Japanese Hatchery-based Stock Enhancement:
Lessons for the Chesapeake Bay Blue Crab

Measurement of Hatchery Contribution

Hatchery based enhancement is typically centered on two goals: (1) Enhancement of a fishery (also called put-grow-and-take strategy) and (2) Enhancement of the population (typically termed stock enhancement). Although the stated overall goal of Japan's Sea Ranching Program is stock enhancement, release and evaluation strategies focus on the issue of whether released juveniles are contributing to catch. Fishery contribution is estimated as the number of hatchery fish harvested as a proportion of the entire (hatchery + wild) harvest. Contribution of released juveniles to a fishery entails two events: (1) Released juveniles grow and survive to a harvestable size ("post-release survival"); (2) After attaining a harvestable size, released crabs are in fact harvested ("harvest recapture rate"). Two general approaches for estimating fishery contribution in this report are: (1) Direct approach, where released juveniles have a unique attribute (e.g., tag or size) that can be identified in harvested crabs; (2) Modeling approach, in which growth and survival into the fishery are directly estimated and coupled with a harvest model that specifies the probability that a harvestable crab is landed.

A common example of the direct approach for estimating fishery contribution is the cohort method, in which a pulse of similar-sized released juveniles makes up a unique size fraction in the size distribution of landed crabs. This cohort of similar sized hatchery crabs can sometimes be distinguished from wild cohorts in the fishery. By tallying hatchery cohort(s) as a proportion of total catch, fishery contribution can be estimated. Problems with the cohort methods arise when size distributions between hatchery and wild cohorts overlap broadly, and when growth rates among individuals are variable. A more rigorous method is to tag released crabs, as done in Hamana Lake, and then sample landed crabs for these tags. Precision in the estimate of fishery contribution rate here will be dependent upon the sampling rate of the fishery (number of crabs examined for a tag versus total number of crabs harvested) and the recapture rate (number of tagged crabs recaptured versus number of crabs tagged).

The modeling approach first determines the probability that a released juvenile crab will survive and grow to a harvestable size and multiplies this estimate by the number of released juveniles to calculate the number of hatchery crabs entering the fishery. Secondly, the modeling approach estimates the likelihood that a hatchery crab, once attaining a harvestable size, will in fact be harvested. This approach is as accurate as the component estimates of survival, growth, and harvest probabilities. Because methods of release, early survival and growth conditions, and fishery circumstances are expected to vary substantially year to year, this method should be verified by using direct methods for estimating fishery contribution. (See discussion of modeling approach for Osaka Bay.)

Fishery contribution is not one in the same as fisheries enhancement. It only specifies that hatchery individuals are entering the fishery and being captured. There is evidence from salmon and flatfish release studies that hatchery fish can in fact displace wild fish in terms of survival and growth. In addition, habitat productivity waxes and wanes, which can lead to large variations in survival and growth of both hatchery and wild cohorts. Thus, fisheries enhancement requires broader-scale information on interactions between hatchery and wild individuals as well as changes in ecosystem carrying capacity. For similar reasons, analysis of fishery enhancement by correlating release numbers with catch levels is problematic. A positive or negative correlation may have more to do with changes in habitat productivity or the status of the wild population than the absolute numbers of hatchery individuals that enter the fishery.

– David Secor