— 21 — LATER STAGES; PLEUR0NECT1DS 



age-groups which lie in succession sea-wards. Probably results of a similar character 

 would be obtained by the selection of any line running seawards from any part of the 

 continental coast from Sandettie to the Horn Reef. 



[f the catch per hour of each age-group be compared, it is seen that in May 1906, 

 the I group are not found further than about 20 miles from the coast, whereas in September 

 1905 they are spread out beyond 80 miles and the other groups also show a relatively 

 greater seaward distribution in the autumn than in the spring, with the inference that 

 there is a general seaward movement of the successive broods. This much was already 

 known from the investigations of previous workers but the author has attempted to give 

 a greater precision to the results by a determination of the Centre of Density of each 

 brood. This is found by multiplying the catch per hour in each case by the mean 

 distance of the haul and dividing the sum of the products by the total catch per hour. 

 A locus for each brood is then obtained, expressed in number of miles from shore. The 

 following centres of density are thus obtained in distances in miles from the Dutch coast. 



II 



III IV 



v + 



I I ! 



May ^1906 15 24 47 46 



September 1905 26 37 35 63 



This shows that the centres of density of the age-groups in September varied from 

 11 to 17 miles seawards ot those of the same age denomination in May, and, if we may 

 assume that in each case a typical season is being dealt with, the figures may be taken 

 as representing the offshore movement of plaice with increasing age. 



Another instructive comparison can be made by referring each brood to its date of 

 origin. 



Average distance from shore (in miles) of each age-group. 



Here it is seen that the 1903 brood had a centre of density of 26 miles offshore in 

 the autumn of 1905 whereas the same brood of young fish lay at a mean distance of 

 only 24 miles offshore in the following spring, when it had become the III group. The 

 older broods show a similar series of figures and one is inclined to consider the apparent 

 approach of the earlier brood to the coast as being really a reflection of the well-known 



