Reduction of harbour porpoise bycatch by use of high-density iron-oxide (IO) gillnets was tested in sea trials in the Danish North Sea bottom set gillnet fishery in September–October 2000. The trials were conducted as a controlled experiment with conventional gillnets as the control group. Eight porpoises were caught in the control nets and none in the IO nets, a highly significant reduction (P < 0.01). Of the four fish species analysed only catch rates of cod (Gadus morhua) were significantly (P < 0.01) different between the two net types, with CPUE in the IO nets being ca. 70% of the CPUE in the control nets. Subsequent investigations in seawater tanks revealed that the difference in acoustic target strength of the two net types was not significant and that the nets behaved similarly under various water flow conditions. Based on laboratory tests of twine samples and analyses of catch composition we conclude that it is the mechanical properties of the IO nets, primarily the measured increase in stiffness, that are the main reasons for the differences in catch rates for cod and for porpoises between IO and conventional nets.