

AuGUST 5, 1915] 
NATURE 
635 

and ‘‘ Mediterranean’ forms, once separated by a land 
ridge from Scotland to Iceland. 
All fish ultimately depend on plant-life for their 
food. In the ocean floating plants are all-important, 
minute unicellular organisms living in incredible num- 
bers in the upper layers of the water, in suitable con- 
ditions daily multiplying themselves, giving a fertility 
to the sea 
many times 
that of the best 
soil. The cycle 
of life of such 
pelagic plants 
in the open 
ocean is simple, 
complicated 
only by — fluc- 
tuations in 
sunshine, rain- 
fall, and other 
m e teorclogical 
conditions. In 
coastal waters, 
especially 
where broad 
shelves extend 
out from the 
land, such 
fluctuations are 
more consider- 
able, and often 
The Oli Tis Ores! 
seasonal 
changes in tem- 
perature and 
salinity have to 
be added. In- 
stead of the 
same species of plants all the year round, and in 
consequence of the smaller animals which feed upon 
them, there may be a whole series of species, each 
suitable to some particular phase of seasonal change. 
Form succeeds form, the old form disappearing, per- 
haps passing into resting stages upon the bottom, until 
suitable physical conditions bring them 
once more into active life. It seems 
possible that this very destruction and 1 
re-creation adds further to the total 
food of our seas. The peculiar richness 
of the fishing grounds is, however, 
mainly ascribed to the abundant land- 2. 
drainage of western Europe with its 
teeming population, to this being due 
much of the nitrogen, phosphorus, and 
silica in forms available for living 3 
organisms. 
Most fish are bottom-feeders, and the 
ground must be suitable to the animals 
upon which they feed. Mud, the de- 
position of which implies the absence 
of movement, is inimical to all life. 
Flat fishes are themselves only suited 
to sand, on which they rest, invisible 
to their enemies. The cod and had- 
dock prefer respectively rock and sand, 
but the hake, ling, coalfish, and whit- 
ing, belong to a family of such remarkable adapt- 
ability in feeding that it is of vast economic importance. 
The herring deposits its spawn on rocky ground, so 
that its eggs get well aerated and are not silted over, 
but most other economic fishes have floating eggs. 
As examples the plaice, cod, herring, mackerel, and 
eel are chosen. Each has its own spawning grounds, 
its optimal depth, salinity, and temperature at each 
NO. 2388, VOL. 95] 

Fic. 2.—Floating plants, mainly Ceratium. 

oe 5 

period of its life, its own wanderings in search of 
food, etc., and its own peculiar development. The 
plaice has its southern limit at the isotherm 10° C. 
The North Sea stock spawns in February, principally 
off our coasts between Dover and Cromer, and the 
eggs and young require to be carried by the currents 
so that the latest larval stages fall on the bottom in 
not too great intensity in a few feet of water, which 
must have a salinity of at least 0-017. Naturally the 
Dutch coast is the great plaice nursery, and from here 
the young fish gradually disperse into deeper and 
deeper waters, at the same time becoming more suit- 
able to the food they produce, until in their fourth 
season they undertake their first breeding migration. 
The eggs, which are somewhat corrugated, have been 
found carried into the Baltic in undercurrents of 
warmer Atlantic water, a phenomenon considered to 
be correlated mainly with viscosity. Growth is a 
matter largely of temperature and salinity. Thus in 
the North Sea a fish of gocm. long is six years old, as 
shown by its otoliths, and in Barents Sea (White Sea) 
about eighteen. The latter rate of growth is found 
in the brackish Baltic, where 4o cm. and eighteen 
years are about the limits as contrasted with 70 cm. 
and fifty years in Barents Sea. 
The herring is a feeder on the floating life of the 
sea, and rises and falls in the water by night and 
day, shunning the light. It exhibits somewhat similar 
rates of growth to the plaice as shown by its scales, 
which form broad, transparent growth-bands in 
summer as contrasted with narrow, opaque bands in 
winter, the fish not shedding its scales during its 
life. By the study of the breadths of the 
summer bands the areas where fish spent their previous 
lives may be ascertained, and thus breeding shoals 
may be analysed. Maturity is reached in different 
localities in from three to ten years. The spawn is 
laid on the bottom and at 3° C. hatches in forty days, 
at 12° C. in eight days. The young are at the mercy 
of the currents, and good years off Norway are con- 
sidered to be those in which spawning is late, the 
currents being more determined and the sea having 
abundance of minufe and suitable plant-life as food. 
Prof. Gardiner concludes with a plea for the con- 
tinuation after the war of the International Council 
on Cp ain 
Fic. 3.—Rates of Growth of Herring. Eight fish of equal age (four years) from: 1. White 
Sea; 2, Lysefjord, Norway; 3, Zuyder Zee; 4. East Coast of Sweden; 5, West Part of 
North Sea ; 6, Atlantic Ocean; 7, Iceland; &, West Coast of Norway (Spring Herring). 
(After Hyort). 
for the Exploration of the Sea, which came into exist- 
ence in 1g02. <Any control of the West European 
fisheries must be by international agreement, for 
which, as he says, there must be a foundation of in- 
contestable evidence. The conferences of the repre- 
sentatives of the twelve countries on the International 
Council have been highly profitable in the investiga- 
tions of the economic conditions of fisheries, depend- 
