Invasion trajectory of Pacific oysters in the northern Wadden Sea

Karsten Reise,Johannes Rick,Christian Buschbaum,Heike Büttger,K Mathias Wegner

doi:10.1007/s00227-017-3104-2

Abstract

Invasion trajectories of introduced alien species usually begin with a long establishment phase of low abundance, often followed by exponential expansion and subsequent adjustment phases. We review the first 26 years of feral Pacific oysters Crassostrea gigas around the island of Sylt in the Wadden Sea (North Sea, NE Atlantic), and reveal causal conditions for the invasion phases. Sea-based oyster farming with repeated introductions made establishment of feral oysters almost inevitable. Beds of mussels Mytilus edulis on mud flats offered firm substrate for attachment and ideal growth conditions around low tide level. C. gigas mapped on to the spatial pattern of mussel beds. During the 1990s, cold summers often hampered recruitment and abundances remained low but oyster longevity secured persistence. Since the 2000s, summers were often warmer and recruitment more regular. Young oysters attached to adult oysters and abundances of >1000 m−2 were achieved. However, peak abundance was followed by recruitment failure. The population declined and then was also struck by ice winters causing high mortality. Recovery was fast (>2000 m−2) but then recruitment failed again. We expect adjustment phase will proceed with mean abundance of about 1000 m−2 but density-dependent (e.g., diseases) and density-independent (e.g., weather anomalies) events causing strong fluctuations. With continued global warming, feral C. gigas at the current invasion fronts in British estuaries and Scandinavian fjords may show similar adjustment trajectories as observed in the northern Wadden Sea, and also other marine introductions may follow the invasion trajectory of Pacific oysters.

Highlights

Transfers of marine organisms in the wake of expanding global markets and seafaring are increasingly changing the species compositions along coasts (Rilov and Crooks 2009)
Four years after the farm started production of Pacific oysters on the island of Sylt, breeding and subsequent larval dispersal led to first settlement of feral Crassostrea gigas (Fig. 3)
In 1995, a first survey was conducted in the List and Hörnum tidal basins of Sylt to assess the extent of spread

Summary

Introduction

Transfers of marine organisms in the wake of expanding global markets and seafaring are increasingly changing the species compositions along coasts (Rilov and Crooks 2009). To compare invasion trajectories between populations or species, Reise et al (2006) suggested an empirical scheme of three phases: after one or more introduction events, an establishment phase of low abundance and minor spread may linger on for several generations. It may be followed by exponential expansion phase leading to dominance with strong impacts. For initial phases we can ask what facilitated or inhibited establishment and expansion, while later phases of the invasion can indicate what has stopped the population boom Answering such questions and providing details from closely monitored invasion processes might provide the necessary data to develop a general model of marine invasions that can be used to implement proper management. Abundant mussel beds up- and downstream of the oyster farm offered ample settling sites for oyster larvae at suitable depth in this sedimentary environment

Methods

Results

Discussion

Conclusion