Stock management strategies to control reproduction and optimize growth

The attainment of sexual maturity during commercial ongrowing by commercially important teleost species brings both a commercial loss due to reduced growth potential and detrimental effects on welfare through increased mortalities and increased susceptibility to disease as well as a potential genetic interaction with native stocks through broadcast spawning or spawning interaction by escapees. It is however possible to inhibit or prevent maturation during commercial ongrowing using one of three main management strategies 1.) photoperiodic regulation of maturation and 2.) production of monosex populations and 3) sterilisation. The strategies to be adopted can be species-specific with certain species lending themselves more favourably to one technique over the other. The Group is therefore heavily involved in studies aiming to improve our basic understanding of reproductive physiology in fish to aid the development of new husbandry protocols for the industry.

Photoperiodic regimes

In most temperate spawning species it is now widely accepted that the pattern of seasonally changing daylength is primarily responsible for the cueing and timing of reproduction with temperature usually acting as an ultimate cue synchronizing the latter stages of reproduction and spawning. The Group`s early work has focused on the control of broodstock spawning at the hatchery and suppression of early maturation during on growing in salmonids. In more recent years, with the development of new species for aquaculture such as cod, haddock, halibut, sole etc. photoperiod manipulation is being further investigated to develop species specific photoperiod protocols that can alter spawning time in hatchery and suppress sexual maturation during ongrowing.

Photoperiod is also known to strongly influence smoltification and is routinely used within the salmon farming industry to produce out of season smolt and boost growth performances. The G&R Group`s early work focused on the most appropriate photoperiod regimes for producing out-of-season smolts. However, the outcome of current husbandry regimes is still unpredictable and importantly there is still a clear lack of simple reliable diagnostic tool that can be used to confirm seawater adaptation. The G&R group is therefore working to better understand the smoltification process and implications on the whole fish physiology which can then lead to the refinement of current regimes and the development of new management practices for the industry.

Central to designing these protocols is understanding how each species interprets seasonally changing daylength to cue and coordinate reproductive development. For example, it has been shown that increasing daylength from winter to summer is the recruiting signal in salmonids whereas it is the decreasing daylength which recruits Atlantic cod in to reproduction. The transfer of such knowledge leads to the development of scientifically-based husbandry practises and industry guidelines and specifically the implementation of lighting regimes in the industry that can enhanced both growth and reproductive performance in commercially important species (salmon, trout, and cod).

Perhaps surprisingly, it also appears that photoperiodic manipulations can also be important in tropical species such as Nile tilapia in which long day photoperiod can increase egg production and growth performances. The Group is further investigating light induced physiological effects in tropical species.

New lighting technologies

While standard submersible metal halide lights are routinely used in the salmonid farming industry to good effect, these systems are neither environment- nor species-specific. Therefore the next generation of submersible lighting will be required to offer greater flexibility in the management of the cage lighting fields to assist in the suppression of maturation in the ever expanding range of farming environments and cultured species. This “new generation” will include technologies like Cold Cathode lighting (CCL) or Light Emitting Diodes (LED). Both can be characterised as having significantly lower capital investment and running costs than metal halide systems, and furthermore offer greater flexibility in the arrangement of lighting fields within the cage environment. Both can be manufactured to output specific wavelengths and thus can be matched to environment and species requirements with low power requirements and electrical running costs. Large scale trials are ongoing to test these technologies as part of a collaborative network including lighting manufacturers and other EU research Institutes.

Monosex populations

However, in other species, photoperiod manipulations have limited value as to date the tested protocols did not prevent maturation as in rainbow trout or the production cycle is too long as in Atlantic halibut. Thus, production of single sex populations became the management strategy of choice. As an example, the production of all female stocks in trout and halibut remove any concerns of sexual maturation in production cycles as females are harvested well in advance of sexual maturation. This result in female stocks performing better. Single sex population are already used commercially in both the trout and tilapia industries but is under investigation by the Group in halibut.


The third management strategy under investigation by the G&R Group is sterilization. Triploidy induction is the only means known to date to produce sterile fish. Other methods are technically feasible but require the use of chemicals or Genetic Modification that may be unacceptable to consumers. The G&R Group is currently investigating this avenue in salmon and cod as a means to reduce the negative environmental impact of farmed escapees such as in salmon or broadcast spawning such as in cod.


graph eggs in mouth

light lights