16 



south, on March 13th. I do not intend at present to seriously 

 consider it, for the reason that I heard of a skipper of a trawler who 

 caught one of our marked specimens, and threw it overboard on 

 his way to Hull. If this should come to his notice I should be glad 

 to hear from him as to whether he took a note of the number on 

 the label, and if he remembers where and when he caught it, and 

 where he set it free again. 



This and the previous experiments make it evident that during 

 the last three months of the year many of the females migrate from 

 the Northumberland coast to the north, those of the Beadnell region 

 reaching the St. Abbs district and even Dunbar. A comparison of 

 the experiments for a number of } T ears shows also that the time of 

 migration is subject to seasonal variation. There is also individual 

 variation, for while some females marked at the same time are being 

 caught in the Berwick and St. Abbs regions, others have crossed 

 the Firth of Forth, and some have actually got as far as Forfarshire, 

 Kincardineshire, and even the Moray Firth. At the same time, 

 as has been noted, a large number of females are recaptured during 

 the period of migration in or near the district of liberation. 



In such a crab-fishing district as Beadnell the assemblage of 

 crabs of both sexes in the autumn gradually moves out into deeper 

 water, and returns during the early months of the year towards the 

 coast. During the period of the outward migration a varying pro- 

 portion of the females, which have in the same season cast, segregate 

 themselves from their companions towards the north, and the 

 migration is continued in this direction during the months when the 

 crab population generally is approaching the shore. The time when 

 they actually leave the district of Beadnell, and also the degree of 

 migration, are evidently matters of individual variation. Both 

 however, are susceptible of explanation if the factors were known. 

 There can be no question to my mind that the principal factor is 

 the ripening of the gonads. But while we know that the ova of 

 the migratory females are approaching the fully developed condition 

 we do not yet know for comparison the condition of the gonads of 

 the females which have remained in the Beadnell district. The 

 question is one to which I hope to have an opportunity of returning.* 



* Since this was written I have had the opportunity of examining the ovaries and sper- 

 mathecae of crabs which have not migrated. The spermatheese have been charged with sperms, 

 but the ovaries are not so large as those of the females which have migrated, and the ova are 

 only half the size of those of the latter. It may be concluded, therefore, that migration is in 

 response to the ripening of the ova. It is a spawning migration which takes place in the same 

 or the next season after casting, and, in the case of the older crabs, may be postponed for 

 still another year. 



