Hosting carps and predatory species in polyculture
The production cycle typically takes three years. In the first year the product is 60-150 g fingerlings, while in the second year it is 500-800 g juveniles. Market size carp of 2-3 kg are produced in the third year. The area requirements of the age classes are different, thus in those farms which produce all three age classes yearly, 5% of their net water surface area is used for fingerling rearing, 20% is used for juvenile rearing and 75% is used for market size fish. Fish pond production is typically managed in polyculture, where common carp is produced in combination with other fish species of the same age class. Common carp is the most widely produced species accounting for 80% of the total pond based fish production in 2018 (AKI 2019). Besides common carp typical pond fish species are silver carp, grass carp, and predatory fish species including catfish, pikeperch and pike.
Fish pond production is a semi-intensive farming technology. Organic manure is used to augment the natural feed present in the pond. In addition, the natural feed — zooplankton mostly — can be complemented with the use of cereals and plant based feeds of high protein content (e.g. oil-extracted sun flower seeds, lupine, pea). The ratio of the yields received from naturally available feed and from feeding are typically similar, 50-50%, however this varies significantly between farms, based on the way they are managed. Over the last 10 years, pond production has become more extensive in Hungary as farmers seek to minimise costs and increase profitability.
Ponds and natural wetlands have many similarities
Pond fish farming, besides producing common carp, creates a fish pond ecosystem that is closely related to natural wetland habitats. Although this is an artificial system, the nature of nutrient cycling is identical to that of natural wetlands. The fish pond ecosystem is similar to the natural aquatic ecological systems in complexity. The larger homogeneous habitat patches (e.g. open water, dry pond bottom, reeds) allow specific taxa to be more diverse than in natural habitats, however, on the whole the biodiversity of fishponds is lower than that of natural wetlands.
An attribute of the fish pond ecosystem is the dominance of planktonic organisms which feed on the dissolved nutrients in the water. Unless the proper amount of common carp juveniles are added to the system the pond will be converted into a shallow aquatic habitat with homogeneous marshy vegetation (reed communities, willow-shrub vegetation) in only three-four years. The increased nutrient input enhances the population sizes at all levels of the food web, so that fish ponds maintain notably higher quantity of organisms compared to natural ecosystems. With a shift to a more extensive technology, the nutrient input decreases or ceases completely, the nutrient sources in the pond diminish, and there is a decrease in the population sizes of all the organisms in the pond. The appropriate strategies for farming fish are vital for production levels and for the health of the pond itself. Another unique attribute of fish ponds is the seasonality of water coverage. Due to the technological processes, the different (aquatic, semi-aquatic, terrestrial) stages of ponds are simultaneously present in a relatively small area, thus allowing different habitat complexes to coexist.
Besides contributing to the sustainability of aquatic and wetland habitats, fishponds sustain wildlife of European importance. Their most significant impact is the support of waterfowls connected to wetland habitats; they provide nesting, resting and feeding habitats for these bird species. Fish ponds also support the populations of the Eurasian otter (Lutra lutra). The natural fauna of fishponds includes various amphibian and reptile species, as well as protected and endangered fish species in ponds and canals. Surveys of Hortobágy Fishponds, the largest fish farm in Hungary with ca. 5,500 ha, have identified more than 300 bird species, nine amphibian, three reptilian and eight protected fish species. A detailed assessment of the wildlife associated with fish ponds has not yet been made at a European level. Some attempts have been made to monetise the ecosystem services of fishponds. The value of the habitat service of the Hortobágy Fishponds in Hungary, based solely on the occurrence of bird species, was calculated at 32,000 euro/ha. Regarding Polish fish ponds the total non-productive value was estimated at 52,858 euro/ha.
Farmers pay a price for their support to wildlife
Supporting protected animal species — mainly birds and the otter — reduces yields and increases costs for farmers. Species depending on fishponds can be grouped according to their economic impact: into species with direct economic impact, consisting of fish consumers (great cormorant, pygmy cormorant, Eurasian otter), and fish feed consumers (mallard, common pochard, Eurasian coot). Then there are species which indirectly cause yield loss or increased costs. This category includes all the protected species that inhibit fish pond operations. Fish farming activities that are affected are usually the filling, draining, fishing, waterweed control, and reed cutting processes and the constraints are due to the nesting, breeding, migrating or simple presence of these protected species. These indirect effects reduce the efficiency of farming and result in lower yields. The most important species with direct economic impact is the great cormorant which is responsible for losses of 10-20% of the fish production value. The intensity of the damage differs between barrage and paddy ponds, and between smaller and larger ponds.
A good indicator of the importance of fishponds for nature protection is that 49% of used fish pond area is protected as National Parks and 66% are in NATURA 2000 areas. Nature protection legislation contributed to making fish farming operations more extensive. The results of the AQUASPACE project highlighted that the impact on production caused by nature protection regulations is the most marked in larger ponds located in protected areas.
Mitigating the effects of flooding and drought
Besides providing habitat for natural fauna and flora, pond aquaculture has an important role in water management. Pond production is well adapted to the natural seasonal fluctuation of surface water resources and the rate of consumption is tailored to these hydrological conditions. Fish ponds can retain substantial amount of water. Climate change is causing the occurrence of extreme water levels (both low and high) to increase. Fish ponds act as buffers to a certain degree functioning as reservoirs when there is excess water, and returning it to the source, when water levels fall.
Water quality at the inlet, outlet, and in the pond is an important factor in pond production. According to the measurements, the quality of surface waters is good and does not limit production. The artificially increased nutrient levels in ponds used for fish production means that these levels are typically higher than in natural wetland habitats. However in the ponds nutrients are transformed into fish biomass. After the production period the used water is discharged seasonally to natural surface waters again. The quality of the discharged water is defined by environmental legislation. Besides the fish production, the discharged water is affected by the inflow water quality, which can be quite diverse, especially considering that ground water can also enter the pond system. The nutrient content of the inlet water is also recycled during fish production. The discharged water from a fish production pond usually has higher concentrations of organic material (COD) and a higher content of suspended solids, while nitrogen and phosphorus compounds are at similar or lower levels, compared to the inflow water. In conclusion, fishponds often act as biological filters rather than polluters in the natural surface water network.
Béla Halasi-Kovács, PhD