•6 GEORGE V .SESSIONAL PAPER No. 38a A. 1916 



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REPORT ON THE LIFE HISTORY OF THE OOD AS DETERMINED FROM 



THE SCALES AND OTHER DATA. 



By R. p. Wodehouse, B.A., University of Toronto. 



This investigation was carried ouit in the summer of 1914: at the Marine Biology- 

 Station, St. Andrews, from the beginning of June till the beginning of September, 

 the object being to test the method of determining the age of a cod (Gadus callarias) 

 from its scales, to determine the rate of growth of the cod in these regions (Passa- 

 maquoddy bay), and the comparative frequencies of the different "year-classes" and 

 of the different " length-classes." Knowing these facts and the relation between the 

 weights and lengths, it is possible to decide at what *age it is most profitable to kill 

 the fish. Recent experiments and the experience of fishermen, in injudicious exploita- 

 tion of their stock, show that though the sea seems limitless, the stock of fish is by 

 no means inexhaustible. The tagging experiments of Hjort and others show that the 

 annual catch represents a very considerable proportion of the whole stock. In most of 

 these experiments rarely less than 20 per cent of the tags were recovered, and usually 

 •considerably more, and it is reasonable to suppose that the number of tags recovered 

 iears about the same relation to the number of fish tagged as the total number caught 

 does to total stock in the sea. 



Before proceeding to the results obtained, it will be necessary to explain the 

 xnethod of investigation. The greater number of measurements were taken at Gardi- 

 ner and Doon's fish market, St. Andrews. The firm were cordial and kindly in allow- 

 ing us to go there and examine their fish whenever tliey had the kinds we wanted, and 

 "were always ready in proffering information as to the locality and method of making 

 the catches. Two members of the staff of the Station co-worked, one to take the 

 notes and the other to make the measurements and take the scales. The cod were 

 selected as nearly at random as possible, laid on a board and measured with a centri- 

 anetre rule, and the length called 'out to the man keeping notes. 



The method of measuring was found to be slow and awkward, so a measuring 

 board was devised on which the fish could be laid and the measurement read off at 

 •once. The fish were always measured from the tip of the snout to the end of the 

 vertebral column, reading to the nearest centimetre. It was while on this measuring 

 ocard that the scales were -removed, these (with few exceptions) being taken from the 

 shoulder (usually the right) above the lateral line, forward of the first dorsal fin. The 

 slinie and loose scales were carefully removed from the part of the body from wliich 

 the scales were to be taken, then a few scales .(about 50 to 100) were removed with a 

 •clean scalpel and placed on a small piece of paper on which the number and length 

 'of the fish were marked by the note-keeper. The papers were then folded once and 

 put in the back of the note-book until we returned to the laboratory, when they were 

 allowed to dry until needed for mounting. 



In the laboratory the scales so obtained were removed from the papers on which 

 they were collected and soaked in water. There is always a great deal of slime, dirt, 

 ■and pigmented epidermis, that must be removed. The method found quickest and 

 most satisfactory by the author was as follows : After the scales had soaked for from 

 ■one to three days in fresh water, the water was poured off and replaced with a weak 

 •f olution of KOH (about 1 :4) in water. Tlie scales must be very carefully watched 



