Increased Net Depth
WHAT IS INCREASED NET DEPTH?
Similar to the way in which increasing sink rates of lines – by using line weighting or safe leads, for example – ensures that gear is beyond the diving range of seabirds or seabirds, increased net depth can place nets out of the range of diving seals, thus reducing the risk of entanglement. Altering net depth has also been shown to reduce cetacean bycatch, in the case of a drift gillnet fishery in the Indian ocean.
CURRENT USE & RESEARCH
A study by Cosgrove et al. (2013) on seal depredation in Irish set net fisheries suggested that net depth could influence the rate of seal depredation – and therefore bycatch and entanglement. Depredation was higher in pollack nets (set at a depth of 78 ± 44 m) than in the deeper hake nets (set at a depth of 152.04 ± 24.51 m). The depth of the hake nets exceeded the average seal dive depth in the area; and it was suggested that depredation in the shallower pollack nets was higher, due to easier accessibility. In a separate study in Cape Cod, Massachusetts, seal depredation generally only occurred between 25 – 57m. Understanding the average diving depths of seals, and setting nets below these depths, could therefore be a technique to reduce entanglement and bycatch.
Increasing net depth has been shown to reduce seabird bycatch in Monterey Bay (USA) and the Japanese squid fishery in the North Pacific, although in both cases, target catch was also reduced. While research has not yet been undertaken in the UK on the potential effectiveness of this mitigation, the RSPB is currently analysing time-depth recorder data from UK-breeding seabirds, which is expected to help determine the potential applicability of the mitigation to UK fisheries.
For cetaceans, there is evidence from a collaboration between WWF-Pakistan and drift gillnet fishers in the northern Indian Ocean that moving surface gillnets to the subsurface (2 metres down) can substantially reduce bycatch without significantly affecting target catch rates. However, this trial was opportunistic and preliminary in nature, and further research is needed.
This page was last updated on 11.05.21.
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