Historical Distribution of the European Flat Oyster in the German Bight and Causes of Extinction

Dr. Jens Gercken Institut für Angewandte Ökosystemforschung GmbH 18184 Neu Broderstorf

BfN - European Flat Oyster Workshop – Isle of Vilm 15.-16. November 2012

1 Outline • Introduction • North Frisian Wadden Sea • East Frisian Wadden Sea • Helgoland oyster bank • North Sea oyster ground

• Synopsis - historical oyster distribution - reasons for decline

• Outlook -recent oyster activities

K. Reise Introduction

Different areas of former German oyster banks and fisheries • East Frisian Wadden Sea (A) • North Frisian Wadden Sea (B) B • North Sea oyster ground (C) • Helgoland oyster bank(D)

C D Wadden Sea

• World largest tidal flats (14.700km²) • 1978: Trilateral Wadden See Coop. A • 1985-1990: 3 German National Parks • 2009: UNESCO World Heritage North Frisian Wadden Sea

North Frisian Oyster Beds

• Most important German coastal oyster fishery. • 11th century: Danish King Canute introduced oysters from England to west coast of Schleswig-Holstein. • Since 13th century oysters were brought on the market in Hamburg. • 1587: Decree by Danish King Fredric II: unauthorized harvesting of oysters will be punished. • 1627: Fishing rights transfered to single lease holders. • Management strategies: eg. minimum landing size, closed season, stock assessment, relaying and restocking. • 1695: 10 oyster banks between Römö and Föhr were ruined. • Surveys of condition of oyster beds since 1709. From 1730 to 1852 10 surveys to assess the state of oyster beds. • Since 1873 steady decline of yields. No recovery even after closed season. • Unregular long lasting variations of oyster stocks. Strong negative effects of harsh winters. • Continuous overfishing of the banks by different lease holders. Möbius, 1877 • Invention of motorized boats increased fishing pressure. North Frisian Wadden Sea

Investigation of fiscal oyster beds by Karl Möbius

• Professor of Zoology at the University of Kiel (1868-1888).

• Prussion agricultural ministry commissioned Möbius to investigate the oyster beds.

• 1868-1870: study of the ecology of oyster banks.

• 1877: „The oyster and oyster culture“. Karl Möbius, 1825-1908 • Möbius took oyster beds of Sylt as an example to develop his concept of a living community (biocoenosis).

• Wadden Sea area were oysters exist: 74 x 22km.

• Oyster settle on only <1% of the sublittoral Wadden Sea area.

• The only natural limiting factor is the unfavorable sediment in most parts of the Wadden Sea. North Frisian Wadden Sea

Karl Möbius (1877) Location of an oyster bank in a tidal channel

• 47 oyster banks of very different size exist. The largest one was about 3km long.

• Preferred water depth: from 2m below low tide to 6-9m.

• Estimated 5 M. oysters live in S.-H. area. Möbius, 1877 • Oyster banks in a good state: 1-3 adult oysters per m²; mainly single not in clumps.

• Oyster beds east of Sylt and near the islands of Amrum and Föhr produced the best oysters.

• Reproduction from mid June till end of August/early September.

Reise, 2005 North Frisian Wadden Sea

Survey of Prussion fiscal oyster banks (Hagmeier & Kändler, 1927)

• 1924/25: survey about the state of oyster banks. • Most banks were in very bad condition. • Oysters, shells and stones covered with epifauna, almost no juveniles. • Since 1873 steady decline of harvest. • Decline of Wadden Sea banks coincided with decline of North Sea Oyster ground and Helgoland oyster bank. • Standing stock of oysters capable of reproduction were likely to be low already in the 1870s. • 1884 – 1930: oysters from Netherland, France and Norway introduced. North Frisian Wadden Sea

Categories of oyster banks (Hagmeier & Kändler, 1927) • Banks at slopes of tidal channels (Strombänke): • Hard substrate at channel slopes • Strong water exchange • No danger of harsh winter temperatures • Production of larvae feeds other banks • 0,17-0,43 oyster/m²

• Shallow banks (Flachbänke): • Low current velocity • Large areas • Minor water depth • Slow oyster growth; • Negative factors: silt, epifauna, Mytilus • 0,13-0,24 oyster/m²

• Inner banks (Binnenbänke): • At slopes of smaller tidal channels. Usually deeper than shallow banks. • Good conditions for larval settlement. Negative factors: temperature, epifauna, Mytilus • AWI, 2009 • Smallest fraction of fiscal banks. • 0,5-3,4 oyster/m² North Frisian Wadden Sea

Long term variation of oyster populations (Hagmeier & Kändler, 1927)

• Population fluctuations are likely due to a short increase and a longtime decline.

