CLARK and BROWN: CHANGES IN BIOMASS OF FINFISHES AND SQUIDS 



TABLE 8. — Stratified mean catch per tow (kilograms) for selected species of finfish and squid, 

 Albatross IV autumn bottom trawl survey data, 1967-74, Middle Atlantic, southern New 

 England, Georges Bank, and Gulf of Maine (strata 61-76, 1-30, and 36-40). 



'Less than 0.05. 



2 Does not Include data for tunas, sharks, swordfish, American eel. or white perch. 



TABLE 9. — Stratified mean catch per tow (kilograms) for selected species of finfish and squid, Albatross IV autumn bottom trawl 

 survey data, 1963-74, southern New England, Georges Bank, and Gulf of Maine areas (strata 1-30 and 36-40). 



'Less than 0.05. 



2 Does not include data for tunas, sharks, swordfish. American eel, or white perch 



3 Data not recorded. 



"Squid catches for 1964-66 prorated by species according to relative percentages caught in later years. 



not reflect the relative magnitude of various 

 species within the biomass as a whole. For in- 

 stance, herring and mackerel together appear to 

 have constituted over 50% of the biomass present 

 during this study (Edwards 1968; International 

 Commission for the Northwest Atlantic Fisheries 

 1974e, footnote 17) yet account for less than 1% of 

 the weight taken in autumn bottom trawl surveys. 

 Furthermore, the aggregated distribution of 

 finfishes and squid in nature, and the behavior of 

 the gear employed, insure that catch data for 

 individual species will seldom be normally dis- 

 tributed but rather will tend to conform to the 

 negative binomial or some other contagious form 

 (Taylor 1953). In the following sections, we utilize 

 selected transformation and weighting procedures 

 in attempts to correct for these factors. 



^International Commission for the Northwest Atlantic 

 Fisheries. 1975. Report of the herring working group, April 

 1975. ICNAF Annu. Meet. 1975, Summ Doc. No. 19, Serial No. 

 3499 (mimeo.), 31 p. 



Weighted Analyses 



Catchability differences among species imply 

 that trends in biomass as defined in this study will 

 be primarily determined by trends for species most 

 vulnerable to the survey gear unless adjustments 

 in terms of catchability are made. Accordingly, we 

 developed catchability coefficients by year for the 

 species and species groups in Tables 8 and 9 for use 

 in computing weighting factors by relating 

 stratified mean catch per tow by stock to available 

 estimates of stock size, all computations being in 

 terms of weight. Annual estimates of stock size 

 (weight at the beginning of year i) were required 

 for this purpose for each individual stock for which 

 TAC's have been established (International 

 Commission for the Northwest Atlantic Fisheries 

 1975c); thus, separate estimates were required for 

 cod in 5Y 18 and 5Z, haddock in 5Ze, silver hake in 



18 Alphanumeric designations refer to divisions and sub- 

 divisions of SA 5 and 6 given in Figure 1. 



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