LIFE-HISTORY OF THE PLAICE 89 



a million eggs. What percentage of these are actually fertilized 

 in nature and what are the losses in the egg stage and early 

 larval stages are not known. The losses are certainly heavy. 

 At the Trondjhem Plaice Hatchery an average of 298 females 

 produced an average of 99,000 post-larval fry per female over 

 a period of five years.^ The annual averages varied from 

 13,000 in 1910, to 91,000, 209,000, 89,000, and 132,000 in the 

 succeeding years. The figures show that some factors not yet 

 discovered produce enormous fluctuations in the output of fry 

 bred in captivity. It is not improbable that similar factors '^ — 

 to which must be added actual attacks by enemies — affect the 

 output year by year on the spawning grounds, and during the 

 early drift from the spawning grounds to the nurseries. 



Dr. C. G. J. Petersen, the Danish biologist, has invented an 

 instrument by which he can take up one square foot of the 

 sea bottom. By means of this he has proved that the ' broods ' 

 of molluscs, and other bottom animals on which plaice feed, 

 fluctuate from year to year as do the ' broods ' of herring, cod, 

 and haddock. It is clear that if a large number of baby 

 plaice are hatched out at a time when the free swimming 

 larvae of these molluscs are scarce, many of them will die of 

 starvation (or indigestion ?) as soon as the yolk-sac is absorbed. 

 And the same result will follow if there is a shortage of the 

 minute plants we call diatoms. To what extent does the 

 success of a ' hatch ' of plaice depend on the presence or 

 absence of these organisms ? 



Movements of Plaice 



The main feeding-grounds are anywhere south of a line from 

 the Tees to the 8kaw. Marking experiments have proved that 

 plaice roam freely over these great feeding-grounds, the four 

 most prolific of which are in the 36,000 square miles between 

 the 10 and 30-fathom lines in the southern bight of the North 

 Sea.^ The general movements of these big fish are summed 

 up by Garstang as a general deployment northwards and off 

 shore in summer, and concentration (generally southward) to 

 the various spawning-grounds in winter. The causes which 

 impel big plaice to move about among the feeding-grounds are 

 not known ; but fishermen say that fish will be on certain 

 banks at some states of the tides and not at others. Where 

 they disappear to or why they go is not yet known. 



^ Experiments along these lines in English and Scotch hatcheries might 

 throw light on the subject. 



2 e. g. prevalence of easterly (unfavourable) or south-westerly (favourable) 

 winds during the fry-stage period. ^ Cf. below, p. 51. 



