Effects of photoperiod manipulation on growth and reproduction in Atlantic cod (Gadus morhua L.)

Abstract

Sexual maturation during commercial culture of Atlantic cod (Gadus morhua L.) represents a significant production bottleneck restricting the profitability of the industry. Such problems in other species have traditionally been addressed by artificial manipulation of photoperiod cycles, however little research exists in this field in cod. This thesis therefore investigates the interactions between artificial photoperiod manipulation, sexual maturation and somatic growth in this species. In the first experiment, populations of Atlantic cod (hatched, spring 1999) were maintained on either a simulated natural photoperiod (SNP) or continuous illumination (LL) from approximately 15 months post hatch (MPH) (July 2000) in an enclosed tank system. Growth performance was recorded monthly along with observations of reproductive activity over the subsequent 2 years (up to July 2002). At both 2 and 3 years of age the entire population raised under SNP matured and spawned, during which time mean weight reduced by 13% and 24% respectively. No spawning individuals were recorded at 2 years of age in the LL population and only 18% were observed to spawn at 3. However, observations of both changes in gonadal morphology (observed via ultrasound scanning) and a suppression in growth rate at 2 years of age in the LL population alluded to a maturation “dummy run” regulated by an endogenous clock. Despite this phenomenon, the LL treatment realised a 39% and 43% improvement in wet weight following 1 and 2 years of exposure to LL respectively. When the diel cycle of plasma melatonin was compared between the treatments in February 2001 (23MPH) the SNP population displayed an A-profile diel rhythm ranging between 20 and 50 pg/ml while the LL treatment did not display any rhythm. In the second experiment of this work, two populations of cod (hatched, spring 2001) were reared in commercial open cage systems, one of which experienced continuous additional artificial illumination between July 2002 (15MPH) and October 2003 (30MPH) provided by four, 400W submerged lighting units. Growth and maturation were assessed in both populations throughout. In March 2003 (24MPH) it was apparent that spawning individuals were present in both the SNP and LL populations though a significantly lower number of spawning individuals in the LL treatment suggested that the peak in spawning activity was delayed by about 1 to 2 months. With both populations apparently maturing at 2 years of age, there was no significant difference in weight between the populations at the end of the trial. In comparison to experiments I and IV of this work, these results would suggest that in comparison to salmonids for example, Atlantic cod appear to have a heightened sensitivity to light allowing individuals to differentiate the ambient photoperiod signal from the application of continuous artificial light. In the third experiment, 6 populations of approximately 20 tagged individuals (hatched spring 1999) were maintained, from December 2000 to July 2002, under either SNP, LL or one of four, out of season “square wave” photoperiod regimes (repeating cycles with a 12 month period, consisting of a 6 month window of LL followed by six months of short day lengths [SD, 7L:17D] which had been staggered to start over a six month period). Each individual was monitored monthly for maturation status. Out of season “square wave” photoperiods were demonstrated to successfully entrain maturation and hence significantly alter the spawning profiles in these populations. Application of LL from December 2000 failed to inhibit maturation in the spring of 2001 and, in fact, advanced the spawning season by 1 month while those that experienced SD from the same date showed significant extension of the subsequent spawning season. Interestingly, the males maintained on LL throughout the experiment matured both in the spring of 2001 and one year later in the spring of 2002 while females under the same treatment only matured and spawned in 2001. In the fourth experiment, a total of 830 tagged individuals were raised either under SNP or one of 7 photoperiod treatments, consisting of 5 groups transferred from SNP to LL at 3 monthly intervals between 6 and 18 MPH where they remained and a further two groups maintained on LL from 6 to 15MPH and 6 to 21 MPH respectively before being returned to SNP. Both the gonadic and somatic axes were monitored at the physiological and endocrinological level at three monthly intervals from 6 to 27 MPH. The results demonstrated that it is the falling autumnal photoperiod signal after the summer solstice, more specifically after October, that is responsible for recruiting individuals to enter the sexual maturation cycle. Furthermore, in all treatments where this signal was masked i.e. those which experienced LL starting at or prior to 15MPH, except for some restricted spermatogenic activity in the males testis observed at 27MPH, there was no significant reproductive activity and growth was improved by up to 60% at 27 MPH. While providing evidence for direct photic stimulation of somatic growth, the growth results were also correlated with the measurement of plasma IGF-I and demonstrated its potential as a tool to assess growth rates in the species. Plasma melatonin measured at 15MPH, as in experiment I, was suppressed in all populations which were under LL photoperiods. By identifying the photoperiod “window of opportunity” which recruits individuals into the sexual maturation cycle, this work was able to conclude that the application of LL from the summer solstice prior to maturation is the most efficient photoperiod strategy to be adopted by the aquaculture industry to realise maximum growth potential from their cultured stocks.