Stocking, Enhancement, and Mariculture of Penaeus orientalis and Other Species in Shanghai and Zhejiang Provinces, China.
XU, J. ; XIA, M. ; NING, X. 等
Introduction
Fish as a Source of Food
Marine and freshwater landings from capture and culture fisheries
in the People's Republic of China (PRC) have steadily increased
since 1982 (Fig. 1A). In 1991 the combined landings of [is greater than]
21,000,000 t were dominated by capture landings (61.9%). Per-capita
consumption of fish of all kinds in the PRC was 18.1 kg/yr in 1991, with
6.9 kg/yr from culture and 11.2 kg/yr from capture fisheries. Landings
from mariculture in 1991 provided the smallest contribution: only 3.2
kg/yr (17.9% of total landings).
[Figure 1 ILLUSTRATION OMITTED]
Culture landings are dominated by freshwater fish (48.7%; Fig. 1A),
mostly from small farms and aquaculture facilities. Mollusks and
crustaceans (almost entirely marine) provide important contributions to
culture landings (2.2% and 15.3%, respectively, Fig. 1B), while marine
fish form a very small portion of the total (only 0.6%). Algae ([is
greater than] 90% Laminaria japonica) provide 34.1% of culture landings,
but are harvested directly, generally without any significant associated
culture or farming activities. Table 1 provides a more detailed
breakdown of culture landings by species.
Table 1. -- Aquaculture landings (1,000 t) by species in the PRC
and in percent total culture landings, 1993 (FAO, 1994).
Landings Percent
Species (1,000 t) of total
Freshwater fish
Carassius carassius 254.3 2.4
Cirrhinus mulitorella 810.0 0.8
Ctenopharyngodon idella 12,318.0 11.8
Cyprinus carpio 7,061.0 6.8
Hypophthalmichthys molitrix 1,540.7 14.8
Hypophthalmichthys nobilis 7,704.0 74.0
Mylopharyngodon piceus 51.8 0.5
Parabramis pekinensis 1,815.0 1.7
Oreochromis (Tilapia) niloticus 157.2 1.5
Subtotal 4,974.9 478.0
Marine Fish, Subtotal 58.7 0.6
Marine Crustaceans
Eriocheir sinensis 9.5 0.1
Penaeus chinensis 206.9 2.0
Other marine crustaceans 8.6 1.0
Subtotal 225.0 2.2
Marine mollusks
Crassostrea gigas 123.0 1.2
Mytilidae 538.9 5.2
Pecten yessoensis 338.0 3.2
Anadara granosa 41.6 0.4
Solen spp 198.6 1.9
Venerupis japonica 270.5 2.6
Other mollusks 86.9 0.8
Subtotal 1,597.4 15.3
Algae
Laminaria japonica 2,964.8 28.5
Porphyra tenera 159.0 15.0
Other algae 429.4 41.0
Subtotal 3,553.2 34.1
Grand Total 104,091.0 100.0
Attempts at fisheries stocking and enhancement have been made in
the PRC but have contributed only a negligible proportion of the
landings. This paper describes results of attempts to increase landings
by stocking shrimp, Penaeus orientalis (also referred to as P.
chinensis), and enhancing jellyfish, Rhopilema esculenta, landings in
Zhejiang Province, which landed [is greater than] 1,300,000 t from
capture fisheries in 1991, compared to [is less than] 40,000 t from
mariculture landings.(1) Information is also provided about some of the
mariculture activities in Zhejiang Province.
