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Common names: Yesso scallop, Ezo scallop, Giant scallop, Japanese scallop, Russian scallop, Primorsky scallop and Common scallop.
Yesso scallop M. yessoensis is a Pacific Asian low-boreal species (Fig. 9) of commercial value, the biggest of all Pectinidae. It occurs along the northern coastline of the Korean peninsula, the coastline of Primorye, near the shores of the Sakhalin islands, South Kuriles and Hokkaido and on the northern coastline of Honshu Island (Scarlato, 1981).
Total populations and biomass.
In 1932, about 40 million scallops inhabited the area of 16-17 thousand hectares along the coast of Primorye. In 1940's-1960's, the scallop population in most settlements decreased and in some places the mollusk completely disappeared. From 1932 to 1959, the stocks in Peter the Great Bay had reduced thrice. In the following decade, the abundance remained almost the same i.e. about 5'703 million specimens on 906 hectares with biomass of 1'708 tons (Biryulina and Rodionov, 1972).
A significant drop in abundance of the scallops was also observed in the same period in the north areas of Peter the Great Bay (Olga Bay), where the scallop population in 1932 was four times as much as in 1975 (Silina and Bregman, 1986). In addition, according to V. A. Skalkin (1971), the biomass in Aniva Bay (southern Sakhalin) in 1969 was twice as low as compared to 1961-1962. The scallop population became ten times as low in Terpenie Bay (northern Sakhalin) and some areas of the Kuriles. Most investigators believe that intensive industrial fishing mainly caused the overall drop in scallop stocks. At present, commercial reserves of natural Yesso scallop along the coast of Primorye are exhausted because of irrational catching during the latest ten years.
Distribution in Primorye
Distribution over depths
Most of the mollusks were found along the coastline at a depth of 20-25 m near rugged shores. This is apparently due to the corresponding range of spat subduction horizons in the region i.e. underwater rocks are substrate carriers for attaching scallop larvae. In Peter the Great Bay, the scallop was recorded at a maximum depth of 82 m (Scarlato, 1981).
Age structure of scallop settlements
The scallop shell grows isometrically to retain its initial form. L. G. Makarova (1985) calculated the general equation for the linear growth of Yesso scallop:
Ht = (160.92 ± 18.7) × (1-e(-0.378 ± 0.04) × t),
where Ht is the shell height, mm; and t is scallops age, years.
The scallop grows at a temperature ranging from -2°C to +26°C. The optimal growth temperature is 4-6 °C. In Primorye, the temperature optimum is in May-June and in September-October (Silina, Pozdnyakova, 1986). Within the initial three years, the scallop height reaches 90-110 mm, then its growth slows exponentially. The largest specimens occur in settlements on silty-sandy soil in sites with good water exchange conditions and relatively stable temperatures approximately at a depth of 20 m. There they become 190-195 mm height and even longer at the age of 16 and older. In the South Kurile shallows specimens older than 20 with a shell height of 220 mm are met (Skalkin, 1966). In shallow silty inlets, scallops seldom exceed 150 mm and live not more than 10-12 years. In Posjet Bay, we recorded the largest scallop specimen, whose dimensions were as follows: shell height 222 mm, height 202 mm, and width 37 mm. The scallop mass varies proportionally to its linear dimensions (Fig. 12; Silina, Pozdnyakova, 1986).
The share of muscles in the scallop total mass amounts to 10-18 % in various settlements of Peter the Great Bay (Belogrudov, 1981).
Sex structure of settlements
Development in plankton
On the eve of scallop cultivation, the number of sexually mature specimens in Minonosok Inlet in 1972 was 70'000. Ten years later, the number has increased up to 650'000 because of sowing culture (Fig. 13). The number of annually collected spat showed instability of larvae settling throughout all period. Nor was there any sign of increase in the number of spat during the year. Apparently, the parent-larvae relationship was indirect because of great dilution of larvae pool by water mass (Kalashnikov, 1986). Water exchange with adjacent bodies of water (about 10 % of waters every day is replaced by tidal) disturbed the parent-larvae relationships.
A comparison of the age composition (Fig. 13) and the results of spat collection in various bays and inlets showed that intensity of replenishment changes asynchronously. So, bonanza generations in one bay do not necessarily correspond to the same in another bay. For instance in 1980, the lowest spat settling (2-20 specimens per collector) was noted in Posjet Bay, but in Vostok Bay it amounted to 400-500 specimens per collector. It is of interest to note that at that year, red tides absent in Vostok Bay were observed in Posjet Bay. However these distinctions were also noted in other bays without red tides, and this showed that intensity of replenishment in some settlements was essentially a local characteristic.
Recent investigation (Silina, 1996) also shows the highest mortality when scallop were less than 2 years while the mortality from 2 to 5 years of age is minimal. And quite the contrary, at 6-7 years of age (probably the beginning of the senile period of scallop development) and upwards to 9-10 (transition to the old-aged stage), scallops mortality increased sharply.
