ICES mar. Sei. Symp., 199: 40-46. 1995

Distribution and biology of commercial scallops (Chlamys) in the Northwest Pacific

V. G. Myasnikov and K. A. Zgurovsky

Myasnikov, V. G ., and Zgurovsky, K. A. 1995. Distribution and biology of commer­cial scallops (Chlamys) in the Northwest Pacific. - ICES mar. Sei. Symp., 199: 40-46.

D ata on the three main commercial species of scallops (genus Chlamys) in the Northwest Pacific were collected from 1980 to 1988. Chlamys behringianus forms local concentrations in the Olyutorsky Gulf and off Karaginsky Island (Bering Sea), while C. albidus forms the densest concentrations off the northern Kuril Islands. Some stocks of C. rosealbus were found off North Primorye. Commercial concentrations of these species occur at depths of 50 to 200 m. Densities can reach 200 individuals m 2. Underwater observations showed that scallop concentrations occur on level or slightly sloped surfaces. C. rosealbus can live as long as 28 to 30 years, with maximum increase in the linear increment occurring in 3- to 4-year-old scallops. Growth from this age is satisfactorily described by the von Bertalanffy equation. Time of spawning varies from March in the Sea of Japan through July in the Bering Sea. Individual fecundity varies from 2000 to 164000 oocytes. The greatest portion of the stomach contents is detritus (up to 70% ), but diatoms, peridineas, remnants o f crustacean larvae, algal spores, and sand were also found.

V. G. M yasnikov and K. A . Zgurovsky: Pacific Research Institute o f Fishery and Oceanography, 4, Shevchenko Alley, Vladivostok 690600, Russia [fax: (+7) 257 783],

Introduction

Scallops are distributed in practically all seas and oceans. In the seas of the Russian Far East, three genera of the Family Pectinidae occur in commercial quantities: Patinopecten (Mezuhopectert) yessoensis (Jay, 1856), Swiftopecten swifti (Bernard, 1858), and five species of Chlamys: C. albidus (Dali, 1906; cited in Scarlato, 1981), C. behringianus (Middendorf, 1849; cited in Scar­lato, 1981), C. farreri (Jones and Preston, 1904; cited in Scarlato and Kafanov, 1988), C. rosealbus (Scarlato, 1981), and C. asiatica (Scarlato, 1981) (Scarlato and Kafanov, 1988).

P. yessoensis is not abundant and only limited fishing is carried out. No concentrations of S. swifti have been found and C. asiatica is extremely rare in this area. In contrast, C. farreri forms dense concentrations in shal­low waters in some southern Primorye bays, but com­mercial exploitation is difficult because of the shallow depth at which it is distributed and its attached mode of life; it has more potential as an aquaculture species (Bregman, 1982).

The remainder of the Chlamys genera form dense concentrations in many areas of the Northwest Pacific. For example, off the northern Kuril Islands, C. albidus

has been commercially harvested since 1973 and C. rosealbus has been exploited off the Primorye coast (the Sea of Japan). This article describes the distribution and main biological features of the white scallop (C. albi­dus), the pink scallop (C. rosealbus), and the Bering scallop (C. behringianus), i.e. those of most interest in the Northwest Pacific.

Materials and methods

The material for this article was obtained in research cruises conducted between 1979 and 1989 in the Okhotsk, Japan, and Bering seas and off the Kuril Islands. Sampling was done with bottom trawls and drag nets. Analysis of the catch included determination of the size, weight, and sex, and identification of the species. Observations were made from a manned submersible (TINRO-2) on the distribution, density, behaviour, accompanying species, and ground and bottom relief. Data from trawl catches and midwater observations were compared and from these mollusc distribution charts were generated. When determining the relation­ship between raw mass and shell height, the equation of degree of dependence was employed: y = axb.

ICES mar. Sei. Symp., 199 (1995) Distribution and biology o f scallops in the Northwest Pacific 41

A Bering Sea im » 11111"

Sea of Okhotsk

O

- 200 miles zones of the USSR

I C. behringianus

III C. albidus

§ C. rosealbus

Figure 1. Areas where concentrations of scallops (Chlamys) are found in the Northwest Pacific.

