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Lessons Learned
Chapter 5
Lessons Learned 5.1. Lessons for Restocking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Lessons for Stock Enhancement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3. Lessons for Both Restocking and Stock Enhancement. . . . . . . . . . . . . . . . . . . . . . . . . . .
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The broad range of case studies in this review provides a clear message for fisheries agencies considering the use of restocking and stock enhancement as management tools, that is, there are no generic methods for such interventions. Even relatively small diVerences in life-history traits among species (e.g., larval duration, nursery habitats, vulnerability to predation, age at first maturity, feeding areas) can have major eVects on the approach required, and costs involved, for releasing marine invertebrates in restocking and stock enhancement programmes. These diVerences have necessitated approaches as diverse as: (1) hatchery production of juveniles; (2) collection of wild spat; (3) provision of more settlement habitat; (4) redistribution of settled juveniles and (5) translocation of adults. Associated measures include construction of additional habitat to increase carrying capacity, removal of predators and installation of temporary artificial shelters to reduce the high levels of predation that can occur during release of juveniles. The variations in life history among species, the range of possible ways of increasing the numbers of juveniles, and the numerous failures, lead to the inescapable conclusion that there are no shortcuts to targeted research to identify whether a particular form of restocking or stock enhancement is likely to be cost-eVective. A number of key lessons emerge from the accounts presented for the 11 species/groups described in Chapters 2 and 3. We summarise these lessons below to provide guidance for the design, development and implementation of future restocking or stock enhancement programmes.
ADVANCES IN MARINE BIOLOGY VOL 49 # 2005 Elsevier Ltd. All rights reserved
0065-2881/05 $35.00 DOI: 10.1016/S0065-2881(05)49005-7
222
BELL ET AL.
5.1. LESSONS FOR RESTOCKING Lesson 1: The costs and time frames involved in restocking programmes for some species can be prohibitive. These costs will be unpalatable enough for developed countries, but beyond the capacity of developing countries unless they receive long-term assistance. Restocking cannot, therefore, always be relied on to correct abuse of the resource, so management agencies should take great care to avoid depletion of populations to chronically low levels. Long-lived sessile species are particularly vulnerable to depletion to low densities, and such mismanagement is likely to result in local extinction because the remnant population may be dispersed too widely to reproduce eVectively. Lesson 2: The costs of restocking can be reduced greatly for some species simply by relocating a proportion of adults to form a viable spawning biomass. This method is likely to be eVective only for species with multiple, largely self-replenishing populations at relatively small spatial scales and/or very limited larval dispersal (e.g., topshell, abalone and sea cucumbers, but also scallops and other bivalves in some situations). Even then, this measure will probably only be successful at sites with good larval retention. It will also depend on transferring suYcient adults to establish a viable breeding population and a moratorium on fishing until the spawning biomass reaches a level that can support regular, substantial harvests (see Chapter 6, Section 6.2). Lesson 3: The high costs involved in releasing some species at a size where they escape most predation can be defrayed by combining the culture of animals intended for restocking with other forms of aquaculture. For example, giant clams for release in the wild could be reared on farms supplying the aquarium trade (Chapter 2, Section 2.1.8), and the sea cucumber Holothuria scabra could possibly be produced in combined culture with shrimp in ponds (Chapter 2, Section 2.3.2).
