Shark 'drop-out' is a term used by fishermen to describe the phenomenon where sharks fall out of shark gillnets during hauling operations. The term is distinguished from shark 'escapement' which is the phenomenon where sharks _struggle to free themselves from gillnets with a high probability of survival.

Sharks dropping out of gillnets and not surviving is not only a source of wastage but can cause fishing mortality to be under-estimated and natural mortality to be over­estimated in stock assessments. Such biases, if large, can contribute to overly optimistic scenarios for the status of the stocks.

The FRDC funded 'Southern Shark Drop-Out Project, designed to estimate rates of drop-out through application of a 'remotely operated vehicle' (ROY), was abandoned after preliminary trials of an ROY on board FV Lincoln when it became apparent that the approach was impractical.

It was originally proposed that an ROY be deployed from a shark fishing vessel operating under normal commercial fishing conditions for the purpose of identifying species and estimating the length of captured sharks in gillnets immediately before the nets are hauled off the seabed and aboard the vessel. Subsequently identifying and measuring the sharks after hauling the gillnets would provide appropriate data for determining the rate at which sharks drop-out of the nets and for determining whether the rate of drop-out varied with species of shark, length of shark and mesh-size of the gillnets.

An alternative method to viewing the gillnets while being hauled was for the ROY to travel the full length of the nets on the seabed to count and estimate the lengths of the captured sharks. This approach was avoided because there would have been too much time between when most of the nets were viewed and when they were hauled. This approach would have underestimated the drop-out rate.

The original experimental design involved using gillnets of several mesh-sizes for 20 days at sea aboard a commercial shark fishing vessel fishing at various depths in the Great Australian Bight. In addition there was to have been several days of laboratory processing of data on film, statistical analysis of the data and preparation of a report. However because of difficulties associated with deployment of an ROY as proposed, the project was terminated before any quantitative data for estimating rates of 'drop­out' were collected.

The ROY was tested initially beside the pier in the Thevenard harbour on 26 October and then three times in the vicinity of St Francis Island near Thevenard on 26 and 27 October 1995 with varying lengths of shark gillnet. No sharks were viewed by the ROY during these trials, and because of difficulties operating the ROY under sea conditions of waves less than 1 m and to avoid the risk of damage to the ROY no attempt was made subsequently to deploy the ROY under normal shark fishing conditions.