Decline of the North Sea houting: protective measures for an endangered anadromous fish

Niels Jepsen, Michael Deacon, Anders Koed

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Once an abundant fish species in the rivers of the Wadden Sea in northwest Europe,
the North Sea houting Coregonus oxyrinchus (NSH) was at the brink of extinction 25 yr ago. The
very last stronghold for this species was in the small Danish River Vidaa. In an attempt to preserve
this anadromous whitefish species, juveniles were hatchery-reared and stocked in 6 Danish rivers
flowing into the Wadden Sea. These stocking efforts were fairly successful, and the houting established
populations in at least 2 of the rivers, but the underlying problem of habitat degradation and
migration obstacles was not solved. Since a national management plan was issued in Denmark in
2003, large-scale river restoration measures have been undertaken, including the removal of 13
large obstacles in 3 NSH rivers and reestablishment of meanders in the lower river reaches. These
measures were all intended to increase the spawning area for NSH and reduce juvenile mortality
of NSH. The generic effect of these recent and ongoing changes is, however, not yet known. The
NSH population has been irregularly monitored under various programs. Results from 15 yr of
field investigation using passive integrated transponder (PIT)-tagging showed that the NSH population
in the River Vidaa remains stable, but in the other 2 rivers populations are decreasing.
However, due to the recent physical improvements in river habitat and connectivity, we expect a
substantial improvement in the population status of the NSH in the near future. Once they reach
sexual maturity, NSH grow relatively slowly (mean: 2.55 cm yr−1, ranging from 0 to 13.8 cm yr−1)
and can reach an age of 10 to 12 yr. The number of repeated recaptures year after year indicates
low mortality for adult fish. Six individuals were recaptured in rivers other than the one in which
they were initially PIT-tagged, indicating some exchange/straying between adjacent rivers. Overall
there is a good chance that this unique species was saved in the 11th hour by intervention from
managers and the EU
Original languageEnglish
JournalEndangered Species Research
Pages (from-to)77-84
Publication statusPublished - 2012

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