Polyculture is an art and science of growing two or more compatible fish species in a pond to maximize production by taking advantage of different spatial distribution and feeding habits (Zimmermann, Nair, & New, 2009). Polyculture concepts rely on the complete utilization of various spatial niches and trophic of a pond to acquire a maximum production per unit area. Carp polyculture is the most common, popular, and successful aquaculture system in Nepal. Carp polyculture is the major aquaculture system contributing to more than 90% of total aquaculture production in Nepal. In 2018/19, total fish production, aquaculture, and productivity were 91,832 t, 70,832 t, and 4.92 t/ha, respectively (CFPCC, 2019), which is insufficient to fulfill the growing demand for fish in Nepal. Therefore, it is essential to increase production and productivity.
Keyword: Carp, Polyculture, Productivity
Polyculture is a production system where two or more species of fish with different ecological habitat and food preferences are cultured together in such densities that there will be almost no competition for space and food (Shrestha & Pandit, 2012). Polyculture is characterized by low investment, quick return, high profit, and rapid growth in yield, and meet the needs for raising the living standard of the people (Lin, 1982). The two main aspects that significantly influences the level of intensities of operation of polyculture systems are stocking densities and species combination (Jena, Ayyappan, Aravindakshan, Dash, Singh, & Muduli, 2002). The monoculture production is much more feed dependent than polyculture unless low stocking densities are practiced (Woynarovich et al., 2010). Besides, the choice of species in a polyculture system should be based on the following criteria: availability of fish seeds, availability, and cost of other inputs such as feeds, fertilizers, and productivity of the system (Guerrero, Guerrero, & Ala, 1988).
Carp polyculture is the most viable and common aquaculture system in Nepal. There are different types of carp polyculture, which is practiced to enhance productivity. The major and established system of Nepal is a semi-intensive carp polyculture. The main species under culture are rohu, naini or mrigal, bhakur, silver carp, bighead carp, grass carp, and common carp (Hussain & Yadav, 2016).
Principle of carp polyculture
The motivating principle is that a combination of different species can maximize fish production. Different species combinations in a polyculture system effectively utilize available natural food produced in a pond and contribute to improving the pond environment. Polyculture management is based on the relationship between organisms at different levels of the food chain (Singh, Maqsood, Samoon, Verma, Singh, & Saxena, 2020). Therefore, the selection of species plays a vital role in the polyculture system because all of the species should benefit from the available food without competing with one another (Yin, Zhu, Zhou, Li, Wang, & Liao, 2017). In this system, food niches are enriched by using fertilization or supplemental feeding, but only with a proper combination and densities will utilize it efficiently. A suitable combination of species will maximize the synergistic and minimizes antagonistic fish-fish and fish- environment relationships.
Synergistic interaction is based on two processes: improved environmental conditions and an increase in food resources. The mechanisms through which different fish species contribute to the improvement of environmental conditions depend on the specific levels of the food chain at which they feed (Milstein, 1992). An example of increased availability of food resources is illustrated by the Chinese proverb 'feed one grass carp well, and you feed three other fish (Opuszynski, 1986). At the macrophytic level, grass carp ingest many plant materials and decrease excess plant growth, which prevents nocturnal oxygen deletion. Moreover, its feces are the source for detritivorous fish and have a fertilization effect on phytoplankton, which is grazed by silver carp at the planktonic level. Besides, feces of silver carp which have partially digest phytoplankton are being eaten by common carp, which otherwise would not have been utilized. At the benthos level, common carp stir the mud, which would recirculate nutrients helps the development of phytoplankton, thus food for silver carp. As grazing on algae, it will control bloom and avoid the risk of O2 depletion, thus improving the heterotrophic food chain, improving bottom-feeding fish. At the heterotrophic level, detritivorous fish helps in the improvement of water quality. Tilapia feed on sediment, preventing an increase in organic load and increasing ammonia level (Milstein, 1992).
In unbalanced conditions, the system is affected in different ways according to the level of the food chain where the imbalance occurs. At the phytoplankton level, excessive silver carp may lead to ichytyo-eutrophication caused by overgrazing of large algae promoting small algae. Overstocking of bighead will lead to the consumption of copepods, chironomid larvae, which are also basic food of common carp. Bottom fish might affect others by interfering with other food and reproduction by destroying nest during food search. Overstocking of grass will eliminate macrophytes and a concurrent increase in feces resulting in phytoplankton bloom. The principal tool for maximum production and polyculture management is the knowledge of fish-fish and fish-environment quantitative relationships. This helps in selecting appropriate combinations of species, stocking density, and other decisions according to the specific conditions. Polyculture is the appropriate technique when the goal is the production of low-cost fish or when fish feeds are not available (Milstein, 1992).
Principal requirements of different species in combination (Shrestha & Pandit, 2012)
- They should have complementary feeding habits.
- They should occupy different ecological niches.
- They should attain marketable sizes at the same time.
- They should all be non-predatory and tolerate each other.
Pros and cons of carp polyculture Pros of polyculture
- Better utilization of the space and feeding niche.
- Full utilization of natural and formulated feeds.
- Low water quality and disease problems.
- Product diversity for household consumption and marketing in the same period.
- Risk diversification and more economic return than monoculture.
Cons of polyculture
- Skilled technical knowledge, expertise, and experience are required.
- Difficult to procure and synchronize fingerlings of the right size at the right time.
- Difficult to maintain the food for all species.
Difficult in harvesting and low production.
The stocking rate depends on the biological productivity, supplementary feeding, and water surface area of a pond. So, to maximize the capacity of the pond, four to seven species are stocked at the same time, but sometimes, due to interspecific competition, stocking time can be altered. In contrast, the species ratio depends on pond nutrients, fish seed accessibility, consumer, and market demand. Fingerling has a high chance of survival and better production than a smaller size despite being a little expensive (Rahman, Varga, & Chowdhury, 1992).
The desirable stocking rate is one fish/m2 and the desirable socking ratio is as below: Table 1. Species combination in carp polyculture system in Nepal
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Source : (CFPCC, 2020).
- Quote paper
- Saurav Lamichhane (Author)Biplov Shrestha (Author)Parbati Kandel (Author), 2021, A Retrospective Review of Carp Polyculture in Nepal, Munich, GRIN Verlag, https://www.grin.com/document/1014217