Fishery bycatch is among the most important threats to the European population of Greater ScaupAythya marila

Abstract

SummaryFor Greater Scaup Aythya marila, classified as ‘Vulnerable’ on the European Red List of Birds, the south-western Baltic Sea is one of the most important wintering sites in Europe. In this area, a large concentration of gillnet fishery temporally overlaps periods of the most abundant occurrence of foraging diving birds. The aim of the article is to show how bycatch can impact the population of a diving duck. To assess this, we calculate the Potential Biological Removal (PBR) for the studied Greater Scaup population and we model the population change according to age-structured matrix models. Summing all the available recent figures on Greater Scaup bycatch in north-west Europe yields an estimated mean annual total of 3,991 individuals (2% of the flyway population). For a baseline stable population, an age-structured matrix model indicates that at this bycatch level the Greater Scaup population that winters in north-west Europe will decrease by 36% over the next 30 years, qualifying the status of the population as ‘Vulnerable’ according to IUCN criteria. As this population also experiences decline prior to bycatch, this decrease will be 57%, which qualifies the status as ‘Endangered’. PBR as an indicator of population vitality does not work in our case because the PBR-informed allowable bycatch values have a significantly negative impact on the population. Our results indicate unambiguously that fishery bycatch is among the most important threats responsible for the Greater Scaup’s decline. While recent data suggest that some improvement has taken place in the species’ status over the last 10 years, measures to protect Greater Scaup from bycatch are required. The solution should involve the prohibition of gillnet fishing in selected key sites and the use of mitigation techniques in other areas.

Effectiveness of the European Natura 2000 network to sustain a specialist wintering waterbird population in the face of climate change

“…We suspect that this may be partly the result of the significant decrease in the Scaup bycatch in the Netherlands 29 – 31 . The added mortality from fisheries bycatch represents one of the most important threats to the relatively long-lived Scaup 32 . Evidence showed that drowning mortality was extremely high between 1985 and 1994, when an estimated average of 17,672 birds died annually in fishing gear (6% of the total population of the time), but this has declined since the 2000s 32 .…”

“…The added mortality from fisheries bycatch represents one of the most important threats to the relatively long-lived Scaup 32 . Evidence showed that drowning mortality was extremely high between 1985 and 1994, when an estimated average of 17,672 birds died annually in fishing gear (6% of the total population of the time), but this has declined since the 2000s 32 . Of all Scaup from this flyway population that drowned in fishing nets in years 1978–1990, up to 65% died at the most important wintering site at the time—the Dutch IJsselmeer 32 .…”

“…Evidence showed that drowning mortality was extremely high between 1985 and 1994, when an estimated average of 17,672 birds died annually in fishing gear (6% of the total population of the time), but this has declined since the 2000s 32 . Of all Scaup from this flyway population that drowned in fishing nets in years 1978–1990, up to 65% died at the most important wintering site at the time—the Dutch IJsselmeer 32 . However, our highly uneven knowledge of the extent of the Scaup bycatch throughout its winter range should be taken into account here.…”

“…Exceptionally detailed estimates from IJsselmeer during the earlier period 14 contrasts our lack of data or poor estimates from elsewhere, which may result in a bias that implies a greater importance for Scaup bycatch at the IJsselmeer for the population than was actually the case. Current estimates of bycatch levels throughout the flyway suggest that Scaup death in fishing nets has decreased, amounting to c. 4000 individuals yearly, partly explained by the substantial decrease in the Dutch bycatch 32 .…”

“…During spring migration, extremely large Scaup concentrations can occur in these important sites, which provide for other biological functions such a communal courtship, displaying, pair-bonding etc. 32 . Given that Scaup are among the most vulnerable of diving ducks to bycatch 34 (constituting more than 50% of diving birds drowned in fishing nets in the Polish Odra Estuary 35 ) potentially high mortality during the prelude to the breeding season is likely to have severe adverse effects on the entire population.…”