Reviewing hook degradation to promote ejection after ingestion by marine fish.
McGrath SP, Butcher PA, Broadhurst MK and Cairns SC (2011) Reviewing hook degradation to promote ejection after ingestion by marine fish. Marine & Freshwater Research, 62: 1237–1247.
[early online publication available: http://dx.doi.org/10.1071/MF11082]
SummaryDuring the past 10 years, considerable research has been done to assess the fate of fish that are angled and released. For the majority of species, their short-term (< 10 days) survival rates have been estimated at > 70%, with the few mortalities typically attributed to quite severe treatments, and especially gut hooking.
A widely recommended strategy for improving the survival of gut-hooked fish is to quickly release them with their lines cut. The utility of this approach is based on the premise that the fish will eventually eject the hook, once it has sufficiently oxidised, and often broken into smaller pieces. While such ejection has been observed among several species (often within a few weeks), very little is known about the factors influencing hook degradation and the extent to which it can be promoted via simple modifications. We sought to address this issue by quantifying the technical parameters affecting the percentage weight and point remaining and tensile and compression strengths for more than 800 small hooks comprising 23 designs (used to target fish < 40 cm in length) after immersion in seawater for 24 days.
The results showed that initially the examined hooks were, on average, 13 times stronger than that required to catch the desired sizes of fish; which means that all could be modified to reduce their resilience and therefore increase their degradation once ingested. The key factors affecting hook degradation were the wire material and diameter, and to a lesser extent, the gape and bend lengths.
We conclude that the degradation of hooks could be significantly promoted by choosing carbon-steel designs, either with a wire diameter of < 0.9 mm for the examined sizes, or alternatively, bait-holder barbs (or similar modifications) along the shaft, and narrow bend and gape lengths. By rapidly oxidising and weakening, such designs could help to reduce some of the negative impacts to released hook-ingested fish.
Ongoing work is being done to test the effectiveness of some of the above recommended characteristics for promoting ejection and improving survival among snapper, yellowfin bream and mulloway that have ingested hooks. Ultimately, this research should facilitate improvements to the design of conventional fishing hooks.