RAPPORTS. XIV: MASTERMAN , — 22 - 



spring shorewards migration of the young fish. In any case, the position seawards of 

 any age-group of plaice must be determined, at certain seasons not only by the tendency 

 to progressive offshore migration but by the now well-known equinoctial movements 

 inshore of the younger and smaller fish. The claim of these figures to representative 

 value is of course open to serious criticism unies their use is confined to very broad 

 deductions such as is done by the author, e. g. for contrasting the general locus of the 

 broods in two widely-apart periods of the year. It is a little unfortunate that one series 

 of samples (the later) were affected by a beam trawl and the earlier by an otter trawl 

 as it cannot be determined with precision how far the retention of smaller fish or other 

 differential actions affect the comparison. The experiment, as far as it goes, gives data 

 for each brood with respect to (l) distance from shore, (2) number of individuals per 

 unit of area, or density and (3) average size at each distance. A comparison of these 

 data from year to year or from one period to the other would give(i) extent of migratory 

 movements, (2) variations in density and (3) average rate of growth, of each brood. The 

 variations in density can be obtained by dividing the sum of the products, distance from 

 shore x catch per hour, by the total catch per hour, giving an average catch per hour 

 for each brood. 



The following table is thus obtatned: — 



The average catch per hour of plaice in each age-group. 



Year of Origin. 1904. 



1903 



1902 



1901 



1900 



I ! 



September 1905 10.7 10.5 5.7 



11 m iv v 



May 1906 7.5 3.3 2.3 



I 



It is seen that the 1903 brood had an average catch per hour of 10.7 kilos, in September 

 1905, when they were of the II group, whereas the same brood in May 1906, eight 

 months older and now of the III group, were represented by a catch per hour of 7.5 

 kilos; presumably the difference between these two represents the loss due to natural 

 causes and to fishing though the fact that a beam trawl was used in 1906 and an otter 

 trawl in 1905 may have had a considerable influence. Similarly the 1902 brood was 

 reduced from 10.5 to 3.3 kilos, per hour and that of 1901 from 5.7 to 2.3 kilos. The 

 greatest reduction was effected amongst the 1902 brood, representing the III and IV groups 

 in which it is already known that the most intense fishing takes place. It is well to 

 consider that if the reduction in density of a brood is so great as is here indicated, 

 any inequality in application of the reducing agent (e. g. a more intense fishing at the 

 seaward extremity of the brood) would radically affecl the centre of density. 



Under the head of "Relation of Size to Age on different Grounds" the average length 

 of each age-group is compared for the three series of grounds defined as the West Bay 

 (English Channel), the South Dogger (Northern North Sea) and Southern Bight (Southern 

 North Sea). 



