Weak Hooks

'Weak hooks' refers to weak circle hooks, constructed of thinner gauge stock wire than standard circle hooks of the same size, and designed to straighten at a lower strain (pull) level than standard hooks, thereby allowing large hooked animals to escape while retaining the target catch [10]. It is important that the hook is the weakest component of the terminal tackle and thus straightens before any other element of the branch line fails (leaving the terminal tackle attached and increasing the risk of post-release mortality) [9].

 

The following is an extract from Fader et al (2021)

If avoidance of depredators or minimizing contact with gear is not possible, modifying the terminal gear to release hooked animals or facilitating shedding of entangled gear may be the only option to mitigate bycatch impacts (Werner et al., 2015; Zollett and Swimmer, 2019; Swimmer et al., 2020). In longline fisheries, this strategy generally entails guidelines to encourage fishermen to remove gear from hooked or entangled animals, or the use of hooks with a targeted bending strength, such that hooks are weak enough to straighten and release toothed whales but sufficiently strong to retain target catch (Bayse and Kerstetter, 2010; Bigelow et al., 2012). This “weak-hook” strategy has been used successfully to reduce bycatch of large, non-target bluefin tuna (Thunnus thynnus) by 46% in the U.S. Gulf of Mexico pelagic longline fishery, with no statistically significant impact on catch rates of yellowfin tuna (Thunnus albacares) (Walter, 2017).

Controlled mechanical tests of bending strengths and behavior of hooks under strain in the lip tissue of dead odontocetes have helped identify candidate weak hooks for minimizing cetacean bycatch (McLellan et al., 2015). Field trials in the U.S. Atlantic large pelagics longline fishery and Hawai‘i deep-set longline fisheries have tested similar hook designs under controlled conditions (Bayse and Kerstetter, 2010; Bigelow et al., 2012). The bycatch of cetaceans was too rare to determine whether weaker hooks had a positive influence on the outcome of such events, but weaker hooks were returned straightened more often than strong hooks in each study and one pilot whale was observed released by a straightened hook in the Atlantic (Bayse and Kerstetter, 2010; Bigelow et al., 2012). Comparable rates of target catch were recorded in each study, although the Hawai‘i study was not carried out during the season when the largest tuna are caught (Bigelow et al., 2012) and the size of swordfish was slightly smaller on weak hooks in some Atlantic trials (Bayse and Kerstetter, 2010).

One obstacle to implementation of such measures is the understandable reluctance of fishermen to modify their terminal tackle, particularly if such changes might reduce the catch rates of large and valuable target species (e.g., Bigelow et al., 2012; Ayers and Leong, 2020). The post-release survival rates of animals hooked or entangled in pelagic longline gear are not well understood but have obvious and important implications for understanding population-level impacts (Garrison, 2007; Werner et al., 2015).