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Fish stock are released into the wild.
Fish stock are released into the wild.
Courtesy of UNSW, Australia


The production of the marine wild stocks is oscillating around its feasible maximum of 80-100 million tonnes and improvements, if any, can only come from sustainable enhancements of the habitat (e.g. through artificial reefs), productivity (providing for limiting elements such as iron) or recruitment (e.g. through restocking/ranching). However, while experience in fisheries enhancement is already well established in inland fisheries, it is still in its infancy and raising issues in the marine environment.

For various reasons, the effectiveness of human enhancements in an open environment are still very uncertain. For decades, emphasis was put on the politically rewarding action (e.g. of putting reefs or fingerlings in the ocean) and on the successful rearing of the animals in captivity for release, little has been done to effectively demonstrate the biological or economic performance of the interventions. Indeed, the late discovery of the inefficiency of many such operations have tended to discredit the whole field as a waste of public resources.

The difficulty of tagging the reared animals to identify their contribution to harvest has contributed to the paucity of information available to assess stocking impact. The fact that governments tended to favour politically “easy” enhancement while refraining from taken needed but politically more difficult management action (e.g. blocking fishing capacity or allocating rights) has also helped discrediting the approach among fishery scientists.

Interest might be renewed because:

  • governments are now more decisively addressing the core issue of overcapacity;
  • most high value resources are declining;
  • the recent proliferation of modern tagging technology and progress in visual assessments of reefs stocks provide a way to more convincingly assess the impacts of stock enhancements or artificial reefs.

With the progress now at hand in rearing marine animals, the potential is growing but the experience, protocols, guidelines, etc., on how to combine fisheries enhancement and management are still missing.

Approaches available

Improvements in productivity may come, inter alia, from sustainable enhancements of the habitat (e.g. through artificial reefs), primary productivity (providing for limiting elements such as iron) or recruitment (e.g. through artificial propagation and restocking/ranching).

Artificial propagation would be a solution to consider: (i) for areas in which a species cannot reproduce naturally due to irremediable loss of critical habitat; (ii) to maintain fisheries despite recruitment overfishing in places and situations where conventional management do not work; or (iii) to boost stock rebuilding.

Physical modifications are likely to be acceptable when they aim in fact at re-establishing natural conditions (e.g. reopening coastal lagoons and wetlands, replanting mangroves or seagrass beds).

Artificial reefs made with using redundant vessels, old tyres, quarry rocks and concrete structures may might be adequate to create additional habitats (e.g. in soft bottoms ecosystems) in order to increase biodiversity; attract predators; develop refuges for adults and juveniles; provide reproductive, feeding or nursery space; or establish biofilters (e.g. with oyster or mussels to re-establish water quality when nutrient loads are too high). Artificial reefs have also been used as “anti trawl” devices.

Potential sources of failure
Six main causes for trouble in marine enhancement programmes have been identified (see Waters and Martell in information sources):

  1. failure to produce fish that successfully recruit into the harvested and spawning population;
  2. aggravating the wild stock problems of recruitment by exploiting its residual spawning stock as broodstock;
  3. post-release competition between hatchery and wild juvenile fish;
  4. increase in predation and disease risk for the wild fish fishery due to mass release of hatchery reared fish;
  5. inadvertent selection, under hatchery conditions, of inappropriate traits that decrease the survival rate of released fish or may negatively impact the wild counterparts (e.g. through reproductive advantages);
  6. attraction of fishing effort from unregulated fisheries, aggravating the overfishing situation.

A strong risk exist that enhancements be considered as a “cool” quick fix to long standing fisheries problems. At best, the technology could find its place into a well implemented ecosystem approach to fisheries. At worst, it will be paying lip service to sustainability at high cost to society, delaying real management action.


To develop a sound marine stock-enhancement, a number of considerations are essential.

First, as a premise, be doubtful about the efficiency of enhancement in general and take “successful” experiments with a grain of salt, examining the claims in light of the steps suggested below.

Second, recognize from the onset that the number of species and individuals released or the extent of artificial reefs built are irrelevant without a detailed assessment of the impact.

Third, when considering the possibility to integrate stock enhancement in the fishery management package:

  1. explicitly consider management priorities and goals and all acceptable trade offs before deciding for enhancement programmes. Clearly identify the specific objectives of enhancement and specify measures and indicators of success to facilitate ex ante and ex post performance assessment;
  2. assess carefully the problem being faced and the contributing factors in order to decide between, reproduction or habitat enhancement. Artificial propagation might be adequate in case insufficient reproductive capacity. Habitat restoration or artificial reefs would be preferable in case of insufficient critical habitat for shelter or food. In any case, assess whether conventional methods (e.g. reduced fishing capacity, increased age at first capture, area closures, increased pressure on predators) could not be more efficient;
  3. demonstrate the effectiveness of the approach at pilot scale. Demonstrate for example that artificially reared fish can successfully establish in the wild, e.g. by tagging all fish before release. Demonstrate that the type of reef can lead to the settlement or protection of the target species;
  4. demonstrate that the enhancement is intended to complement and not to substitute, for fisheries management measures and habitat-protection. Show, in particular, that the current fishery regulations are sufficient to ensure the long term sustainability (avoid overfishing) of the enhanced stock;
  5. show that the enhancement is sustainable in the long term, e.g. that the reef life span is acceptable or that there are resources to maintain the hatchery system indefinitely;
  6. undertake a full ex ante environmental risk assessment of the proposed enhancement, on habitat, biodiversity, genetic pools, food chain, introduction of diseases, etc. within an ecosystem and precautionary approach to the fishery;
  7. implement enhancement programmes following an adaptive approach, starting with pilot operations and proceeding stepwise to larger scales only after testing, aware of the uncertainties attached in scaling up.


It is not easy to foresee the future of marine fisheries enhancement. The demand for it should be expected to increase steadily with the growth and development of the world population. Progress in the area is likely to be slow considering the inherent difficulties and the strong a priori dubious or negative attitude of many fishery scientists, ecosystem specialists and environmental NGOs in relation to large scale ecosystem modifications. Considering in particular the complex nature of social-ecological systems, assessing, ex ante, or demonstrating at pilot scale, the expected effects remains a high order challenge.  

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