Fish learn the dangers of the speargun

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Reef fishes have learned to fear the speargun, according to a study published in the latest issue of the journal Conservation Biology.

David Feary and coauthors investigated the effects of marine areas closed to fishing by customary laws, and in doing so, found that fish that have been regularly hunted by speargun fishers are more wary and take flight much earlier on the approach of a diver compared with those living in protected zones.

The primary aim of the study was to examine the effectiveness of marine protected areas and the effects of closure on fish behaviour, and the authors chose to do this by measuring the flight distance (i.e. the distance to it that the fish will allow a diver to approach before it flees) in seven species of coral reef fish in protected and non-protected areas in Karkar Island, off the coast of Papua New Guinea.  

The seven species studied were the Orange-lined triggerfish (Balistapus undulatus), Striated surgeonfish (Ctenochaetus striatus), Daisy parrotfish (Chlorurus sordidus), Bleeker's parrotfish (Chlorurus bleekeri), Yellow-barred parrotfish (Scarus dimidiatus), Dusky parrotfish (Scarus niger), and Humpback red snapper (Lutjanus gibbus).

In the area under study, the most commonly-used fishing gear was the speargun (both rifle- and sling-type guns).

The researchers slowly approached the focal fish in a horizontal swimming position at the depth of the fish. The position of the fish was marked by noting the features on the seabed at where the fish was at the start of the observation.  

Once the fish began to flee, as indicated by an increase in swimming speed and usually followed by an abrupt change in direction, the researcher dropped a weighted marker.  Measuring the distance between this marker and another one placed at the position of the fish when the study was initiated gave the flight distance of the fish.

The authors found that the flight distances of the fish found in the protected areas were significantly shorter than those outside the areas, i.e. fishes that had not been subjected to fishing pressure by speargun fishers allowed divers to approach more closely.  

The differences in the flight distances varied for the different species, ranging from 64 cm for the striated surgeonfish to 2.8 m for the daisy parrotfish.

According to lead author David Feary: "Fish which are regularly targeted appeared to have a pretty fair idea of the 3m range of the typical rifle-style speargun used by the local PNG fishers. Inside protected areas, the fish tended to move off when the diver closed to within 2–3m of them. However those outside the protected zone, where hunting was common, mostly fled when the diver came within 4–5m of them.  

"Quite simply, the fish in areas that were fished regularly were warier and stayed further away – just far enough that it would be difficult to hit them with the spear gun technology used locally."

The authors found that closing an area to fishing allowed the fish to recover their boldness, allowing divers to once again approach them within speargun range.  

Therefore, the authors conclude that closing an area to fishing to allow fish stocks to recover may actually be counterproductive, as the fishes may become more susceptible to spear fishing when a closed area is reopened to fishing.

The authors argue that to fully protect fish stocks, the effect of temporary closures on fish behaviour have to be factored in when these reserves are reopened.  They propose the gear-use restrictions or short reopenings to avoid a heavy kill of fish that have become used to the relative safety of a closed area.

For more information, see the paper: Feary. DA, JE Cinner, NAJ Graham and FA Januchowski-Hartley (2011) Effects of customary marine closures on fish behavior, spear-fishing success, and underwater visual surveys. Conservation Biology 25, pp. 341–349.