306
J. H. 8. BLAXTER AND F. 0 . T. HOLLIDAY
clupeids, too, the maximum length attained seemed to be positively
correlated with the life span and larger alupeids eeemed to live to a
greater age than fish of other species of comparable size.
Few attempts have been made to estimate the natural mortality
rates of clupeids resulting from old age, internal " stress ", predation,
disease and parasites. Presumably old age is of little importance in
h e a d y fished stocks, though it is known, for instance, that the AtlantoScandian herring lives a t least to an age of 20 years, from following the
progress of the 1904 year class in the Norwegian fishery. The Iifespan
of other herring races and of other clupeid species seems to be considerably less, nearer 10-12 years, while in lsprattw 8prattua it may be
5-6 years (see review by Beverton and Holt, 1959).
Estimates of natural mortality rate may be made by plotting the
relationship between fishing intensity and total mortality and extrapolating back to zero fishing intensity. Independent estimates of
natural mortality would be of value in confirming such extrapolations,
but unfortunately this approach has been completely negleoted in the
past due to the extreme difEculty of making a census of predators and
their food intake. However, in view of the fact that clupeids are so
heavily preyed on, one might expect that relatively high estimates of
natural mortality would be obtained.
2. Predation
In particular, Russian workers have stressed the need for more
importance to be attached to studies on predation. Awerinzew (1935)
described the mammals, birds and fish preying on the Barents Sea
herring, and their effect on the migration pattern and capture by drift
nets. Mitchell (1864, p. 37) reported the possible effect of the gannet,
Sula alba, a diving bird, on herring stocks round the Scottish Coast.
It was assumed that 200,000 of these birds near St. Kilda were feeding
for 7 months of the year, taking five fish each per day on average. The
total taken by this one colony of sea birds would thus have amounted to
214 million fish. Gunter and Christmas (1960) gave details of predation
by sea birds, cetaceans and fish such at3 tuna on Brevm&&z ptrmw.
The accumulation of guano on the Galapagos Islands from ma birds
feeding on Sardinqs sagax off the Peruvian coast is a sign of the high
level of predation there. This source of fertilizer haa now been replaced
by fish meal obtained by direct capture of the fish, but predation by
the birds continues.
The behaviour of captive herring kept with cod (Qdw eallarias) in
a tank was described by Schiifer (1965). "he fish usually reacted
80-90 cm away from the cod, the avoiding movement of the shod
J. H. 8. BLAXTER AND F. 0 . T. HOLLIDAY
clupeids, too, the maximum length attained seemed to be positively
correlated with the life span and larger alupeids eeemed to live to a
greater age than fish of other species of comparable size.
Few attempts have been made to estimate the natural mortality
rates of clupeids resulting from old age, internal " stress ", predation,
disease and parasites. Presumably old age is of little importance in
h e a d y fished stocks, though it is known, for instance, that the AtlantoScandian herring lives a t least to an age of 20 years, from following the
progress of the 1904 year class in the Norwegian fishery. The Iifespan
of other herring races and of other clupeid species seems to be considerably less, nearer 10-12 years, while in lsprattw 8prattua it may be
5-6 years (see review by Beverton and Holt, 1959).
Estimates of natural mortality rate may be made by plotting the
relationship between fishing intensity and total mortality and extrapolating back to zero fishing intensity. Independent estimates of
natural mortality would be of value in confirming such extrapolations,
but unfortunately this approach has been completely negleoted in the
past due to the extreme difEculty of making a census of predators and
their food intake. However, in view of the fact that clupeids are so
heavily preyed on, one might expect that relatively high estimates of
natural mortality would be obtained.
2. Predation
In particular, Russian workers have stressed the need for more
importance to be attached to studies on predation. Awerinzew (1935)
described the mammals, birds and fish preying on the Barents Sea
herring, and their effect on the migration pattern and capture by drift
nets. Mitchell (1864, p. 37) reported the possible effect of the gannet,
Sula alba, a diving bird, on herring stocks round the Scottish Coast.
It was assumed that 200,000 of these birds near St. Kilda were feeding
for 7 months of the year, taking five fish each per day on average. The
total taken by this one colony of sea birds would thus have amounted to
214 million fish. Gunter and Christmas (1960) gave details of predation
by sea birds, cetaceans and fish such at3 tuna on Brevm&&z ptrmw.
The accumulation of guano on the Galapagos Islands from ma birds
feeding on Sardinqs sagax off the Peruvian coast is a sign of the high
level of predation there. This source of fertilizer haa now been replaced
by fish meal obtained by direct capture of the fish, but predation by
the birds continues.
The behaviour of captive herring kept with cod (Qdw eallarias) in
a tank was described by Schiifer (1965). "he fish usually reacted
80-90 cm away from the cod, the avoiding movement of the shod