What Are The Larval Arms Of Sea Urchins Used For?

What Are The Larval Arms Of Sea Urchins Used For?

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What Are The Larval Arms Of Sea Urchins Used For?

Brood Stock Protection

Brood stock protection is one of the oldest fishery management approaches to maintaining adequate larval supply. In the early days of the red sea urchin harvest in California, fishermen focussed on relatively few areas, typically those close to ports and international airports. Unfished areas continued to produce larvae. With the expansion of the fishery in the last few years, California can no longer count on unfished stocks to repopulate harvested areas.

While red sea urchins become sexually mature between 40 and 50 mm (Bernard and Miller, 1973; Kato and Schroeter, 1985; Tegner and Barry2), small animals contribute few gametes for the next generation. A plot of gonad weight as a function of test diameter (Fig. 3) shows that the increase in gonad production is exponential with size; the largest animals contribute a disproportionately large share of gametes to the population. Thus, animals smaller than the legal minimum size of 76 mm are not capable of maintaining larval production at previous rates.

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To ensure brood stock protection of their sea urchin populations, the Japanese have taken a threefold approach (Mottet, 1976a). There are minimum size regulations to allow reproduction and to ensure adequate product size. The actual spawning season is closed to fishing, ensuring that reproduction will take place and that sea urchins will not be harvested when the roe quality is poor. Finally, many sma]) sites are designated as "no fishing" areas, an action which also protects recruits. Some sites, known as good recruitment areas from which young sea urchins migrate into fishing areas as adults, are permanently closed. Other sites are fished in 3-year cycles. Harvestable sea urchins are fished out in the first year of the cycle, allowing luxuriant algal growth and presumably minimizing competition of juveniles with the adults for food while maximizing growth rates. These animals are taken at the beginning of the next 3-year cycle. Mottet (1976a) reports that the resulting high density of legal-sized sea urchins greatly increases the efficiency of harvesting.

What Are The Larval Arms Of Sea Urchins Used For

Implications for red sea urchins: Assuming that the existing minimum size limit will not provide adequate larval production, what measures are potentially available under the category of brood stock protection? The State of Washington has both a minimum and a maximum size limit (Sloan, 1986), and Kato and Schroeter (1985) have suggested that this may also be useful in California. Furthermore, the roe of the largest sea urchins (> 125 mm) has been reported to be of generally inferior quality3. The usefulness of a maximum size limit to provide some degree of brood stock and nursery (see the "Habitat Improvement" section on p. 12) protection depends upon the number and density of sea urchins that have been left behind by the fishery. Significant densities are critical because fertilization efficiency declines rapidly over distances as small as 20 cm between spawners (Pennington, 1985), but where the numbers are adequate, this may be a simple and effective approach.

Because the actual period of spawning is highly variable in both time and space, this may be impractical to legislate but could potentially be controlled at the level of the processor. This approach works in Japan because of the high degree of local control over fisheries (Mottet, 1976a). The State of Washington has also adopted a simple system of rotating fishing closures; sea urchin grounds are divided into three zones, one of which is closed each year to foster repopulation (Kato and Schroeter, 1985). The Japanese approach of closing sea urchin recruitment habitats permanently would not work with S. franciscanus because of the spine canopy association; juveniles are abundant in the same habitat as adults (Tegner and Dayton, 1977).

Wild Seed Collection

When natural recruitment rates appeared to be the major factor limiting sea urchin population size, the Japanese began experimenting with a variety of collectors on which larvae would settle directly, thus presumably bypassing sources of mortality associated with settlement in their natural habitat. Some of the earliest collectors were apparently slate surfaces covered with tufts of nylon thread (300 denier thread at a density of 30 threads/cm2) (Mottet, 1981). Eight months after being placed in the sea, these collectors averaged 1,200 sea urchins of about 2 mm test diameter per m2. Long (80 mm) tufts were found to be less susceptible to overgrowth by fouling organisms than short (10 mm) tufts.

Also Read: How To Farm Raise Scallops

Biologists in Hokkaido, one of the three major sea urchin producing areas in Japan (Takagi, 1986), conducted extensive studies on the collection of wild seed of S. intermedius, their in-termediate culture, and release back into the natural habitat (Department of Mariculture, Hokkaido Central Fisheries Experimental Station, 1984; Kawamura, 1987).

Previous efforts in sea urchin propagation in Hokkaido had included policies for resource control, transplantation, and creation of fishing grounds, but it was felt that further increase required large numbers of seed. After observing that sea urchins settled abundantly on scallop culturing facilities, a decade-long research project was initiated to pursue collection of wild seed. Extensive studies related the spawning period to the fall drop in water temperature, determined the distribution of larvae of various stages in time and space, investigated the direction and velocity of currents, and assessed annual variability in larval availability.

They found that settlement was from December to January, and that by June, the density of young-of-the-year sea urchins in natural habitats on the coast was consistently below 151m2 but usually greater than 100/m2 on a collecting apparatus. Furthermore, there was a sharp decrease in abundance (average mortality, 65 percent; range, 43-96 percent; n = 5 years) in the natural habitat from spring to fall which is attributed to predation by crabs and seastars and, when temperatures exceed 22°-23°C, to thermal stress.

A large variety of materials and designs for collectors were field tested using different suspension systems and periods of installation in several locations.

The most successful collector was constructed from transparent vinyl chloride plates with a corrugated surface called wave plates (Fig. 4). The transparent plates allowed algae to grow on both sides, increasing the surface available for settlement. Three square plates, 30 cm on a side, were suspended vertically in a layered configuration with the corrugations running horizontally. Arrays of this arrangement (Fig. 5) led to the adherence of an annual average of 197 sea urchins per collector (range 42836; n = 9 years) in January-February at one location in western Hokkaido. The numbers adhering were strongly affected by depth of the installation; 20-30 m bottom depths with the plates suspended at intermediate levels appeared to be optimal. The plates were most successful when installed 3 months before the settling season began.

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