Study Area
The study area, located in the Zhoushan Islands, is situated in
Hangzhou Bay, on the East China Sea (Fig. 2). Coastal waters are
generally turbid and of low salinity ([is less than] 25%??) in Hangzhou
Bay and around the Zhoushan Islands, and show a temperature range of
4-11 [degrees] C in February and 26-28 [degrees] C in August (Ning,
1994). Oceanographic structures in Hangzhou Bay and the East China Sea
are very complex, with water from the Changjiang (Yangtze) River, which
has an inflow of [is greater than] 28,000 [m.sup.3]/sec, providing an
important and perhaps dominant influence in coastal waters. Smaller
inflows come from the Qian Tang (1,100 [m.sup.3]/sec), the Huangpoo ([is
greater than] 300 [m.sup.3]/sec), and other rivers entering Hangzhou Bay
from the west such as the Casejiang and the Yongjiang (230
[m.sup.3]/sec). Depending on the time of year, coastal waters in the
study area are also influenced by the mixing of different water masses
coming from the plume of the Changjiang and the Taiwan Warm Current, an
inshore branch of the Kuroshio Current (Second Institute of
Oceanography, unpubl. rep.). In summer, incursions of warm, clear, and
salty Taiwan Warm Current water may move westwards from the East China
Sea, bringing clearer, more transparent waters across the Zhoushan
Archipelago and into Hangzhou Bay. These incursions may cause red tides
in some years, impacting aquaculture activities. Red tides are probably
related to nutrient flows into the study area from the Changjiang River
basin, and to agricultural and industrial pollutants entering the study
area from the west. A full description of the study area and its
characteristics is given in ADB(2) and in WB.(3)
[Figure 2 ILLUSTRATION OMITTED]
Zhejiang Marine Fisheries Research Institue (ZMFRI)
ZMFRI was established and started its research program in the early
1950's. Since then it has carried out fisheries research on a
variety of stocks including the large yellow croaker, Pseudosciaena
crocea: the small yellow croaker, P. polyactis; and the edible
jellyfish, Rhopilema esculenta, in Zhejiang Province and especially in
the Zhoushan Archipelago. Mariculture research started in the early
1960's, and in 1972 an important mariculture program was started in
ZMFRI's laboratories in Shenjiamen (Fig. 2) on Zhoushan Island.
Work continued there until 1984. By 1979 the effects of increasing
industrialization of Hangzhou Bay on the general conditions in and
around the Zhoushan Archipelago, together with the establishment of
important shipyards, a large fishery, freezer plants, and related
industrial developments in Shenjiamen, led to reduced water quality at
the ZMFRI's laboratories. This impacted mariculture development.
Therefore in 1984 the Xixuan Aquatic Experimental Station was
established on Xixuan Island, about 10 n.mi. and 30-45 minutes by sea
from Shenjiamen (Fig. 2). Xixuan Island has an area of 0.45 [km.sup.2],
of which 015 [km.sup.2] is devoted to culture ponds, principally to
Penaeus orientalis spawning and grow-out. As the techniques for
large-scale mariculture and restocking of P. orientalis were
successfully mastered, additional tanks have been built to allow
mariculture research, grow-out, and restocking of the following species:
jellyfish, Rhopilema esculenta; abalone, Haliotis diversicolor and H.
discus; Gulf scallop, Agropecten irradians; black porgy, Mylio
macrocepahalus czerkskii; and grouper, Epinephelus awaara.
An important part of the Xixuan Island culture facility is the
capability to produce live food for cultures, based on large-scale
phytoplankton and zooplankton production of Phaeodactylum tricornutum,
Chaetoceros calcitrans, Skeletonema costatum, and Brachionus plicatilis.
Up to about 1,000 t/yr of phytoplankton is produced at various
densities, usually at about 4 ml/l. These organisms are used to feed the
newly hatched and young stages of the cultured organisms.
Stocking and Enhancement
The crustacean fishery in the East China Sea landed about 131,000 t
in 1956 (the first year for which landings data are available),
increasing to about 884,000 t in 1994. The species composition of these
landings is not monitored but is known to be dominated by crabs and
palaemonid shrimp, with [is less than] 1% being composed of penaeids.
The only penaeid fished in commercially important amounts is P
orientalis which is taken regularly from a very small stock located near
the mouth of the Changjiang. This stock has provided a fishery with
significant but small ([is less than] 1,000 t/yr) landings over the last
20-30 years ([WB.sup.3], [Chen.sup.4]). This stock has been used by
Chinese scientists in Zhoushan and Shanghai as a source for broodstock
when needed.
Stocking in Xiang Shan Bay
P. orientalis was stocked by releasing large numbers of 3.0-4.0 cm
TL (total length) shrimp cultured at the Shenjiamen Laboratory (until
about 1983) and at the Xixuan Mariculture Station from 1985 onwards, in
Xiang Shan Bay, an area where P. orientalis never occurred prior to
stocking. Catches of P. orientalis resulting from stocking were
estimated by two different methods: direct interviews with fishermen
provided estimates of the numbers of P. orientalis recaptured; and
landings data compiled by local Fisheries Management Bureaus, through
weighing of catches at the landing places and estimating the catch in
metric tons fresh weight (Table 2). Low releases and catches occurred
until 1986 when the new facilities at the Xixuan station came fully on
line. Numbers of P. orientalis released exceeded 100,000,000/ yr for the
first time in 1986, and declined from about 1991 when numbers stocked
were decreased. By 1993 when stocking ceased, landings of P. orientalis
were again very low, with negligible landings in 1994.