5.2. LESSONS FOR STOCK ENHANCEMENT Lesson 4: Very large numbers of juveniles are needed for eVective stock enhancement. Even where hatcheries can produce large numbers (tens of millions) of fit individuals at reasonable cost, releases will not have an economic impact on catches unless the supply of recruits is generally limiting, the cultured juveniles represent a large proportion of recruitment, and fishing is regulated appropriately. The release programmes for hard clams on
LESSONS LEARNED
223
Long Island, New York (Chapter 3, Section 3.2.9), and for abalone and sea urchins in Japan (Chapter 3, Sections 3.3.9 and 3.8.9) help illustrate this point. It is important not to underestimate the scale of releases needed for stock enhancement programmes. The two groups of marine invertebrates for which stock enhancement has been most successful, scallops and shrimp, have been released in hundreds of millions to billions of juveniles each year. The releases of abalone and sea urchins in Japan, which have been an order of magnitude lower, have not met the goals of increasing harvests, although they may have stabilised catches at lower levels by arresting further declines. Lesson 5: Excessive releases of juveniles cause density-dependent mortality. This is evident at both large and small scales. In Japan and China, the population size of shrimp seems to have been reduced after releases of very large numbers of juveniles (Chapter 3, Section 3.5.8). For abalone, releases of low numbers of juveniles have been as successful as releases of higher numbers at limited spatial scales (Chapter 3, Section 3.3.11). This highlights the fact that a sound understanding of the ecology of the target species, and the carrying capacity of the ecosystem, is needed to optimise the number of animals released (see Chapter 6, Section 6.4). Lesson 6: Wild spat can provide an abundant, low-cost source of juveniles for some species. The most successful form of marine invertebrate stock enhancement (scallops) is based on the collection of wild spat (Chapter 3, Section 3.1.2). However, caution is needed here, because variation in abundance of spat may be too great for establishment of stock enhancement programmes. This was the case for sea urchins in Japan (Chapter 3, Section 3.8.2) and is a firm reminder that development of methods for collection of spat (or postlarvae) cannot always be translated into a cost-eVective way of supplying juveniles for stock enhancement. Lesson 7: Artificial habitats can be used to increase the carrying capacity for target species by providing space and/or food for additional released animals. Such man-made structures have provided additional shelter and feeding surfaces for abalone (Chapter 3, Section 3.3.5) and increased areas for development of the algal communities needed to improve roe weight in sea urchin fisheries (Chapter 3, Section 3.8.5). There is also a strong indication that habitat is limiting for adult lobsters in some places. Where this occurs, any increase in the supply of juvenile lobsters will have little eVect without provision of additional habitat for the adults (Chapter 3, Section 3.7.5). Lesson 8: Yields of some species can be increased simply by providing suitable settlement habitat. This has been demonstrated for clam species on the west coast of the United States where gravel beds have ‘harnessed’ juveniles that otherwise would not have settled at the same locality (Chapter 3, Section 3.2.2). Provision of artificial shelter with crevices of the appropriate
224
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size has also increased settlement success of spiny lobsters up to six-fold in the Caribbean (Chapter 3, Section 3.6.2). Lesson 9: Harvests of some species can be improved by redistributing some juveniles from areas of heavy settlement to areas where spat failed to settle. The twin benefits of this intervention are: (1) that growth and survival are likely to improve in areas with abundant juveniles because of reduced intraspecific interactions and (2) harvests can be taken from areas that would otherwise have yielded little because larvae generally failed to arrive or settle there. This has been demonstrated for razor clams on the west coast of the United States (Chapter 3, Section 3.2.9). However, great care must be taken not to alter the conditions needed for successful future settlement when juveniles are collected and transplanted. The destruction of oyster reefs in Chesapeake Bay is a salutary reminder of the economic losses that can occur if the underpinning ecosystem is degraded (Chapter 3, Section 3.2.1). Lesson 10: Large areas are needed for stock enhancement of some species of marine invertebrates. Access to such areas will need to be negotiated before investment in large-scale releases and then protected if the stock enhancement programme is to be successful and sustainable in the long term. Scallops are a case in point (Chapter 3, Section 3.1.11). Lesson 11: The most successful stock enhancement programmes for marine invertebrates are run by co-operatives and the private sector. This has occurred because fishermen have been given access or property rights to the resource, and therefore the incentive to invest in application of the technology. The best examples of this are the scallop stock enhancement programmes in Japan and New Zealand (Chapter 3, Section 3.1.9, see also Chapter 6, Section 6.5). The corollary to this is that, ultimately, large-scale releases for stock enhancement commenced by governments will not be sustainable unless the beneficiaries bear the costs involved. This is evident from the massive reductions in the releases of shrimp in China since the implementation of new economic policies (Chapter 3, Section 3.5.9). Lesson 12: Successful stock enhancement may change the marketing conditions for some species. This has occurred for scallops in Japan, where increased production from stock enhancement and aquaculture meant that fisheries co-operatives had to add value to the product, engage in wholesaling and make concerted eVorts to raise awareness of the quality and availability of their scallops to maintain prices (Chapter 3, Section 3.1.10). By contrast, the great increase in production of the sea cucumber Apostichopus japonicus in China through ‘put-and-take’ sea ranching has not yet lowered prices. Rather, the price has increased because of greater prosperity in the country, which has resulted in many more consumers and higher demand (Chapter 2, Section 2.3.11).
