Early maturation of Atlantic salmon has, and continues to be, a major source of economic loss for farmers , as precociously developed fish often exhibit decreased growth and feed conversion efficiency, reduced product quality, and increased susceptibility to opportunistic microorganisms. The initiation of sexual maturation can be difficult to prevent, given the numerous factors, such as photoperiod, temperature, fish size, growth rate, nutritional status, and genetics, that can influence the onset of puberty. Early sexual maturation has been particularly problematic in land-based, closed containment recirculation aquaculture system (RAS) facilities raising Atlantic salmon, with reported early maturation ranging from 2% to 100% in salmon populations raised at seven surveyed RAS facilities. With the massive investment in land-based salmon farms within the past several years, the issue of early maturation appears to be largely ignored; however, it will likely be of profound economic significance for land-based salmon producers unless reliable remedial methods are identified and utilized.
Among other variables influencing maturation, water temperature appears to be a critical factor, and could explain why RAS-raised salmon, typically cultured at higher temperatures than those in sea cages, exhibit comparatively higher rates of early maturation. In particular, smolts and post-smolts experiencing an increase in water temperature while in land-based RAS appear to be at increased risk for early maturation prior to sea transfer. T o further understand this problem, we sought to investigate whether there is a size threshold after which post-smolts will sexually mature in response to an increase in water temperature . To this end, we incubated and hatched m ixed-sex diploid eyed eggs an d reared the fry / parr in flow-through freshwater at 12 oC up to 40 g in weight, at which point 5 0% of the salmon received an S0 winter photoperiod (i.e., 6 weeks LD12:12) to induce smoltification. All fish were then transferred and comingled in 15 0.5 m3 flow-through tanks, to be raised to 400 g final size. Treatment groups (with n=3 replication) included: i ) 12 o C for the entire study duration ; and ii-v) t emperature elevation to 14 oC at 100 g, 150 g, 250 g, and 350 g , respectively. At the time of abstract submission, the study is ongoing with anticipated final data collection in July 2021. At study termination, all fish will be humanely euthanized, and maturation will be assessed within each treatment group via gonadosomatic index quantification, to determine i ) if there is evidence of a size-based threshold related to maturation in response to temperature elevation, and ii) whether receiving an S0 winter photoperiod or not impacts maturation within each treatment group. Final study results will be presented at Aquaculture America 2021.