5=4 



NA TURE 



[April 2, 190S 



Rcibisch's discovery of the importance of these rings as 

 un indication of the age of the individual fish, Redecke, 

 Johansen, and Wallace have all independently proved the 

 correctness of his discovery. 



In the present paper Dr. Wallace applies the method to 

 determine the rate of growth of the plaice, its distribution 

 in relation to age, and other points of interest with regard 

 lo the habits and life of the species. 



Previous to the application of this method the only 

 means of getting approximately at the age of the fish 

 was by measuring the length of large numbers of 

 individuals and obtaining a frequency curve, but, as Dr. 

 Wallace points out, this method had many disadvantages 

 which are avoided by the otolith method. 



By examination of the otoliths, not only is the average 

 length for any age determined, but also the maximum 

 and minimum lengths are obtained. The only assump- 

 tion made is that all plaice emerge from the egg on 

 ."Vpril I ! Since the plaice only spawns once a year, and 

 the spawning season at the most lasts three months, the 

 error, owing to this assumption, is negligible. 



Having determined the " age-groups," Dr. Wallace 

 discusses their distribution in the area investigated, and 

 in connection with this is brought out the existence of 

 a " selective migration," that is, the distri- 

 bution or sorting out of individuals according 

 to size. The larger fish of the year tend 

 to move into deeper water than the smaller. 

 It is not a new observation that large fish 

 are found in deeper water, but age is a factor 

 which has not previously been taken into con- 

 sideration. At any depth are to be found fish 

 representing the larger ones of a recent year, the 

 average sized ones of an earlier year, and the 

 smaller ones of a still earlier year, and so on, 

 but the bulk of the fish of any one size will 

 belong to one particular year, so that at any 

 given depth we shall find the fish of one par- 

 ticular year dominant. 



Although this is the rule up to three or perhaps 

 four years old, these older fish seem to be affected 

 by other factors not yet sutificiently understood. 

 as on apparently similar grounds the average size 

 of the fish may differ considerably. 



In discussing the average growth-rate of the 

 plaice in the southern North Sea, Dr. Wallace 

 points out that there is a different average 

 growth-rate in the two sexes, and also that the 

 longevity of the female is greater than that of 

 the male. Up to five years of age the average 

 number of males and females is apparently the 

 same, but after that the males rapidly fall off in 

 numbers, and in plaice of eight years old and 

 upwards gi per cent, are females. The males 

 arrive at maturity one or even two years earlier Views c 



than the females, and after seven years prac- 

 tically cease to grow. 



We reproduce one of the excellent series of photographs 

 of otoliths taken by Mr. R. A. Todd, representing the 

 right otoliths of twenty plaice. The fish were 27 cm. 

 long, and the majority were four years old, as can be seen 

 from the otoliths, which nearly all show four white rings. 



Dr. \\'allace's paper is somewhat difficult reading owing 

 to the short paragraphs and frequent interpolation of 

 tables, diagrams, and references. A short summary cover- 

 ing all the ground would, we think, in the circumstances 

 have been specially useful. 



Mr. R. A. Todd has continued his researches on the 

 food of fishes, and some most valuable results are brought 

 out by his laborious work. First he notes the fact that 

 the younger fish of every species examined (thirty-four in 

 all) chiefly depend upon Crustacea for their sustenance, 

 amphipods, cumacea, and decapod larva; forming the chief 

 food supplies. 



Secondly, he shows that although in the young fish 

 competition between all species may be great, in the adults 

 competition is not nearly so keen, except in certain cases 

 which he mentions. The chief food of all fishes appears 

 to be fish, molluscs, and crustaceans, but a few species 

 seem to favour echinoderms and coelcnterates as supple- 

 NO. 2005, '^'<^L. 77] 



mentary supplies. Competition is avoided by each species 

 preferrmg some few particular species, which often seem 

 to form its staple diet. 



Mr. Todd's researches have led to some interesting 

 observations as to a fasting period peculiar to one or two 

 Pleuronectidae. It seems that this is connected with the 

 spawning period, as in the salmon, but in the case of the 

 plaice it is chiefly the male which abstains from food, 

 since on the spawning grounds " the greater the excess 

 of S over 9 the greater the proportion of empty 

 stomachs." 



The report upon the plankton of the English Channel, by 

 Dr. L. H. Gough, contains some interesting observations 

 as to the causes of distribution. In spite of the fact that 

 the minute organisms constituting the plankton are 

 primarily dependent for their distribution on the water 

 movements, there are two characteristic classes, the 

 oceanic, found only away from land, and the neritic, found 

 in shallow waters. Salinity was at first supposed to be 

 the determining factor, it being thought that the oceanic 

 forms could not survive in the fresher waters near the 

 coast, but Dr. Gough points out that these forms are 

 capable of withstanding a greater range of salinity than 

 is to be met with in the region explored. The most 



recent theory to account for this distribution is that the 

 decomposing organic matters in the shallower waters 

 where bottom fauna and flora flourish are poisons to the 

 oceanic species, w'hereas they are either innocuous to in' 

 possibly necessary for the neritic species. 



The fourth paper in the report deals with the surface 

 waters of the North .\tlantic Ocean south of 60° N. lat. 

 from September, 1904, to December, 1905. Mr. D. J. 

 Matthews explains that the paper is almost entirely de- 

 scriptive, giving an account of the distribution of surface 

 salinity and temperature over a period of sixteen months. 

 Samples of water have been obtained over a large area 

 through the assistance of the captains of a number of 

 steamers. 



Even during the short period covered certain striking 

 facts have been observed as to the movements of the 

 waters. For instance, a distinct waxing and waning of 

 the Labrador current has been detected. 



Monthly charts showing temperature and salinity for the 

 whole sixteen months add grcatlv to the interest of the 



form; 



a valuable contribution to science. 

 Frank Balfour Browne. 