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5.3. LESSONS FOR BOTH RESTOCKING AND STOCK ENHANCEMENT Lesson 13: Establishment of a successful restocking or stock enhancement programme at one site is no guarantee that the methods can be transferred to other countries or to other sites in the same country. For example, few nations have been able to implement the methods for collecting scallop spat that are so beneficial in northern Japan, and the technology has only been successful in one of the areas where it has been applied in New Zealand (Chapter 3, Section 3.1.8). Lesson 14: Small-scale experiments to test methods for restocking and stock enhancement can give misleading results. Increases in the scale of releases often reveal problems not apparent in initial experiments (e.g., for scallops in Australia and Canada [Chapter 3, Section 3.1.8] and geoduck clams in subtidal habitats in the United States [Chapter 3, Section 3.2.4]). Therefore, decisions to invest in large-scale releases of juveniles should not be made on the basis of small-scale, albeit well-designed, experiments. Large, ‘commercial’ scale experiments should be done to determine whether the assumptions stemming from initial experiments hold over sizable management areas. Lesson 15: Predation is the single greatest hurdle to the successful establishment of released juveniles in restocking and stock enhancement programmes. The need for a thorough understanding of the ecology of the species and the ecosystem at the release site, and field experiments to identify the best way to release animals to maximise survival, are essential steps to reduce predation. In general, the chances of success can be improved greatly by releasing animals at low density in areas where predators are in relatively low abundance and at a size where the juveniles escape most predation. For some species, culling predators, and/or providing structures that impede predation, can also improve survival of released animals substantially. Lesson 16: Aquaculture will create opportunities for restocking and stock enhancement. Although restocking and stock enhancement programmes were among the prime reasons for development of hatchery technology for many species (e.g., giant clams, sea cucumbers, topshell, abalone, sea urchins), large-scale releases of other species (e.g., shrimp) have only been made possible through investments in aquaculture. ‘Spin-oV’ benefits from aquaculture, in terms of methods for culturing juveniles for release, can be expected to continue. One of the most promising developments in this regard is the capture and culture of puerulus larvae of spiny lobsters (Chapter 3, Section 3.6.2). Lesson 17: Availability of methods to rear larvae in hatcheries does not necessarily mean that they can be applied cost-eVectively to produce the large numbers of juveniles needed for eVective restocking and stock enhancement
226
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programmes. This problem has been particularly severe for giant clams, topshell, queen conch and lobsters, where the costs involved in rearing juveniles to a size large enough to escape most predation have been punitive. However, it has also occurred to some extent for most other species except shrimp. When there are severe limits on production of juveniles because of high costs and restrictions on capacity, use of hatchery-reared juveniles should be considered only for restocking, when the initial high cost of rebuilding spawning biomass may be justified. Even then, managers need to confirm that restocking will add value to other forms of management and that the supply of cultured juveniles is adequate to meet the aim of the programme (see Chapter 6, Section 6.1.2). Lesson 18: Large-scale releases of hatchery-reared juveniles can aVect genetic diversity of wild populations. The evidence for this is still equivocal for some species. However, it has been reported for sea urchins in Japan (Chapter 3, Section 3.8.6), and the reduced diversity of Penaeus chinensis in the coastal waters of China, compared with Korea, is thought to be due to very large releases of cultured shrimp for stock enhancement (Chapter 3, Sections 3.5.5 and 3.5.8). Whether the shrimp released in China were of more limited genetic diversity than the wild population because of intentional or unintentional selective breeding, or whether there were diVerences in diversity between P. chinensis in China and Korea originally, remains to be determined. Only then will it be possible to gauge the full impact of stock enhancement on the gene frequencies of these shrimp. Lesson 19: The potential benefits of releases of hatchery-reared juveniles will be reduced unless the releases address the most urgent needs of the fishery and appropriate measures are used to manage the intervention. The release of lobsters in Norway is a case in point (Chapter 3, Section 3.7.9). There, legislation to prevent fishing for the released animals, and the remnant wild population, until replenishment occurred would have delivered greater benefits than allowing the cultured animals to be caught when they reached the minimum legal size. Given the current status of the fishery, releases of lobsters in Norway funded from the public purse would be managed better as restocking rather than stock enhancement. Lesson 20: Adaptive management can reduce the cost of restocking and stock enhancement. This is epitomised by the Challenger Scallop Enhancement Company in New Zealand, where the quantity of spat collected, grown and released each year is based on accurate annual estimates of the size of the population and the natural supply of juveniles (Chapter 3, Section 3.1.9). The quantity of cultured scallops needed has also been reduced further by improved ways of preparing collected spat for release (Chapter 3, Section 3.1.4).
