Chondrostoma nasus - its reproduction strategy and possible reasons for a widely observed population decline - a review

Summary

Under convenient environmental conditions and normal population structure and density, the nase, Chondrostoma nasus, exhibits the following typical spawning behaviour:

• Spawning run is conspicuous, massive and often conducted over long distances.

• Spawning is synchronized, both in terms of gonadal ripening for individual fish (i.e., synchronous ripening of all oocytes in the ovaria) and in terms of demecological dimension, resulting in all members of a population or its greater part ripening and spawning at the same time.

• Both sexes are spatially isolated during the culminating period of the spawning season.

• The ripening females occupy deeper microhabitats, with a less rapid current, outside of the spawning sites, usually nearby and downstream. After completing ovulation, they enter the spawning site occupied by a shoal of mMes for a very short time only, and immediately begin to spawn, releasing the eggs and leaving the spawning ground to return to nearby less rapid waters downstream. This act, despite synchronous oocyte ripening and despite uniportional type of spawning, could be repeated with a new batch of eggs several times within a short period.

• The mating system is promiscuous (mixture of polyandry and polygamy). Each female spawns regularly with a number of males, one male may consecutively mate several females.

• The choice for spawning substrate is strictly lithophilic.

Recently, a drastic decline in population densities has been observed, over almost the entire distribution area of the nase. This has included a considerable change in spawning behaviour (loss of migration intensity, less abundant fish aggregations on spawning sites, substitution of spawning substrates, transition from polyandric types of mating to pairing, loss of spawning synchronisation, possible lower fertilization rate due to lower sperm concentration and quality, lower genetic exchange and erosion of the gene pool). These factors result in secondary effects and additionally reduced reproduction success and the accelerate retreat of the nase. However, the primary reasons for this adverse situation are still not satisfactorily known and a complex of more anthropogenic factors (mainly engineering activity, pollution, change in hydrological regime and reduction of food base) are commonly considered as responsible for this situation.