Introduction
Many fish populations are under pressure due to overfishing and environmental deterioration. They are frequently supported by release of cultured individuals. These releases are common practice in fisheries management, especially in waters that are subjected to angling pressure and for stock enhancement for instance in ocean ranching. Furthermore, in species conservation practices where the reproductive population is declining steadily, and effective population size has become critically low, ex situ practices are utilized to safeguard the remaining genetic diversity until the adverse impacts that led to the decline of the populations have been overcome, while hatchery release of progeny can be needed to allow self-sustaining populations to reestablish.
While the general suitability of the introduction of captive reared or transplanted fish has been increasingly questioned due to poor survival and adverse impact upon native populations, conditions exist where no alternatives for releases are available, and must be employed (for instance in reintroduction measures).
Results
The adverse impacts of confined rearing on post-release fitness and survival can strongly influence morphological, genetic, behavioral and physiological responses. Fish fitness as the ability to survive and reproduce in the harsh natural environment does not develop in the well-protected (disease and predator-free) culture environment. The monotonous nature of traditional hatchery practices, including high stocking densities, and predictability in feeding regimes, often results in a lack of environmental variability depriving behavioral responses linked to behavioral and neural deficits. As a result the fish reveal reduced survival in novel conditions. This has been exemplified in Gulf sturgeon (A. oxyrinchus desotoi) where survival following release reached only 20% of the fish that were originating from natural recruitment.
Over the past years, the adverse effects of such release practices have been the target of research and several means for the improvement of the rearing practices as well as the adaptation of release strategies have been considered wherever the releases have been determined as indispensable. For sturgeons, a first guideline to improve culture methods for release has been suggested.
Fish exhibit plasticity in neurogenesis depending upon the environment that they are reared in. While brain size is reported to be a result of the early life experiences with regard to the diversity of the environmental stimuli the neuronal interlinkage and brain structure adapts constantly. As such, transfer from monotonous environments into diverse ones even later in ontogeny has the potential to alter their cognition and adapt their physiological responses to challenges. Consequently, fish are sensitive to changes in environmental conditions and social interactions and thus are able to adopt skills to develop coping mechanisms provided by previous experience, i.e. do learn.
Discussion
Strategies to cope with behavioral depreviation - or even better - to prevent it at an early phase of development are strategies that have shown promising results in laboratory tests and following release. Such prevention or mitigation strategies can be applied as early as during reproduction, allowing mate choice and near natural incubation to optimize the resulting offspring. Among the mitigation strategies applied during embryonic development and rearing, streamside hatcheries are applied utilizing river water from the release site which are providing near natural conditions with regard to the environmental fluctuations while at the same time preventing excessive mortalities through predation. Alternatively, training schools prior to release have been shown to increase the adaptability of juveniles. Here, different stimuli or a combination of these can be used over a period of some 10 days to enhance the responsiveness of the fish to changing environments. It has been shown in Baltic sturgeon A. oxyrinchus that moderate environmental fluctuations such as temperature or light during the training phase alter the response of the fish with regard to the expression of heat shock proteins and increase the swimming performance. In current research it was shown that enhanced rearing conditions result not only in significant differences in behavior in laboratory experiments but also produce a significant difference in performance habitat choice and migration patterns) following release.
As such the adaptation of rearing conditions to near natural variability or even the short-term exposition to environmental fluctuation improves the performance of the fish by enhancing their responsiveness and as such have the potential to increase effectiveness of releases and subsequent survival. Further studies are needed to develop methodologies to prepare hatchery-reared juvenile fish for fitness for survival in a highly competitive world, such as early recognition of potential predatory and effective escape reactions as well as early measures to support the development of the immune defense against pathogens known to occur in the natural habitat.