Lessons Learned 5.1. Lessons for Restocking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Lessons for Stock Enhancement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3. Lessons for Both Restocking and Stock Enhancement. . . . . . . . . . . . . . . . . . . . . . . . . . .
222 222 225
The broad range of case studies in this review provides a clear message for fisheries agencies considering the use of restocking and stock enhancement as management tools, that is, there are no generic methods for such interventions. Even relatively small diVerences in life-history traits among species (e.g., larval duration, nursery habitats, vulnerability to predation, age at first maturity, feeding areas) can have major eVects on the approach required, and costs involved, for releasing marine invertebrates in restocking and stock enhancement programmes. These diVerences have necessitated approaches as diverse as: (1) hatchery production of juveniles; (2) collection of wild spat; (3) provision of more settlement habitat; (4) redistribution of settled juveniles and (5) translocation of adults. Associated measures include construction of additional habitat to increase carrying capacity, removal of predators and installation of temporary artificial shelters to reduce the high levels of predation that can occur during release of juveniles. The variations in life history among species, the range of possible ways of increasing the numbers of juveniles, and the numerous failures, lead to the inescapable conclusion that there are no shortcuts to targeted research to identify whether a particular form of restocking or stock enhancement is likely to be cost-eVective. A number of key lessons emerge from the accounts presented for the 11 species/groups described in Chapters 2 and 3. We summarise these lessons below to provide guidance for the design, development and implementation of future restocking or stock enhancement programmes.
ADVANCES IN MARINE BIOLOGY VOL 49 # 2005 Elsevier Ltd. All rights reserved
0065-2881/05 $35.00 DOI: 10.1016/S0065-2881(05)49005-7
222
BELL ET AL.
5.1. LESSONS FOR RESTOCKING Lesson 1: The costs and time frames involved in restocking programmes for some species can be prohibitive. These costs will be unpalatable enough for developed countries, but beyond the capacity of developing countries unless they receive long-term assistance. Restocking cannot, therefore, always be relied on to correct abuse of the resource, so management agencies should take great care to avoid depletion of populations to chronically low levels. Long-lived sessile species are particularly vulnerable to depletion to low densities, and such mismanagement is likely to result in local extinction because the remnant population may be dispersed too widely to reproduce eVectively. Lesson 2: The costs of restocking can be reduced greatly for some species simply by relocating a proportion of adults to form a viable spawning biomass. This method is likely to be eVective only for species with multiple, largely self-replenishing populations at relatively small spatial scales and/or very limited larval dispersal (e.g., topshell, abalone and sea cucumbers, but also scallops and other bivalves in some situations). Even then, this measure will probably only be successful at sites with good larval retention. It will also depend on transferring suYcient adults to establish a viable breeding population and a moratorium on fishing until the spawning biomass reaches a level that can support regular, substantial harvests (see Chapter 6, Section 6.2). Lesson 3: The high costs involved in releasing some species at a size where they escape most predation can be defrayed by combining the culture of animals intended for restocking with other forms of aquaculture. For example, giant clams for release in the wild could be reared on farms supplying the aquarium trade (Chapter 2, Section 2.1.8), and the sea cucumber Holothuria scabra could possibly be produced in combined culture with shrimp in ponds (Chapter 2, Section 2.3.2).
