T-POD use in fisheries

The earliest POD study of fisheries was made by the Sea Mammal Research Unit, now based in St. Andrew's University, Scotland. The first version of the POD was deployed by offshore fishermen from Newlyn, Cornwall, UK, on bottom set gill nets in the Celtic Sea. The whole deployment and record-keeping aspects of the study were managed by the fishery, with the support of the Cornish Fish Producers Organisation. The aim was to better understand the mechanism of porpoise bycatch in hake nets.

The results of this study were unexpected and valuable. Firstly, it was clear that porpoises were around the nets without becoming entangled much more frequently than was generally supposed by cetacean scientists at the time - it was thought that the nets were so difficult for a porpoise to detect that they would generally blunder into the net and become entangled.

Secondly, it was found that a voyage with an unusually high porpoise bycatch rate had a low level of acoustic detections, suggesting that silent porpoises might be particularly at risk of entanglement. That voyage had another unusual feature - a significant catch of mackerel, a fish that gives a poor echo as it has no swim bladder and this might be a circumstance in which porpoises do not use their echo-location, but simply listen to the noise of the fish swimming.

Several studies have used T-PODs to investigate the effect of fishery pingers on porpoises. In every case the T-POD data has shown that pingers have an aversive effect and, where visual data has also been collected, the results have concurred.

Julia Carlström and Per Berggren from the University of Stockholm used T-PODs as part of a study of the impact of fishery pingers on porpoises. This study included theodolite tracking. The results showed a good match between visual and T-POD data and pinger effects over a wider range than had been previously supposed. Return to normal porpoise distribution occurred over a few hours after the pingers were switched off. See publication details...

A T-POD study by Silvia Scali in Mallorca, Balearic Islands, Spain, showed that bottlenose dolphin visits to nets correlated with holes in the nets. See publication details...

Giancarlo Lauriano, from the ICRAM, Italy, showed in an unpublished study using T-PODs, that click rates in bottlenose dolphin click trains were significantly higher around gill nets than around fish traps, suggesting that the dolphins were foraging around, or from, the nets but not the traps.

Paqui Diaz de Len used T-PODs to demonstrate the diel pattern of bottlenose dolphin visits to commercial fish farms in Tenerife, Canary Islands.

Limited attempts have been made to use T-PODs on pelagic trawls to identify the timing of cetacean interactions. So far these have not given meaningful results. A major problem is the presence of net sonars, that often work at 50kHz like the dolphins. Further work in this area may be justified.