Table 2.--Numbers of P. orientalis released and landings in Xiang,
Shan Bay.
Released Catch Released Catch
Year (x10,000) (t) Year (x10,000) (t)
1982 239.2 5.0 1988 15644.3 329.0
1983 164.5 7.0 1989 16,218.0 228.0
1984 793.9 15.8 1990 14,874.2 265.0
1985 0.0 4.4 1991 9,158.0 158.0
1986 13,873.2 135.0 1992 10,950.0 163.0
1987 22,483.0 398.0 1993 0.0 27.0
Releases of small shrimp occurred in May-June, and recaptures of
juveniles. in the commercial fishery occurred from July to December when
shrimp reached 10-12 cm TL (Table 3). Larger, mature shrimp were taken
in small numbers, usually in the mouth of Xiang Shan Bay in March and
April of the following year. Mature shrimp left the Xiang Shan mouth
fishing grounds or died out of the population by May. No mature prawn were ever taken in the fishery from May onward when recruitment would be
expected in a natural population. Conditions in the sea were suitable
for grow-out but, in spite of the occurrence of some mature individuals
in the population, the restocked P. orientalis population was not
reproductively successful, nor were large numbers of ripe individuals
taken in or near Xiang Shan Bay at any time. Ripe individuals provided
only 0.05-0.15% of all P. orientalis taken in the commercial catches.
Table 3.--Relation between numbers of juvenile P. orientalis
released and numbers recaptured by the fishery.
Releases (x10,000)
Year April/May July August
1986 13,873.2 709.0 125.0
1987 22,483.0 1,083.0 167.0
1988 15,644.3 1,707.0 208.4
1989 16,218.0 1,174.0 167.0
1990 14,874.2 1,005.0 132.0
Catch of juveniles (x10,000)
Year Sept.-Oct. Nov.-Dec. Mar.-Apr.
1986 67.0 35.0 7.0
1987 156.7 243.0 24.0
1988 67.2 75.0 12.0
1989 112.0 75.0 15.0
1990 120.0 63.0 12.0
Catch of matures (x10,000)
Total % of the Matures Matures
No. releases as % of as % of
Year landed landed recaptures releases
1986 943.0 6.8 0.00 0.000
1987 1,673.4 7.4 1.43 0.1067
1988 2,069.6 13.3 0.58 0.0767
1989 1,543.0 10.5 0.97 0.0925
1990 1,332.0 9.1 0.90 0.0807
The lack of any P. orientalis in Xiang Shan Bay prior to the
initiation of restocking in 1982, the decline to nearly zero landings in
1994 after restocking was suspended in 1990, together with results of
the tagging experiments, show conclusively that the landings were
obtained solely from the stocking operation. Stocking studies reported
outside of China are mostly hampered by the need to estimate the amount
of shrimp landed that originated from the natural and the restocked
shrimp. Such separation of shrimp taken in commercial catches is
technically demanding and often leads to difficulties not encountered in
the Xiang Shan Bay project as noted by Kurata (1981) working on Penaeus
japonicus in the Inland Seto Sea of Japan.
The success of P. orientalis restocking in Xiang Shan Bay over more
than a decade is shown in Table 2: when numbers of released shrimp
exceeded 100,000,000/yr, landings were over about 100 t/yr. Figure 3
shows the least squares regression of total P. orientalis catch in Xiang
Shan Bay on the numbers of restocked juveniles of the same cohort: the
regression is highly significant (r = 0.9637 at 9 df; t = 11.4159, P [is
less than] 0.00001 for the slope; t = 0.1791, P = 0.8614 for the
intercept, which was not significantly different from zero; F ratio for
the model was 130.3/6.71, P [is less than] 0.00001; [R.sup.2] = 92.87%).
The residuals showed no pattern; the largest deviations from the
predicted values occurred in 1986 and in 1988. In 1986, unusually low
landings occurred (135 t observed and about 245 t predicted from the
model), and were probably caused by technical difficulties associated
with training staff during the first massive releases of small juvenile
P. orientalis. In 1988, landings were about 65 t higher than predicted
by the model: survival was the highest on record in that year (Table 3),
probably because of unusually favorable environmental conditions and/or
an earlier than usual opening of the Xiang Shan Bay fishery in that
year.