5.2. LESSONS FOR STOCK ENHANCEMENT Lesson 4: Very large numbers of juveniles are needed for eVective stock enhancement. Even where hatcheries can produce large numbers (tens of millions) of fit individuals at reasonable cost, releases will not have an economic impact on catches unless the supply of recruits is generally limiting, the cultured juveniles represent a large proportion of recruitment, and fishing is regulated appropriately. The release programmes for hard clams on
LESSONS LEARNED
223
Long Island, New York (Chapter 3, Section 3.2.9), and for abalone and sea urchins in Japan (Chapter 3, Sections 3.3.9 and 3.8.9) help illustrate this point. It is important not to underestimate the scale of releases needed for stock enhancement programmes. The two groups of marine invertebrates for which stock enhancement has been most successful, scallops and shrimp, have been released in hundreds of millions to billions of juveniles each year. The releases of abalone and sea urchins in Japan, which have been an order of magnitude lower, have not met the goals of increasing harvests, although they may have stabilised catches at lower levels by arresting further declines. Lesson 5: Excessive releases of juveniles cause density-dependent mortality. This is evident at both large and small scales. In Japan and China, the population size of shrimp seems to have been reduced after releases of very large numbers of juveniles (Chapter 3, Section 3.5.8). For abalone, releases of low numbers of juveniles have been as successful as releases of higher numbers at limited spatial scales (Chapter 3, Section 3.3.11). This highlights the fact that a sound understanding of the ecology of the target species, and the carrying capacity of the ecosystem, is needed to optimise the number of animals released (see Chapter 6, Section 6.4). Lesson 6: Wild spat can provide an abundant, low-cost source of juveniles for some species. The most successful form of marine invertebrate stock enhancement (scallops) is based on the collection of wild spat (Chapter 3, Section 3.1.2). However, caution is needed here, because variation in abundance of spat may be too great for establishment of stock enhancement programmes. This was the case for sea urchins in Japan (Chapter 3, Section 3.8.2) and is a firm reminder that development of methods for collection of spat (or postlarvae) cannot always be translated into a cost-eVective way of supplying juveniles for stock enhancement. Lesson 7: Artificial habitats can be used to increase the carrying capacity for target species by providing space and/or food for additional released animals. Such man-made structures have provided additional shelter and feeding surfaces for abalone (Chapter 3, Section 3.3.5) and increased areas for development of the algal communities needed to improve roe weight in sea urchin fisheries (Chapter 3, Section 3.8.5). There is also a strong indication that habitat is limiting for adult lobsters in some places. Where this occurs, any increase in the supply of juvenile lobsters will have little eVect without provision of additional habitat for the adults (Chapter 3, Section 3.7.5). Lesson 8: Yields of some species can be increased simply by providing suitable settlement habitat. This has been demonstrated for clam species on the west coast of the United States where gravel beds have ‘harnessed’ juveniles that otherwise would not have settled at the same locality (Chapter 3, Section 3.2.2). Provision of artificial shelter with crevices of the appropriate
224
BELL ET AL.
size has also increased settlement success of spiny lobsters up to six-fold in the Caribbean (Chapter 3, Section 3.6.2). Lesson 9: Harvests of some species can be improved by redistributing some juveniles from areas of heavy settlement to areas where spat failed to settle. The twin benefits of this intervention are: (1) that growth and survival are likely to improve in areas with abundant juveniles because of reduced intraspecific interactions and (2) harvests can be taken from areas that would otherwise have yielded little because larvae generally failed to arrive or settle there. This has been demonstrated for razor clams on the west coast of the United States (Chapter 3, Section 3.2.9). However, great care must be taken not to alter the conditions needed for successful future settlement when juveniles are collected and transplanted. The destruction of oyster reefs in Chesapeake Bay is a salutary reminder of the economic losses that can occur if the underpinning ecosystem is degraded (Chapter 3, Section 3.2.1). Lesson 10: Large areas are needed for stock enhancement of some species of marine invertebrates. Access to such areas will need to be negotiated before investment in large-scale releases and then protected if the stock enhancement programme is to be successful and sustainable in the long term. Scallops are a case in point (Chapter 3, Section 3.1.11). Lesson 11: The most successful stock enhancement programmes for marine invertebrates are run by co-operatives and the private sector. This has occurred because fishermen have been given access or property rights to the resource, and therefore the incentive to invest in application of the technology. The best examples of this are the scallop stock enhancement programmes in Japan and New Zealand (Chapter 3, Section 3.1.9, see also Chapter 6, Section 6.5). The corollary to this is that, ultimately, large-scale releases for stock enhancement commenced by governments will not be sustainable unless the beneficiaries bear the costs involved. This is evident from the massive reductions in the releases of shrimp in China since the implementation of new economic policies (Chapter 3, Section 3.5.9). Lesson 12: Successful stock enhancement may change the marketing conditions for some species. This has occurred for scallops in Japan, where increased production from stock enhancement and aquaculture meant that fisheries co-operatives had to add value to the product, engage in wholesaling and make concerted eVorts to raise awareness of the quality and availability of their scallops to maintain prices (Chapter 3, Section 3.1.10). By contrast, the great increase in production of the sea cucumber Apostichopus japonicus in China through ‘put-and-take’ sea ranching has not yet lowered prices. Rather, the price has increased because of greater prosperity in the country, which has resulted in many more consumers and higher demand (Chapter 2, Section 2.3.11).
