Circle hooks

See Swimmer et al 2020 for a summary of recent research findings regarding bycatch species and circle hooks, including cross-taxa effects.
For illustrations of circle hooks, see Beverly, S. 2009. Longline terminal gear identification guide. Secretariat of the Pacific Community. The following description of circle hooks is an extract from FAO. 2009. Guidelines to reduce sea turtle mortality in fishing operations. FAO Fisheries Department, Rome., except where noted otherwise.
Hook Design
Circle hooks, J-hooks and tuna hooks are three types of hooks in use in pelagic longline fisheries. Circle hook shape is rounded with the point oriented perpendicular to the shank, while a J-hook is shaped as its name implies, with its point oriented parallel to the hook shaft. In shape, a tuna hook is in between a circle and a J hook, but the point of the tuna hook is not guarded by the shaft, as is the case for J hooks. The point on a circle hook is turned in, towards the hook shank.  
Sea Turtles
Experiments suggest that circle hooks are effective at reducing captures of hardshelled turtles because they are wider at their narrowest point than J hooks and tuna hooks. Therefore, they are too wide to fit into the mouths of sea turtles. On the other hand, the circle hook may be effective at reducing leatherback captures because of its shape; hard-shelled turtles tend to get caught in longline gear because they bite a baited hook, while leatherbacks tend to get caught because they are foul-hooked on the body or entangled in the line.
Circle hooks and fish bait   
Sea Turtles
There are a growing number of experiments that provide information about the effects of hook and bait combinations on both sea turtle capture rates and target species catch rates in pelagic longline fisheries. For example, in the United States North Atlantic longline fishery for swordfish, the use of 18/0 circle hooks and squid bait reduced loggerhead and leatherback bycatch rates by 86 percent and 57 percent, respectively compared to fishing with J hooks and the same bait. When combined with mackerel bait (rather than squid bait), the 18/0 circle hook reduced loggerhead and leatherback bycatch rates by 90 percent and 65 percent, respectively, without compromising catch rates of swordfish. Similar results have been observed in the Hawaiian longline swordfish fishery: capture rates of leatherback and loggerhead turtles declined substantially - by 83 percent and 90 percent respectively - after switching from a J hook with squid bait to a wider circle hook with fish bait.  
In addition to reducing sea turtle capture rates, the use of circle hooks has been shown to reduce the number of turtles that are deeply-hooked i.e. the hook is swallowed into the esophagus or deeper, rather than being hooked in the mouth or foul hooked on the body. Mouth-hooked turtles probably have a greater chance of surviving a hooking than deeply hooked turtles. Moreover, gear removal is more commonly accomplished with lightly hooked turtles. For example, in the United States North Atlantic longline fishery for swordfish, the use of circle hooks rather than J hooks substantially reduced the proportion of deeply hooked sea turtles landed by the fishery. Similar effects were observed in the Hawaiian longline swordfish fishery: after switching from J hooks and squid bait to wider circle hooks and fish bait, there was a significant reduction in the number of turtles that swallowed hooks (into the esophagus and deeper) and a significant increase in the numbers of turtles that were released after the removal of all terminal tackle, both of which are outcomes that may increase the likelihood of turtles surviving the interaction.
In some fisheries, the use of circle hooks and fish bait has been shown to improve catch rates of certain target species. For example, after a requirement was instituted for vessels in the Hawaiian longline fishery for swordfish to use 18/0 circle hooks with fish bait - in place of 9/0 J hooks with squid bait - the swordfish catch rate increased significantly by 16 percent. However, catch rates of combined tuna species and catch rates of combined mahi mahi, opah, and wahoo declined significantly, by 50 percent and 34 percent respectively. Similar results were observed in the US Atlantic longline swordfish fishery. The reduction in catch per unit effort (CPUE) for tuna species is likely due to the size of the fish bait being used in these fisheries. Other studies have shown increases in CPUE for tuna species when circle hooks were used in combination with smaller sized fish. Reduced CPUE for the other fish species is likely due to the size of the circle hook used.
Sharks and marine mammals.
To be added.
Different hook shapes show different results
Circle hooks come in a variety of shapes and sizes. However, there is no uniform system of hook measurements. This is problematic when reporting research results and comparing results between experiments and may be compounded by the fact that the different manufacturers of hooks use different terminology.
Sea Turtles
Different shapes can change the performance of individual hooks. For example, a circle hook with a larger gap between the point and the shank, or greater than a 10 offset, may affect the hook's interactions with sea turtles. Other differences in hook designs, such as the material from which the hook is manufactured, may also affect sea turtle capture rates and position of hooking.
Offset circle hooks are similar in shape to non-offset circle hooks, but the point is not in line with the shank. When laid on a flat surface, a non-offset hook would lie flat, but the point of an offset hook would be slightly elevated.
Research has shown that using offset circle hooks with 10 degrees or less offset, rather than non-offset circle hooks in longline fisheries, does not affect sea turtle capture rates. Furthermore, the use of less than 10 degree offset circle hooks does not seem to affect the location of turtle hooking. Circle hooks with more than a 10 degree offset behave similarly to J hooks and increase turtle capture rate and increase the proportion of caught turtles that are deeply hooked when compared to non offset circle hooks. It may be possible that offset hooks result in increased injury to turtles relative to non-offset hooks when a hook is ingested because the offset hooks may be more likely to embed internally instead of passing through. The use of circle hooks results in less foul hooking than J hooks. Leatherbacks most often are foul hooked; it is likely that any size circle hook with minimal offset will result in a reduction in leatherback bycatch.
  1. FAO. 2009. Guidelines to reduce sea turtle mortality in fishing operations. FAO Fisheries Department, Rome.
  2. Gilman, E., Clarke, S., Brothers, N., Alfaro-Shigueto, J., Mandelman, J., Mangel, J.,  Petersen, S., Piovano, S., Thomson, N.,  Dalzell, P.,  Donoso, M., Goren, M. and Werner, T. 2007a. Shark depredation and unwanted bycatch in pelagic longline fisheries: Industry practices and attitudes, and shark avoidance strategies. Western Pacific Regional Fishery Management Council, Honolulu, USA.
  3. Gilman, E., Kobayashi, D., Swenarton, T., Brothers, N., Dalzell, P. and Kinan-Kelly, I. 2007b. Reducing sea turtle interactions in the Hawaii-based longline swordfish fishery. Biological Conservation 139(1-2): 19-28.
  4. Patterson, H.M. and Tudman, M.J. 2009. Chondrichthyan guide for fisheries managers: A practical guide to mitigating chondrichthyan bycatch. Bureau of Rural Sciences and Australian Fisheries Management Authority, Canberra.
  5. Watson, J.W., Epperly, S.P., Shah, A.K., and Foster, D.G. 2005. Fishing methods to reduce sea turtle mortality associated with pelagic longlines. Canadian Journal of Fisheries and Aquatic Sciences 62:965-981.