The problems of reducing small cetacean bycatch in fishing nets are many and complex, and acoustic solutions need to be tailored to suit individual species and net types. Active deterrent devices or alarms have recently been shown to be beneficial in reducing the bycatch of harbour porpoises Phocoena phocoena in some carefully monitored trials in North American sink-gillnet fisheries (Kraus et al. 1995). The methodology employed to date is still at an early stage of development; typical devices generate simple 10 kHz tonal pulses from small battery powered packages distributed at intervals along the fishing net. The long term effectiveness of this approach has already been questioned, as small cetaceans are known to habituate quite quickly to novel stimuli. A more sophisticated approach, now at the trials stage, uses higher frequency, wideband signals which have been determined to be more aversive to this species (Goodson et a1. above). To minimise habituation effects over time, such 'beacon alarm' signals need only be transmitted occasionally if the acoustic activity of an approaching echolocating animal can also be detected and used to trigger the device into a transponder type of operation. Such a device has now been designed at Loughborough University and implemented using digital micro-controller technology. As this circuitry is programmable almost any complex waveform can be generated to suit specific applications. The use of a digital micro-controller permits a number of additional features to be implemented in software without a significant increase in the overall component count and, despite the sophistication, the cost per device remains acceptably low. It may be argued that a silent animal could remain at risk. However, in the context of bottom set nets (sink-gillnets) this should not apply, as the harbour porpoise swimming close to the bottom in deep water is there to forage for prey and hence actively employing its echolocation sense. This interactive approach reduces spurious acoustic emissions which waste battery energy and, as most responses are triggered by the approach of an animal at risk of colliding with the net, the deterrent effect is expected to be longer lasting. The alarm response ceases quickly once the animal turns away. Provided that these devices are spaced apart along the net within detection range of each other, additional benefit is obtained if they also respond to a neighbour's alarm signal as this will result in a 'ripple-fire' of activity along the net. This linking of emissions provides better orientation information than can be given by simple randomly timed pingers. lnteractive devices are also intended for application to large pelagic trawls, where the delineation of the net's boundary by such a ripple-fire transmission is expected to give a clearer indication of the extent of the hazard to an animal which may have followed fish into the net. Since triggering can also be stimulated by a ship's echosounder, these devices should make the location and recovery of lost nets a relatively simple exercise.