268 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



the vessels had spent all or a large portion of their 

 time seeking this species. Therefore, all trips con- 

 sisting of over 40 percent ocean perch were elim- 

 inated from the comparison. This amounted to 

 less than 10 percent of tlie catcli in Area XXII. 



The coefficient of correlation between the aver- 

 age catch per day of the 25 vessels and their gross 

 tonnages, -f-0.4033, was not statistically signifi- 

 cant. What correlation exists is undoubtedly due 

 to the linkage of gross tonnage to other factors, 

 treated below. 



Since these vessels all employ the same type and 

 size of otter trawl, regardless of differences in the 

 sizes of the vessels, the absence of a significant 

 correlation between size of vessel and fishing ability 

 is not surprising. Obviously, more important 

 factors are the amount of sea bottom covered by 

 the net at each tow and the number of tows made 

 each day. 



The amount of ground covered in a tow will 

 depend largely on the speed and power of the 

 vessel. Therefore, the catch per day was corre- 

 lated with the power of the vessels. Instead of 

 correlating catch per day directly with horsepower, 

 the ratio of horsepower to length was used as the 

 criterion of power, since the horsepower of a vessel 

 depends more on length than on tonnage. Also, 

 the use of horsepower directly, instead of the ratio, 

 does not give a true estimate of towing ability. 

 This correlation gave a statistically significant 

 coefficient of correlation of +0.75. 



Since the newer vessels take advantage of all 

 improvements in design and usually obtain tlie 

 best crews, it was suspected that age of the vessel 

 might play a part. The correlation of age of 

 vessel and catch per day gave a significant coeffi- 

 cient of —0.6643, showing that the newer vessels 

 were superior. 



However, as the newer vessels were often better 

 powered than the older vessels, it was necessary 

 to eliminate the effect of the other variable in 

 comparing the catch per day with either horse- 

 power-length ratio or age of vessel. 



The coefficient of partial correlation of catch per 

 day and horsepower-length ratio, with age of 

 vessel fixed, was +0.686. The coefficient of par- 

 tial correlation of catch per day and age of vessel, 

 with horsepower-length ratio fixed, was —0.497. 

 Squaring the two partial-correlation coefficients 

 shows that 47 percent of the variability in catch 

 per day was due to differences in the horsepower- 



length ratio of the vessels and an additional 25 

 percent of the variability was due to dift'erences in 

 age of the vessels, leaving only 28 percent of the 

 variability in catch per day unaccounted for. 



In obtaining a more accurate method of rating 

 each boat according to its fishing ability, both age 

 of vessel and horsepower-length ratio were taken 

 into account. For each boat, the amount in 

 standard deviations that it varied from the mean 

 of the horsepower-length ratio was obtained. 

 The same was done for age of vessel. The two 

 figures were then combined, but the age ratio was 

 weighted by 0.52, the ratio of its influence on the 

 catch to the influence of power. 



The correlation of this adjusted rating of the 

 individual boats with their catch per day of fishing 

 gives a correlation coefficient of +0.817. Squar- 

 ing the coefficient shows that the differences in the 

 adjusted ratings of the vessels accounts for 67 

 percent of the variability in the catch per day. 



This accounts for all but 33 percent of the vari- 

 ability in catcli per day, agreeing closely with the 

 28 percent shown by tlie two coefficients of partial 

 correlation. 



Because such a large proportion of the varia- 

 bility in catch per day is due to the age and power 

 of the vessel, it was obviously incorrect to intro- 

 duce new boats into the calculation. Therefore, 

 it was decided to reject the data from all vessels 

 except 16 that fished continuously from 1932 

 through 1938. The use of the same boats every 

 year meant that variations due to age and power 

 of vessel could be held to a minimum. Whether 

 the correlation between age of vessel and catch 

 per day was due to obsolescence or to the increased 

 efficiency of the newer boats cannot be deduced 

 from the correlation. It is safe to say, however, 

 that at least a large share of it is due to improve- 

 ments otlier than power in the design of the newer 

 boats. 



ADJUSTMENT FOR CATCHING ABILITY OF TWO 

 GROUPS OF OTTER TRAWLERS 



As a preliminary step in analyzing the catch 

 per unit of fishing effort in various areas and at 

 various seasons it was desirable to determine the 

 relative catching ability of the two groups of 

 otter trawlers. This was to make possible the 

 pooling of their catches so that one final curve of 

 abundance could be obtained for each area. 



