OCEAN SEASONS 249 



These seasonal changes in the quantity of plankton are 

 reflected in the growth of all the larger animals in the sea. 

 A very great number of the small invertebrates living on the 

 sea floor might be termed annuals. Spending their 

 early life in the drifting plankton community in the spring, 

 they soon settle to the bottom and reach maturity in the 

 course of the summer. The countless young that never 

 survive to maturity have nourished other growing animals, 

 which in turn have fallen prey to the larger creatures. 

 It is probably largely for this reason that we find that most 

 of the fish in our northern waters make the greater part of 

 their growth during the summer. Plankton is then present 

 in greatest quantity for those pelagic fish such as the 

 herring and the mackerel which feed directly on it. Then, 

 too, at its greatest is the toll of life among the young 

 molluscs and Crustacea at the hands of such bottom-living 

 fish as the plaice. It is natural that during the period 

 of greatest feeding most growth takes place and this is 

 shown very distinctly on the scales of many fish. A reliable 

 index to the age of these fishes is afforded by the number 

 of wide and narrow zones shown on their scales, the wide 

 zones corresponding to the period of greatest growth 

 which generally takes place in the summer, and the narrow 

 zones to the period of poor growth in the winter. 



Such then is the cycle of life in our northern seas. But 

 it remains to be discovered how the animal and plant 

 life in the tropical waters varies with the time of year. 

 Here there are not the marked temperature changes 

 that occur in temperate seas neither does the strength of 

 the sun's light vary to such a marked extent. At any 

 rate it is probable that at all periods of the year it is 

 sufficiently strong to allow active growth amongst the drift- 

 ing plants. On account of this uniformity in temperature 

 and light it is thought that such marked changes in the 



