Apparent abundance of some pelagic marine fishes off the southern and central California coast as surveyed by an airborne monitoring program Fishery

Abstract

From September 1962 through December 1969, commercial aerial fish spotter pilots esti­ mated tonnage of species observed during flights off the southern and central California coast. Observations of fish and the aircraft's flight route were recorded on special charts. These data were analyzed using 10-minute-longitude by 10-minute-latitude ,areas. A total of over 17,593 flight hours was involved, surveying 57,628 block areas-37,186 during the day and 20,442 during the night. Data from each block area were used to compute diurnal and nocturnal variation in apparent abundance and an annual index of apparent abundance. Pacific bonito, Sarda chiliensis, and yellowtail, Seriola dorsalis, were observed in greater frequency and quantity during the day, and the northern anchovy, Engraulis mordax; jack mackerel, Trachurus .~ymmetricus; and Pacific mackerel, Scomber japonicus, were observed in greater frequency and quantity during the night. Pacific barracuda, Sphy­ raena argentea, was observed in greater quantity at night but more frequently during the day. Between 1963 and 1969 indexes of apparent abundance declined for jack mackerel, Pacific mackerel, Pacific sardine, Pacific bonito, Pacific barracuda, and yellowtail and increased slightly for the northern anchovy. The index closely follows estimates of total abundance for the Pacific mackerel, a species for which reliable estimates of total abundance are available. From observations of the catch trends in the bonito fishery, the index appears to be little affected by changes in economic demand. Its trends in apparent abundance are evident before they are reflected in catches and are useful in the evaluation of catch variations in underutilized resources.