Unanswered Forum Topics

The selectivity of the Skagerrak standard trawls were compared with a trawl differing only in the belly length, being 37% shorter. The trawls fished shrimp above 19 mm carapace length equally, while catch rates of shrimp below 15.5–16 mm carapace length in the shorter trawl were more than halved. Bycatch of Norway pout (Trispterus esmarkii) was slightly reduced in the shorter trawl, unrelated to fish length.

In south-eastern Australia, baited, round traps (comprising 50–57-mm mesh netting) are used to target giant mud crabs (Scylla serrata) and blue swimmer crabs (Portunus armatus) in spatially separated fisheries.  Two experiments assessed the utility of (1) traps partially or completely covered in larger mesh (91 mm to match the minimum legal size of the smaller P. armatus), and (2) any cumulative benefits of fitting species-specific escape gaps.

This study developed fishing traps made with plastic that included adjustable escape gaps for the release of undersized fishes. The unique improved selective fishing traps operate in shallow water of 1-2 meters depth. Traps are set against the flow of water current to aid movement of fishes into the traps. The average catch per month of matured fishes of table sizes caught by the invention is approximately 61% compared with approximately 23% fishes usually caught by the traditional traps.

A software tool was developed to assess observer coverage with respect to several objectives for documenting or estimating rare-event bycatch. The ObsCovgTool predicts observer coverage performance for a given total fishery effort in relation to three metrics: (1) the conditional probability of observing any bycatch given that bycatch occurred in the fishery and the probability of any bycatch in the total fishery effort, (2) the upper confidence limit for total bycatch when none is observed, and (3) precision (coefficient of variation) of the bycatch estimate. 

The study tested the vocalization response of harbor porpoises (Phocoena phocoena) to two different acoustic pingers, the AQUAmark 100 (20-160 kHz) and the AQUAmark 300 (10 kHz). Trials in Scotland and Denmark found that the AQUAmark 100 resulted in a significant reduction of porpoise vocalizations at 0,200, and 400 meters distance from the pingers. The vocalization behavior reveal no signs of habituation. Studies of the AQUAmark 300 resulted in a significant reduction of vocalizations at 0m and no significant reduction at 300m.

A long-term study assessed the efficiency of acoustic pingers (AQUAmark 100, 20-160kHz) in reducing the encounter rates of finless porpoises (Neophocaena spp.) with fishing nets. The study used a passive recorder to obtain acoustic encounter rates of echolocating finless porpoises over two eight-month periods. Encounter rates were significantly lower in periods when pingers were in use, but this effect decreased over time. By the end of each study period, the number of encounters was greater than those during periods without pingers, suggesting that habituation had occurred.

The study combined fisher questionnaires, acoustic monitoring, and participatory experiments to investigate the occurrence of bottlenose dolphins (Tursiops truncates) in the gillnet fisheries of Northern Cyprus. Dolphins were present in fishing grounds throughout the year, and were detected at 28% of net sets. Net damage was six times greater when dolphins were present during sets. Use of an acoustic pinger (AQUAmark 200, 5-160kHz) was tested, but had no significant effect on dolphin presence.