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sideration of protective measures the fishery catch-samples and the market samples 



are the most important; these two last-named being indeed, in principle, of equal value. 



Examination ot the catch-samples shows us the composition of the quantities of plaice 



actually caught by the fishermen, with regard to age and size, while the market-samples 



on the other hand, give the corresponding composition of the quantities of plaice brought 



to market and utilised for human consumption. From the diff'erence between the two 



analyses we learn the quantities and sizes of plaice which though caught, are thrown 



overboard again as valueless, and thus for the most part uselessly destroyed. 



The investigation of plaice samples consists in determination of the length in 



centimetres, the weight in grammes, sex, and age of each individual fish. And with regard 



to this, the following points should be borne in mind. The actual weighing can, in 



the case of large samples with many specimens, be omitted, being then replaced by a 



theoretical calculation of the weight {^) from the length (/) (full length from snout to 



Fk 

 tip of tail) the weight g (in grammes) being on an average = where k represents 



a coefficient, varying somewhat according to the time of year, which can in most 

 cases, for fresh entire plaice, without any essential error be reckoned as ^ i. The 

 determination of sex of the plaice is very easily made; it is, however, unfortunately 

 seldom done in the case of market samples. The age of the plaice can be determined 

 with considerable accuracy by means of the annual rings of the otoliths and of the 

 bones; this would however, in the case of large samples, often take too much time to 

 permit of its being calculated for each individual plaice. 



Any further calculations based upon plaice samples thus dealt with will depend 

 upon how far the composition of a sample can be regarded as equivalent to the com- 

 position of the whole quantity of plaice from which the sample was taken, or, to use 

 the current phrase, whether the sample is representative of the whole. The necessary 

 conditions for such representative value in a sample are: i) that all the various size- 

 classes were well mixed together in the quantity from which it was taken, 2) that the 

 sample is not too small, and taken in such a way that no particular selection of certain 

 sizes could occur; the last condition being the most important. Thus a sample taken, 

 for instance, by the ordinary trawl, on a fishing ground where many small plaice of the 

 second and third years are found, i. e. of 10 — 20 cm. in length, cannot be regarded as 

 a representative sample of the stock of plaice, as the net here makes a selection, 

 allowing the greater number of the small plaice to escape. If on the other hand, a 

 market sample for instance, is to be regarded as representative for the composition of 

 the landings at a certain port from a particular area in a given month, say at Grimsby 

 from Area B4 in June, then such market-sample must contain the various market- classes 

 of plaice, as "large", "medium", "small", in the same relative quantities and mixed to 

 the same extent as they appear in the total quantity of the landings. 



A proper selection of the plaice samples to be examined is thus of decisive im- 

 portance for the reliability of all our calculations bearing on the plaice question. If 

 there is reason to suppose that a good selection of samples has been made, these can 

 then be regarded as having a considerable representative value, the extent of which 

 again depends upon the size of the sample, and can be determined, by means of ma- 

 thematical calculation, from the composition of same. 



