Seasonal and year-to-year variability of baroclinic volume
transport of the　Tsushima Warm Current, off the west
of Hokkaido

Hokkaido Central Fisheries Experimental Station has been conducting
routine hydrographic　observation bimonthly to clarify seasonal and
year-to-year variability of baroclinic volume transport of the Tsushima
Warm Current off the west of Hokkaido （TWCH）. Using the data from October
1988 to August 2001, seasonal mean northward volume transport is
shown to have the minimum　value （0．85Sv） in April and the maximum
value （1．59Sv） in August. The transport in October
decreases with sea temperature decrease below 100m depth near the coast.
The transport increases　once again in December. The annual mean transport
is 1．20Sv. Year-to-year variability of the　transport of TWCH seems to
have 5－6 years period. Long-term trend of the transport is shown to
decrease since 1988. Average bifurcation ratio of the transport
between TWCH and the Tsugaru　Warm Current at the west of the Tsugaru
Strait is estimated to be 4：5.

Age-dependent bathymetric pattern of walleye pollock,
 Theragra chalcogramma,off the Pacific coast of eastern Hokkaido

The distribution of walleye pollock, Theragra chalcogramma, off the Pacific coast
of eastern Hokkaido was examined using data from echo-integration and trawl
surveys conducted in May and November 1997. Acoustic data were collected
using a quantitative Furuno FQ-70（50kHz）echo-sounding system during the day.
Walleye pollock were distributed in waters shallower than 300 m with
temperatures ranging from 0 to 9℃.
Pollock aggregations consisting of 1 or 2 age groups of fish were located
in continuous bands along isobaths.
A trend to increasing age and size of pollock further offshore was obvious.
Briefly, age 0 to 2 fish were distributed mainly in the continental shelf
region（＜150 m）, age 2 fish in November and age 3 fish were located from the
continental shelf to the slope region, and age 4 and older fish were
distributed mainly in the slope region（＞150 m）.
The water temperature in the shelf region changed from 0-1℃ in May
to 4-9℃ in November.
By contrast, the water temperature in the slope region was relatively
constant at 1-4℃.
I discuss this age dependence of the pollock bathymetric pattern in
relation to age-specific differences in temperature preference and
ontogenic and seasonal variation in their diet.

Growth and age of Pointhead Flounder Hippoglossoides pinetorum
in the coastal waters off southwestern Hokkaido
This paper described that growth and age of Pointhead Flounder
Hippoglossoides pinetorum were analyzed with otoliths of them, which
collected in the coastal waters off southwestern Hokkaido
from 1975 to 1984 and from 1996 to 2000.
A transparent zone and an opaque zone were formed from June to
July and from September to April, respectively.
As flounder became older, the month in which the percentage of
otoliths with an opaque edge was highest, became earlier.
The month of the highest percentage of otoliths with an opaque
edge was June for mature fish and July for immature fish（＞4-age）.
Age was determined by counting the inner margin of an opaque zone.
As the inner margin of an opaque zone was formed from May to August,
the birthday was set to 1st of August.
Average body length was estimated from samples of fish collected
by the beam trawlnet, the sledge net
（＜2-age）and Age-Length Keys （≧2-age）. Monthly growth rates of 0-age
and 2-age fish were the largest from spring to summer,
while ≧3-age fish grew most quickly from winter to spring.
The period of the largest growth rate almost agreed with the period of opaque
zone formation. The von Bertalanffy growth curves presumed from mean
standard lengths by age in May-June were:
lt＝30．262×｛1-e－0．199（t＋0．313）｝for male and
lt＝42．816×｛1-e－0．132（t＋0．322）｝for female, where
lt is standard length
（cm）at estimated age t.

Movement and growth of hatchery-reared Japanese flounder 
Paralichthys olivaceus tagged and released at northern Hokkaido
coast in 1999

To examine movement and growth, a total of 997 hatchery-reared
Japanese flounder Paralichthys olivaceus juvenile,
116‐282 mm in total length, were tagged and released at
the west coast of Wakkanai facing the Sea of Japan,
northern Hokkaido in May 1999.
First recovery of released flounder occurred around the release
location in August 1999; distribution of flounder tended to
expand toward the south from around October 1999.
Flounder recovery concentrated during the period from August to
October in 1999 and from May to October in 2000;
those fish comprised 25．3% and 50．7% of
the total of 71 fish recaptured. Relationships between days after release
and growth increment in length, or total length at recapture
indicated that flounder would grow rapidly between July and November.
Also, the individual growth increment, especially for 420‐560 days
post-release, suggested that the flounder daily growth rate was
0．94 mm per day, ranging from 300 to 500 mm in total length at recapture.

Formation and maintenance of the natural Laminaria bed on
the coast of Iwanai,Hokkaido, Japan.

Macroalgal and sea urchin distribution of the large-scale natural Laminaria-bed
were investigated by means of aerial photographs and SCUBA from
1989 to 1999 on the coast of Iwanai, Hokkaido,Japan.
The Laminaria-bed was located off the western brakewater of Iwanai Port.
Its area approximated from 22 to 26ha. Vertical distribution of
the Laminaria-bed extended to about 15m deep.
Sea bottom topography of the Laminaria-bed was gentle slanted ridge-like feature.
Sediment composition of the bed was mainly cobble and boulder
from 5 to 100cm in diameter. Macroalgal attached substrata were
mainly 5 to 45cm in diameter.
Sea urchin density was low and algal species diversity was greater in the bed
compared to the “Isoyake”（crustose coralline and sea urchin dominated）area.
Low sea urchins density and widening gaps in the bed could be caused by
physical disturbances induced by wave action.

Effects of supplements of Nannochloropsis oculata to rearing water
on initial feeding and survival in larval barfin flounder
Verasper moseri

The present study examined the effects of supplements of unicellular alga
Nannochloropsis oculata to rearing water on feeding and survival in larval barfin
flounder Verasper moseri.
Feeding characteristics after rotifer addition to 8 hours later were
investigated in 13- and 20 day-old larvae kept in Nannochloropsis-supplemented
water at density of 50×104 cell/ml.
The larvae reared without inflow of seawater showed active feeding immediately
after rotifer addition, and the number of rotifers ingested per fish increased
linearly, reaching peak values 6 hours later.
However, with continuous inflow of fresh seawater, a decrease of rotifer
uptake was apparent by 2 or 4 hours after rotifer addition,
in accord with the decreased density of Nannochloropsis in rearing water.
In second experiment, 10 day-old larvae were reared in water
supplemented with Nannochloropsis at a different density of
0, 40, 160 and 320×104 cell/ml, for 25 days.
Both feeding incidence of larvae and the number of rotifer
ingested per fish were higher in groups provided with at least
40×104 cell/ml than in non-supplemented group throughout
the experimental period. Especially, early on in the trial,
when larvae were between 13 and 16 day-old, there was a
tendency for higher feed uptake by larvae maintained in high
densities of Nannochloropsis. In almost all Nannochloropsis-supplemented
groups, larval survival rates were high, ranging between
70．5 to 90．7%. In contrast, in non-supplement group,
large mortality occurred while 18- to 22 day-old,
resulting in survival rates of only 7．1 to 34．0%.
Moreover, upon observing the intestine of these dead larvae,
the mortality could be tentatively attributed to a shortage of
feed intake early on in the trial.
These results demonstrate that rearing of barfin flounder larvae
in Nannochloropsis-supplemented water is an effective means to accelerate
initial feeding and to increase survival.