- 19 — LATER STAGES: PLEURONECTIDS 



to be at a certain ground at a certain time of the year in order to find the animal. For 

 these reasons he is extremely doubtful whether the migrations of plaice are connected in 

 any way with search for food. 



He considers that plaice migrations are dependent on hydrographical conditions — 

 periodic changes in salinity and more especially temperature, whilst depth also is a factor 

 later on. During their migrations they doubtless stay longer on good feeding grounds 

 than on bad. 



It is possible that interesting correlations may be possible later on between the nature 

 of the food on different grounds and the condition and growth of the fish, especially in 

 the case of the plaice, in which a large no. of determinations of length and weight have 

 been recorded. 



The vitality of trawl-caught plaice. 



The experiments described in this report by Borley (2 a) constitute a contribution 

 towards ascertaining what proportion of the small plaice caught by trawlers would survive 

 if returned to the sea. Their import is thus mainty economic, bearing on the operation 

 of a size limit in cases in which such a limit does not result in trawlers leaving the 

 ground unworked. The main conclusions drawn have therefore been left to the consi- 

 deration of the Editor. 



In the course of the examination of the data, however, it was found necessary to 

 investigate the comparative proportions of the two sexes and of various sizes which 

 survived the injuries incident on Trawling and exposure on deck. The writer of the re- 

 port considers that the percentage of any given category remaining vigorous after the 

 treatment received, when compared with percentages similarly ascertained from other 

 categories, furnishes a means of comparing the general stamina, vitality, or general vigour 

 of the groups considered; and in this respect the experiments are not without interest of 

 a biological nature especially in regard to the features of age or sex. 



For the purposes of comparison the plaice are first classified in 2 cm. groups. The 

 proportion in each group remaining vigorous and apparently unhurt is then expressed by 

 a fraction whose numerator is the number of vigorous fish, and whose denominator is 

 the total number of fish in the group. Groups are then compared by dividing the fraction 

 representing the proportion surviving in one group by that appropriate to the other. 

 Where the first group is the more vigorous the quotient of course exceeds unity. No 

 group of less than 5 fish is considered, and the members of pairs of groups compared 

 have been under identical conditions during observation. 



The following table shows the result of comparing fish of the same size but of dif- 

 ferent sex. It will be seen that although in the case of very small fish the 'emales appear 

 to be the more vigorous, in the case of the fish between 14 and 25 cm. the reverse is 

 the case. The more rapid growth in vigour with increasing size thus indicated in the 

 males is not however continuous, and at greater sizes than 20 — 2i cm. the vitality of 

 the males approaches gradually the vitality of the females. It is shown that any errors 

 in determination of sex of the very small fish do not vitiate the indications of the table. 



