Caught in the net

Bycatch in gillnet fisheries may threaten waterbird populations

Photo: M. Vetemaa
Somewhere between 100,000 and 200,000 waterbirds are killed every year after being caught and tangled up in fishing gear. A new report recently published in Biological Conservation sheds new light on the critical consequences of bird bycatch in coastal gillnet fisheries in the Baltic Sea and the North Sea region.
 
- Although bird bycatch in gillnets has been recognised in many places, its magnitude and significance remains largely unknown, centre researcher Henrik Österblom says. He is one of the authors behind the report along with Ramúnas Žydelis, Jochen Bellebaum and others.
 
Main cause for bird mortality
The introduction of synthetic gillnets in the 1960s revolutionized world fisheries by providing inexpensive, long-lasting and easy to handle gear. Consequently, gillnets became widely used in small-scale coastal fisheries.
 
The ecological flip side of things were waterbirds that got entangled in the gillnets when diving into the water.
 
Österblom and his colleagues found that the seasonality of seabird bycatch depends directly on temporaral overlap of fisheries and bird presence. Basically, bycach is higher in winter and during migration. They also found that some birds are more prone to risk than others.
 
- Piscivorous birds, which pursue their prey underwater, are more susceptible to bycatch than the so-called benthivorous ducks, which typically dive straight to the bottom and forage on sessile organisms swimming relatively little in a horizontal plane, Österblom says.
 
He also stresses the significance of certain net characteristics, including net visibility (thickness and colour), droppiness, height and the net mesh size.
 
Zero bycatch not realistic, but...
The researchers see no point in aiming for a zero bycatch, but still believe that action must be taken to reduce it.
 
- Ideally we should strive at zero bycach, but given the economic and social importance of many coastal fisheries, it is more realistic to aim that bycatch at least does not negatively affect birds populations, leading to the approach that human-induced bird mortality levels that exceed intrinsic population growth rates should be treated as unacceptable, co-author Ramúnas Žydelis says.

As much as knowledge about survival rates, reproductive performance and delineation of population segments is sparse or unavailable, the researchers call for an elaborated effort to reduce the bycach.
 
- Crucial to the solution of the problem is the willingness of fishermen and authorities to tackle the problem and to promote the co-existence of fisheries and bird populations, the researchers conclude.

Reference

Citation

Žydelis, R., Bellebaum, J., Österblom, H., Vetemaa, M., Schirmeister, B., Stipniece, A., Dagys, M., van Eerden, M., Garthe,S. 2009. Bycatch in gillnet fisheries — An overlooked threat to waterbird populations. In Biological Conservation, Volume 142, Issue 7, Pages 1269-1281.
Henrik Österblom is a joint theme leader for Governance and ecosystem management of coastal and marine systems. Henrik has a PhD in marine ecology from Stockholm University and his thesis work was mainly focused on the effects of fisheries and fish stock dynamics on marine birds, and interest he gained from Triangle Island field station in the North East Pacific.

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