Author Valerio Sbragaglia and his colleagues guide us through a recent study which advances the understanding of spearfisher-fish behavioural interaction by integrating ecological indicators (i.e., flight and post flight behaviour of fish) with spearfishers’ likelihood to catch a fish. Through modelling and simulating scenarios, their study sheds light on management implications in exploited fish populations.
Exploring spearfishing and fish behaviour
If you ask spearfishers, they will likely tell you that they already know about what we show in our paper. Spearfishers (and fishers in general) always observe and interpret fish behaviour because behaviour is intimately linked to catchability and, consequently, to fishing success. We are passionate about spearfishing ourselves and we did what any other spearfisher would have done – but, instead, we used an explicitly scientific approach.
By looking at the accumulating scientific evidence on fish behavioural reactions to underwater human presence we realized that the elements researchers were using to represent spearfisher-fish behavioural interactions omitted essential parts of the interaction, and this over-simplification may result in incorrect conclusions drawn from the research. Moreover, we changed our point of view by looking at spearfisher-fish behavioural interactions not only from the fish perspective, but also from the spearfishers’ perspective. This led to us deciding to integrate another element of reality into our study: ‘spearfishers fail…and a lot’ as we explain below.
What did we do?
Spearfisher-fish behavioural interaction can be conceptualized as a predator-prey system. With this in mind, we considered two ecological indicators previously used to measure fish behavioural responses to underwater human presence and combined them into one single integrated model.
The first indicator was flight initiation distance (the distance at which animals decide to escape from an approaching predator), while the second indicator was post-flight behaviour, which looks at what fish do immediately after fleeing from an approaching predator (for example, fish may flee but remain in the proximity which could still make them vulnerable to the approaching threat). There is evidence that these two indicators can be used to estimate fish behavioural reaction to underwater human presence, but their functional integration into one cohesive model representing spearfisher-fish behavioural interactions has not been explored.
Moreover, despite previous studies suggested that fish behavioural responses to spearfishing pressure may be used to model fishing mortality (an important parameter to assess dynamics in exploited populations), no studies estimated and integrated the role of spearfishers’ efficiency. We did this for the first time by estimating the decreasing probability to catch a fish at increasing distance from a speargun, and integrating it into the model together with the two ecological indicators mentioned above (flight and post-flight behaviour).
What did we find?
We showed that the two ecological indicators for flight and post-flight behaviour provide complementary information to quantitatively measure the response of fish to the risk associated to being caught by spearfishers. Once we ran our integrated model combining the empirical observations on fish behaviour with the estimates obtained from spearfishers (related to the probability to catch a fish at increasing distance from the speargun), we realized that the results were providing a new view of spearfisher-fish behavioural interactions with important ramifications for management.
Specifically, our integrative model drastically reduces estimates of likelihood of capture outside marine protected areas compared with previous estimates by solely using flight initiation distance as a single indicator of catchability.
Why it is important?
Despite spearfishing being practiced by only a small proportion of recreational (about 5%) and subsistence fishers around the world, there are important ramifications of our study. First, when managing spearfishing through temporal restrictions (e.g., seasonal or periodic closures) it is important to use an integrative ecological indicator that captures more realistic changes of fish behaviour in response to spearfishing – such as the one presented in our study. This would allow managers to, for example, open and close areas in a more functional way that is sensitive to actual fishing dynamics.
Second, the estimates of fishing mortality suggested by the model may be crucial to understand population dynamics and to plan long-term management actions with the aim to keep stock productivity high and to a sustainable level of harvesting.
This research received funding from the Spanish Ministry of Science and Innovation with the “Ramón y Cajal” research fellowships (RYC2021-033065-I) granted to Valerio Sbragaglia.
Read the full article “Advancing the understanding of spearfisher-fish behavioural interactions and its management implications” in Journal of Applied Ecology.
