Uncommon life history and winter spawning of common carp (Cyprinus carpio) in a natural thermal spring, under temperate climate.

Common carp female generally matures at age 4-5 years old and spawns between April and July under the temperate climate. Contrary to a range of 0-28 °C of temperate freshwaters, the water temperature of Lake Hévíz (Hungary, Central Europe), the largest natural bathable thermal lake in the world, varies between 26 and 35 °C seasonally. The specific environmental conditions (continuously warm water and its individual chemical composition, special nutrient base, lack of natural lakeside spawning substrate compared to usual spawning grounds, continuous high human disturbance, etc.) suggest that the carp population here may also differ in reproductive characteristics from their counterparts in surrounding waters. Our findings suggest that the self-sustaining dwarf common carp population of Lake Hévíz matures 2 to 4 years earlier (at the age of one) and spawns 1 to 3 months before (between February and April, at 27-30 °C water temperature) than carp typically do in the temperate zone (16-20 °C). Successful winter spawning was verified by rearing larvae from the collected eggs and in situ induced propagation.

Removal of dead fish eggs by Asellus aquaticus as a potential biological control in aquaculture.

The objective of the present experimental study was to gain a better understanding of the foraging activity of Asellus aquaticus during fish egg incubation. A. aquaticus were introduced into experimental setups of dead eggs, viable eggs and hatched larvae of zebrafish (Danio rerio), a commonly used model organism. The amount of A. aquaticus and the duration of their exposure to the eggs significantly affected the proportion of consumed dead eggs in each experimental cycle. A. aquaticus belongs to the group of aquatic detritivores, and no predatory behavior was observed during the experiments. These crustaceans could distinguish between the dead eggs and those containing living embryos. Furthermore, zebrafish larvae remained unharmed by A. aquaticus, even in the absence of alternative food source. These findings underscore the potential sanitary role of these crustaceans in natural waters and offer new perspectives on their possible use as biological control organisms in aquaculture hatcheries. Additionally, our results suggest a potential application of A. aquaticus in combating pathogens by reducing the growth substrates for bacteria and fungi.