Wim J. Wolff

Introduced marine species of the North Sea coasts

About 80 non-indigenous species are assumed to have been introduced into the North Sea by transoceanic shipping and aquaculture. The number is certainly underestimated as most small organisms received insufficient attention at the species level. Also, the seafaring tradition of the North Sea countries is much longer than our biological surveys are. Most exotic invertebrates originate from the western Atlantic and were introduced by shipping, while most algae stem from the Pacific and came with the introduced oysters. A peak of newcomers was observed in the 1970s. Most of the arrivals became established in brackish environments, at harbor sites and in the vicinity of oyster farms, fouling on hard substrates or living as epibionts. A few live in sediments, are holoplanktonic or are parasites. At the open coast, approximately 6% of the macrobenthic species are exotics, while in estuaries their share is up to 20%. Most exotics have been encountered in the southern North Sea first, and many did not spread further north. About 25% of the established non-natives are widespread and attain locally high abundances. As a consequence, some inshore habitats are entirely dominated by exotics. The overall effect on the ecosystem seems to be more additive than one of displacement. This suggests that the coastal biota of the North Sea are quite capable of accommodating newcomers. However, this is no guarantee that the next introduced species may not cause severe ecological change or economic harm. There is a need to minimize the risk of unintentional introductions by ballast water treatment and by adhering to quarantine procedures in aquaculture. Current research on exotics in the North Sea is regarded as inadequate for proper evaluation and management requirements.

Oyster Imports as a Vector for the Introduction of Alien Species into Northern and Western European Coastal Waters

In western and northern Europe there have been deliberate introductions of European flat oyster (Ostrea edulis), American oyster (Crassostrea virginica), Pacific oyster (C. gigas, including the so-called Portuguese oyster ‘C. angulata’), New Zealand oyster (Tiostrea lurida), hard clam (Mercenaria mercenaria), and Manila clam (Tapes philippinarum). Between about 1870 and 1939 tens of millions of Crassostrea virginica were introduced from the Atlantic coast of North America. However, C. virginica has been unable to establish itself in Europe. For 5 other species it is very likely that they have been introduced with American oysters. Between 1964 and about 1980 C. gigas was imported on a large scale from Japan and the Pacific coast of Canada and the USA. It has established itself in Europe permanently. C gigas brought its own parasites and the imports were accompanied by the import of more than 20 species of animals. Most of these observed imports failed, however, and only about 5–6 species seem to have established themselves in European waters. As a vector for the introduction of exotic species into the North Sea area, oyster imports are slightly more important than transport on ship’s hulls, and clearly more important than introductions through ballast water. In the Dutch Oosterschelde estuary Japanese oysters interfere with the recreational use of the estuary because of their razor-sharp shells. They also seem to have changed the ecological conditions in the estuary: coinciding with the increase of the oysters, mussels and cockles decrease, as does the oystercatcher (Haematopus ostralegus). It is not yet clear if this is a causal relationship. In the Wadden Sea near the island of Sylt, C. gigas established itself as an epibiont on mussel beds, and seems to be at the verge of transforming mussel beds into oyster reefs.

Exotic invaders of the meso-oligohaline zone of estuaries in the Netherlands : why are there so many?

The numbers of exotic species introduced into brackish waters (5–20 psu) and high-salinity waters (> 20 psu) in the Netherlands are hypothesized to reflect species richness in such waters elsewhere in the world. Notwithstanding the fact that species numbers in brackish waters all over the world are lower than in high-salinity waters, the numbers of introduced species in these waters in the Netherlands are about equal. Alternative hypotheses to explain this phenomenon are: (1) because most ports are situated in brackish regions, brackish-water species stand a better chance of being transported; (2) because brackish-water species are more tolerant of conditions in ballast water tanks, these species have a better chance of being transported alive than high-salinity species; and (3) because brackish waters have few species, it is easier for an introduced species to establish itself in brackish waters. None of the latter three hypotheses can be rejected and probably they all play a part in explaining the phenomenon. The third hypothesis, however, seems most likely.