[Figure 3 ILLUSTRATION OMITTED]
The relation between catch and numbers of 3.0-4.0 cm TL juveniles
released (Fig. 3) was:
Catch = 3.0696 + 0.000001679 x N (1)
where Catch = total catch in t/yr of P orientalis from July to the
following April, and N = the number of 3.0-4.0 cm TL juveniles released
in May-June. The linear relation between Catch and N shows that catches
do not decline with increasing numbers of shrimp into Xiang Shan Bay,
i.e. diminishing returns did not occur over the observed range of
numbers released. Equation (1) also estimates that around 596,000
juveniles must be released to provide about 1.0 t of P. orientalis. This
compares with about 200,000 fry needed to produce 1.0 t of P. japonicus
in the Inland Seto Sea restocking of P. japonicus (estimated from data
in Kurata (1981)). The lower number of fry required to produce 1.0 t of
P. japonicus is probably due to use of large, specially designed nursery
grounds that were used to allow the shrimp to become accustomed to life
in the sea and to protect them from high initial predation in Japan.
This step was not taken in China.
The cost:benefit (C:B) ratio increases in a roughly linear fashion
with the numbers of juvenile P. orientalis released (Fig. 4); although
the data are sparse, they suggest clearly that large catches and high
profits could be obtained from a larger operation. At some level [is
greater than] 225,000,000 juvenile P. orientalis, diseconomies from
scale will occur (i.e. the C:B ratio will cease to increase, and may
even decrease, with number of juveniles released; Fig. 4). At some
unknown level of releases, the catch will cease to rise proportionately
to the numbers of shrimp released (Fig. 3). However, available data do
not allow any estimate of the levels at which these limitations on
stocking will occur. Perhaps stocking would become uneconomic in the
range from 2-3 times to 10 times the present levels, giving a possible
range for stocked landings in the region of 800-4,000 t/yr. A strategy
of slowly increasing stocking numbers would allow identification of a
level of releases that can produce high landings combined with a
profitable C:B ratio.
[Figure 4 ILLUSTRATION OMITTED]
Enhancement of Jellyfish Landings in Zhoushan
Jellyfish were abundant in the 1960's and 1970's but
landings in Zhoushan fell drastically from a mean of 8,800 t/yr in the
1960's to a mean of 800 t/yr in the 1980's and to [is less
than] 200 t/yr in 1991-92. This decline was thought to be caused by a
combination of overfishing and pollution.
Because of the long-term trend toward lower landings, ZMFRI
established a jellyfish restocking project in 1983. Several years of
fundamental research provided the basic biological knowledge needed.
Growth from the fertilized egg to about 1.0 cm disc size takes about 1.0
month (Fig. 5), with a survival of about 90%, and 1.0 cm jellyfish are
restocked in February or March. By May they reach 10-20 cm and by August
about 50 cm. Fishermen reported significantly greater landings in
1990-92 following experimental releases of up to 100,000,000 1.0 cm
jellyfish at the beginning of each year. Preliminary cost-benefit
analysis shows that C:B ratios for jellyfish restocking range from 1:2
to 1:4, depending on numbers released, mortality rates, and the size at
harvest.
[Figure 5 ILLUSTRATION OMITTED]
Mariculture for Seed and Grow-out
Penaeus orientalis
Restocking was phased out at the Xixuan Island mariculture
facility, starting in 1990 and was suspended in 1993. Culture tanks were
then used to hold a limited number of shrimp and grow them from the 3-4
cm TL size at which they were previously released into Xiang Shan Bay in
May-June, to about 12 cm TL which is reached around October or November;
they are then harvested and sold commercially. In November of 1993 [is
greater than] 40 t of P. orientalis were harvested and sold for about
Yuan 1,600,000.(5) Profits ([is greater than] Yuan 1,000,000) were used
to finance mariculture and assessment research.
Haliotis diversicolor, H. discus
Abalone are very much appreciated in all parts of China and a
combination of expanding demand, increasing prices, and falling catches
from abalone capture fisheries has led to interest in abalone
mariculture. H. diversicolor and H. discus are being produced at pilot
culture levels at the Xixuan Island mariculture facility. Spawning
occurs in May, and growth to about 2.0 cm TL takes about 1.0 yr (Fig.
