336
J. H. 9. BLAXTICR AND F. 0. T. HOLLIDAY
established connections betwecn zooplankton, particularly Calanw,
and herring, both in the North Sea and north-east Atlantic. Sometimes,
however, the correlation fails and it is clear that the physiological state
of the fish as well as the size and type of plankton need to be taken into
consideration. Blooms of Phaeocystis seem often to be avoided, perhaps
partly or wholly due to the absence of large zooplanktons in such
blooms. However, the possibility of the phytoplankton being unpalatable or of gills becoming clogged by such blooms cannot be
disregarded and could be tested by experiment. Pavshtiks also discussed how annual fluctuations in plankton abundance could shift the
feeding grounds of herring. It is difficult to do justice to the enormous
amount of work done in correlating zooplankton and herring. Glover
(1957) described the possible effect of oceanic influence and its concomitant plankton changes on the Scottish East Coast and Shetland
fisheries, suggesting broadly that an increased oceanic (Atlantic)
influence in the northern North Sea might cause a shift of fish southwards. Williamson (1 961), by means of statistical analysis, suggested
some relationship between “ northerliness ” of the stock, oceanic
influence and the abundance of certain zooplankton groups. The
suggestion here is that oceanic conditions are avoided by North Sea,
herring. Fraser (1961) has put forward the hypothesis that pure
oceanic water is not conducive to good growth of plankton and this,
and gossibly an element of ‘ I distastefulness ”, might cause the avoidance. On the other hand, mixed water appeared to give good conditions
for zooplankton production, which might partly account for herring
being found along hydrographical fronts. These fronts might also
delimit areas of unfavourable water, thus acting as migratory blocks.
Steele (1961) showed that euphausiids were to be found along the Baltic
water front off the Norwegian coast in the spring, possibly partly cawing the collection of herring there. Devoid (1961), too, reported on the
feeding of Norwegian herring along the “ Polar Front ”. Jakobsson
(1961) showed very interesting correlations between the abundance of
zooplankton and the concentration of herring off the north coast of
Iceland (see Fig. 16) ; he suggested that some zooplankton would tend
to cause the formation of larger shoals where it was abundant.
FIG. 16. a. 4-11/7/1957. 6 . 7-19/7/1958. The relationship between herring shoal8 off
Iceland and the concentration of zooplankton. (Re-drawn with permiseion from
Jakobsson, 1961.)
continuous line-path of research vessel
transverse lin-oncentration
of herring trecea
0
--zooplankton concentrations
X
-phytoplankton concentrations
J. H. 9. BLAXTICR AND F. 0. T. HOLLIDAY
established connections betwecn zooplankton, particularly Calanw,
and herring, both in the North Sea and north-east Atlantic. Sometimes,
however, the correlation fails and it is clear that the physiological state
of the fish as well as the size and type of plankton need to be taken into
consideration. Blooms of Phaeocystis seem often to be avoided, perhaps
partly or wholly due to the absence of large zooplanktons in such
blooms. However, the possibility of the phytoplankton being unpalatable or of gills becoming clogged by such blooms cannot be
disregarded and could be tested by experiment. Pavshtiks also discussed how annual fluctuations in plankton abundance could shift the
feeding grounds of herring. It is difficult to do justice to the enormous
amount of work done in correlating zooplankton and herring. Glover
(1957) described the possible effect of oceanic influence and its concomitant plankton changes on the Scottish East Coast and Shetland
fisheries, suggesting broadly that an increased oceanic (Atlantic)
influence in the northern North Sea might cause a shift of fish southwards. Williamson (1 961), by means of statistical analysis, suggested
some relationship between “ northerliness ” of the stock, oceanic
influence and the abundance of certain zooplankton groups. The
suggestion here is that oceanic conditions are avoided by North Sea,
herring. Fraser (1961) has put forward the hypothesis that pure
oceanic water is not conducive to good growth of plankton and this,
and gossibly an element of ‘ I distastefulness ”, might cause the avoidance. On the other hand, mixed water appeared to give good conditions
for zooplankton production, which might partly account for herring
being found along hydrographical fronts. These fronts might also
delimit areas of unfavourable water, thus acting as migratory blocks.
Steele (1961) showed that euphausiids were to be found along the Baltic
water front off the Norwegian coast in the spring, possibly partly cawing the collection of herring there. Devoid (1961), too, reported on the
feeding of Norwegian herring along the “ Polar Front ”. Jakobsson
(1961) showed very interesting correlations between the abundance of
zooplankton and the concentration of herring off the north coast of
Iceland (see Fig. 16) ; he suggested that some zooplankton would tend
to cause the formation of larger shoals where it was abundant.
FIG. 16. a. 4-11/7/1957. 6 . 7-19/7/1958. The relationship between herring shoal8 off
Iceland and the concentration of zooplankton. (Re-drawn with permiseion from
Jakobsson, 1961.)
continuous line-path of research vessel
transverse lin-oncentration
of herring trecea
0
--zooplankton concentrations
X
-phytoplankton concentrations