The von Bertalanffy equation was used to describe growth. Age was determined from the “annullii” formed by the concentrations of scales at the shell sur­face. Age determination using the sculptural marks on the surface of the shell was done by calculating the number of dark bands on the ligament and by determin­ing the magnesium concentration in the shell samples; minimum levels of magnesium correspond to slow growth in the winter, while maximum levels reflect the rapid growth that occurs during the summer (Silina et al., 1976; Silina, 1981; Myasnikov and Kochnev, 1988; Silina and Pozdnyakova, 1990). Spawning time was esti­mated from changes in gonad index.

Results and discussion

The largest scallop stock discovered was C. albidus off the northern Kuril Islands. Concentrations were also found off the central and southern Kurils and in the northwestern Sea of Okhotsk. The second largest scallop stock is the C. rosealbus concentration off the Primorye coast. The most abundant stock of C. behr­ingianus was found in the area of Karaginsky Island and in the Olyutorsky Gulf of the Bering Sea (Fig. 1). Distri­butions of these Chlamys species overlap somewhat and there are transitional forms between the species, which

makes it difficult to identify some of them. The morpho­logical variability of the Pectinidae was noted earlier by Scarlato (1981) and Woodburn (1987).

In all areas studied, the character of the distributions was similar, but densities differed. The frequency of scallops in the trawl catches varied from 40 to 60% between years. The bottom relief in areas where mol­luscs occur is usually a plain with flat uprisings, and the distribution of individuals is either random or grouped. An even distribution is characteristic of younger individ­uals. Fields of adult scallops contain scallop aggre­gations varying from only 3-5 individuals up to several hundred or even several thousand.

On the Onekotan Island shelf (northern Kuril Islands), C. albidus aggregates either as “heaps” (with scallops forming several layers one upon the other), “rosettes” (with individuals gathered in a hollow, touch­ing each other with their shell locks), or as “brushes” (with the scallops positioned on the bottom with their apertures up and their locks close to each other) (Fig. 2). In this last case, densities can reach 200 individuals m -2. Maximum density for C. albidus was difficult to calcu­late, as the scallops in the “heaps” form layers one atop the other. For C. rosealbus, maximal density was esti­mated at 25 individuals m~2 and for C. behringianus at 10-15 individuals m~2. In areas with complex relief (base rock), only individual scallops were observed,

42 V. G. M yasnikov and K. A . Zgurovsky

U niform d i s t r i b u t i o n

ICES mar. Sei. Symp., 199 (1995)

"Heaps"

Figure 2. Types of distribution of the white scallop off Onekotan Island.

although large scallops are able to concentrate in non­stable groups, sometimes attached to rocks (usually in silted places). Juveniles are almost always attached to pieces of wood, bivalve or gastropod shells, rocks, or stones (Zgurovsky and Myasnikov, 1986).

In the northern Sea of Okhotsk, white scallop juven­iles occur profusely among hydroids and moss animals which are most abundant at a depth of 30-40 m. Dense scallop concentrations in the Bering Sea occur in areas dominated by pebble grounds with silt; off the Primorye coasts, scallops concentrate on silt and silt-and-sand grounds. In the Sea of Okhotsk, scallops prefer the silt- and-sand grounds with gravel and pebbles. In all these areas, undersea currents of 0 .1 -0 .3 m s-1 are always present; maximum current speed was observed off Onekotan Island, where it reached 0 .5m s-1.

Chlamys genera inhabit depths of 15 to 400 m, but C. rosealbus in the Bay of Peter the Great (Petra Velikogo Bay) (Sea of Japan) occurred at a depth of up to 2030 m (Scarlato, 1981). In the Sea of Japan, the main C. roseal­bus concentrations occur at depths of 50-150 m; in the Bering Sea the largest catches of C. behringianus are

taken from a depth of 75 to 175 m . Off the northern Kuril Islands, C. albidus is found at 50-200 m, and on the Pacific side they are found down to a depth of 250m.

