- Most scientifically supported theory – learned behaviour
- Struggling/fatigued fish on a line, or shortly after release are easy prey
- Use of berley can exacerbate the effect
The bulk of the scientific research conducted in recreational and commercial fisheries indicates that the depredation fishers experience is a result of learned behaviour – fishing boats/gear mean easy meals (Mitchell et al., 2020; Ryan et al., 2019; Mitchell et al., 2018). Experiments run in areas where fishing is prohibited have demonstrated that sharks can start associating fishing gear with food after seeing it as little as 16 times over a period of six days (Mitchell et al., 2020).
Some of the cues suspected of exacerbating the issue include:
- Prolonged fight times
- The use of berley
- Leaving fish in the water
- Pulsating, low frequency sounds (e.g. running motors)
Shark depredation – where it occurs
- Popular reefs for recreational fishing
- Sometimes, even that isn’t necessary (cite papers)
- Offshore while game fishing
- Inshore areas with large numbers of juvenile sharks
Among line fishers, depredation is most often reported by game fishers, and people fishing on popular reefs.
However, it appears that sharks learn to target fish attached to fishing lines very quickly, even in waters where recreational fishing is not allowed (Mitchell et al., 2020; Ryan et al., 2019; Mitchell et al., 2018). In the instance of spear fishers, sharks and rays quickly learn to pick fish off float-lines and spear tips, and have been known to harass spear fishers themselves.
Depredation also often occurs in-shore, especially during those times of year when sharks breed. Here, the sharks that attack/take fish being fought by recreational fishers tend to be juveniles, and the depredation is mostly only a significant issue up until those juvenile sharks leave the in-shore areas.