The red sea urchin, Stringy locentrotus franciscanus, is the largest echinoid in kelp forest communities along the west coast of North America and is fished commercially from British Columbia to Baja California (Sloan, 1986). The California sea urchin fishery began in the early 1970's and grew very rapidly to total landings of more than 20,000 metric tons (t) in 1987 (Kato and Schroeter, 1985; Parkerl ). By 1987, this was the second most valuable fishery in California waters with a landed value of $13,693,000. There are no unfished stocks left in southern California; only the development of a northern coastal fishery in the mid 1980' s allowed the continued increase in landings.
With the eventual elimination of unfished stocks state-wide, and potentially throughout the range of the fishery, the alternatives are a reduced harvest and contraction of the industry, or stock enhancement to maintain populations above what is possible with current fishery practices.
The spectacular successes achieved with some finfishes, notably the salmonids, as well as the many recent advances in aquaculture, have led to widespread interest in the second and more economically appealing alternative. Here, I examine potential options for the enhancement of red sea urchin stocks, consider the conditions under which each might be appropriate, and make relative estimates of the costs and effort involved.
The concept of stock enhancement for valuable invertebrate fisheries is not new. Transplantation of abalones, Haliotis spp., to better habitats for growth dates to the 19th century (lno, 1966), and lobster, Homarus spp., juveniles were cultured for release on fishing grounds as early as the 1880's (Van GIst et aI., 1980). The Japanese in particular have explored a variety of methods to enhance fishable stocks. The success of all attempts to enhance benthic invertebrate stocks by the release of hatchery-reared juveniles will be reviewed, as well as other methods relevant to sea urchins.
Because the success of many Japanese approaches is related to the structure of their fishing industry, this and the implications of the American tradition of open access fisheries for stock enhancement will be discussed.
The success of any enhancement effort will depend upon the answer to a critical question: What stages in the life history of the organism limit the production of fishable stocks? For red sea urchins, the limiting factors could be larval supply, problems associated with settlement, survival of newly settled to mid-sized animals, or adequate food to support growth and/or gonad production. Enhancement efforts which do not address the stage which limits production are not likely to be successful; e.g. seeding of juveniles will do little for a population limited by survival of mid-sized animals or food supply.
Furthermore, these factors are likely to vary on local and regional scales, e.g. the survival of mid-sized animals correlates negatively with the abundance of two warm temperate predators, spiny lobsters, Panulirus interruptus, and California sheep head, Semicossyphus pulcher (Tegner and Dayton, 1981; Tegner and Barry2). The northern range limit of these two predators is normally Point Conception, and both are characteristic of bottoms with considerable vertical relief; they will have little impact on red sea urchins north of southern California or in pavement habitats. Finally, enhancement methods will vary considerably in costs, in the length of time required to achieve results, and in how long the results will last.