ESTIMATING ABUNDANCE OF GROUNDFISH ON GEORGES BANK 



269 



The date for this comparison were ohteinc<l by 

 determining the ratio of the catches of the larger- 

 sized vessels (group B) to the smaller-sized vessels 

 (group A) for each month in each individual sub- 

 area and depth zone for each year during the 

 period 1932-38, whenever each group was repre- 

 sented by not less than 20 days of fishing. The 

 result gave us 52 ratios for comparison. Because 

 the final desideratum was a measure of abundance 

 for all years and seasons, regardless of the amount 

 of fishing conducted therein, these ratios were 

 used without weighting. 



For Subarea XXII J (medium depth), ratios 

 were available for 6 years (all except 1934) for 

 the months of July, August, and September. 

 Testing the variance between the means of the 3 

 months against the variance within months (see 

 Snedecor, 1940) showed no statistical difference 



in variance (F= 



198.4 



=6.30, whereas P of 0.05 = 



31.5 



19.42). 



This indicates no seasonal difference in the ratio of 

 catching ability of the two groups of vessels dur- 

 ing this period. 



A further test was applied to the same data 

 (Snedecor 1940), in which the variance between 

 the means of the 6 years was compared with the 

 variance remaining after accounting for that be- 



tween the means of the 3 months (i^= 



303.5 



=2.08, 



145.9 



whereas /' of 0.05=4.75), 



which showed that the differences between the 

 years were not significantly greater than could be 

 expected from random sampling. 



As a final test, all 52 ratios were grouped into 

 7 annual samples. The proposition that the dis- 

 tribution of all years could have been drawn from 

 the same population by random sampling was 

 then tested by cx)mparing the variance between 

 the means of the annual samples with the variance 



355 

 within the samples (^'=,r— =1.41, whereas P of 



Zoo 



0.05 = 2.31). 



The result showed that there was no significant 



difference between the mean ratios of different 



years. 



As the data do not indicate any significant 

 seasonal or annual differences in the ratios of 

 fishing ability of the two groups, the average 

 ratio of all 52 comparisons, 0.887 ±0.0217, has 



been used as the ratio. In order to pool the 

 catches for the two groups of otter trawlers, the 

 number of days fished by group-B boats has been 

 decreased by 11 percent to make the fishing days 

 comparable to group-A fishing days. 



DEPTH ZONES 



SELECTION OF THE ZONES 



The depth of water sharply limits the distribu- 

 tion of many species. Obviously, the abundance 

 of such a species cannot be accurately estimated 

 by using the unweighted average catch per fishing 

 day if the species varies in abundance according 

 to depth. For this reason, the data have been 

 analyzed by depths. 



It was impractical to divide the banks bj' nar- 

 row depth bands, because the vessels usuallj- fish 

 over a range of several fathoms. After making 

 preliminary plots of the depths fished by large 

 otter trawlers, it was decided to employ three 

 depth zones: shallow, to 30 fathoms; medium, 

 31 to 60 fathoms; and deep, 61 fathoms and more. 



As a check on the validity of the three bands 

 selected, the depths that the captains or mates 

 hailed as having been fished were plotted for 1933 

 and 1938 for a selected group of lai^e otter 

 trawlers in Area XXII South (Geoi^es Bank and 

 South Channel). The number of days fished (to 

 the nearest tenth) was allocated to each 5-fathom 

 zone. Thus, if a boat fished for 5 days in water 

 from 55 to 65 fathoms, 2% days were credited to 

 the 56- to 60-fathom depth category and 2% days 

 to the 61-to-65-fathom depth. An overlap of 

 only 1 fathom into another 5-fathom zone was 

 disregarded, because the liailcd depths, being only 

 estimates, are not accurate within narrow limits. 

 Figures 2 and 3 show, for these two samples, both 

 the total number of days fished (as hailed) in each 

 5-fathom depth category and the number of days 

 classified as shallow, medium, and deep, as used 

 in our study. 



Variations between the depths hailed and the 

 depths used arc due to three factors: (1) Minor 

 inaccuracies in depth hails; (2) the fact that a 

 vessel occasionally fished chiefly in one zone but 

 also fished for a small portion of the time in 

 another depth zone, without being able to give 

 sufficient information about the species caught at 

 each depth to permit splitting of the trip into 



