DR. PETERSEN'S REPORT OF THE DANISH BIOLOGICAL STATION. 83 



industry, are comparatively new-comers, and belong, not to one of 

 tlie stunted Baltic races, but, as Petersen incidentally shows, to the 

 largest of the North Sea races, viz., the one familiar to Grimsby 

 fishermen. It is, moreover, most important to note that the plaice 

 of the Lira fjord does not as yet appear to have become a true native, 

 the stock of fish annually taken being merely those which have entered 

 the fjord in the early stages of their career. Petersen evidently holds 

 the conviction that the fish does not breed there at all ; but rightly 

 guards himself against an absolute statement to that effect until his 

 evidence shall be more complete. It is equally evident that the 

 contrary opinion is not unknown in the locality ; but certainly all 

 that we know in this country about the breeding of the plaice 

 must be taken as evidence of the correctness of Petersen's views 

 on this matter. 



To what extent the reproductive activity may be affected by con- 

 ditions of mere salinity is a question to the solution of which we 

 are not helped by the recorded observations of any writer ; and, as 

 it happens, no details of specific gravity, etc., are given in the present 

 Eeport, though we learn, incidentally, that the salinity, if that be really 

 a desideratum, must be sufficient for the reproduction of the rockling, 

 sprat, and flounder. That these fish will spawn in narrow waters 

 we know from our own experience at home, but except where, as 

 on the west coast of Ireland, the rapid declivity limits the habitable 

 area to a comparatively narrow strip, I cannot call to mind any record 

 of plaice spawning close inshore. In the Lim fjord the fish is limited 

 to an area of 614 square miles, of which only 283 square miles contain 

 three or more fathoms of water ; and, as Petersen shows, the Liv and 

 Thisted broads, which comprise about one-half of the whole area, are 

 seldom, if ever, reached at all by plaice under natural conditions. 



That a few fish are occasionally found with ripe ovaries is, I think, 

 rightly held by Petersen to be no proof that actual breeding takes place 

 there, since we know, from our experience of aquaria, tliat fish will 

 yearly develop spawn, but will not shed it until thoroughly acclimatised. 

 Even in the feral condition one occasionally meets with fish in which 

 the ripe ova have remained to decompose within the ovaries, and I can 

 recall at least one instance in which such a condition appeared to be 

 directly due to the creatures having returned too soon, or having never 

 left the estuarine waters. 



That the fish are not actually land-locked in the fjord is evident from 

 the fact that they got in there, but very few seem to get out again, for 

 the sufficient reason that the fishermen catch practically all that are 

 marketable ; and it is worth while to note that it only takes about 

 twelve days' fishing in each broad to exhaust the annual supply. 



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