Constraints in Aquaculture-Based Ocean Replenishment

1) Agency involvement level

  • Adequate integration of stocking with fishery management is a crucial mediator of the potential for stock enhancement success. Linear management of stocking, without integrated co-management with the other forms of fisheries management will always under perform as a fishery management strategy. With strong agency involvement, fisheries management decisions have the potential to achieve synergistic effects on stock biomass by combining several tools for managing fisheries along with several disciplines for monitoring results. Integrated agency involvement also fuels adaptive management by allowing appropriate responses to new information gained through monitoring the results of management actions.

2) Cost effectiveness

  • The cost effectiveness of stock enhancement needs to be compared with that of other management strategies. This has rarely been done by stocking programs. Costs may well exceed the value of gains made. But change in costs over time should be tracked, as the costs of enhancement of marine organisms may decrease over time as new and cheaper aquaculture and more effective stocking technologies come on line.

3) Density dependence

  • The current "question of the day" in fisheries enhancement is whether displacement of wild fish by hatchery fish can be realized and avoided. Walters & Juanes (1993) hypothesize that refugia (habitats that afford shelter from predators) are the principal microhabitats occupied by young-of-the-year recruits and that fish abundance is strongly mediated by food availability within and near refugia, with higher mortality of fish that move out of refugia; density-dependent mortality rates result primarily from increased activity with increased density leading to higher predation rates, which are greater on prey that leave refugia in search of food (foraging-arena theory). The problem for managers of stocking programs is to understand the relationship between juvenile density and survival at least well enough to know how much juvenile abundance can be increased before recruitment success in wild stocks starts to drop.

4) Habitat availability

  • Attempting to increase abundance by stocking fish into areas where abundance is habitat limited makes little sense. Only for stocks that are limited by recruitment failures will stocking have the potential to increase abundance (i.e., only those stocks whose abundance is limited by amount of spawning success, or by weather, or by habitat limitations to younger stages of the life cycle than the stage being stocked, etc.). When declines in fish abundance are due to habitat degradation or loss of essential habitats needed for certain stages of the life cycle, then adding fish smaller than the stage(s) that are habitat limited should have no effect on abundance.

5) Stakeholder considerations

  • In mixed stock fisheries (hatchery stocks and wild stocks), increasing depleted fish populations with hatchery releases will increase fishing mortality of wild stocks. How can you support a hatchery program to stock large numbers of hatchery fish and yet prevent overfishing of the weaker (less abundant) wild stock when overall catch quotas are increased in response to the increase in fish population size caused by stocking hatchery fish? The weak (wild) stock needs to be protected.
  • What is the expected fishing effort response to stocking and how will this affect the ability to achieve stocking goals, particularly increasing catch per unit effort (CPUE)? CPUE is proportional to fish abundance divided by fishing effort. Won't increases in fish abundance result in greater fishing effort, negating expected increase in CPUE by each angler?

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