Fertilized Chondrostoma nasus eggs were incubated at 10, 13, 16 and 19 C until full resorption of the yolk sac. High survival was observed at 10-16 C (89-92% at the onset of external feeding), whereas at 19 C survival was depressed (76%). The time at which 5, 50 and 95% of individuals had hatched, filled the swim bladder, ingested the first food and fully resorbed the yolk sac was determined. An increase in temperature accelerated development and made it more synchronous. Within the period from fertilization to hatching embryonic development was theoretically arrested (t 0 dev ) at 8·8 C, and growth was arrested (t 0 gr ) at 8·86 C. For the whole endogenous feeding period (from fertilization to full yolk resorption) the amount of matter transformed into tissue was temperature independent between 10 and 19 C. Respiration increased exponentially with age; the respiration increase was faster at higher temperatures, but, in general, metabolic expenditures of C. nasus were low. As a consequence, the efficiency of utilizing yolk energy for growth was high as compared with other fish species (57% during the whole endogenous feeding period); it was temperature independent. However, time was used less efficiently at low temperatures, increasing a risk of predation. Within the endogenous feeding period a shift from lower to higher temperatures for optimal yolk utilization efficiency was observed. The temperatures optimal for survival and energetic performance seem to be 13-16 C for egg incubation and 15-18 C for rearing of yolk-feeding larvae. Chondrostoma nasus is a potential candidate for aquaculture for restocking purposes. 1998 The Fisheries Society of the British Isles

Survival was generally high, 94-100%, for newly hatched larvae of the nase Chondrostoma nasus held at 10, 13, 16, 19, 22, 25 and 28 C up to day 66 post-fertilization. The developmental rate decreased with age and increased with temperature. Specific growth rates increased with temperature; within one temperature range growth rate decreased with ontogenetic development. Food consumption and respiration increased with temperature and body size. A temperature increase from 25 to 28 C resulted in slightly reduced survival, minor acceleration of developmental growth and respiration rates, and impeded skeleton formation. Growth efficiency of consumed energy decreased throughout the larval period from 55 to 67% at the first larval stage (L 1 ) to 36-48% at the first juvenile stage (J 1 ). A similar trend for assimilation efficiency and its utilization for growth was observed. The constant temperatures required by larval nase ranged from a minimum 8-10 C to a maximum 25-28 C. A shift of optimum temperatures, 8-12, 13-16, 15-18, 19 and 22 C for nase spawning, embryonic development, yolk feeding larvae, early externally feeding larvae and, late larvae and juveniles, respectively, paralleled the spring rise in the river water temperature. Larval and juvenile nase show high survival, growth and energy conversion efficiencies compared with other fish species. On the other hand, low survival rates and growth can be attributed to external perturbations; thus, young nase may be considered a good indicator of the environmental and ecological integrity of river systems. 2001 The Fisheries Society of the British Isles

Intraspecific differences in female attributes influence egg quality and, as a consequence, offspring viability. To further investigate this hypothesis, we compared female attributes, egg size, biochemical egg composition, and survival potential of offspring from 20 female spawners of an endangered rheophilic cyprinid, Chondrostoma nasus. Egg size was strongly related to female age and size. The chemical composition of egg dry matter was influenced by female size to a lesser extent. No significant relationship between egg size and egg dry matter composition was observed. Mortality curves revealed three distinct periods of elevated mortality: early mortality, hatching mortality, and starvation mortality, separated by periods of reduced mortality. No significant correlations between embryonic mortality (early mortality and hatching mortality) and egg size were found. Starvation mortality was size selective: resistance to starvation correlated significantly with egg size and egg energy content. The direct and indirect relationhips between female attributes, egg quality, and offspring viability show that the two main components of offspring viability (embryonic mortality and larval resistance to starvation) are not interrelated and that the sequence female attributes - egg size - larval resistance to starvation is the main pathway along which size selectivity operates.

Exposure of embryos of Chondrostoma nasus to a strongly reduced oxygen content (about 10% of air saturation) enhanced mortality and depressed hatching success. Viable larvae hatched only from the controls and from the group exposed to reduced oxygen during the earliest embryogenesis (from fertilization to gastrula). In the groups exposed from gastrula to eyed stage and from eyed stage to hatching, a high percentage of fish hatched, but all were deformed and almost all died during the following 1-5 days. Hatching period was prolonged in the groups exposed to reduced oxygen content. At a spawning area C. nasus had deposited a large portion of eggs in areas of reduced oxygen content. On the other hand, eggs placed at the substratum surface were exposed to high predation pressure from Barbus barbus, Lota lota and ducks (Anas platyrhynchos). The trade-off between predation pressure at the substratum surface and reduced oxygen conditions in the interstices resulted in a narrow space for successful development of embryos of this endangered rheophilic fish species under natural conditions. 1996 The Fisheries Society of the British Isles

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