OBJECTIVES: a. To determine the efficiency of Beam, Triple and Prawn
trawling methods for the harvesting of scatlops.
b. To investigate the difference in efficiency between the
sputnik dredge and the Japanese Keta-ami dredge.
c. To investigate the difference in bottom damage between the
sputnik and Keta-ami dredge.
d. To study dredge and trawl net behaviour using CSIRO
underwater surveillance equipment.

The principal aims of this study were to evaluate whether the locally­ produced Whayman-Holdsworth trap provides an effective method for minimising Asterias amurensis infestations on shellfish farms, and to objectively assess the value of seastar traps when used in commercial applications.

In an initial fishdown experiment, the efficiency of seastar traps was assessed at sites with low/ moderate and high densities of seastars. Intensive trapping effort directed at the low site failed to control the seastar population within the trap field, even though 1160% of the original population held been removed during the 51 day period of study. Furthermore, analysis of trapping and length-frequency data showed that catch rate did not decline towards the centre of the array, and that the mean size of seastars collected from within the low density array increased. A. amurensis clearly immigrated rapidly and persistently into the trapping array, precluding attempts to control seastar numbers within the trap field.

The proportion of the initial array population removed by trapping at the high density site during the initial fishdown was approximately 53%, considerably lower than that recorded for the low density site. However, a significant decrease in seastar density occurred over the period of fishdown at this site. Analysis of catch data showed that catch rate did not decline towards the centre of the array, and that seastars predominantly immigrated from a north easterly direction. Trap collected length­frequency distributions showed a decline in mean size, indicating the initial stages of trapping impact on the population. Immigration at this site therefore appears to have been considerably less than at the low density site.

Although no strong soak time related catch trends were apparent, traps were generally saturated after 24 to 48 hours. Few animals were caught as bycatch during the fishdown, with the five main bycatch species consisting of three crabs, an introduced seastar and an ascidian.

Following the initial experiments an attempt was made to counteract rapid immigration rates by pulling and resetting traps over several consecutive days (sequential 24 hour soak times), thereby maximising the numbers of animals removed from each trapping array. Seastar densities in both the high and low density arrays did not change significantly over time: however, a reduction in mean size of seastars following intensive trapping occurred, presumably because the mean size of animals removed in traps was greater than that of immigrant individuals.