266 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



was calculated to the nearest tenth of a day. 

 The time spent away from port was consumed 

 partly in the voyage to and from the bank. To 

 discount this, the groups of otter-trawl vessels 

 selected for study were questioned about their 

 average speed under working conditions, and in 

 many cases this was checked for various voyages 

 by means of radio reports. Tables were made to 

 show the time (to the nearest tenth of a day) that 

 it would take a vessel to make the trip from port 

 to each statistical subarea (see fig. 1.) at the 

 average speed of the selected group. The time 

 the vessel was absent from port, minus this running 

 time, and also minus any time lost by bad weather, 

 . . ., gave the calculated number of fishing days 

 for each trip. On the average, these calculated 

 fishing times were found to agree with the esti- 

 mated fishing times obtained from the interviews 

 but they were used instead of the estimated times 

 given in the interviews because they wei-e con- 

 sidered less subject to personal judgment. 



In the preceding calculations, the distance was 

 measured from port to a point in the subarea 

 empirically selected as being nearest to the avei'age 

 position fished in that subarea (as shown by plots 

 of many fishing positions over several years). 

 Wlien the vessel fished in two subareas that 

 extended in the same direction from port, only the 

 voyage to and from the most distant of the two 

 subareas was used. When more than one subarea 

 was fished and the subareas were not in line, the 

 running time was taken from port to one subarea, 

 then between subareas, and finally from the last 

 subarea back to port. 



When a vessel fished in more than one subarea, 

 the calcu'ated fishing time was divided between 

 the subareas in the same proportion as the esti- 

 mated fishing time given in the interview, except 

 that when the estimated and calculated times did 

 not agree and the estimated time in a certain sub- 

 area was only 1 day or a fraction of a day, this 

 estimated time was considered correct, and adjust- 

 ment was made in the time for the subarea or sub- 

 areas in which more fishing was conducted. Al- 

 though this approach is not easily susceptible of 

 statistical proof, it is obvious that the estimates 

 of the shorter periods of time are much more apt 

 to be correct than those of the longer periods. A 

 mate may easily be uncertain whether they fished 

 6 days or 7 days in a subarea, but an estimate of 

 12 hours is seldom far off. 



In some cases, the mate did not remember the 

 number of hours spent in a subarea in which the 

 vessel did little fishing but knew the number of 

 tows made by the otter trawl. In these cases, 

 each tow was considered as an estimated one-tenth 

 of a fishing day. This estimate is predicated on 

 the number of tows per day by large otter trawls, 

 as indicated by careful notes and logs kept by 

 several vessels for W. C. Herrington. These data 

 showed that on the average there were 10 tows per 

 day. 



SELECTION OF OTTER-TRAWL VESSELS FOR 

 DETERMINING RELATIVE ABUNDANCE 



The first step in obtaining the catch per day 

 was to select two groups of Boston otter trawlers, 

 each group fairly homogeneous with respect to 

 size of vessels. The first group of 12 large (over 

 150 gross tons) otter trawlers ranged in size from 

 163 to 173 gross tons, with an average of 167 gross 

 tons. The second group of 13 vessels ranged from 

 229 to 262 gross tons, with an average of 247 gross 

 tons, or 48 percent larger than the first group in 

 average size. However, after the data on catch 

 per day were tabulated, it was found that the 

 selection of these groups on the basis of gross ton- 

 nage was apparently erroneous. In order to de- 

 cide on the proper basis for selection, all 25 boats 

 were compared for the year 1938. 



The levels of fish abundance differ considerably 

 between the New England and Nova Scotia banks; 

 therefore the comparison of fishing ability was con- 

 fined to the New England banks, which accounted 

 for 57 percent of the season's catch. 



In making tliis comparison, it was found that 

 some of these boats did considerable fishing for 

 ocean perch, while others did little or none. As 

 this is a specialized fishery that yields a far greater 

 poundage per unit of fishing effort, it was necessary 

 to eliminate this cause of variability in order to 

 obtain a valid comparison. Tabulation of the 146 

 trips or portions of trips made in the deep waters 

 (more than 60 fathoms) of Subareas XXII, F, G, 

 and H in wliich ocean perch were taken, showed 72 

 instances in whicli over 80 percent of the catch 

 consisted of ocean perch; these trips averaged 95 

 percent ocean perch. Another 29 trips had be- 

 tween 41 and 80 percent ocean perch and averaged 

 58 percent, and 45 trips had from 1 to 40 percent 

 ocean perch and averaged 16 percent. Obviously, 

 on the trips vvitli a higli percentage of ocean perch 