Scallops do migrate and we have repeatedly observed high scallop activity from the submersible. Migrations to some extent influence their bathymetric distribution. Thus, if the distribution of young C. behringianus and C. albidus does not depend on depth, as the available data indicate, juveniles of C. rosealbus and C. albidus (from the northern Sea of Okhotsk) prefer depths of less than 50 m - although they occur at all depths. As they grow, they migrate to the shelf areas at depths of 65 to 125 m, where the main concentrations of C. rosealbus and C. albidus occur.

In areas of mass concentrations in the studied areas, fairly high oxygen levels are observed (up to 4 .7 - 7 .5 m gl-1) and the salinity varies from 33.05 to 34.10ppm. The temperature range at which these scal­lops occur is fairly broad (from —1.9 to 13.03°C), but maximum concentrations are observed at a temperature of 1-3°C.

It may be necessary for scallops to concentrate in

i c e s m ar. s d . Symp., 199 ( 1995 ) Distribution and biology o f scallops in the Northwest Pacific 4 3

Table 1. Parameters of the von Bertalanffy growth equation for scallops (Chlamys) in the Northwest Pacific.

Species H„„ ± m K ± m 0 1+ 3

C. rosealbus 112.5 ± 3.2638 0.0912 ± 0.0087 -0 .9326 ± 0.3746

C. behringianus 94.2 ± 1.1095 0.1275 ± 0.0064 -0 .1385 ±0.0215

C. albidusOnekotan IslandOkhotsk Sea coast 109.0 ± 3.2678 0.2220 ± 0.0206 0.6242 ± 0.0446

Pacific coast 84.9 ± 1.6288 0.1995 ±0.0186 0.1483 ± 0.2358

C. albidusNorthern Okhotsk Sea 79.3 ± 0.6952 0.2010 ± 0.0089 -0 .0433 ± 0.1178

order to spawn successfully. It has been suggested that decreases in catches of Placopecten magellanicus are related to a reduction in the number of juveniles caused by intensive fishing of the adult population (Gaudet and Belanger, 1987).

The life span of the species we have studied is 28 to 30 years (Myasnikov and Kochnev, 1988), considerably higher than reported for Chlamys in other areas. In Broom’s review (1976), the maximum age of Atlantic scallops was cited as 2 -6 years, and according to Denisenko (1979) the life span of C. islandicus off the eastern Murmansk coasts (Barents Sea) does not exceed 15 or 16 years. Other members of the Pectinidae family frequently reach 20-25 years or more (Hennick, 1970; Gruffydd, 1974; MacDonald, 1986).

Scallop growth, starting from the age of 2 or 3 years, is well described by the von Bertalanffy equation. In Table 1, the calculated coefficients of the von Bertalanffy equations are given. As noted earlier, growth rates in the white scallop differ not only between populations, but also among habitats and year classes (Myasnikov and Kochnev, 1988). Figure 3 shows scallop growth curves in some areas of the Northwest Pacific. Because of different rates of growth at different life stages, three stages of linear growth were distinguished: juvenile, mature, and senile, as has been used with other bivalves (Zolotaryov and Selin, 1983; Zolotaryov, 1989). The variation in annual linear increments is clearly seen in Figure 4. Scallop growth is greatest from approximately the third through the fifth year, after which there is a decrease in linear increment. After reaching about 10 years of age, the increment remains almost constant and does not exceed 0 .5-1.5m m yr_1. Thus, growth in shell height practically stops; there is continued thickening of the shell’s edge, making the shell more convex. These features are characteristic of the presence of the senile stage. This change in the shape of the shell of the white scallop takes place at 6-18 years of age (average of 10 years of age), at a shell size of 70-100mm (average about 90 mm). Both valves are affected, so the shells of those individuals are quite protuberant. This senile stage represents 23% of the entire catch of C. albidus off Onekotan Island (Myasnikov and Kochnev, 1988).