6). In 1993, about 10,000 abalone of 2.0 cm TL, surplus to research
requirements, were sold to fishermen/small-holders at 1.0
Yuan/individual, and smaller numbers have been sold over the last few
years. The small abalone are held in grow out tanks for up to two years
by small-holders and will reach the commercial size of about 5.0 cm TL
in about 2 years (i.e. at a total age of about 3.0 yr) when they are
worth about Yuan 10-15 each; survival rates are usually about 80%. An
average profit of about Yuan 70,000, on a sale of 10,000 individuals,
may be made in 2 years: (10,000 x 10 x 0.8) - 10,000 = Yuan 70,000. This
provides a satisfactory margin for small-scale grow out by farmers. The
commercial-scale viability of the Haliotis spp. cultures will be
determined when large-scale sales of seed are carried out during the
next 3-5 years.
[Figure 6 ILLUSTRATION OMITTED]
Gulf Scallop, A. irradians
A. irradians reaches 6-7 cm TL in nature but landings from wild
stocks are now very low. The preferred commercial size is about 5.0 cm
TL. A. irradians spawns in March-April at temperatures of about 20-25
[degrees] C and reaches commercial size by about November, when they are
worth about Yuan 6.0-10.0/kg. Spat settle a few hours after spawning and
reach about 0.1 cm TL in less than a month, i.e. around April-May, when
they are sold to fishermen for Yuan 0.02/ individual. Around 20,000,000
small A. irradians were sold in April-May 1993 for a total of Yuan
400,000. Survival during grow-out to market size by small-holders varies
from 10 to 20%. The small-holders who grow the A. irradians are mostly
low-income farmers and fishermen who operate very small, part-time
enterprises, so that a substantial (cash) addition to family income for
many families is made by a small grow-out operation.
Experimental Cultures
In addition to the larger scale activities described above,
experimental cultures have been started on two species of fish: M.
macrocepahalus and Epinephelus awaara. The former porgy grows to a
maximum of around 3-4 kg total weight (TW) and the latter to around 10
kg TW, although cultured fish would be harvested at a smaller size. Work
is currently aimed at achieving:
1) routine spawning on as large a scale as desired in a controlled
environment,
2) grow-out to desired sizes in controlled conditions, and
3) determining in detail the technology required for successful,
large scale restocking and growth to market size.
Technology obtained through systematic research in this way will be
used by the end of the next decade or so as the basis for commercial
production of these species.
Discussion
Constraints on P. orientalis Stocking
The mean C:B ratio from P. orientalis restocking in Xiang Shan Bay
was 1:5.2, i.e. Yuan 5.2 earned for each Yuan 1.0 spent (Fig. 3). This
compares favorably with the C:B ratio for P japonicus restocking in the
Inland Sero Sea of Japan, which was only 1.0:1.79 (i.e. Yen 1.0 spent on
restocking per Yen 1.79 earned from the commercial fishery (Kurata,
1981)). The cost of restocking in China was probably reduced by cheap
labor, and perhaps also because there was no need to construct the
special nursery grounds used in Japan for releasing P. japonicus into
the sea. Chinese and Japanese data cited here include all costs of
production and so are comparable.
The new economic policy in the PRC, based on the market economy,
led to withdrawal of financial support from the Xiang Shan Bay stocking
project in 1993. This occurred in spite of the technical and economic
successes, largely because there was no practical way for government
funding agencies to recover the costs of stocking from the income
generated by the fishermen who harvested the shrimp. Many of the
fishermen operating in Xiang Shan Bay are, however, organized into
cooperatives, each of which has access to certain fishing areas. It is
conceivable that a new stocking project might eventually be financed
through cooperatives which, in collaboration with the Provincial
Government and by means of co-management structures, could control
access to trawling and gill netting in Xiang Shan Bay. Funding for P.
orientalis stocking could eventually be provided by the people
benefiting from the landings. It is likely that financing the
re-stocking of other species, such as jellyfish, could be tackled in
this way. This approach cannot be undertaken successfully without a
complete biological and socioeconomic survey; nevertheless, it is
already clear that the constraints on successful P. orientalis stocking
are likely to be mainly financial, rather than technical or economic.