Predation on 0-group and older year classes of the bivalve Macoma balthica: interaction of size selection and intertidal distribution of epibenthic predators

The bivalve Macoma balthica is a common species in the Wadden Sea and North Sea. Juveniles temporarily use nurseries in the high intertidal. To explain this nursery use, predation pressure was examined for both juvenile and adult Macoma at low and high tidal flats. The study was carried out in the eastern Dutch Wadden Sea. Shrimps Crangon crangon, adult crabs Carcinus maenas, gobies Pomatoschistus and juvenile flatfish were more abundant and larger on low than on high tidal flats, but 0-group Carcinus was more abundant on the high tidal flats. Crangon and 0-group Carcinus stomachs frequently contained Macoma remains. These predators selectively preyed on small 0-group Macoma, both in the field and in laboratory experiments. The effect of predation by epibenthic animals and birds, on the low and high tidal flats, was examined in exclosure experiments (2 mm mesh). There was no effect of epibenthos exclosure on adult Macoma. For 0-group Macoma, densities were higher in exclosures than in the controls where predators had normal access. The density reduction by epibenthic predators was much larger in the low than in the high intertidal. We found no effect of bird predation on densities of 0- and 1+group Macoma. Thus, 0-group Macoma is under high predation pressure by epibenthos in the low intertidal, especially by shrimps, while they are relatively safe in the high intertidal. However, most of the shellfish outgrow their epibenthic predators during their first summer. Therefore, it becomes safe for the bivalves to redistribute to locations where epibenthic predators are abundant, during their first winter. On the other hand, it did not become clear from this study why many of the larger Macoma leave the high intertidal. Concluding, the nursery use of Macoma-spat in the high intertidal is probably, at least partly, an adaptation to avoid epibenthic predation.

The south-eastern North Sea

At least 31 species of marine mammals, marine and coastal birds, and marine and anadromous fish have disappeared temporarily or permanently from the coasts of The Netherlands and in most cases also from the south-eastern North Sea (south of 54 degrees N) during the past 2000 years-In 18-22 cases, the disappearance was probably due to overexploitation. For 9-12 species, physical destruction of their habitat was involved and, for 3-5 species, pollution probably played a part. Five species have returned to the area; these are doing very well. Three species may return through expansion of populations elsewhere. Anadromous fish and demersal fish species that have disappeared because of bottom-trawling in the North Sea have little chance of returning under the present conditions. For the gray whale (Eschrichtius robustus) and the Dalmatian pelican (Pelecanus crispus) suggestions are made to investigate the possibilities for re-introduction

Introduced aquatic species of the North Sea coasts and adjacent brackish waters

Introduced aquatic species have received more attention in north-western Europe following the summaries from the German North Sea coast (Gollasch 1996; Nehring and Leuchs 1999), Britain (and Ireland) (Eno et al. 1997; Minchin and Eno 2002), Norway (Hopkins 2002) and a more general account for the North Sea (Reise et al. 1999). Since then, several inventories have appeared: for the German coast (Nehring 2005), the Dutch coast (Wolff 2005) and the Danish coast (Jensen and Knudsen, 2005). In this account we review, summarise and update all those previous accounts. We have also included NIS (=non-indigenous introduced species) which were known from the North Sea but most probably are extinct in this area today, and species that have been recorded, but for which we have no proof of self-sustaining populations. For the purpose of this account:

Predation of intertidal infauna on juveniles of the bivalve Macoma balthica

Juveniles of the bivalve Macoma balthica live on tidal flats in the Wadden Sea. This study examined the interaction of Macoma with the infaunal polychaetes Arenicola marina and Nereis diversicolor and the gastropod Retusa obtusa. The distribution of M. balthica spat on the flats, shortly after settlement in April, showed a positive correlation with the Arenicola distribution and a negative correlation with Nereis distribution. There were no locations where Macoma spat and Retusa occurred together. In August, Macoma spat had grown too large for predation by intertidal infauna. Small individuals of Macoma spat were found in stomachs of Arenicola (0.14 worm 1 ) and Nereis (0.05 worm 1 ). Laboratory experiments showed that Nereis and Retusa could reduce Macoma spat abundance, both in the absence and presence of sediment and alternative prey. Arenicola reduced the abundance of small Macoma (<1 mm) in sediment without, but not with, alternative prey. In field experiments, we manipulated the density of Arenicola in 0.25–1 m 2 plots and of Nereis in 0.03 m 2 cages and examined the effect on Macoma density several weeks later. We found a significant negative relation between densities of polychaetes and Macoma spat for both polychaete species in these experimental plots. Peculiarly, we found a significant positive relation between manipulated Nereis density and adult Macoma density in the cages; we cannot explain this. Consumption rates, calculated both from stomach contents and from field experiments, were 45 to 102 Macoma m 2 d 1 for Arenicola and 5 to 116 Macoma m 2 d 1 for Nereis. These values are higher than recorded consumption rates by epibenthic predators in the same area. Nevertheless, between-year differences in year-class strength could not be explained by differential abundance of these polychaetes. In conclusion, Arenicola and Nereis had a negative effect on the abundance of Macoma <1.5 mm, which was at least partly caused by direct consumption. Retusa obtusa can eat juvenile Macoma, but probably did not so in the study area, because there were no locations where Retusa and Macoma spat occurred together in the period that Macoma was <2 mm. D 2002 Elsevier Science B.V. All rights reserved.

