Daily net emigration from a spawning concentration of chokka squid (Loligo vulgaris reynaudii d'Orbigny, 1845) in Kromme Bay, South Africa

The net emigration from a spawning concentration of the loliginid squid, Loligo vulgaris reynaudii d'Orbigny, 1845, was investigated quantitatively using a combination of tagging and hydroacoustic techniques, midwater trawling, purse seining and SCUBA diving. Aquarium experiments were used to supply additional information on tag loss and mortality from tagging. The number of squid in a concentration containing T lagged individuals was assessed hydroacoustically and then c squid were caught by means of a midwater trawl or purse seine. It was assumed that the t(n) tagged squids in these catches were the result of natural mortality, tagging mortality and net stability of the concentration (the result of emigration and immigration) n days after tagging. The average net emigration was then calculated to be 0.2 per day for 7.5 days of observations. This result is a first attempt to solve the so-called "Hilborn's problem", i.e. the problem of how to measure the aggregation dynamics of nektonic organisms in the sea (instead of modelling). (C) 1998 International Council for the Exploration of the Sea.