MODELING EFFECTS OF FADS AND ISLANDS ON MOVEMENT OF SKIPJACK TUNA (KATSUWONUS-PELAMIS) - ESTIMATING PARAMETERS FROM TAGGING DATA

From an experiment with ordinary dart tags, we have found evidence of the effect of fish-aggregating devices (FADs) and of islands on the mo vements of skipjack tuna (Katsuwonus pelamis) around the Solomon Islan ds. By fitting a fish movement model to the tag data, we were able to estimate mortality and movement parameters (including diffusivity), pa rameters of a function that models FAD attraction, and a separate para meter of island attraction. Diffusivity was high enough to effectively distribute fish throughout the island archipelago (approximately 150 000 km(2)) within a few months. Estimates of FAD parameters indicate t hat the presence of up to four or five FADs in an area approximately 5 0 x 50 km can reduce the propensity for skipjack to leave that area by approximately 50%, but that deploying additional FADs in such an area does not significantly increase their effectiveness in holding skipja ck. Estimates of the island attraction parameter imply that the propen sity of skipjack for movement away from the archipelago is less than h alf the propensity for movement within it.