Use acoustically reflective fishing gear materials
Overall effectiveness category Likely to be beneficial
Number of studies: 5
Background information and definitions
Using acoustically reflective fishing gear materials, such as nylon infused with chemicals or metal oxides, may increase the detectability of gear for marine and freshwater mammals that use echolocation. This may allow mammals to avoid fishing gear reducing the risk of entanglements. However, it should be noted that using gear materials that are acoustically reflective can also change the density, stiffness and/or colour of the gear, making it difficult to determine which mechanism is causing a deterrent effect (Larsen et al. 2007, Mooney et al. 2007).
Larsen F., Eigaard O.R. & Tougaard J. (2007) Reduction of harbour porpoise (Phocoena phocoena) bycatch by iron-oxide gillnets. Fisheries Research, 85, 270–278.
Mooney T.A., Au W.W.L., Nachtigall P.E. & Trippel E.A. (2007) Acoustic and stiffness properties of gillnets as they relate to small cetacean bycatch. ICES Journal of Marine Science, 64, 1324–1332.
Supporting evidence from individual studies
A controlled study in 2000 of a pelagic area in the Bay of Fundy, Canada (Cox & Read 2004) found that fishing nets made from acoustically reflective materials (barium sulfate) had similar harbour porpoise Phocoena phocoena echolocation activity around them compared to conventional nets. The average occurrence and rate of porpoise echolocation clicks were similar at barium sulfate nets (18–54 intervals with clicks/h; 32–52 clicks/h) and conventional nets (17–57 intervals with clicks/h; 18–54 clicks/h). Average catches of target groundfish species did not differ significantly between barium sulfate nets (0.41 fish/h) and conventional nets (0.38 fish/h). In July–August 2000, nine barium sulfate and 14 conventional gill net strings were deployed across a fishing area (same study site and nets as Trippel et al. 2009). All strings (comprising 3 x 100 m nets, 15 cm stretched monofilament mesh) were deployed on the ocean bottom at depths of 100–130 m for 24–72 h. Four acoustic detectors attached to each of the 23 net strings continuously recorded porpoise echolocation activity at 10 second intervals for a total of 225 h on barium sulfate nets and 366 h on conventional nets.Study and other actions tested
A controlled study in 2003 in a fjord in the Fortune Channel, Vancouver Island, Canada (Koschinski et al. 2006) found that fishing nets made from an acoustically reflective material (barium sulfate) were approached to similar distances and for similar durations by harbour porpoises Phocoena phocoena compared to conventional nets, but porpoises used fewer echolocation clicks at barium sulfate nets. Harbour porpoises approached to similar distances and spent similar amounts of time within 50 m of barium sulfate nets (average 18 m; 24 seconds) and conventional nets (average 18 m; 20 seconds). At barium sulfate nets, echolocating porpoises used fewer clicks (average 23 clicks/interaction) and had longer click intervals (average 51 ms) than at conventional nets (average 56 clicks/interaction; click interval: 45 ms). Two surface gill nets (one barium sulfate, one conventional; both 45 x 9 m, 0.62 mm diameter mesh) were deployed in August 2003. Barium sulfate nets were a mix of high-density barium sulfate and nylon dyed green. Conventional nets were semi-transparent blue nylon. A theodolite was used to track porpoises during six deployments (14 h over four days) with the barium sulfate net and nine deployments (26.5 h over eight days) with the conventional net. A click detector suspended in the middle of each net at a depth of 4.5 m recorded echolocation activity.Study and other actions tested
A replicated, controlled study in 2000 of six pelagic sites in the North Sea, Denmark (Larsen et al. 2007) found that fishing nets made from an acoustically reflective material (iron-oxide) had fewer entanglements of harbour porpoises Phocoena phocoena than conventional nets. No porpoises were found in entangled in iron-oxide nets, whereas a total of eight porpoises (average 0.1 porpoises/km/day) were entangled in conventional nets. Average catch rates of target cod Gadus morhua were lower in iron-oxide nets (6–15 fish/km/day) than conventional nets (8–32 fish/km/day). Each of six sites was fished for three days with 4–8 strings (50 x 60 m gill nets) of each of two net types: high-density iron-oxide nets and conventional nylon nets. The authors did not find a significant difference in acoustic target strengths between the two net types (see original paper for details) and suggest that other factors (e.g. net colour, stiffness, mechanics) may have reduced porpoise entanglements. An observer on board a chartered commercial fishing vessel recorded the number of entangled porpoises and fish catches as the nets were hauled in September–October 2000.Study and other actions tested
A controlled study in 1998 and 2000–2001 of a pelagic area in the Bay of Fundy, Canada (Trippel et al. 2009) found that fishing nets made from an acoustically reflective material (barium sulfate) had fewer harbour porpoise Phocoena phocoena entanglements than conventional nets. Entanglement rates of harbour porpoises were lower in barium sulfate nets than in conventional nets (data reported as statistical model results). For target fish species, catch rates of haddock Melanogrammus aeglefinus were lower in barium sulphate nets than conventional nets, but catches of Atlantic cod Gadus morhua, pollock Pollachius virens and spiny dog fish Squalus acanthias were similar (see original paper for data). In July–September 1998, 2000 and 2001, gill net fishery vessels deployed a total of 590 strings of barium sulfate nets and 815 strings of conventional nets. Barium sulfate nets were made from nylon containing particles of barium sulfate (3% volume, 10% weight) and dyed pale blue. Conventional nets were transparent nylon. All nets (300 m long, 4 m deep, stretched mesh size of 15 cm) were deployed at depths of 60 m for 24 h (same study site and nets as Cox & Read 2004). Onboard observers or fishers recorded porpoise entanglements and fish catches as the nets were hauled.Study and other actions tested
A controlled study in 2009–2010 of two pelagic areas in the South Atlantic Ocean, off the coast of Buenos Aires, Argentina (Bordino et al. 2013) found that fishing nets made from an acoustically reflective material (barium sulfate) had a similar number of Franciscana dolphin Pontoporia blainvillei entanglements to conventional nets. Entanglement rates of Franciscana dolphins did not differ significantly between barium sulfate nets (0.1 dolphins/haul) and conventional nets (0.08 dolphins/haul). Catch rates of the three main target fish species also did not differ (whitemouth croaker Micropogonias furnieri, striped weakfish Cynoscion guatucupa, king weakfish Macrodon ancylodonalso; see original paper for data). Monofilament nylon gill nets of two types (nets infused with barium sulfate and conventional nets; number of each not reported) were deployed in 150 locations across two fishing areas. The nets were sampled 1–19 times resulting in 255 hauls of barium sulfate nets and 279 hauls of conventional nets. An observer on board each of three fishing vessels retrieving the nets recorded the number of entangled dolphins within each of 534 hauls between October 2009 and March 2010.Study and other actions tested