To stock or not to stock: The paradox of releasing fish and invertebrates in estuaries

Release programs for fish and invertebrates involve the release of hatchery-reared juveniles to increase the number of recruits to a fishery, usually to fulfil objectives including stock enhancement, restocking or sea ranching and more recently conservation stocking. Release programs often focus on coastal and estuarine waters because of their high productivity, relatively low predation rates, valuable ecosystem services and ease of access to fishers from surrounding population centres. However, estuaries are environmentally variable and subject to anthropogenic degradation which makes them potentially challenging environments for released individuals, hence the paradox of stocking in estuaries. We evaluated 10 case studies for teleost and crustacean releases with life histories in three major categories of Estuarine Use Functional Groups (EUFGs): estuarine, diadromous (migratory) and marine. We considered the effectiveness of the releases of ‘fish’ in terms of their potential benefits from the stocking, the likely retention of released fish within the estuary, and the risks of stocking in these highly dynamic environments. The case studies come from three countries and annual releases ranging from several thousands to 150,000 for teleosts in Australia and the United States and millions to hundreds of millions for a penaeid prawn (=shrimp) and portunid crab in China. Successful release programs were evident in each of the three broad EUFGs, albeit to different extents. The productive and sheltered waters of estuaries enables high survival, however, the retention of estuarine species within an estuary makes them more vulnerable to environmental perturbations such as unusually cold conditions and episodic, aseasonal rainfall leading to salinity declines and hypoxia/anoxia and associated mortalities. Although diadromous and marine species disperse more widely than estuarine species, their releases in estuaries can lead to substantial positive returns to fishers, particularly in those systems where there are severe recruitment bottlenecks.