My thesis work investigated the acoustic and physical properties of gillnets.  Cetaceans are entangled  in gillnets world-wide.  A major problem seems to be that regular nylon nets are particularly difficult for echolocation marine mammals to detect.  This is because the nets are often set at depth, where light levels are low and because the nets are of low reflectivity.  That is, they reflect dolphin and porpoise echolocation signals poorly.  I investigated a new type of net that was hypothesized to be acoustically more reflective than regular nylon nets.  These nets had fillers such as barium sulphate or iron oxide, which would increase the density of the nylon and theoretically reflect sound better than regular nylon nets. 

Using broadband dolphin and porpoise-like clicks, I measured the target strength (TS), or acoustic reflectivity ,of regular nylon monofilament nets and the new experimental 'acoustically reflective nets.  The new nets are basically more reflective than the regular nylon nets.  However, whether these experimental barium sulphate and iron oxide nets are reflective enough remains to be determined.  They are now being commercially made by Atlantic Gillnet Supply for field tests and to distribute to fisherman.

In recent experimental trials the barium sulphate nets have shown to reduce harbor porpoise bycatch.  This may be due to the enhance acoustic reflectivity of these nets but also to other physical properties of the nets.  The other major variable in reducing marine mammal bycatch seems to be the stiffness of the fishing nets.  It is simply more difficult to become entangled in a stiffer material.  So we devised a method to test stiffness of net line.  It seems that the 'acoustically enhanced' nets are also stiffer nets.  Therefore, this may have a connection to the effectiveness of the experimental nets in reducing bycatch.