Experimental Investigation of the Success of Brown Trout Stocking and its Effect on Wild Brown Trout Populations in Upland and Lowland Rivers.
The thesis describes an experimental study of the success of stocking either domestic brown trout (DBT) adults or fry, and the effects on wild brown trout (WBT) populations in upland rain-fed rivers and lowland spring-fed chalk streams in the United Kingdom. The effects of adult stocking on the biomass, growth rate, displacement rate and abundance of adult WBT have been investigated. Also, the post-release growth of DBT adults and fry and the quantity of them remaining in their original release sites was examined. Experimental sites were located on either upland rain-fed rivers or lowland spring-fed chalk streams. The experimental design used replicated, randomised blocks of river containing three treatment sites and a control. The results of the adult study showed that the stocking of adult DBT had no significant impact on the biomass, growth rate, displacement rate or abundance of resident adult WBT. The stocked adult DBT exhibited negligible growth and few remained in their original release site. During the fry stocking study, an evaluation of calcein-marking showed the method to have great potential. The results on the impact of fry stocking were similar to that found in the adult study. No significant effects of fry stocking on the abundance of WBT fry were found despite considerable numbers of stocked fry remaining in their original release sites in some instances. However, the growth of resident WBT fry in upland rivers was negatively affected by the introductions. It is demonstrated that the provenance of DBT fry and adults can significantly affect their post-release performance. Overall, it is considered that the results of both studies provide useful information on the success and impacts of DBT stocking which will help inform fishery managers and guide Environment Agency policy particularly with regard to their National Trout and Grayling Fisheries Strategy.