LESSONS LEARNED
225
5.3. LESSONS FOR BOTH RESTOCKING AND STOCK ENHANCEMENT Lesson 13: Establishment of a successful restocking or stock enhancement programme at one site is no guarantee that the methods can be transferred to other countries or to other sites in the same country. For example, few nations have been able to implement the methods for collecting scallop spat that are so beneficial in northern Japan, and the technology has only been successful in one of the areas where it has been applied in New Zealand (Chapter 3, Section 3.1.8). Lesson 14: Small-scale experiments to test methods for restocking and stock enhancement can give misleading results. Increases in the scale of releases often reveal problems not apparent in initial experiments (e.g., for scallops in Australia and Canada [Chapter 3, Section 3.1.8] and geoduck clams in subtidal habitats in the United States [Chapter 3, Section 3.2.4]). Therefore, decisions to invest in large-scale releases of juveniles should not be made on the basis of small-scale, albeit well-designed, experiments. Large, ‘commercial’ scale experiments should be done to determine whether the assumptions stemming from initial experiments hold over sizable management areas. Lesson 15: Predation is the single greatest hurdle to the successful establishment of released juveniles in restocking and stock enhancement programmes. The need for a thorough understanding of the ecology of the species and the ecosystem at the release site, and field experiments to identify the best way to release animals to maximise survival, are essential steps to reduce predation. In general, the chances of success can be improved greatly by releasing animals at low density in areas where predators are in relatively low abundance and at a size where the juveniles escape most predation. For some species, culling predators, and/or providing structures that impede predation, can also improve survival of released animals substantially. Lesson 16: Aquaculture will create opportunities for restocking and stock enhancement. Although restocking and stock enhancement programmes were among the prime reasons for development of hatchery technology for many species (e.g., giant clams, sea cucumbers, topshell, abalone, sea urchins), large-scale releases of other species (e.g., shrimp) have only been made possible through investments in aquaculture. ‘Spin-oV’ benefits from aquaculture, in terms of methods for culturing juveniles for release, can be expected to continue. One of the most promising developments in this regard is the capture and culture of puerulus larvae of spiny lobsters (Chapter 3, Section 3.6.2). Lesson 17: Availability of methods to rear larvae in hatcheries does not necessarily mean that they can be applied cost-eVectively to produce the large numbers of juveniles needed for eVective restocking and stock enhancement
226
BELL ET AL.
programmes. This problem has been particularly severe for giant clams, topshell, queen conch and lobsters, where the costs involved in rearing juveniles to a size large enough to escape most predation have been punitive. However, it has also occurred to some extent for most other species except shrimp. When there are severe limits on production of juveniles because of high costs and restrictions on capacity, use of hatchery-reared juveniles should be considered only for restocking, when the initial high cost of rebuilding spawning biomass may be justified. Even then, managers need to confirm that restocking will add value to other forms of management and that the supply of cultured juveniles is adequate to meet the aim of the programme (see Chapter 6, Section 6.1.2). Lesson 18: Large-scale releases of hatchery-reared juveniles can aVect genetic diversity of wild populations. The evidence for this is still equivocal for some species. However, it has been reported for sea urchins in Japan (Chapter 3, Section 3.8.6), and the reduced diversity of Penaeus chinensis in the coastal waters of China, compared with Korea, is thought to be due to very large releases of cultured shrimp for stock enhancement (Chapter 3, Sections 3.5.5 and 3.5.8). Whether the shrimp released in China were of more limited genetic diversity than the wild population because of intentional or unintentional selective breeding, or whether there were diVerences in diversity between P. chinensis in China and Korea originally, remains to be determined. Only then will it be possible to gauge the full impact of stock enhancement on the gene frequencies of these shrimp. Lesson 19: The potential benefits of releases of hatchery-reared juveniles will be reduced unless the releases address the most urgent needs of the fishery and appropriate measures are used to manage the intervention. The release of lobsters in Norway is a case in point (Chapter 3, Section 3.7.9). There, legislation to prevent fishing for the released animals, and the remnant wild population, until replenishment occurred would have delivered greater benefits than allowing the cultured animals to be caught when they reached the minimum legal size. Given the current status of the fishery, releases of lobsters in Norway funded from the public purse would be managed better as restocking rather than stock enhancement. Lesson 20: Adaptive management can reduce the cost of restocking and stock enhancement. This is epitomised by the Challenger Scallop Enhancement Company in New Zealand, where the quantity of spat collected, grown and released each year is based on accurate annual estimates of the size of the population and the natural supply of juveniles (Chapter 3, Section 3.1.9). The quantity of cultured scallops needed has also been reduced further by improved ways of preparing collected spat for release (Chapter 3, Section 3.1.4).