Advancing the understanding of spearfisher-fish behavioural interactions and its management implications

11 pages, 3 figures, 1 tables, supporting information https://doi.org/10.1111/1365-2664.14796.-- Data Availability Statement: Data and code are available via the Dryad Digital Repository https://doi.org/10.5061/dryad.qbzkh18sk (Sbragaglia et al., 2024)

Fish behaviour is intimately linked to fishing catchability. Measurements of fish behaviour may provide quantitative indicators that inform about the impact of fishing pressure and help design effective management strategies. The implications of spearfishing pressure on fish behaviour have mainly been characterized using single quantitative indicators such as flight initiation distance (FID), which provide incomplete views of spearfisher-fish behavioural interactions. We used an integrative approach to advance the understanding of spearfisher-fish behavioural interactions by complementing FID estimates with post-flight responses (i.e. individual fish behaviour after fleeing), and the decreasing probability to catch a fish at increasing distance from a speargun. We first collected empirical observations of white seabream (Diplodus sargus) FID and post-flight behaviour in response to an approaching human (spearfisher or snorkeler) inside and outside marine protected areas. We then combined these observations with simulations based on an innovative model representing how catchability changes during a spearfisher-fish behavioural interaction. Empirical results showed that the likelihood of a still-vulnerable post-flight response after fleeing was inversely related to increasing exposure to spearfishers. This demonstrates that post-flight response can be used as an indicator of spearfishing pressure. Moreover, post-flight responses provided complementary information to that provided by FID, suggesting that these two indicators should be integrated for a holistic view of spearfishing-fish interactions, such as in the model and simulations presented here. The simulations demonstrated that: (i) accounting for post-flight responses may increase likelihood of approach by spearfisher within speargun range by up to 20%; and (ii) accounting for post-flight responses and decreasing probability of capture with increasing distance from speargun drastically reduces estimates of likelihood of capture of white seabream at minimum catch length limit outside marine protected areas compared with using FID as single indicator of catchability. Synthesis and applications. First, integrating flight initiation distance (FID) measurement with post-flight responses provides a more realistic indicator to measure fish behavioural reaction to spearfishing pressure. Second, the model presented here showed that spearfishing mortality may drastically decrease when accounting for both prey behaviour (i.e. FID and post-flight responses) and spearfishers effectiveness (i.e. probability to catch a fish at increasing distance from a speargun). This must be considered to understand population dynamics and potential changes in catchability when spearfishing is managed through temporal restrictions (e.g. seasonal or periodic closures)

VS is supported by a ‘Ramón y Cajal’ research fellowships (RYC2021-033065-I) granted by the Spanish Ministry of Science and Innovation, and he also acknowledge the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S). EC was funded with the support of the Erasmus+ program of the European Union. FAJ-H acknowledges the support of Sêr Cymru ERDF Cofund Fellowship (80761-SU-135)

Peer reviewed