A comparison of catches of swordfish, Xiphias gladius, and other pelagic species from Canadian longline gear configured with alternating monofilament and multifilament nylon gangions

Canadian pelagic longline fishermen have been using monofilament gangions for over a decade now and generally agree over its ability to outperform the older tarred multifilament nylon gangions in terms of swordfish catchability, although this theory has never been tested scientifically. In this note, the results of a collaborative science and industry study involving the Canadian Department of Fisheries and Oceans and the Nova Scotia Swordfishermen's Association are presented. Our study was designed to compare the catchability of swordfish and other pelagic species on commercial longline gear configured with alternate monofilament and tarred multifilament nylon gangions. It was based on the premise that some species can avoid capture on the tarred multifilament gangions which are thicker, darker, and possibly more "visible" than the monofilament nylon gangions.

The purpose of our analysis was to examine differences in pelagic longline catch by species for two different types of gangion and was based on the premise that monofilament nylon gangions currently used by Canadian pelagic longline fishermen yield higher catches than the tarred multifilament nylon gangions used in the past. Although only a small data set from a limited geographic area was available for this analysis, it was apparent in the case of swordfish and blue shark that catches were significantly higher on monofilament gangions, which yielded double the catch of the multifilament gangions for some sets. Although only the upper half of the multifilament gangion was made from braided nylon material (based on a configuration used by fishermen in the past), the differences in catches for these two species between this gangion and one constructed entirely of monofilament nylon were striking. A similar trend for the other pelagic species was not as evident on a set-by-set basis owing to lower catches; however, for combined sets, more captures occurred on the monofilament gear overall. Although these results clearly indicate differences in catches between gangion types, the influence of oceanographic conditions off Georges Bank, such as water temperature and thermocline depth, likely influence the availability and catchability of all species. Therefore, it is important to point out that results could differ among geographic areas with different oceanographic regimes.