Monthly estimates of fish numbers using a long-range sonar

Abstract

Sonar records are available for the Perranporth area for 1963 to 1968, showing many wriggling tracks due to shoals of fish. The numbers of shoals, assumed to be composed of sardines, have been estimated for a sector between 4 and 37 km from the array. There is a marked seasonal dependence, with the largest numbers usually appearing from May to October. The density of shoals reached in the summer is typically three per km along the sonar beam and also three per km ² , though the lower limit of shoal size is ill defined. This translates with certain assumptions into about 2 × 10 ⁻³ sardinesper m ³ when dispersed, which compares well with other estimates. The pros and cons of long-range sonar for fish population surveys are contrasted with those of conventional echo-sounding surveys.

Author notes

present address: 77 Wyke Road, Weymouth, Dorset DT4 9QN, England

present address: 9 Beechtree Close, Cattistock, Dorset, England