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Behavioural differences between and within individuals can greatly affect the outcome of behavioural studies. In addition, behavioural interactions between individuals can compromise the health and welfare of captive fish. In paper I, I investigate the relationship between locomotory activity, boldness and aggressive behaviour in ~2000 hatchery-reared Baltic salmon parr (Salmo salar L), with the aim to predict aggression level from activity and boldness displayed in the open field test. We found that activity and boldness were positively correlated while they were not correlated with aggression level measured in the mirror stimulation test. Surprisingly, medium and low aggressive fish were the most active, while highly aggressive fish showed only average activity. We conclude that the open field test, although efficient, does not accurately predict aggressive behaviour. However, the mirror stimulation test can be used for high-throughput aggression profiling of juvenile salmon. In paper II, I tested a subset of the salmon parr for a second time, to quantify behavioural consistency between trials and to investigate if phenotypic plasticity was related to aggression level. Our results show that activity was the most stable behavioural variable between trials. Even though aggression was not consistent between tests, we found that the fish displaying a low level of aggression in the first test were less consistent in their behaviour than highly aggressive fish. In paper III, we compared the behavioural development of zebrafish larvae of two strains, the AB strain and 5th generation offspring of wild-caught zebrafish from India. Individual larvae were screened for activity and boldness at the age of 5-, 7-, 12- and 30-days post fertilization using an open field test with alternating light and dark cycles. Furthermore, we analysed mRNA expression of genes encoding serotonin, dopamine, galanin and opioid receptor subunits, as well the peptide neurotransmitter spexin in whole brain samples from juveniles, with the aim to investigate potential neuroendocrine mechanisms of divergent behavioural profiles. Our results show that larvae from the wild strain had higher activity and greater variance in their behaviour than AB larvae, under both light and dark conditions. Wild larvae also had significantly higher expression of dopamine receptor subunit drd2b at 30 days post fertilization, a difference that could be related to difference in activity. In conclusion, the results presented in this thesis contribute to our understanding of animal behavioural profiles, at both an intraspecific and intraindividual level.