Reproductive performance of farmed Arctic charr

Abstract: The production of farmed Arctic charr (Salvelinus alpinus) has increased rapidly over the last decade, and the industry is predicted to continue to grow in the coming years. One major bottleneck for this future expansion concerns the supply of viable eggs and juveniles. Hatching rates of Arctic charr eggs in aquaculture are generally much lower than for other farmed salmonids, and exhibit a large variation between individuals and years. The aim of this thesis is to evaluate current hatchery conditions and to deepen the understanding on how environmental and biological factors affect the reproductive performance of Arctic charr in routine farming. The thesis comprises four separate studies. In paper I, records of biological and environmental variables and individual egg survival data from an Arctic charr hatchery, covering a period of 12 years, were analysed in an attempt to find single and combined factors that can explain some of the variation in egg survival. Rearing temperature during summer was identified as the most critical factor for reproductive success of the current broodstock. The study also revealed strong positive relationships between female age, egg size, and egg viability, most likely enhanced by the thermal stress experienced by the broodstock. Paper II assessed broodstock rearing temperature and egg viability over a period of 28 years with focus on thermal stress in a changing climate. Mean summer water temperatures in the hatchery increased by approximately 2°C from 1986 to 2010. The temperature increase was most evident in July but was also apparent in May, August, and September. Egg survival was most closely linked to September temperatures, which indicates that it is not necessarily the warmest days that are the most detrimental. Paper III is a study on egg incubation temperature, and the first, to my knowledge, to show that the initial stages of embryogenesis in Arctic charr are much more cold-sensitive than later in the incubation period. Incubations initiated at low temperature (2.3°C to 2.8°C) resulted in significantly higher mortality and deformity rates. Paper IV is a case study which assessed hormonal status and gamete quality of male and female Arctic charr during routine artificial fertilisation. Female plasma levels of maturation inducing hormone during stripping indicated that post-ovulation aging of oocytes can explain part of the egg loss. The results also suggest that the paternal effect on reproductive performance of the current broodstock is considerable.