6o THE SEA FISHERIES 



and other valuable marine fish are now available. 



With regard to the rate of growth of the plaice and its length at 

 different ages, it might be as well to explain that there are in general 

 three methods of determining the rate of growth of fish. These are :— 



1. Determination of the size at different ages by means of 

 observations of the rings of groAvth on the scales. 



2. Similar observations on the otoliths (ear-stones). 



3. To measure the whole of the fish caught in certain hauls where 

 the fish are numerous, and to deduce the growth rate from the 

 grouping of individuals of different lengths. 



Another method is to keep the fish in aquaria and observe for 

 as long a period as possible the increase of the body length. This 

 method is open to serious objection, since the fish are living under 

 artificial conditions, it is not relied on in determining the growth 

 of sea fish. 



Of the above methods the second and third have been extensively 

 employed in the case of the plaice, ^ whilst the first method has been 

 used for the salmon* and herring. ^ 



According to Wallace, in the southern part of the North Sea, 

 a plaice usually adds as much to its weight in the fourth year of 

 its life as in the first three years combined. The greatest average 

 increase in weight probably occurs in the fifth and sixth years, but 

 in the South Dogger region there is a marked diminution in the 

 annual average weight -increment after the fifth year in the case 

 of males ; the females, on the contrary, show no diminution in the 

 sixth and very little, if any, in their seventh year. In the Southern 

 Bight of the North Sea the average length of the three year old 

 plaice is approximately 21 cm. for males and 22 for females (about 

 8| in.). In the Great West Bay the average length of plaice of the 

 same age is 27 cm. for males and 28 for females (lof inches). The 

 average annual growth is therefore about 7 cm. in the first three years 

 in the Southern Bight, and 9 to 9-5 cm. in the western part. In the 

 fourth year the average increase is 5-5 cm. in the Southern Bight, 

 and from 3-5 to 4 cm. in the West Bay ; so that while plaice grow 

 more rapidly at first in the West Bay, they grow less rapidly subse- 



1 There is an enormous amount of " literature " on the rate of growth of fish. 

 Reference may be made to W. Wallace, " Report on the Age and Growth-rate of 

 Plaice in the Southern North Sea as determined by the investigation of Otoliths." 

 Marine Biol. Assn. Internat. Fishery Investigations. Second Report (Southern 

 Area), 1907. Cd. 3837, p. 1. See also Heincke und Henking, tfber SchoUen und 

 Schollenfischerei in der sudostlichen Nordsee. Aus die Beteiligung Deutschlands an der 

 internationalen Meeresjorschung. Berlin, 1907. 



2 See Board of Agriculture and Fisheries. Fishery Investigation. Series I, 

 Vol. I. " Report on Investigations upon the Salmon with special reference to age- 

 determination by study of scales." 



^ For a criticism of this method see Rosa Lee, " An Investigation into the Methods 

 of Growth Determination in Fishes." Publications de Circanstance. Cons. Perm. 

 Internat. Explor. de la Mer, No. 63, November, 1912. 



