A METHOD OF ESTIMATING ABUNDANCE OF GROUNDFISH ON 



GEORGES BANK 



By George a. Rounsefell, Fishery Research Biologist 



In studying the fluctuations in iitjundiinc.e of 

 various species tliat comprise the catcli, it is of 

 paramount importance to know how tlie abun- 

 dance of each species usually vaiies from l)ank to 

 bank and from depth to depth. When vessels arc 

 fishing chiefly for a particular species, they seek 

 the grounds and the depths at which that species 

 is most easily taken in abundance. For such a 

 species, the catch per unit of fishing effort will 

 measure the relative abundance with considerable 

 accuracy, since the vessels will shift to grounds 

 and depths yielding the highest catches. For 

 other species, however, the fluctuations in actual 

 abundance cannot be measured without sufficient 

 knowledge of their average relative density in 

 different depths and on different grounds. There- 

 fore, a study of the distribution of these other 

 species by depth and fishing grounds is a necessary 

 preliminary to a study of their annual fluctuations 

 in abundance. 



Knowledge of the relative density of each species 

 by fishing grounds is of considerable value from 

 other standpoints. What effect is a fishery in any 

 certain area likely to have on the stock of each 

 species? In certain cases the question arises: 

 What effect will a change in the size of the mesh 

 of the trawl have on the catches? Only by 

 knowing the density of each species by areas and 

 depths can these questions be answered. 



For many species not extensively sought for 

 economic reasons, it is desirable to know whether 

 there is a possibility of the catch being increased, 

 should it become desirable to increase production. 

 There is also the problem whether the range of a 

 species is wholly covered by the fishery or may 

 extend to areas beyond. 



Tlic methods developed in this paper have been 

 followed by the haddock investigation of the N'oi'th 

 Atlantic Fishery Investigations in computing 

 indexes of abundance from 1931 to 19.'):i. 



MATERIAL 



To obtain a measure of the relative density of 

 each species it was necessary to ascertain the 

 quantity caught by certain units of fishing effort. 

 Collection of the data necessary for this study was 

 started in the fall of 1931, at the Boston Fish Pier, 

 and IS continuing. In 1942 this collection was 

 extended to the ports of Gloucester and New- 

 Bedford, Mass.; in 1953 it was extended to 

 Provincetown, Mass., and Rockland and Portland, 

 Maine. A full description of the methods of 

 collection is given by Rounsefell (1948). 



The essential data collected for each vessel 

 interviewed are as follows: 



1. Name of the vessel, and type of gear employed. 



2. Day and hour of departure and of arrival at port. 



3. Positions fished, by "unit" areas, each unit comprising 

 a rectangle of 10' of latitude and 10' of longitude, or about 

 10 miles by 7-plus miles. 



4. Depth, in fathoms, at each fishing position. 



5. E.'itimated amount of the catch, in thousands of 

 pounds, at each fishing position. 



6. Estimated proportion of each species taken on 

 different fishing grounds. 



7. For line-trawl vessels, the number of tubs of gear set 

 out at each fishing location. 



8. For otter-trawl vessels, an estimate of the time spent 

 on each fishing ground. 



9. For otter-trawl vessels, the total amoimt of time 

 lost on the trip (other than the usual running time to and 

 from the banks) because of such occurrences as torn nets, 

 engine trouble, or stormy weather. 



CALCULATION OF CATCH PER FISHING DAY FOR 

 OTTER-TRAWL VESSELS 



In order to obtain for otter-trawl vessels a 

 measm-e of fishing effort more or less independent 

 of weather, distance traveled . . ., it was found 

 desirable to calculate the amount of time the 

 vessel actually spent in fishing while on the fishing 

 grounds. P'rom the data available, the actuid 

 number of days the vessel was absent from port 



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