110

EE

70jzo>CDI

Q)

2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

Years

Figure 3. Growth curves of scallops in areas of the Northwest Pacific. 1 = C. albidus off the Okhotsk Sea coast of Onekotan Island; 2 = C. albidus off the oceanic coast of Onekotan Island; 3 = C. behringianus in Olyutorsky Bay in the Bering Sea; 4 = C. rosealbus in the Sea of Japan; 5 = C. albidus, the northern part of the Sea of Okhotsk.

The highest growth rate (linear increments up to 18 mm yr” 1) among the far eastern scallops is observed in C. albidus from the Sea of Okhotsk coast of Onekotan Island. The smallest increments in this species (up to 12 mm yr-1) are found in scallops off Simushir Island (the central Kurils) (Myasnikov and Kochnev, 1988). The relationship between body weight and shell height and the parameters of the growth equations for the Chlamys scallops in the Northwest Pacific are given in Figure 5 and Table 2. The dependence character for the majority of species is similar. Only C. behringianus is any different, having an unusually high regression coef­ficient (3.75).

In most areas, it is usual for large adults to dominate the catch (Fig. 6). This is partially related to the poor catchability of juveniles with the fishing gear that is used. However, it may also be indicative of weak recruitment. The large number of dead scallops found in areas with­out live individuals testifies to uneven recruitment.

44 V. G. M yasnikov and. K. A . Zgurovsky

(mm)

ICES mar. Sei. Symp., 199 (1995)

. 3

When examining the age structure of the catch, it was also found that mature individuals dominated. In C. rosealbus concentrations off the Primorye coast (Sea of Japan), 7- to 12-year-olds prevailed. Off Onekotan Island, where C. albidus is distributed, 2- to 10-year-olds are dominant, probably because of the removal of the adults in the commercial fishery. The proportion of the animals older than 15 years in all populations is insignifi­cant (Fig. 7).

The sex ratio in the populations of C. albidus varies considerably - from 0.6:1.0 to 2.0:1.0 (females to males). In C. behringianus and C. rosealbus, the ratio is close to 1.0:1.0. Sexual maturity in these scallops occurs by the age of 3-5 years at a shell height of 30 -70mm. Spawning in C. rosealbus takes place in March and April. C. albidus spawns in June off the northern Kurils and C. behringianus spawns in late June and July. The fecundity of this latter species (from 2000 to 164000 oocytes) is considerably lower than that of P. yessoensis, whose gonads contain an average of 100-170 million oocytes (Yamamoto, 1964). Average individual fe­cundity is greatest for C. rosealbus (47500 oocytes) and smallest for C. behringianus (32 300 oocytes).

An examination of stomach contents revealed mostly detritus (up to 70% of the contents); Diatomea and Peridinea also occur frequently, but remnants of crus­tacean larvae, seaweed spores, and sand were less common.

Underwater surveys showed that in areas of dense scallop habitation the specific composition of the macro-

250 -

200

-j? 150

£çna)

^ 100

50

10

Height (mm)Figure 5. Relationship between body weight and height of shell. 1 = C. albidus off the Okhotsk Sea coast of Onekotan Island; 2 = C. albidus off the oceanic coast of O nekotan Island; 3 = C. behringianus in Olyutorsky Bay in the Bering Sea; 4 = C. rosealbus in the Sea of Japan.

benthos is poor compared to other areas of the shelf. The scallops normally dominate, but Ophiactidae, Ophiolepididae, Gorgonocephalidae, and sea urchins of the genus Strongylocentrotus also occur. Off the Pri­morye coast, the pink scallop is common in the areas almost completely occupied by Brachiopoda, whose density is an order of magnitude higher. Molluscs occur less often with sea lilies (Heliometra sp.), sponges, star­fishes, holothurias, and hydroids. In the Bering Sea and off the Kuril Islands, in the zone of scallop concen­trations, large cup sponges frequently occur, as well as holothurians and starfishes of the Asteriidae, Astropec- tinidae, and Solasteridae families.

