ESTIMATING ABUNDANCE OF GROUNDFISH ON GEORGES BANK 



275 



of a unit of fisliiiifj cfrort may give close estimates 

 of the iihutulaiu'e. However, for the species that 

 are eau<rlit incidentally while fishinjj for anotlier 

 species, no such assumption can he made without 

 careful study. Shifts in the depth or locality fished 

 in pursuit of the principal species may yield 

 changes in the catch per unit of fishing effort of the 

 minor species that cannot be interpreted as changes 

 in abundance witliout a knowledge of the depth 

 and areal distribution of those species. 



In order to discover the relative abundance of 

 each species in different depth zones, the catch per 

 unit of fishing effort was calculated for each 

 species, in each depth zone, for each year from 

 19:^2 to 19.38. In this analysis, only "pure" trips 

 were used — that is, trips in which the entire catch 

 Mas taken in the same depth zone and subarea. 

 This was essential, because in "mixed" trips one 

 must depend wholly on the fisherman's recollection 

 of what proportion of each species was caught in 

 each ilepth zone. Because of the variety of fishes 

 in a normal catch it is impractical to obtain this 

 information for all of the minor species. 



In analyzing these data, only the Georges Bank 

 area was used for two reasons: (1) The data 

 from other areas were too scattered; and (2) it 

 was hoped that, by confining the analysis to a 

 relatively small area, variations in the data 

 arising from including various populations of the 

 same species would be reduced to a minimum. 



For the shallow zone (0-30 fathoms), all of 

 the shallow fishing in Subareas XXII H, J, M, N 

 is included. This forms the shallow areas of 

 Georges Bank. 



For the medium depth zone (31-60 fathoms), 

 Subareas XXII H, J, and M are included. The 

 fishing on Subarea XXII N was only occasional, 

 so this subarea was excluded. The medium-depth 

 fishing in Subareas XXII G and XXII O was 

 excluded because of differences in conditions and 

 populations. For instance, the "blackbacks" in 

 Subareas XXII H, J, and M are young lemon 

 sole, while in Subareas XXII G and XXII O 

 they are the true blackback. 



For the deep zone (more than 60 fathoms), 

 .Subareas XXII G, H and J are included. 



In order to discount changes in abundance or 

 availability of different species during the 7-year 

 period (1932-38), it was decided to average the 

 ratios between the catch per day in each depth 

 zone, not the actual catch per day. The following 



tabulation shows the number of standard days 

 fished and the tola! catch in each depth zone 

 during each year. 



Since the medium-depth zone was weU repre- 

 sented in each year, it was used as a standard, 

 and the ratio of the catch of each species in both 

 shallow and deep zones to the catch in the medium - 

 depth zone was calculated for each j-ear. 



The geometric means of the ratios were tiien 

 calculated. Thus if 



a = catch of a species in shallow (or 



deep) zone, 

 6 = number of standard days fished in 



shallow (or deep) zone, 

 t=catch of a species in medium-depth 



zone, 

 m=number of standard days fished in 



medium-depth zone, 

 !/=any individual year, and 

 j"=geometric mean ratio of availability 

 of a species in shallow (or deep) 

 zone to the availability in the 

 medium-depth zone. 



then 



log 



+ 



+log 





y = l 



There might be some question about the ad- 

 visability of using an unweighted average instead 

 of a weighted average in obtaining these geometric- 

 mean ratios. At this point it must be remembered 

 that in such a chronological series, weighting of 

 the data (thus giving much more weight to certain 

 years) may introduce a bias which we cannot 

 measure. Using the logaritlims of the ratios, an 

 imalvsis of variance was made, which showed no 



