A review of the literature confirms that human activities have caused genetic changes in some wild fish populations, with most of these changes being adverse. These genetic effects include a reduction in growth rate and/or possibly in age/size at sexual maturity in some heavily fished populations. There was also considerable evidence of hybridization between wild and released populations (sometimes resulting in the loss of the wild population) and the extinction of some wild populations due to habitat changes and to competition and/or predation from introduced species.