THE AUSTRALIAN SCALLOP DREDGE - ESTIMATES OF CATCHING EFFICIENCY AND ASSOCIATED INDIRECT FISHING MORTALITY

The catching efficiency of the Astralian scallop "mud" dredge was examined in two experiments on plots seeded with scallops (Pecten fumatus) of known size and abundance. Catching efficiency was found to be low: on average only 11.6% of the reseeded scallops in the tow path were caught. Size selectivity ranged from 1% for scallops of 57 mm shell height, to 28% for scallops of 86 mm shell height. The efficiency of the dredge was not affected by either the dredge mesh size, or the direction of tow with respect to orientation of ripples and sandwaves on the sea-bed. To determine the mortality of scallops resulting from the use of this dredge, changes in the relative proportions of live, damaged and dead scallops on the Banks Strait grounds before and after the start of the 1986 fishing season were measured by assigning scallops from subsamples of catches to one of the three categories. At the start of fishing, both scallop density and levels of shell damage due to dredging were high. Although the proportion of damaged scallops in catches declined over time, a high mortality rate of scallops continued after commercial fishing had ceased. This rate was such that almost all the remaining scallops on the bed were dead within 8 months of the closure of the grounds. A general theoretical model describing changes in the proportions of live, damaged and dead scallops as a consequence of dredging is presented. The model indicates that only 12-22% of the initial stock in Banks Strait was landed as catch, with the rest of the stock wasted through direct and indirect mortality resulting from dredging.