THE BEIIAVIOUR A N D YHYSIOLOOY OF HERRING AND OTHER CLUPEIDS 329
southern North Sea that herring shoals in strong tidal streams were
cigar-shaped with their long axes parallel with tho tidd strcnms.
Whether this shape is brought about by thct pattorn of tidal currciits or
whether it in somo way reflects tho switiitning abilit,y of tho fish, a few
being faster, a few Rlowcr, tho majority bciiig average, is not ktwwti.
Runnstr~m (1941b) also found herring shouls off the Norwegian coast
in the form of a long narrow streak, and Khoals of Sardina pilchardus
may also be cigar-shaped (Cushing, 1960).
Glover (1957) called for a study of the factors causing dispersion or
aggregation in herring. Jakobsson (1961) has given an account of the
factors which might determine shoal size off Iceland. The formation of
large shoals seemed to be positively correlated with the numbers of
some zooplankton organisms such as Calanus, with the mixed water in
the border areas between warm and cold water masses, and with shallow
and irregular bottom configurations. There seemed to be a negative
correlation between large shoals and fast-swimming food organisms
such as fish larvae, and with deep and uniformly cold water. These
gross factors which may control shoal size must be accompanied by
other more intimate stimuli between individual fish.
6. Maintenance of shoals
It has been extensively reported that shoals tend to break up to
feed (see Section VI, B) and that shoaling behaviour ceases in darkness. Loukwhkin and Grant (1959) cited many authors who had
reported this.
The effect of low light intcntiticfi on clupoidfl ifl shown in Tal)lc: IX.
Apart from some sea obRervationR and in Anchnviellu, an c : t i ~ r t ~ t ~ l i d ,
there seem8 to be diRpcrHion of shoalH at low light iritcmnition, niiggwtirq
the extreme importance of viHiori in kceping shoals togothcr. Tho
observation of Zusser (1968a), Radakov and Solovyev (1959) and
Radakov 11960) of some " schools " existing in darkness needs to be
further examined, in particular as to whether the fish were in true
schools or unpolarized groups, and what effect the means of observation
had on behaviour. It is always possible that phosphorescence in the
water may help fish to keep together at night (Cushing, 1960).
Jones (1962) has also suggested that the breaking up of clupeid
shoals, as observed especially in aquaria, might not necessarily be
invariable. He watched the movement8 of herring 8 h f J d H in tho
southern North Sea by echo-sounder. Both duna: trmwi and " h k "
traces were probably attributablo to hurring; tho " ( I d "
traces (small groups of fish) dkappeard :It night, whilr; thc: f h M 9
traces (large groups) became more diffuw. I!(: ~ i ~ g g 0 ~ t . 4
t h n L while f,ttcl
southern North Sea that herring shoals in strong tidal streams were
cigar-shaped with their long axes parallel with tho tidd strcnms.
Whether this shape is brought about by thct pattorn of tidal currciits or
whether it in somo way reflects tho switiitning abilit,y of tho fish, a few
being faster, a few Rlowcr, tho majority bciiig average, is not ktwwti.
Runnstr~m (1941b) also found herring shouls off the Norwegian coast
in the form of a long narrow streak, and Khoals of Sardina pilchardus
may also be cigar-shaped (Cushing, 1960).
Glover (1957) called for a study of the factors causing dispersion or
aggregation in herring. Jakobsson (1961) has given an account of the
factors which might determine shoal size off Iceland. The formation of
large shoals seemed to be positively correlated with the numbers of
some zooplankton organisms such as Calanus, with the mixed water in
the border areas between warm and cold water masses, and with shallow
and irregular bottom configurations. There seemed to be a negative
correlation between large shoals and fast-swimming food organisms
such as fish larvae, and with deep and uniformly cold water. These
gross factors which may control shoal size must be accompanied by
other more intimate stimuli between individual fish.
6. Maintenance of shoals
It has been extensively reported that shoals tend to break up to
feed (see Section VI, B) and that shoaling behaviour ceases in darkness. Loukwhkin and Grant (1959) cited many authors who had
reported this.
The effect of low light intcntiticfi on clupoidfl ifl shown in Tal)lc: IX.
Apart from some sea obRervationR and in Anchnviellu, an c : t i ~ r t ~ t ~ l i d ,
there seem8 to be diRpcrHion of shoalH at low light iritcmnition, niiggwtirq
the extreme importance of viHiori in kceping shoals togothcr. Tho
observation of Zusser (1968a), Radakov and Solovyev (1959) and
Radakov 11960) of some " schools " existing in darkness needs to be
further examined, in particular as to whether the fish were in true
schools or unpolarized groups, and what effect the means of observation
had on behaviour. It is always possible that phosphorescence in the
water may help fish to keep together at night (Cushing, 1960).
Jones (1962) has also suggested that the breaking up of clupeid
shoals, as observed especially in aquaria, might not necessarily be
invariable. He watched the movement8 of herring 8 h f J d H in tho
southern North Sea by echo-sounder. Both duna: trmwi and " h k "
traces were probably attributablo to hurring; tho " ( I d "
traces (small groups of fish) dkappeard :It night, whilr; thc: f h M 9
traces (large groups) became more diffuw. I!(: ~ i ~ g g 0 ~ t . 4
t h n L while f,ttcl