Spiny Dogfish Mortality Induced by Gill-Net and Trawl Capture and Tag and Release

Abstract

The spiny dogfish Squalus acanthias was once classified as an underutilized species along the U.S. East Coast, but it constituted a lucrative fishery in the 1990s until recruitment overfishing caused stock collapse. Coastwide restrictions currently apply; federal stock assessment models use bycatch mortality estimates of 50% for trawling, 75% for gill netting, and 100% for hook-and-line fishing. This study examined mortality at the southern end of commercial fishing operations caused by trawling for 30- and 90-min periods and by gill nets of various mesh sizes set for 19- to 24-h periods. Both experiments used tagged and untagged fish placed in rectangular cages anchored to the seafloor for 48 h. Tags were the Floy SS-94 single-barb nylon dart with a stainless steel wire insert. A total of 635 spiny dogfish were captured by trawl and all were alive, for a 0% initial mortality rate. A total of 2,284 spiny dogfish were collected by gill net for an initial mortality rate of 17.5%. There was no additional mortality in the 480 trawl-caught fish held for 48 h, but there was 33.3% mortality among the 480 gill-net-caught fish held under the same conditions, for an overall gill-net mortality rate of 55.0%. Examination of subsampled catches indicated that 88.6% of gill-net-caught fish had gill-net marks on the head and 41.2% had gill-net marks on the girth but, interestingly, 26.1% of trawl-caught fish had the same markings, indicating prior gill-net capture and release. Female spiny dogfish caught by gill net had a 3.6% abortion rate, compared with zero incidences of those caught by trawl. There was no significant difference in mortality between tagged and untagged fish caught by trawl or by gill net. Tag loss after 48 h was less than 1%.