Semi-natural incubation techniques for Atlantic salmon, Salmo salar: Opportunities and limitations for restocking

Extremely high mortality rates of hatchery reared salmonid fry after stocking into a river made it imperative to have a fresh look at substrate incubation techniques as an alternative to a traditional technique which based on hatchery troughs. Four different types of semi-natural incubation units, analogous in utilizing a gravel matrix for incubation and a natural intra- and supra gravel water flow, but differing in size and shape, were developed. The new units required evaluation to establish efficiency of incubation, with existing hatchery troughs acting as control. Trials were carried out in a replicated design during five consecutive years using a range of genetically different Atlantic salmon (Salmo salar L.) broodstocks. Comparison at the fry stage showed the substrate incubation units produced significantly larger and heavier fry than the troughs. Semi-natural incubation resulted in a reduction in deformities by 420%. Mean eyed-embryo-to-fry (EETF) survival from semi-natural incubators (89.0 ± 4.9%) matched EETF survival from hatchery troughs (94.3 ± 3.3%) and greatly exceeded published data for egg-to-fry survival in the wild. During a period of unusual high water temperatures all fry died in hatchery troughs whereas fry survived seemingly unaffected in semi-natural incubators. Fry emergence from the semi-natural incubation units on a seasonal and diurnal time scale was similar to emergence patterns reported for wild populations. It was concluded that fry from the new semi-natural units are apt to have an increased survival rate when stocked in to a river. Low costs of production make the new incubators an economically viable alternative to traditional incubation systems.