Fróðskaparrit - 01.01.1987, Page 78

82
SPAWNING OF ATLANTIC HALIBUT
more than 200 cm where observed both
above and below 700 m depth, but with
most large fish in the deeper strata.
No apparent change seemed to occur in
size composition from month to month, yet
such comparisons are complicated by the
low number of individuals recorded in cer-
tain months.
Age composition
From Fig. 4 it is evident that while the shal-
low strata (<700 m depths) were domina-
ted by young fish (mainly 3-7 years old for
both sexes), the fish from the deeper
waters (>700 m) were generally older with
ages ranging from 2-3 years to more than
25 years. It also appears from Fig. 4 that
females caught at depths below 700 m were
generally older than males.
Sexual maturity
In the shallow strata above 700 m depths,
the catches were predominated by imma-
ture fish (males as well as females) in No-
vember-January, although the number of
fish caught in December and January were
rather small (Fig. 5). In February, how-
ever, considerable numbers of mature fish
( with ripe or running gonads) were capt-
ured above 700 m depths.
Below 700 m depths immature fish were
rather scarce, and the catches were domi-
nated by sexually mature fish whose spaw-
ning preparedness became more and more
evident from evident from month to month
(Fig. 5).
Hydrography
In February 1987 the hydrographical con-
ditions in the middle of the protected spaw-
ning area (the southmost CTD-station in
Fig. 1) were very homogeneous with tem-
peratures (c. 8°C), salinities (c. 35.25%o)
and sea water densities (c. 27.5) varying
only very little from surface to bottom (Fig.
6). Further north (the northmost CTD-sta-
tion in Fig. 1), however, there was a strong
vertical gradient between 450 and 700 m
depth (Fig. 7), where the temperature
changed from c. 8°C to below 0°C, the sali-
nity from c. 35.3%o to 34.9%o, and the sea
water density from 27.5 to 28.1 with in-
creasing depth.
Usually, the bottom temperatures, as
measured during the whole February 1987
cruise by the temperature sensor on the
bottom trawl, ranged between 7-8°C. On
16 February, however, when two bottom
trawl hauls were carried out in the northern
parts of the protected spawning area (Fig.
1), very sudden changes occurred in tempe-
ratures during the hauls (Fig. 8). During
the first of these hauls (from north to
south, Fig. 8a), the temperature was below
0°C when the trawl hit the bottom, but inc-
reased to c. 8°C over a very short time in-
terval and within a distance of less than 1
nautical mile. During the second haul
(from south to north, Fig. 8b), the tempe-
rature decreased from a little over 7°C to
less than 2°C during the haul.
Egg-surveys
In the egg surveys, 21 and 7 Tucker Trawl
hauls were carried out south and north of
61°30’N respectively at depths varying be-