and to control disease occurrence. The average amount of lime used is around 1 000 kg /ha and agricultural lime is most commonly used.

Substrate/periphyton based aquaculture
In the Indian sub-continent, fertilization is widely used to increase pond fish production. Hickling (1962) and Hepher (1988) demonstrated that fish production in fertilized ponds does not increase in direct proportion to increased fertilizer application. In other words, fish yield does not increase beyond a certain level of fertilization. It has also been shown that only about 30 percent of manure / fertilizer input is converted into harvestable product and the rest is lost to the sediments, effluent water and the atmosphere (Acosta et al., 1994; Beveridge et al., 1994; Olah et al., 1994). Azim et al. (2001a) reported that conversion of nutrients to harvestable products in ponds can be improved by adoption of periphyton based production. Periphyton production in ponds is enhanced through the provision of substrata such as bamboo poles (Figures 5a and b), and rohu has been found to be the most suitable species for periphyton based production systems (Azim et al., 2002a).

Two mechanisms have been proposed for higher levels of fish production in periphyton based systems. Firstly, that the additional shelter provided by the substrata allows for a reduction in metabolic cost and a greater proportion of resources to flow into fish production and secondly, that the additional primary and benthic production fostered by the artificial substrata support a new food web, part of which ends up in fish biomass (Miller and Falace, 2000). Further, it has been suggested that filter feeding on phytoplankton is unlikely to provide the micro-nutrient and energy requirements of most herbivorous fishes (including rohu) and that larger food items, such as benthic algae and algal detritus that grow on the bamboo poles are required to provide the additional complement of nutrients required by the fish (Dempster et al., 1993; Dempster et al., 1995). Further, it has been demonstrated that periphyton improves the water quality in ponds (Umesh et al., 1999; Ramesh et al., 1999; Azim et al., 2002b; Milstein et al., 2003; Mridula et al., 2005).

Several studies have demonstrated that production of rohu in periphyton based pond systems is significantly higher (18–200 percent) in comparison to control ponds (Wahab et al., 1999; Umesh et al., 1999; Ramesh et al., 1999; Azim et al., 2001a). Azim et al. (2001b) reported that fish production levels of 5 000 kg/ha can be achieved without supplementary feeding by providing substrata for periphyton  that is equivalent to the pond surface area and fertilizing with cow manure at 4,500 kg/ha, urea at 150 kg/ha and Triple Super Phosphate at 150 kg/ha. In a later study, Azim et al. (2004) showed that by adding periphyton substrata equivalent to 50 percent, 75 percent and 100 percent of the pond surface area increased fish production by 114 percent, 168 percent and 209 percent, respectively, compared to the control. Wahab et al. (1999) and Milstein et al. (2003) concluded that the use of periphyton based systems, using suitable material such as bamboo poles, and stocking the pond with a bottom feeding species in conjunction with filter feeders hold significant advantages for farmers in resource poor areas or countries. However, Wahab et al. (1999) added the caveat that the economics of the technology should be evaluated before recommending it to resource poor farmers.