162 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



this varies, of course, according to the time of year. During 

 the winter months, November-March, the fish are smallest, 

 because then they belong mostly to those hatched in the 

 previous year. The prevalent length is about 7 cms. (2f ins.), 

 and 50 per cent, of all are between 6 cms. and 8 cms. in length 

 (2 \ ins. to 3 \ ins.). During the months May to July there are 

 three maximal lengths, or prevalent sizes : about 5 cms. 

 (2 ins.), 9-5 cms. (3| ins.), and about 14 cms. (5J ins.). That 

 means that a great number of the plaice caught in May to 

 July are those that have been hatched in the same year (they 

 are two to four months old). Then there are plaice that are 

 one and two years older (that is, about 1J and 2 \ years old). 

 It is impossible to be more precise as to the ages of fish caught 

 in the shrimp-trawl (on pp. 195-6 we discuss the general 

 question of the growth rate of the fish), but the following 

 results are useful : half of all the plaice caught during May to 

 July are from 11 J to 16 J cms. long (that is, 4 J to 6 J ins.), and 

 half of all those taken during the months August to October 

 are from 11 to 16 cms. long (that is, 4 \ to 6 \ ins.). This will 

 give a good idea of the kinds of plaice caught in the course 

 of shrimp-trawling. 



Next, as to the numbers caught. A summary of the results 

 of the Mersey experimental hauls is given by R. J. Daniel,* 

 and this shows the actual numbers per haul, per hour's fishing, 

 etc., taken between 1898 and 1920. The number per haul 

 varies between 14,697 (in \\ hour's drag) and 0. The average 

 number per hour's fishing per annum varies between 1,197 

 (in 1911) and 64 (1904 and 1916). There is a very evident 

 periodicity in abundance of young plaice on this ground, and 

 to this question we return later in the report (p. 200). 



The short statement made here will show, however, what an 

 extremely heavy toll shrimp-trawling makes on the plaice 

 population of the inshore nursery grounds in the Irish Sea. 



* Ann. Rept. Lancashire Sea-Fish. Laby. for 1919 pp. 51-71. 



