crustaceans, isopods, and schizopods. 



Among these latter Meganyctiphanes norvegica and Euphausia Kronii 

 Brandt (so diffused in the Mediterranean as to influence notably in many 

 places the total quantity of macroplanktonj Jespersen) have a special 

 importance; shoals of these traverse the Strait of Messina in the autumn 

 and winter and the tuna appear so glutted on them that under these 

 conditions they let themselves be easily approached and harpooned by the 

 fishermen. 



It appears therefore likely that these and other forms, widely 

 distributed in the Atlantic, forming regular shoals, can actually 

 influence by their fluctuations and migrations from the Atlantic to 

 the Mediterranean the routes, the concentrations, and the distribution 

 of the tuna in the latter sea. 



I note the analogy with Thunnus alalonga , which, in the Atlantic, 

 is distributed in relation to the shoals of an amphipod, Buthemisto 

 bispinosa Boek (Joubin and Roule; Le Danois)o 



The planktonic feeding of the tuna probably also facilitates its 

 exodus from the Mediterranean and contributes to its enormous range, 

 which would be very restricted if its food habits were strictly linked 

 to certain pelagic fishes, divided into races of limited geographic 

 habitat, as for example the Clupeidi or the Scombri of the Mediterranean, 

 which are distinct from those of the Atlantico 



It further appears that the tuna does not pursue indefinitely the 

 same schools of fish. In the Strait of Messina it gives chase successively 

 to the fishes which are passing through there, in autumn to the young 

 anchovies, in winter to the eels, and then to the " costardelle" ( Scombresox ), 

 etc. 



ll) In regard to the preceding, we can ask whether the fluctuations 

 in the balance of tuna between the Atlantic and the Mediterranean may be 

 such as to affect the production of the tuna fisheries. But this discus- 

 sion involves the whole complicated study of the fluctuations in the catch 

 of the fisheries and of their causes. It must not be forgotten that the 

 tuna traps catch fish in spawning condition arid that therefore we should 

 expect to find the production of the fishery influenced especially by 

 oceanographic conditions, both local and general, of a physical and 

 chemical nature. 



More than by the abundance of tuna present at a given moment in the 

 Mediterranean (which reflects also, at a distance in time, the conditions 

 under which the generations of tuna in different years develop), the 

 catch of the tuna trap fisheries is influenced by the greater or lesser 

 degree in which the tuna come close in to the coast and by the geographical 

 dispersions which occur in the areas of maturation. 



11 