Storms. Storms are another factor, which causes mass deaths of scallops in coastal shallow settlements. Depending on the floor relief, they can increase settlement dispersity over vast flat areas or make them denser at the foothills of cliffs. In either case, a considerable number of mollusks are buried under moving soil. Near shallow coasts, particularly beaches, the majority of scallops die in storm debris. Trivial storms deform settlements; however, the loss is compensated by annual replenishment. While the impact of typhoons (annual frequency about 50 %) are natural calamities for the floor coastline population, including the scallop, for they destroy most of the species around open waters at about 20 m deep. By counting the number of shells in the breaker zone of only one beach following the typhoon "Ellis" in 1983, researchers revealed the simultaneous death of 10,000 specimens of different age. In a similar count in 1986 after the typhoon "Vera", we discovered as many as 72,000 specimens in debris (Kalashnikov, 1984). The joint effect of various factors on the sea floor population showed in perennial changes in density of artificial scallop populations, which regularly declined in all cases during the first cultivation season. The decline was greater in more open and unprotected waters (Fig. 14; Kalashnikov, 1985).
Predators. The first hours and days of life on the sea floor from the moment the mollusks become one year old (shell height about 30 mm) appeared to be the most dangerous ones. During this period, natural death is maximal and young scallops are preyed upon by various starfish species such as Asterias amurensis Lütken, which can grow up to 165 mm in a radius and weigh 450 g. Another species, Distolasterias nipon (Döderlein) is even larger growing up to 250 mm with a weight of 1000 g. These species attack the scallops, age of which is less or the same as that of starfish. One-time ration of one starfish changed from 1 up to 8.5 g of fish and annually amounts 400-450 g (Biryulina, 1972). In view of great abundance of these predators (density can amount up to 15 specimens·m-2), the damage of population can be considerable. The death rate of young scallop in super dense aggregations (over 100 specimens·m-2), in which predators temporarily eat only scallop, is especially high.
When storms are so strong that they reach the sea floor, scallops become weaker and readily accessible to predators. Joint effects of storms and starfishes have destroyed an artificial scallop settlement (about two hectares) with a population of over 200'000 specimens for several weeks. In stable conditions, starfish and scallops were noted to co-exist peacefully when they occupied a single habitat for several years in succession. Generally, however, the number of cultivated mollusks declines more in sites where starfish are more abundant.
After the sowing spat are preyed upon by various benthophages fishes such as flounders and bullheads. It is quite possible that at this period scallops are preyed by fishes rather than by starfishes. There are other predators of sown seed and adult scallops, which are of lesser importance but nevertheless, pose a threat to small seed and juveniles. These include the octopuses, some crabs, first of all King crabs Paralithodes camtschatica (Tilesius), and hermit crabs (Kalashnikov, 1986). Crabs predate mainly on seed scallops especially if present in large numbers, for example during seasonal migration, they can greatly denude newly seeded grounds. Some predatory gastropod mollusks pose a threat to both juveniles and adult scallops. E. A. Belogrudov (1973) reported that drilling Muricidae gastropod Boreotrophon candelabrum (Adams et Reeve) and Tritonia japonica (Dunker) could attack and eat the scallops. At the natural populations, about 14-27 % of adult scallops had drilling marks on the shells. D. D. Gabaev and N. K. Kolotukhina (1999) reported that two-year scallops (shell height up to 73 mm) in the cages are preyed upon gastropod Nucella heyseana (Dunker).
Parasites. In comparison to other cultured bivalves, such as ousters and mussels, little is known about the parasites and diseases of scallops. Also epizootic diseases, like those that have devastated the ouster culture industry in parts of the world, have not been encountered by the scallop culture industry. The relative lack of information on parasites and diseases in scallop may be attributed to the shorter of intensive mussel culture, and comparatively fewer investigations. Moreover, insufficient data on scallop parasites indicate the poorness of parasitic fauna and the low vulnerability of the scallop. Infectiousness of scallop by parasites is quite low, as parasitic fauna is scanty and presented only by potentially pathogenic species. Nor was mass scallop death caused by parasites ever recorded. Now it is known about 17 parasites and commensales in scallops (Kurochkin et al., 1986; Kovalenko, 1990; Plyusnin, 1990; Rakov, 1990; Didenko, 1996).
Four of 17 pathogens found in scallop are shell drillers. Other twelve species may be true parasites. Only three species of them such as Sirolpidium zoophthorum, Myxosporidia gen. sp. and Perkinsus sp. can pose the serious threat for scallops, however they were found only once. In addition, others caused no noticeable pathology in normal conditions. Moreover, the scallop has no parasites that could endanger humans.
Bacterial contamination. According to recent investigations (Avdeeva and Filipchuk, 1988; Kovalenko, 1989; Plyusnin, 1990; Plyusnin and Cherkashin, 1991; Kovalenko, 1994), except of these 17 pathogens, numerous species of bacterial contaminants have been identified from cultivated scallops (Table 5). Totally 29 species of contaminants were identified on scallop farms. Gram-negative bacteria present the most part of them (22 species). Some of them, as specimens of Aeromonas, Vibrio and Pseudomonas, can be a conditionally pathogenous. These bacteria may be pathogenic in situations where environmental conditions are poor.
Epibioses. Natural and farmed scallops are an excellent substrate for the settlement of many other organisms, which are collectively termed fouling. Sea organisms, which occur on scallops' shells, can be competitors for space and food. Furthermore, epizoans can be an additional barrier that reduced flow and food accessibility.