• Population rise: • Optimal hydrographic conditions • Little competion (food and space) • Sustainable harvest

• Population decline: • Adverse hydrographic conditions • Excessive competition (food and space) • Storm/frost • Overfishing and damage by fishing gear

A natural population rise did not occur in the last decades. North Frisian Wadden Sea

Conclusions (Hagmeier & Kändler, 1927)

• Oyster fishery no longer possible. Natural banks should be transfered into oyster cultures. • Total available area for oyster culture: 1.800 ha. • Restocking with imported oysters and cleaning of the banks. • Estimated future oyster harvest could be 4-5 millions.

Oyster lab at Sylt, 1924 East Frisian Wadden Sea

East Frisian oyster banks

• 1642: oyster beds mentioned first in archives of the county of Oldenburg. • 1715: bank near Borkum discovered and others later near Baltrum, Juist and Langeoog. • 1728: Fishery was privilege of the prince of Oldenburg. However, strong illegal catch. • Yield much lower than in North Frisia. • Extremely cold winters in 1740 and 1771-1772. • Yield declined already at the end of 18th century. • Restocking in 1800 and a closed season (1816-1823) did not stop decline. • Möbius (1877): East Frisian Wadden Sea less suitable for growing of oysters compared to North Frisia. • 1869: Near Juist an attempt to settle oysters was performed in 1869. However, it failed!

Wikipedia Helgoland Oysters

Helgoland oyster bank

• 4nm ESE of Helgoland; 0,8km². • 1875–1886: intensive fishery by fishermen from Helgoland. • After 1878 decline of fishery despite a short recovery after a closed season from 1879-1882. • 1910-1923: cessation of commercial fishery. • 1936/37: Survey by Caspers: Oysters have been replaced by Nucula nucleus. Spectrum of species still present. • 2003: Former range of species disappeared. Likely due to intensive trawling activities. • Overfishing is main reason for decline. • 2005: A single native oyster has been caught on a scientific cruise at the former oyster bank. North Sea Oyster Ground

North Sea Oyster Ground • 100-150nm offshore in 40m deep water; appr. 20.000 – 25.000 km² (Berghahn & Ruth, 2005). • Starting as a small strip SE off Helgoland, stretching WNW covering a 15 to 22km broad strip far to the west. Depth mainly about 33-34m (Möbius, 1877). • Much higher spawning stock biomass compared with the Wadden sea (100-1000 times). • Oysters growed bigger than Wadden Sea oysters, however quality was not as good. • Fishing season from November to March. English, Dutch and later German fishermen.

• 1875–1886: fishery by fishermen from Finkenwerder (Hamburg). Olsen, 1886 • Deep-water oyster stocks almost certainly destroyed by the heavy gear of fish trawlers.

• Recruitment and stabilization of Wadden Sea Oyster beds through inflow of larvae from North Sea stocks (?)

Synopsis – historical oyster distribution

Wadden Sea Oyster Bands

References: 1) J. Lozán, W. Lenz, E. Rachor (Hrsg.): Warnsignale aus der Nordsee; Parey1990; ISBN 3-489-64634-7; S. 168 2) A. Hagmeier, R. Kändler: Neue Untersuchungen im nordfriesischen Wattenmeer und auf den fiskalischen Austernbänken; Band XVI Nr. 6; BA Helgoland 1927 3) O.T. Olsen: Priscatorial Atlas; London 1883 4) Th. Neudecker: History of the Former German Oyster Fishery and Mariculture: 400 Years of Crown Law on Oyster; Dt. hydrogr. Z., Erg.-H. B, 22: 518-525 Synopsis – historical oyster distribution

North Sea Oyster Ground Summarising Remarks

Reasons for decline • Overexploitation of stocks (Neudecker, 1990) • Destruction of beds by fishing gear • Harsh cold winters • Change of hydrodynamic forces • High inorganic seston content • Replacement by mussel beds • Overgrowing by epifauna • Predators and pathogens • Water quality (contaminants)

A: Estern Frisia B: Northern Frisia C: North Sea oyster ground D: Helgoland stock Short Outlook

EU FP7 bivalve aquaculture projects • oysterrecover • REPROSEED • SETTLE

Recovery and restoration • Limfjord (Denmark) • Strangford Lough (Northern Ireland) Beck et al., 2011 • Restoration project in South Wales • TNC oyster reef conservation and restoration (USA)

Economic aspects • Ecosystem Services provided by oyster reef/banks