MARKED FISH 61 



be got from another experiment with weighted bottles, 

 which were specially devised by Mr. G. P. Bidder to 

 act as indicators of bottom currents, and were thrown 

 overboard from the Huxley in the winter of 1904-1905, 

 in the southward parts of the North Sea. Out of 

 600 bottles more than 54 per cent, were returned by 

 trawl fishermen within twelve months. If anything 

 like half the adolescent stock of plaice is taken by our 

 trawlers every year on the deep-sea fishing-grounds, 

 the establishment of the fact must profoundly affect 

 our views as to the causes of depletion and the reme- 

 dies to be applied ; for the fishing in these instances 

 seems not to have been on the 'so-called 'small-fish' 

 grounds or nurseries, but in areas which have always 

 been recognized as legitimate fields of work. 



The possibility of determining the age of fish is 

 quite a recent discovery, and is based on the observa- 

 tion that the scales, vertebrae, and especially the 

 ' otoliths * or ear-stones of fish, show alternate dark 

 and light rings of growth, corresponding with the 

 summer and winter seasons of the year, exactly like 

 the rings in the wood of trees. Many difficult prob- 

 lems are likely to be cleared up by a knowledge 

 of the age of fish on different fishing-grounds ; and, 

 to judge from the scale on which this investigation is 

 being pursued, it will not be long before we may 

 expect something in the nature of an age-census. The 

 Council of the Marine Biological Association have 

 reported no less than 12,000 age-determinations of 

 plaice by their North Sea staff up to June last ; and 

 the German and Dutch investigators are working on 

 similar lines. 



To conclude our argument, we should now examine 

 the question whether it is possible to determine to 

 what extent and in what manner the destruction of 

 immature fish, which is admittedly enormous, is in- 

 jurious to the permanent supply. We have already 