The fauna of the organisms encrusting the scallops is also diverse, with the sponges Mycale adhaereus and Myxilla parasitica being the most common; hydroids, barnacles, moss animals, seaweeds, Actinias, bivalves, and gastropods are rarer. The organisms living both on and near the scallops compete not only for space, but for

YearsFigure 4. Curves (1, 2) of individual growth and linear increments of C. albidus off Onekotan Island. Arrows show time and size when variation in the curve of the shells takes place.

i c e s mar. Sei. Symp., 199 (1995) Distribution and biology o f scallops in the Northwest Pacific 4 5

Table 2. Parameters of the dependence equation of body weight on shell height for scallops (Chlamys) in the Northwest Pacific.

Species lna ± m b ± m r n

C. rosealbus -9.6051 ± 0.0024 3.19 ± 0.0005 1.000 1107C. behringianus -11.6542 + 0.0680 3.75 ± 0.0162 0.999 197C. albidus

Onekotan IslandOkhotsk Sea coast -9 .2805 ± 0.2953 3.14 ± 0.0679 0.978 100Pacific coast -8 .6632 ±0.1645 2.97 ± 0.0395 0.992 100

C. albidusNorthern part of the Okhotsk Sea -10.8024 ± 0.0200 3.37 ± 0.0490 0.999 150

50

30

:r~‘10

11070 905010 30

Shell Height (mm)

Figure 6. Size structure of some scallop populations. 1 = C. albidus off the Okhotsk Sea coast of O nekotan Island; 2 = C. albidus off the oceanic coast of Onekotan Island; 3 = C. behringianus in Olyutorsky Bay in the Bering Sea; 4 = C. rosealbus in the Sea of Japan; 5 = C. albidus in the northern part of the Sea of Okhotsk.

26

a - 14

8

2

4 6 8 10 12 14 16 18 20 22 24 26 282

Years

Figure 7. Age structure of scallops in some areas of the North­west Pacific. 1 = C. rosealbus concentrations of the Primorye coast (Sea of Japan); 2 = C. albidus off the oceanic coast of Onekotan Island; 3 - C. behringianus from Olyutorsky Bay in the Bering Sea.

food as well. However, encrusting organisms can also increase local turbulence, which seems to increase the flow of food. Ophiopholis aculeata in some areas of the northern Kurils covers the scallops (C. albidus) and the ground creating a “net” with its rays. Among the most serious competitors of Chlamys opercularis for space are the Ophiurianidae (Askew et al., cited in Broom, 1976). Among the scallop predators in the far eastern seas, the most significant are the paralithodes

crabs, benthophagous fishes, starfishes, octopuses, and gastropods. In other areas of the world, the scallops’ enemies include crabs, starfishes, and flatfishes (Mar- chall, 1960, cited in Broom 1976; Naidu, 1987; Paul et al., 1987). In intertidal waters, many scallops are de­stroyed by seagulls (Gutsell, 1930, cited in Broom, 1976).

Commercial stocks of all types of Chlamys scallops have been found only near the northern and north­western coasts of the far eastern seas, where there are similar environmental conditions: cold currents, water dynamics, a cold intermediate layer at depths of 50- 200 m, similar grounds, etc.

The distribution of the molluscs is characterized by a high degree of aggregation or by more random types of distribution. Scallops are found in aggregations ranging from 3-5 to several thousands of individuals.

Growth of molluscs from the age of 2 -3 years is well described by the von Bertalanffy equation. As the rate of growth of different stages of ontogenesis changes, three stages of linear growth can be separated: juvenile (up to 3-5 years of age), mature (until 10 years of age), and senile (over 10 years of age).

Thus, fairly slow growth and low fecundity are charac­teristic of the Chlamys scallops of the Northwest Pacific. In spite of the high scallop concentrations in some parts of the far eastern seas, it is vital to approach the prob­lems of commercial harvesting with special care. In our opinion, for each population, the volume of the catch and the periods when the areas are fished should be determined. We recommend that fishing be barred from designated areas, so that the populations can be con­trolled.

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