FISHERY BULLETIN: VOL. 70, NO. 3 



amount observed (day or night) in each zone was 

 divided by the number of block area flights (day 

 or night) within the zone. The average number 

 of tons observed per block area flight for each 

 zone, the average number of tons observed for 

 all zones combined, and ratios of day and night 

 tonnages observed are shown in Table 4. Ratios 

 were obtained by dividing the tons per block area 

 flight (day) by tons per block area flight (night). 

 Therefore, ratios greater than 1.00 indicate 

 greater tonnage during the day, less than 1.00 

 indicate greater tonnage during the night. 



AVERAGE WEIGHT OF FISH SCHOOLS 



Average weight of schools was computed for 

 the period September 1962 through December 

 1966 from all data having estimates of individual 

 schools by weight. As previously indicated, 

 some tonnages were given by areas, not by num- 

 bers of schools and tonnages of each school. The 

 average tonnage per school is listed for each spe- 

 cies in Table 5. 



INDEX OF ANNUAL APPARENT 

 ABUNDANCE 



An index of annual apparent abundance was 

 calculated for observations during day and night 

 for each zone and for all zones combined from 

 September 1962 through December 1969 for the 

 northern anchovy. Pacific bonito, jack mackerel. 

 Pacific mackerel. Pacific sardine, yellowtail, and 

 Pacific barracuda. Marr (1951) defined the 

 term apparent abundance as "abundance as af- 

 fected by availability, or the absolute number of 

 fish accessible to a fishery." This definition of 

 apparent abundance most nearly describes the 

 type of index calculated in this paper. 



For convenience in calculating this index, four 

 arbitrary tonnage ranges were selected for each 

 species. Tonnage ranges for each species were 

 selected to cover the entire range of observed 

 tonnages that may occur in any one block area. 

 The midpoint tonnage of each range was divided 

 by 100 for the northern anchovy and by 10 for 

 Pacific bonito, jack mackerel. Pacific mackerel, 

 and Pacific sardine to provide a tonnage range 

 value (X) of convenient size to be used in the 



Table 4. — Day/night differences in tonnage and ratios 

 observed based on average tons observed per block area 

 flight in each zone for the period September 1962-1966. 



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