Guidelines for Reducing Porpoise Mortality in Tuna Purse Seining. NOAA Technical Report NMFS 13
More than a decade has passed since the passage of the Marine Mammal Protection Act of 1972. During that time the U.S. tuna purse seine net reduced its incidental porpoise mortality rate more than two-fold. This was made possible through the development of gear and techniques aimed at reducing the frequency of many low probability events that contribute to the kill.
Porpoise are killed by becoming entangled or entrapped in folds and canopies of the net and suffocating. The configuration of the net, both before and during the backdown release procedure, is a major determinant of the number of porpoises killed. Speedboats can be used to tow on the corkline to prevent net collapse and to adjust the net configuration to reduce net canopies prior to backdown. Deepening a net can reduce the probability of porpoise being killed by pre-backdown net collapse. The effects of environmental conditions and mechanical failures on net configuration can result in high porpoise mortality unless mitigated by skilled vessel manoeuvres or prevented by the timely use of speedboats to adjust the net.
The backdown procedure is the only means to effectively release captured porpoise from a purse seine. It Is also the time during the set when most of the mortality occurs. The use of small mesh safety panels and aprons in the backdown areas of nets reduces porpoise entanglement and increases the probability of an effective release. The tie-down points on the net for preparing the backdown channel must be properly located in order to optimise porpoise release. A formula uses the stretched depth of the net to calculate one of these points, making it a simple matter to locate the other. Understanding the dynamics of the backdown procedure permits a thorough troubleshooting of performance, thus preventing the repetition of poorly executed backdowns and thereby reducing mortality.
Porpoise that cannot be released must be rescued by hand. A rescuer in a rigidly inflated raft can rescue porpoise effectively at any time during a net set. Hand rescue can make the difference between above average kill and zero kill sets. In all circumstances, the skill and motivation of the captain and his crew are the final determinants in the prevention of incidental porpoise mortality in tuna seining.