British Association Notes and Comments. 245 
pages of The Naturalist, protested against this new nomen- 
clature. 
ROOKS. 
In the Zoological Section Mr. A. Roebuck stated that 
rooks have been studied in five midland counties — Leicester- 
shire, Derbyshire, Nottinghamshire, Lincolnshire, and 
Rutland. A census of the whole area, repeated after an 
interval of four years, showed the rook population to be 
constant. About 10 per cent, of the rookeries change their 
sites annually. There is no evidence that migration materially 
affects the problem in the Midlands. The distribution of 
their roosts during the winter is considerably different from 
that of their nesting rookeries, although many remain on the 
same sites all the year round. Except for a brief period rooks 
2 re restricted to a limited area for a feeding -ground, the area 
in the vicinity of the rookery. Those roosting at a distance 
in winter return daily. The census gives the rook population 
when it is at its lowest. 
NORTH SEA FISH. 
Mr. M. Graham pointed out : — It is known that fluctuations 
in the yield of certain fisheries, e.g. herring, plaice and cod, 
in the North Sea are largely determined by the degree of 
success in survival of broods some years previously. The 
proportion of particular broods is estimated when the fish are 
large enough to be taken in trawls. Having found that the 
important fisheries depend on fish of a restricted age, say, 
five to six years old, estimates of the yield at that age can be 
made some years in advance, from the abundance of fish of, 
say, one to two years old. The particular case considered, 
the cod in the North Sea, is interesting in that: (1) the area 
populated by small fish is so extensive that adequate sampling 
of small cod can only be obtained in the trade statistics of 
landings, cause being shown for considering the magnitude of 
certain seasonal changes in the statistics as a measure of brood 
strength ; (2) the age of the cod (by which landings of small 
cod are related to brood years and to subsequent yield of 
large cod) has to be obtained indirectly, from size ; partly 
because of the said sampling difficulty and partly because 
the more usual determinations of age from scales or otoliths 
are unreliable for this fish (in the North Sea). The necessary 
analysis involves somewhat unconventional methods, which 
are, however, justified by the high correlation found. 
TANNING. 
Dr. D. J. Lloyd told us that animal skin consists of a 
tissue of fibres which are the biological units. Each fibre 
1933 Nov. 1 
