Post-release survival and feeding in reared turbot

As part of the Danish restocking program, an experiment was carried out with four groups of turbot Psetta maxima released on two different occasions at the same location in Århus Bay, Denmark. One objective was to analyse the duration of post-release mortality and the magnitude of this mortality. In 2003 a group called Large turbot (17.1 cm total length (LT)) and a group called Intermediate (LT = 11.8 cm) were released, and in 2004 two similar-sized groups called Naive and Conditioned (LT = 9.8 cm) were released. The Conditioned differed from the Naive turbot by being transferred to enclosures at the release location six days prior to the actual release. This experiment was performed to investigate whether such a conditioning period had a positive effect on the survival and hence the success of the stocking. All the groups released were monitored daily until day 8, using a juvenile flatfish-trawl to recapture the fish. The catches were analyzed on the basis of a normal distribution approximation method, founded in diffusion theory, from which daily abundance of the released fish and hence post-release mortality could be estimated. The group of Large turbot suffered negligible post-release mortality, but for the Conditioned, Naive and Intermediate groups the loss varied between 34 and 66% d− 1. The mortality for the Conditioned group was found to be half that of the Naive turbot released simultaneously. The period of high post-release mortality was estimated to be restricted to three days after release. The only active predators observed in the area were birds. Besides estimating mortality the diffusion model provides an estimate on the catchability of the released turbot when fished with a juvenile flatfish-trawl. Catchabilities varied between 38 and 52% for all releases except for the 17 cm sized turbot released, where catchability was only 12%. The feeding performance of the released fish was also analysed and compared with that of wild fish caught at the same location. These results showed that the proportion of stomachs containing food increased not only with time after release, but also with the size of the turbot. However, whether or not fish was included in the diet was not related to size but to time after release and to whether they had been conditioned or not.