SEA- FISHERIES LABORATORY. 233 



decked sailing vessels ; by trawl-nets, seine-nets, stake-nets, 

 trammels, etc., and so we must have some idea what all this 

 variety of catching power means when it is reduced to a 

 " common denominator." A steam trawler will catch more 

 fish per day than a smack and a smack will catch more than a 

 half-decked sailing boat. But does the steam trawler catch 

 more fish per unit of man-power, or per £ invested in her 

 maintenance than does a smack ? And which rate — the rate 

 of catch per day, or per man, or per £ ought we to adopt ? 



The ratio of steam vessels to smacks that work on a certain 

 fishing ground is not always the same and we cannot, usually, 

 neglect the fishing by half- decked sailing vessels and motor- 

 boats. What, then, is to be the "common denominator"? 

 We may calculate how many small trawlers are equal in 

 catching power to one smack, and then how many smacks 

 equal one steam trawler. Thus we can express the catching- 

 power in ideal vessels, or " fishing units," or we might try to 

 calculate the number of hauls made per week or day and then, 

 knowing the average size of the trawl-nets used, calculate the 

 number of square miles of sea bottom swept per day. Any 

 sort of calculation made in these ways would be a rough one 

 since we have not much exact knowledge of the conditions. 

 In practice, what is done is generally to calculate the average 

 quantities of plaice caught, per day's absence from port, of 

 an average steam trawler or smack. If this decreases we say 

 that the productivity of the fishing grounds worked also 

 decreases — noting all the while that our quantity of fish 

 caught is fish of a certain, chosen range of sizes. Obviously 

 the results are rough ones in any case and too much strain 

 must not be put on them. Whatever changes in the results 

 of fishing we observe must be big enough to be much the same 

 (that is, to show much the same tendencies) in whatever way 

 we estimate the change in catching power. Tims the quantity 

 of plaice annually landed in England, from the North Sea 



