Trawling for Herring on the Coasts of Scotland. 117 
vanishing for a long period. The chief reason of these variations 
is probably to be sought in the variation, 1st, of the quantity of 
food of the fish; 2d, in the number and force of the destructive 
agencies at work upon it. 
Any circumstance which increases or decreases the quantity of 
erustacea and of sand eels, for example, must influence in a direct 
ratio the chances of existence of a multitude of herrings; while 
any condition which increases or decreases the number or the 
activity of the enemies of the herring, must influence those chances 
in an inverse ratio. 
Leaving out of consideration the animals which prey upon 
herring fry, and their mere accidental and unimportant enemies, 
their great destroyers are,— 
a. Fish, 
Cod, ling, coal-fish, hake, conger, dog-fish. 
b. Birds. 
Gulls and gannets. 
ce. Marine Mammals. 
Porpoises and other cetacea. 
d. Man, 
The following considerations may help us to form an estimate 
of the relative importance of these different destructive agencies :— 
In 1861, there were in Scotland, and that part of England over 
which the Fishery Board have jurisdiction, 42,751 fishermen and 
boys engaged in the herring fishery. The total take of the year 
would give about 20,000 herrings for each of these persons, or 
near upon 900,000,000 for the whole. 
It seems, and indeed is, a most marvellous drain upon the 
power of multiplication of one animal, to destroy in one year 
a multitude exceeding in number the whole human population 
of the globe. But it may be readily demonstrated that this vast 
number sinks into insignificance if compared with the total 
destruction effected by agencies over which man has no control 
whatsoever. 
Consider the destruction of large herring by cod and ling alone. 
It is a very common thing to find a cod-fish with six or seven 
large herrings in his stomach, of which not one has remained long 
enough to be digested. If, in order to be safe, we allow a cod- 
fish only two herrings per diem, and let him feed on herrings for 
Herring, Days. 
only seven months of the year, then 2 x 210 = 420 herring as 
his allowance during that time; and fifty cod-fish will equal one 
fisherman in destructive power. But the quantity of cod and ling 
taken in 1861, and registered by the Fishery Board, was over 
80,000 cwt. On an average, 30 cod-fish go to 1 ewt. of the dried 
fish, Hence, at least, 2,400,000 cod-fish were caught in 1861, 