Establishment of a P. orientalis Breeding Population
A second reason for withdrawing financial support from the P.
orientalis Xiang Shan Bay stocking project was its failure to establish
a self-perpetuating, reproductive stock of shrimp. The highest captures
of mature P. orientalis from Xiang Shan Bay occurred from the May-June
1987 restocking: 240,000 were taken in March-April of 1988, giving a
survival rate of 0.1% (Table 3). These 240,000 individuals were spread
over an area of around 150 [km,.sup.2] near the mouth of Xiang Shan Bay;
i.e. at an average density of about 1,600 shrimp/ [km.sup.2] (625
[m.sup.2]/shrimp) or (assuming a mean weight of about 20 g/shrimp) about
32 kg/[km.sup.2]. These levels are low compared with the levels at which
mature shrimp occur in well known shrimp fisheries, e.g. about 200-500
kg/[km.sup.2] for P. semisulcatus in Kuwait in the 1960's and P.
stylirostris and P. vannamei in Mexico in the 1970's (estimated
from data in Mathews, 1981), around 100 kg/ [km.sup.2] in Saudi Arabia in the 1980's and 100-200 kg/[km.sup.2] in Kuwait in the
1990's (from Mathews et al., 1993).
Densities of mature shrimp are much greater in most of these stocks
(by up to 10 times) and the proportion surviving to spawn was probably
also much higher. Therefore it is possible that the density of adult P.
orientalis, in Xiang Shan Bay never reached a sufficiently high level
for mating and spawning on a large scale to be feasible. A new stocking
project targeted to increase landings by up to 10 times in Xiang Shan
Bay would probably increase the proportion of mature shrimp by 5-10
times, i.e. to about 250-5.00 kg/[km.sup.2]. If stocking was accompanied
by protection of those fishing grounds characterized by especially high
numbers of adults from excessive effort, it is possible that the density
might become high enough for massive spawning to occur naturally. A
self-maintaining population might eventually be established.
The cost of fisheries enhancement is significant, and the analysis
suggests that it would be prudent to carry out ecological research prior
to any continuation of P. orientalis enhancement in Xiang Shan Bay. This
could include 1) comparative studies of environmental conditions in
Xiang Shan Bay and in areas where P. orientalis is well established and
2) research on the relation between recruitment of young shrimp and the
density of spawners that produce them in areas where endemic populations
of P. orientalis occur.
Table 4.--Cost:Benefit (C:B) ratios,
Releases Landings C:B
Year (x10,000) (t) (Yn/Yn)
1986 13,872.2 135.0 1:3.0
1987 22,483.0 398.0 1:7.8
1988 15,644.3 329.0 1:5.7
1989 16,218.0 280.0 1:5.7
1989 14,874.2 265.0 1:5.9
Mean 1:5.2
Acknowledgments
We are grateful to Wu Jiazhui, Vice Director of the Zhejiang
Aquatic Bureau, and Zen Ping of the Zhejiang Environmental Protection
Bureau, both of whom supported our work. We are, however, insensible for
any errors.
(1) Unpublished data, East China Sea Fisheries Research Institute,
Shanghai; Zhejiang Marine Fisheries Bureau, Hangzhou; Asian Development
Bank, 1993 [unpubl. rep. on the Zhoushan Fisheries, 340 p.]
(2) Asian Development Bank. 1993. Final Report of the Zhoushan
Island Fisheries Project (unpubl.)
(3) World Bank. 1995. Final report of the Hangzhou Bay Project
(unpubl.). This presents a recent multidisciplinary, synthetic review
about the water quality and oceanography of Hangzhou Bay and the East
China Sea fisheries.
(4) W. Z. Chen, East China Sea Fisheries Research Institute, 300
Jun Gong Lu, Shanghai, 200090, PRC.
(5) 40 Yuan/kg, i.e. ca US$4.65/kg; US$1.00 = Chinese Yuan 8.62 in
1995.
J. Xu and M. Xia are with the Zhejiang Marine Fisheries Research
Institute, Shenjiamen, Zhoushan, People's Republic of China; X.
Ning is with the Second Institute of Oceanography, P.O. Box 1207,
Hangzhou, Zhejiang 310012, PRC, and C. P. Mathews (corresponding author)
is with the Zoology Department, University of Reading, Whiteknights,
Reading, England. His present address is Northfield House, Cheselbourne,
Dorset, DT2 7NT, England. Internet: 106337.760@compuserve.com. Views or
opinions expressed or implied are those of the authors and do not
necessarily reflect the position of the National Marine Fisheries
Service, NOAA.
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