The effect of spearfishing on fish behavior

It happens almost all the times…we are snorkeling in a crowded spot and suddenly a huge fish appears in front of us swimming slowly among people without fear of being approached.

Is it possible that it knows that those people are not a danger?

As many spearfishers already know fish are able to discriminate whether or not they handle a speargun. This is something already known in the scientific world; however, there are only few cases where quantitative measures are available to really understand what is the effect of the presence of a speargun on fish behavior.

A study recently published on the scientific journal ICES journal of marine science quantified the behavioral response of five mediterranean fish species in the presence of both snorkelers and spearfishers. The behavior has been measured using the Flight Initiation Distance (FID) that is the distance at which an animal flees from approaching threats, in this case the underwater human presence. Researchers simulated both snorkelers (short blue wetsuit, short blue fins and no speargun) and spearfishers (long black wetsuit, long fins and speargun) approaching the fish from the surface in three different areas of the Mediterranean Sea. In each area the measurements have been repeated inside and outside marine protected area (MPA). Moreover, the size of the fish was estimated to evaluate if the behavior of large fish was different from that of smaller ones.

In order to make the results easier to understand we will report only two of the species that have been studied: The white seabream (Diplodus sargus) and the salema (Sarpa salpa). The results are reported in the figure below:

Before discussing all the results, we invite you to observe the figure carefully. What is the way to read the figure?

1) There are two panels: the one above represents the measurements related to the species S. salpa and the one below is related to D. sargus.

2) On the right you can find the legend related to the 4 lines that appear in each of the two panel:

– Snork-P (green) represents the measurement made by a snorkeler inside MPAs

– Spear-P (blue) represents the measurement made by a spearfisher inside MPAs

– Snork-NP (orange) represents the measurement made by a snorkeler outside MPAs

– Spear-NP (red) represents the measurement made by a spearfisher outside MPAs

3) Now move into the second panel (D. sargus) and look at the red line that represents the FID (measured in meters) in the presence of a spearfisher outside MPAs. If you look at the dotted black line, they indicate that a D. sargus of 15 cm flees at about 5 meters from an approaching spearfisher. Such distance is the double (10 meters) when the fish is at about 30 cm.

As you can see such response is less and less evident when we move inside the MPAs and the approaching threat is a snorkeler. At the same time, the flight initiation response in S. salpa is shorter than the one of D. sargus. This suggests that the fish adjust their flight initiation distance according to three factors: the presence of snorkelers or spearfishers, , the level of pressure they receive from spearfishing and finally according to their size.

Now many of you will probably say: “ok, now what? I already knew that fish change their behavior when I have a speargun in my hands”.

Others could argue: “But, if spearfishing makes fish more timid, it will be prohibited more than it already is?

First of all this study explains a small part of the complexity related to the interaction between underwater human presence and fish behavior, however (in our opinion) it is important for two main reasons:

1) It uncovers many of the potential of spearfishing in understanding predator-prey interactions and provides the basis for future studies aiming to understand the ecological and evolutionary adaptation of wild animals to human predators.

2) The metric used here for measuring fish behavior (flight initiation distance) is a perfect indicator to measure the level of fish disturbance and can be easily applied to different contexts. For example, in other parts of the world it has been proposed to use this metric to manage periodically harvest closures (Goetze et al. 2017). For example, in the coral reef of the Fiji islands it has been demonstrated that after opening a marine area to fishing the flight initiation distance of target fishes rapidly increases. On the contrary, fish biomass does not decrease so rapidly. This makes FID a perfect indicator of fishing pressure and its use for the management of periodic harvest closure could be an alternative solution to total prohibition of spearfishing. Managers could allow spearfishing in a specific area as soon as the fish flight initiation distance does not increase above a security threshold that determines the closure of that specific area until the fish behavior is not fully recovered. However, this requires pilot studies and more knowledge before applying the ideas developed in the studies presented above to local spearfishing communities in the Mediterranean Sea. This would allow an adaptive management that together with other measurements can preserve the ecological equilibrium of local fish stocks and, most importantly, foster social and economic benefits of spearfishing that would be lost with a total prohibition. In fact, spearfishing is a popular recreational activity in Europe and has important historical and social roots. Recreational sea fishing (angling and spearfishing) is estimated to be practiced by 8.7 million Europeans with an annual expenditure of at about €5.9 billion (Hyder et al., 2018). An effective management is not only important for preserving fish stocks but also for the social and economic benefits associated with this activity. In conclusion, the study presented here offers the basis for the implementations of future experiments aiming to understand the most effective social, economic and ecological management of spearfishing.

What do you think about these results? You can leave a comment at the end of the article

REFERENCES USED IN THE TEXT

Goetze, J. S., Januchowski-Hartley, F. A., Claudet, J., Langlois, T. J., Wilson, S. K. & Jupiter, S. D. (2017). Fish wariness is a more sensitive indicator to changes in fishing pressure than abundance, length or biomass. Ecol. Appl. 27, 1178-1189.

Sbragaglia, V., Morroni, L., Bramanti, L., Weitzmann, B., Arlinghaus, R. & Azzurro, E. (2018). Spearfishing modulates flight initiation distance of fishes: the effects of protection, individual size, and bearing a speargun. ICES J. Mar. Sci. 75, 1779-1789.

Hyder, K., Weltersbach, M. S., Armstrong, M., Ferter, K., Townhill, B., Ahvonen, A., Arlinghaus, R., Baikov, A., Bellanger, M., Birzaks, J., et al. (2018). Recreational sea fishing in Europe in a global context-Participation rates, fishing effort, expenditure, and implications for monitoring and assessment. Fish Fish. 19, 225-243.

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