The concept of balanced fish nutrition in temperate European fishponds to tackle eutrophication
Koushik Roy, Jaroslav Vrba, Lenka Kajgrová, Jan Mráz
Abstract
The present work aimed to understand nutrient enrichment and resultant eutrophication caused by carp farming in semi-intensively managed, temperate shallow-lake ecosystems like central European fishponds – combining animal nutrition and plankton ecology group model principles. In the traditional yet predominant pond farming in central Europe, carp stocks start the vegetative season on a ketogenic diet (high in natural food), have a balanced diet shortly in mid-season (cereals introduced as supplementary feed), and end on a starchy diet (high in cereals). Under beginning-season diets, the fish (carp) stock exhibit high but non-bioeconomic N and P retentions. With a surplus of ‘digestible’ N (protein, amino acids) relative to insufficient carbohydrate energy, much of the digested N is pumped back to the environment in algae-reactive forms (NH4–N). A surplus of digestible P per unit of digestible N also triggers renal clearance of digested P; pumped back to environment as PO43−. By the end-of-season, N, P retentions deteriorate significantly due to high metabolic N losses caused by missing digestible amino acids (lysine, isoleucine) and decreased P digestibility, respectively. Little digested P is unutilized and even discarded in tandem with poor N deposition. End-of-season feeding in fishponds is perhaps most polluting and triggers de-novo lipogenesis, instead of protein (biomass) accretion. However, the ratio of reactive losses (to suspended losses) of N, P, which could instantly trigger algal assimilation, is equally high (bad) at the beginning- and end-of-season. We show aggravated N, P loading by carp may occur both under high and low zooplankton-zoobenthos availability, contradictory to prevailing notions. Environmental nutrient loading by carps is most suppressed, including lowest reactive N, P losses, when diet is balanced. Carp farming in regional fishponds could benefit by adopting scientifically sound ‘pond feeds’ and managing carps' satiety to graze (or spare) zooplankton-zoobenthos for prolonging clear-water phase.