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252 
NATURE 
[JANUARY I0, 1907 
ation is contained in Dr. Johansen’s paper on the 
plaice fishery of the Kattegat, and the means whereby 
it may be improved (Rapports et Procés-Verbaux, 
vol. v., p. 45). From a study of statistical data it 
is shown that although the intensity of fishing for 
plaice in the Kattegat, in consequence both of an 
increase in the number of boats and ef improvements 
in the fishing gear, has increased very greatly since 
1885, the actual weight of plaice landed has remained 
practically stationary from that year until 1904, the 
year for which the last statistics are available. It is 
only a rise of about 200 per cent. in the price of 
plaice that has enabled so many boats to continue 
profitable fishing; but although the total weight of 
fish has neither increased nor decreased during the 
period mentioned, there has been a marked decrease 
in the average weight per fish, so that the plaice 
harvest is now composed of a much larger number 
of smaller fishes than was formerly the case. 
Johansen, following out an idea previously developed 
by Petersen, shows that at the present time by far 
the greater number of plaice are captured before they 
have attained the size at which their value per unit 
of weight is greatest. 
Biological investigations carried out in the Kattegat 
have shown that there are important differences 
between the plaice populations in the northern and 
in the southern parts of the area. In the northern 
Kattegat the fish are large and well grown, and attain 
sexual maturity at a later age and larger size than 
those in the southern Kattegat. This change in the 
character of the fish population is correlated with 
the changes in the hydrographical conditions which 
take place as the Baltic is approached. The plaice 
supply of the southern Kattegat consists chiefly of 
small fish already sexually mature, that of the 
northern Kattegat of larger and more valuable fish 
which have not yet attained maturity. 
The principal nurseries for young plaice near the 
Danish coast have been investigated, and the fact 
that the rate of growth of the fishes on some of these 
nurseries, as determined both by marking experi- 
ments and by examination of otoliths, is abnormally 
low suggests that these particular grounds are over- 
crowded with young fish. This condition is not, 
however, found in either the Kattegat or Skagerak, 
and in the southern Kattegat the rate of growth 
during the first two or three years of the life of the 
plaice is as rapid as in the northern part, although 
in later years it becomes much less rapid. The ex- 
periments with marked fishes which have been carried 
out on plaice in the Kattegat have shown that far 
more than 50 per cent. of the plaice are re-captured 
each year, thus indicating a very high intensity of 
fishing. 
After a careful review of the evidence gathered 
from all the different lines of research, Johansen comes 
to the conclusion that the enforcement of a minimum 
size-limit for Kattegat plaice of 30 cm. (12 inches) 
would result in an increase in the value of the fishery 
amounting to from 50 per cent. to 100 per cent., 
although the exact figure can only be determined by 
experiment. Further, since in certain parts of the 
Kattegat only insignificant numbers of small plaice 
are found, but these grow rapidly, he thinks that 
the transplantation of large numbers of small fish 
from more crowded nursery grounds to such places 
would be worth a trial. 
A study of Johansen’s paper can hardly fail to 
leave the impression that a great advance has been 
made towards the solution of the more important 
problems connected with the plaice fishery of the 
Kattegat, and that promising practical schemes, 
based upon a rational understanding of the questions 
NO. T94I, VOL. 75] 
involved, are already in prospect for the improvement 
of that fishery. The Kattegat is a somewhat circum- 
scribed sea-area of moderate dimensions, and it will 
be scarcely surprising if the end to which all scientific 
fishery investigations are directed is first achieved 
there; but the reports before us show that work 
upon quite similar lines is being rapidly done on the 
plaice fisheries of the larger region, which may be 
described as the middle and southern North Sea. 
The summary of the investigations of the German, 
Dutch, and English naturalists, which is contained 
in the report of Dr. Garstang, the convener of Com- 
mittee B (‘‘ Reports of British Delegates,’’ vol. ii., 
p- 191), in that of Dr. Redeke (ibid., p. 265), and in 
the various statistical papers by Henking (ibid., p. 
127), Hoek (ibid., p. 300), and Kyle (ibid., p. 363, 
and especially Bulletin statistique, vol. i.), foreshadow 
as complete and satisfactory a solution of the problems 
in this area as has been, one might almost say, 
already achieved in the Kattegat. 
In the larger area the marked-fish experiments 
have already yielded much valuable information, 
and it ought not to be long before a fairly complete 
account is available of the normal migrations of the 
plaice. The transplantation of small plaice from the 
crowded nursery grounds along the coast to the 
shallow waters of the Dogger Bank, in the middle 
of the North Sea, has been attended with a high 
measure of success, the growth of the transplanted 
fish having been several times greater on the Dogger 
than on the inshore grounds. An experiment in 
transplantation upon a very much larger scale is the 
next step which appears to be called for. 
The plaice, however, is only one amongst the many 
fishes which have received attention. Much interest- 
ing worl has been done on the haddock and on 
the cod, although most of the reports so far pub- 
lished are of a preliminary character only (Hjort and 
Petersen, ‘‘ Reports of British Delegates,’’ vol. ii., 
Pp. 153): . a 
The herring, too, is receiving attention, and the 
statistical data brought together by Kyle (Bulletin 
statistique, vol. i., p. 228), with the accompanying 
charts, give a graphic picture of. the movements of 
the herring fleets, and therefore, presumably, of the 
fish themselves. The attempt already begun to corre- 
late these movements with changes in the hydro- 
graphical conditions will almost certainly yield 
valuable guidance to the herring fishermen, and 
ought to enable them to avoid much fruitless shooting 
of their nets. 
A striking piece of work is Dr. Johs. Schmidt’s 
contribution to our knowledge of the life-history of the 
common eel (Rap. et Proc.-Verb., vol. v., p. 137). 
Grassi and Calandruccio had already followed, from 
specimens taken in the Straits of Messina, the different 
stages in the metamorphosis of the eel larva from 
Leptocephalus brevirostris to the young elver, and 
they. had suspected that the natural home of the 
Leptocephali was in deep water, their occurrence in 
the Straits of Messina being due to the peculiar 
nature of the currents and the upwelling of water 
from the deeps. Schmidt’s researches, carried out on 
the Danish investigation steamer Thor, to some ex- 
tent confirm this view, rendering it at the same time 
more precise, and the spawning grounds of the Euro- 
pean eel and the home of the eel larvae are now for 
the first time made clear. It is along the edge of 
the continental slope, to the west of the British Isles, 
that the young eel larve (Leptocephalus brevirostris) 
are found in large numbers, in regions where the 
depth of the water is about 500 fathoms and the 
bottom temperature is at least as high as 7° C. The 
larvae themselves are not, however, near the bottom, 
