Abundance, dynamics and production properties of populations of edible bivalvesMizuhopecten yessoensis andSpisula sachalinensis related to the problem of organization of controllable submarine farms at the Western shores of the Sea of Japan

Kurzfassung

Die Lebensgeschichte, Ökologie, Populationsdynamik und Produktionsleistungen von zwei Muschelarten,Mizuhopecten yessoensis (Jay) undSpisula sachalinensis (Schrenck), werden im Hinblick auf ihre verbesserte Nutzung durch Kulturmaßnahmen beschrieben. Die beiden Arten sind Borealformen und leben im oberen Sublitoral des südwestlichen Japanischen Meeres.M. yessoensis gehört zur Epifauna; die Vertikalverteilung dieser durch Schwimmen zur aktiven Ortsänderung befähigten Art wechselt mit der Jahreszeit und dem Reifezustand. Die Bestandsstärke wird wesentlich vom Vorhandensein von Algen und Seegras bestimmt, auf denen sich die Jungtiere nach der Metamorphose anheften können.S. sachalinensis gehört zur Endofauna; Größe des Bestandes und Vertikalverteilung hängen besonders von den Witterungsbedingungen ab, da ein erheblicher Teil der Jungtiere vom Wellenschlag ausgespült und an den Strand geworfen wird. Die Untersuchungen über Populationsdynamik und Produktionsleistungen basieren auf regelmäßigen quantitativen Bestandsaufnahmen durch Schwimmtaucher, die während der Jahre 1962 bis 1966 zu allen Jahreszeiten durchgeführt wurden. Die Analyse der Größen- und Gewichtszusammensetzung der Populationen gestattet quantitative Aussagen über die Lebensdauer der beiden Arten, übe das nach Jahreszeiten und Altersgruppen verschiedene Größen- und Gewichtswachstum sowie über die Produktionsleistungen.

Summary

1. The quantitative distribution und the size/weight structure of population of the edible molluscsMizuhopecten yessoensis (Jay) andSpisula sachalinensis (Schrenk) have been studied on the shores of Southern Primorje (Sea of Japan) during different seasons of the year.

2. By counting the size-age groups (which correspond to certain generations) constituting a given population, the age of individuals of different size and the maximum individual life-time were determined.

3. The characteristics of growth (weight increase) and calculation of fluctuations in abundance of generations of different age, have enabled the authors to estimate the nett population growth production (expressed as weight increase of all specimens of a population per year at the time of control). The average annual population growth production was determined by averaging the values obtained for each of the four seasons.

4. The living matter accumulated over a year and retained by the individuals of the population, as well as the matter contained in shells, was calculated by adding up the weight of the young present at the time of control (including specimens less than 1 year old) and the annual weight increase in the older specimens available. The value obtained is designated as supporting production; it constitutes the minimum rate of production of living matter required for maintaining the size of a population at a stationary level. The average annual supporting production is the sum of the productive process for a year. It was determined by dividing the values obtained for each of the seasons by 4 (i. e. by the number of seasons).

5. The average annual population growth production ofMizuhopecten yessoensis in the areas studied, is 131 g/m², and the supporting production 32 g/m²; theP/B indices are 1 and 0.25, respectively. The greatest biomasses and maximum production rates occur in summer; they decrease considerably in winter and begin to increase again in the spring.

6. The average annual population growth production ofSpisula sachalinensis is 451 g/m², and the supporting production 329 g/m²; theP/B indices are 0.55 and 0.39, respectively. Maximum biomass and production rates are observed at the beginning of winter because of winter aggregation of specimens in the study area; the rate of turnover of organic matter is small during the cold period. TheP/B indices reach their highest value in autumn.

7. The hydrological conditions and the biotic background at the shores of the Southern Primorje are favourable for spawning and for the planktonic stages of both species.

8. InMizuhopecten yessoensis the larvae settle on submerged plants. The factors which badly limit the abundance of this species are the shortage of substratum for settling and the predation on young settled individuals especially by sea stars. Artificial increase in abundance of the species may be accomplished by: (a) offering sufficient settling area; this can be done by placing suitable collectors in the habitat; (b) protecting the settled young from predators.

9. Abundance ofSpisula sachalinensis is most appreciably limited by the effect of storm waves, which cast numerous young, weak individuals ashore into the intertidal zone, where they perish owing to unfavourable conditions. Collection of young individuals cast ashore and their transfer into protected farm areas favourable for growth is recommended for artificial increase in the abundance of this species.