The world's fish species are under threat from habitat degradation and over-exploitation In many instances, attempts to bolster stocks have been made by rearing fish in hatcheries and releasing them into the wild Fisheries restocking programmes have primarily headed these attempts However, a substantial number of endangered species recovery programmes also rely on the release of hatchery-reared individuals to ensure long-term population viability Fisheries scientists have known about the behavioural deficits displayed by hatchery-reared fish and the resultant poor survival rates in the wild for over a century Whilst there remain considerable gaps in our knowledge about the exact causes of post-release mortality, or their relative contributions, it is clear that significant improvements could be made by rethinking the ways in which hatchery fish are reared, prepared for release and eventually liberated We emphasize that the focus of fisheries research must now shift from husbandry to improving post-release behavioural performance In this paper we take a leaf out of the conservation biology literature, paying particular attention to the recent developments in reintroduction biology Conservation reintroduction techniques including environmental enrichment, life-skills training, and soft release protocols are reviewed and we reflect on their application to fisheries restocking programmes It emerges that many of the methods examined could be implemented by hatcheries with relative ease and could potentially provide large increases in the probability of survival of hatchery-reared fish Several of the necessary measures need not be time-consuming or expensive and many could be applied at the hatchery level without any further experimentation

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The future of stock enhancements: lessons for hatchery practice from conservation biology

This study refines, clarifies and, where necessary, expands details of the guild approach developed by Elliott et al. (2007, Fish and Fisheries 8: 241-268) for the ways in which fish use estuaries. The estuarine usage functional group is now considered to comprise four categories, that is, marine, estuarine, diadromous and freshwater, with each containing multiple guilds. Emphasis has been placed on ensuring that the terminology and definitions of the guilds follow a consistent pattern, on highlighting the characteristics that identify the different guilds belonging to the estuarine category and in clarifying issues related to amphidromy. As the widely employed term 'estuarine dependent' has frequently been imprecisely used, the proposal that the species found in estuaries can be regarded as either obligate or facultative users of these systems is supported and considered in the guild context. Thus, for example, species in the five guilds comprising the diadromous category and those in the guilds containing species or populations confined to estuaries are obligate users, whereas those in the marine and freshwater estuarine-opportunistic guilds are facultative users.

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The ways in which fish use estuaries: a refinement and expansion of the guild approach

Atlantic salmon salmo salar L. parr were reared for 3 months under standard hatchery conditions or in a structurally enriched tank (containing plants, rocks and novel objects). Half of each of these fish had prior exposure to live prey in the form of live bloodworm while the other half were fed hatchery-pellets. After 12 days all fish were tested on a novel live prey item (brine shrimp). A significant interaction between the two factors (prior exposure to live prey and rearing condition) revealed that foraging performance was only enhanced in fish that had been reared in a complex environment and exposed to live prey. It appears that the ability to generalize from one live prey type to another is only enhanced in fish that had been reared in an enriched environment. The findings support the assertion that the provision of enriched environments in combination with exposure to live prey prior to release may significantly improve the post-release survival rates of hatchery-reared fishes. As both the environmental enrichment and the prior foraging experience procedures were comparatively simple, the provision of such pre-release experiences are likely to prove cost effective to hatcheries.

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Environmental enrichment and prior experience of live prey improve foraging behaviour in hatchery-reared Atlantic salmon

Estuarine fish communities are usually used in an ecological monitoring context to infer the environmental changes ocurring in estuarine environments. In the present paper several analyt- ical tools, namely community analysis indices, species-ranking methods and multivariate ordination techniques, were applied to time series data for the Tagus fish community for 2 periods: 1979 to 1981 and 1995 to 1997. Assessment of the relative importance of anthropogenic versus naturally induced fluctuations is made difficult by the range of natural short-term variations. For the Tagus case study, multivariate ordination techniques, rather than the ranking methods and indices analyzed, provide a good indicator of long-term changes. The major trends in the changes observed in this fish commu- nity were a decrease in abundance for several species that use the estuary as a nursery area and an increase in some resident species, namely Gobius spp. and Halobatrachus didactylus. Several species with southern European affinities have increased their importance within the Tagus estuarine fish community.

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Does the Tagus estuary fish community reflect environmental changes

Atlantic cod have been a primary target for marine stock enhancement since the 1880s. In the early part of this period, hatched larvae were released in Norway, the USA and Canada. The last larval releases were conducted in Norway in 1971, and a century of cod larvae releases were halted without any clear evidence of benefit. Since the early 1980s, the focus has been on production of larger, more viable juvenile cod. Emphasis has been given to the design of tag–release programmes involving large-scale releases and ecosystem analysis in selected ecosystems. Most of this research has been carried out in Norway, where more than one million tagged juvenile cod have been released. Smaller stocking experiments have also been performed in Denmark, Sweden, the Faroe Islands and the USA.



This paper reviews the major findings from these programmes. We include summaries and evaluations of rearing techniques for juvenile cod, methods of tagging and recapture, experimental fishing, migration, mortality and growth rates in the different habitats, genetic analysis, and ecosystem studies that have tried to describe the variation in the cod carrying capacity of selected release areas.



Despite relatively large variation in environmental conditions, in cod production and in fishing mortality along the Norwegian coast, results indicate that, under the conditions experienced during the 1980s and 1990s, releases of juvenile cod did not significantly increase cod production and catches. The biological limitations and future prospects of Atlantic cod stock enhancement are addressed.

The enhancement of cod stocks

Fish samples collected at the cooling water intake screens of West Thurrock power station located 35.5 km downstream of London Bridge on the Thames estuary during the decade 1980–1989 were analysed. Seasonal and long-term changes in the abundance of the 15 most numerous fish species and in several fish community parameters were analysed. The majority of species were highly seasonal in their distribution and abundance. Species diversity was lower in the summer (May–August) compared with spring/winter (October–February). Evidence of long-term changes in species diversity and community structure over the decade is presented. The changes were consistent with a period of relative stability (1980–1984) followed by a period of change (1985–1989) and may reflect a deterioration in water quality in the second quinquennium. It is argued that the monitoring of fish communities in estuaries should be based on a multi-metric approach as no single indicator alone can describe the complex community structure.

Williams

Papers

Seasonal and between‐year variations of fish populations in the middle Thames estuary: 1980–1989