

NA TURE 



[September 9, 1909 



but in the majority of cases Wallace found that the males 

 are sexually mature (four to five years) a year before the 

 female is capable of spawning (five to six years). We can 

 now correlate age with size and with weight. 



The migrations of the plaice and of other fish and their 

 rate of growth depend, amongst many other factors, upon 

 their food supply. And the nature of the food of fishes 

 lias recently been re-investigated in the North Sea. I give 

 some of Todd's results, which were made by the examina- 

 tion of some thousands of fish of thirty-one species. Of 

 these I select three — the cod, the plaice, and the dab. 

 Percentages of stomachs conlaiiung various kinds of food. 



Cod. 



These tables show what, of course, was more or less 

 known before, that as a rule the young fry live very 

 largely, and in many cases solely, on Crustacea. To a 

 great extent the supply of suitable food dominates the 

 movement of the young fry, for nowhere is the truth of 

 the Frenchman's definition of life, " I eat, thou eatest, 

 he eats," with its terrible correlative, " I am eaten, thou 

 art eaten, he is eaten," more true than in the sea. Later 

 in life the fishes' taste alters, and with increased size 

 they can tackle animals the calcareous deposits of which 

 would seem to render them highly indigestible. 



Very careful investigations have been made, and are 

 being made, by Mr. Borley and Mr. Todd as to the dis- 

 tribution of the fauna of the middle and southern parts 

 of the North Sea, and its relation to the depth of water, 

 the varying degree of salinity, and to the texture of the 

 bottom deposits. These results, however, have not been 

 published, but 1 may go so far as to say that the inquiry 

 shows that within the area investigated the texture of the 

 sea floor has, on the whole, more influence on the dis- 

 tribution of the invertebrates of the bottom fauna than 

 has depth, and that depth in the area in question seems 

 to have more influence than salinity. 



With regard to the character of the bottom deposits, it 

 has been found by Mr. "Borjey that off shore and on the 

 gently shelving continental coast the sea bottom is of a 

 uniform character over wide areas, though on the western 

 side it is more patchy ; and it has proved possible to divide 

 the samples taken into some nineteen main types, each 

 characteristic of one or more of the areas into which the 

 region has been split up. Only one or two details of this 

 laborious work can be mentioned. One is that the texture 

 or degree of coarseness of the ground in various parts of 

 tlie sea is such as to suggest that the distribution of the 

 finer grades of material, the finer sands and silts, is 

 greatly influenced by the joint action of currents and tides. 

 It is, for instance, known that in the southern part of 

 the North Sea the main direction of the bottom current 

 is to the north and then to the east ; and examination of 

 the deposits shows a regular diminution in the proportion 

 of the coarser sands, a regular increase in the proportion 

 of finer material, as we proceed from the Straits of Dover 

 in a north-easterly direction. A remarkable fact in this 

 connection is the complete absence of silt from the sandy 

 bottom west of the mouths of the great rivers Rhine and 

 Maas. There can be no doubt that the presence of broad 

 NO. 2080, VOL. 81] 



and shallow stretches of sand on the Continental, but not 

 on the English, side of the North Sea is one of the factors 

 which has determined the distribution of the small plaice, 

 which on the Continental shores are so extraordinarily 

 abundant, and on the English shores are relatively so scarce. 



By means of bottles weighted with shot, so as to have 

 about the same specific gravity as the surrounding sea 

 water, Mr. G. P. Bidder has been able to trace slow 

 currents moving over the bottom of the sea The bottles 

 are closed, and contain a postcard in many languages 

 offering a reward to whosoever returns the postcard, re- 

 cording the latitude and longitude of the place it was 

 trawled at, to our laboratory at Lowestoft. Attached to 

 the neck of the bottle is a copper wire ij feet long. This 

 wire trails along the bottom, the bottle itself floating 

 about ij feet above the level of the ground. Slowly as 

 the bottles are swept along yet the distance they cover is 

 sufficient to sharpen the free end of the wire to a needle 

 point. 



By these and by other methods it has been possible to 

 trace the almost imperceptible but steady flow of waters 

 along the bed of the sea. Without doubt these currents 

 influence the distribution of the larval and young forms 

 of all the creatures which live near the bottom, and 

 especially influence the migration of food-fishes in their 

 younger and less active stages, when they are swept help- 

 lessly along. 



But these bottles have a double lesson to teach us : not 

 only do they enable us to chart the slow streaming of the 

 bottom water, but they give us to some extent a measure 

 of the intensity of trawling in the North Sea. They have 

 been re-fished in really surprising numbers. Commercial 

 trawlers have re-taken them at the rate of 58 per cent, 

 per annum. In one area these bottles cast upon the 

 waters were re-taken, not after many days, but after very 

 few. Out of 390, eighty-five were recovered in six weeks, 

 and fifty out of 270 were trawled in five weeks, represent- 

 ing a local intensity of fishing which, if continued, would 

 give us between 80 per cent, and 90 per cent, of re- 

 captures in a year. 



Marked fish which have been liberated and re-captured 

 tell the same story of intensity of fishing. 



The intensity of fishing as indicated by the percentage 

 of re-captures within twelve months of liberation is shown 

 by the following table ' : — 



Percentage 

 Fish under Over 

 Offshore 25 cm. ?s rm. 



Dutch coast 237 20-? 



Deep water, Southern Bight ... I'^'O 26'6 



Leman Ground (liberattd .\ptil and May) 1S7 I7'4 



Lenian Ground (libeiaied December) ... — 2i'o 



, Horn Reef outer ground 33'3 23^0 



Obviously, since some fish are known to have been 

 captured but not returned to the laboratory, the method 

 gives a minimum estimate. 



By applying the same method to the marking experi- 

 ments of other countries as well as our own, Garstang ' 

 gave the percentage recovered within twelve months of 

 liberation of fish more than 25 cm. in length as from 

 4 per cent, on the Fisher Bank to 56 per cent, in the 

 Skager Rak. 



When we reflect on the chances of these marked fish 

 dying or being eaten or losing their labels, it is surely 

 a most remarkable fact, full of significance to the practical 

 man, that in the North Sea marked fish of marketable 

 size are recaptured at the rate of between 20 and 30 per 

 cent, each year, and sometimes at a greater rate. It 

 would seem that each square yard of the fishing grounds 

 is swept by the trawl, not once, but again and again each 

 year. 



Mr. Borley has conducted a large series of experiments 

 to determine the vitality of fish after they have been cap- 

 tured by both the beam and the otter-trawl. It was 

 necessary to determine the degree of injury caused by the 

 actual trawling, the raising of the trawl, and the sub- 

 sequent exposure on deck. The larger fish of both sexes 



1 Garstang, " North Sea Fisheries Investigation Committee Southern 

 Area," Report No. i- 



- '* Provisional Report on the Natural History of the Plaice " (Com- 

 mittee B). Proct's veyhaHx, vol. ilL. 



