Social behaviour and growth rate variation in cultivated tilapia (Oreochromis niloticus)

This study was undertaken to understand the behavioural causes of growth variation in cultivated fish and to study the genetics of agonistic behaviour vis-a-vis growth rate. The fish studied was a laboratory population of the Nile tilapia, Oreochromis niloticus, being maintained at the Marine Gene Probe Laboratory, Dalhousie University. Ten maternal half-sib families were produced by fertilizing the eggs of each female with the sperm from four different males. The fish within each half-sib family were all pooled together for the behaviour as well as growth experiments, in order to expose each fish to the full range of microenvironments within the half-sib family, and to eliminate replicate variance. Upon termination of the experiments, the male parent of each fish was determined using DNA fingerprinting using microsatellites.; The behavioural observations were made by randomly pairing fish from within each half-sib pool, soon after swim-up, and counting the number of aggressive and submissive behaviours displayed by each member of the pair. Two derived variables (net aggression, viz. aggression minus submission; and total agonistic activity, viz. aggression plus submission) were also constructed for each fish.; Growth experiments were conducted by rearing fish from the same 10 half-sib groups in each of two types of competitive environments, high interaction (HI), and low interaction (LI). Measurements included length, weight, and maturity status. Estimation of variance components and heritabilities was done by ANOVA as well as by DFREML (which can utilize pedigree information that was provided). Genetic correlations were calculated by correlating family mean values and by correlating breeding values.; The sire component estimates from ANOVA gave a low value of heritability for aggression, and moderate values, ranging from 0.240 to 0.391, for submission, net aggression and total agonistic activity. The DFREML estimates were very low for aggression and submission, but 0.131 and 0.258, respectively, for total agonistic activity and net aggression.; Sire component heritabilities from ANOVA as well as DFREML estimates gave high to very high values for all the growth traits from the HI environment (0.843 to {dollar}>{dollar}1.0, and 0.652 to 0.962, respectively, by the two methods) while the estimates from the LI environment ranged from 0.218 to 0.345 (ANOVA sire component) and 0.290 to 0.544 (DFREML). The extremely high values from the HI environment is attributed to the effect of the higher level of genotype-by-social microenvironment interaction in the HI environment.; Net aggression and winner/loser status were found to have significant positive correlations with subsequent growth, in both competitive environments, thus making these behaviours good predictors of growth. A path analysis revealed a strong dependence of growth on net aggression. It also revealed that aggressive behaviour was associated with poor growth--more so in the LI environment than HI. Submission, on the other hand, was associated with good growth more in the LI environment than in the HI environment. Finally, it is speculated that net aggression represents a behaviour that is under inadvertent selection during domestication.