Status of grass carp aquaculture
In last two decades the climate changes and overfishing resulted in the intensificated research on introduction of new species in aquaculture, or the possibilities of increased cultivation of herbivorous and more climate resilient fish species that are already cultivated. In terms of sustainability, herbivorous species that tolerate high water temperatures are the best candidates. Such an example of a species that is already cultivated, but still has a great potential for intensification of farming in Europe is grass carp, Ctenopharyngodon idella (Valenciennes, 1844), a naturally active and fast-growing species. This species tolerates a wide range of temperatures, from 0° to 38°C, salinities up to 10 ppt and oxygen levels down to 0.5 ppm (Frimodt, 1995). Wu et al. (2023) found that juvenile grass carp exposed to different dissolved oxygen concentrations over prolonged time period showed different susceptibility and adaptation mechanisms. Low dissolved oxygen concentration greatly improves blood gas transport capacity of grass carp, while high dissolved oxygen concentration slightly improves aerobic respiration level. Its main feed are higher aquatic plants and submerged grasses, but it also takes detritus, insects and other invertebrates (Frimodt, 1995).
Grass carp was introduced in Europe in the 20th century for aquatic weed control and aquaculture purposes. Today, it is grown as a food fish in polyculture in carp ponds (Frimodt, 1995; Woynarovich et al., 2010), but it is also of special interest for sport fishing (Siyi et al., 2022). In its homeland of China, it is currently the most important freshwater aquaculture species. In this country, it is mostly farmed in monoculture, with yearly production of 5.76 million tons in 2021, what was 18.08% of China’s total freshwater aquaculture production (FAMA, 2022). According to EUMOFA (2021) the main grass carp producers in Europe are Poland, Czechia, Hungary and Croatia, with the 1.761 tonnes. Its European production remained fairly stable, while the amount of all other farmed Asian carp species decreased by 60% over the ten-year period from 2008 to 2018. Although grass carp is one of the world’s most important aquaculture species, it is considered as a pest outside of its native range due to negative impact on the aquatic, water quality and biodiversity (Dibble and Kovalenko, 2009). With regard to legislation related to non-native species in Europe, its cultivation is allowed in most countries in waters where population has already been established. Exceptionally, all non-native species could be farmed, but their spread into open waters must be prevented. One possible solution could be its farming in the closed RAS systems with the complete control of output water. Despite the mentioned facts affect the development of grass carp production in Europe, the results of numerous researches conducted in recent years can justify the intensification of its production, either in mono- or polyculture.
Production intensification possibilities
High stocking density in polyculture, may cause high total ammonia and phosphorus concentrations, and consequently, algal blooms (Dibble and Kovalenko, 2009). Li et al. (2019) showed that the use of artificial substrata could be one of the technological solutions to increase pond farming density of grass carp by increasing the bacteria that participate in nitrogen and phosphorus cycles. Except in the pond culture, grass carp growth was tested in different technological systems, such as in-pond tank culture system for high-intensive fish production and in RAS. In-pond partitioned aquaculture system that was used in last two decades for intensification of the production of the pond farmed species, and for reducing the use of land, showed that intensive high stocking density had a negative impact on the growth performance and muscle quality of grass carp (Lu et al., 2022). Opposite to the in-pond system, survival and growth rates of the juvenile grass carp overwintering in the RAS were significantly higher. At the same time, the RAS ensured production profit when compared to the loss in the in-pond system (Kristan et. al., 2018). Similarly, as for the common carp (Gavrilović et al., 2019), properly designed RAS, used for overwintering of juveniles in combination with pond farming during warm period could possibly provide the basis for the intensification of grass carp production with economically feasible results.
It could be concluded that technological improvement can be applied for the intensification of grass carp culture. Considering European market, parallelly with the production intensification, relevant marketing strategy that would include product diversification and development should be considered.
Acknowledgement
This paper resulted from bilateral research project Croatia-China titled „Physiological effects of grass carp, Ctenopharyngodon idella, adaptation to environments in Croatia and China “.
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