Abstract
Sexual maturation and spawning in sea-cages is a major production and environ- mental challenge for the Atlantic cod farming industry. Altered photoperiod has arrested sexual maturation in cod kept in indoor or covered experimental tanks, but only delayed or caused asyn- chronous maturation in cod kept in sea-cages. To arrest the onset of puberty more efficiently, we reduced the relative differences in light intensity between night and day by lowering 4 sub- mersible sea-cages, each stocked with 400 juvenile cod, to 20 m depth for 14 mo. Two cages were continuously illuminated (LL), and 2 cages served as controls with natural light (NL) conditions. Cod grew well in both treatments and showed very low mortality. Final weights did not differ, although the pattern of growth differed through time. Continuous light did not arrest maturation, but delayed it by 3 to 6 mo; gonadosomatic indices reached 21.2% for NL females during spawn- ing in February to April, but reached only 10.2% for the LL females over a protracted spawning period from May to September. The liver index for NL fish decreased during spawning by 6% (females) and 15% (males). In contrast, the liver index in the LL fish decreased relatively little (females = 5%, males = 3%), suggesting these fish invested less energy in maturation. Submer- gence combined with sufficient artificial lighting will both delay and reduce the extent of matura- tion, which can improve production strategies through better feed conversion ratios and reduce the risks associated with 'escape through spawning'. Deeper submergence, combined with shading and stronger lighting, may even arrest maturation.
Highlights
Sexual maturation is a major production problem during the on-growing phase of Atlantic cod Gadus morhua farming
Submergence without artificial light led to cod maturing in a synchronised manner in February/March, likely driven by the seasonal changes in light levels and other environmental variables, which typically lead to maturation of both wild and farmed Atlantic cod in the spring spawning period in western Norway (Kjesbu & Kryvi 1989, Berg & Albert 2003, Taranger et al 2006)
While we do not understand the mechanisms that drove this difference, the difference itself strongly suggests that fewer eggs were spawned by LL fish compared with the natural light (NL) fish
Summary
Sexual maturation is a major production problem during the on-growing phase of Atlantic cod Gadus morhua farming. Ecological effects through localised peaks in recruitment may be possible following escape through spawning, as has been suggested for sea bream Sparus aurata farming in the Mediterranean (Dimitriou et al 2007, Somarakis et al 2013). Both the production and the environmental problems associated with maturation of cod in sea-cages have led to an increased focus on methods to suppress sexual maturation or limit the likely survival of fertilised gametes released
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