Underwater acoustic tag telemetry was used to assess behavioural differences between juvenile wild‐type ( i.e. non‐transgenic, NT ) and growth hormone ( GH ) transgenic (T) coho salmon Oncorhynchus kisutch in a contained simulated ocean environment. T O. kisutch were found across days to maintain higher baseline swimming speeds than NT O. kisutch and differences in response to feeding were detected between T and NT genotypes....