Invasions of estuaries vs the adjacent open coast: A global perspective

Invasions by alien species have been reported from every marine habitat where surveys have been conducted for them. Conspicuous examples from around the globe include the brown alga Sargassum mangarevense in tropical coral reef systems (Andrefouet et al. 2004), the bivalve Mytilus galloprovincialis along temperate rocky shores (Steffani and Branch 2003), and the reef-building polychaete, Ficopomatus enigmaticus in estuaries (Schwindt et al. 2004). Despite numerous examples of marine invaders from a variety of habitats, little is known about how invasion rates of entire assemblages of organisms compare between different marine habitat types. And indeed most marine habitats have not been thoroughly surveyed – the majority of our understanding of marine invasions comes from shallow near-shore environments. Some studies have attempted to quantify habitat differences in marine invasions, examining assemblages (both natives and aliens) at different scales. Within estuarine ecosystems, focus has been on comparisons between different salinities and substrates. (In this chapter an estuary is considered to be a ‘partly enclosed body of water by the coast in which sea water and fresh water mix‘ (Little 2000).) Wolff (1973) examined the benthic macroinvertebrates of four major estuaries in the Netherlands. He found that in the high salinity parts of these estuaries about 2% of the species were alien, in the brackish part about 20%, and in the tidal freshwater part about 8%. In non-tidal brackish waters the share of alien species was about 28%. Wolff (1999) re-analyzed these data and included three more estuaries in the northern Netherlands and Germany. He found that tidal and stagnant low salinity habitats of seven Dutch and German estuaries harbored a higher proportion of alien species (about 20%) than estuarine high salinity habitats (about 6%). This pattern was not clearly related to propagule pressure (harbors and aquaculture were not focused in the middle salinity). Lee et al. (2003) found that patterns of invasion varied along an estuarine gradient in San Francisco Bay; soft-bottom benthic communities at estuarine salinities were more invaded than communities at either brackish or marine salinities. Wasson et al. (2005) found hard substrates to be more invaded than soft substrates, and a site near the mouth of an estuary to be less

Feeding current characteristics of three morphologically different bivalve suspension feeders, Crassostrea gigas, Mytilus edulis and Cerastoderma edule, in relation to food competition

Introduced Pacific oysters (Crassostrea gigas) have shown rapid expansion in the Oosterschelde estuary, while stocks of native bivalves declined slightly or remained stable. This indicates that they might have an advantage over native bivalve filter feeders. Hence, at the scale of individual bivalves, we studied whether this advantage occurs in optimizing food intake over native bivalves. We investigated feeding current characteristics, in which potential differences may ultimately lead to a differential food intake. We compared feeding currents of the invasive epibenthic non-siphonate Pacific oyster to those of two native bivalve suspension feeders: the epibenthic siphonate blue mussel Mytilus edulis and the endobenthic siphonate common cockle Cerastoderma edule. Inhalant flow fields were studied empirically using digital particle image velocimetry and particle tracking velocimetry. Exhalant jet speeds were modelled for a range of exhalant-aperture cross-sectional areas as determined in the laboratory and a range of filtration rates derived from literature. Significant differences were found in inhalant and exhalant current velocities and properties of the inhalant flow field (acceleration and distance of influence). At comparable body weight, inhalant current velocities were lower in C. gigas than in the other species. Modelled exhalant jets were higher in C. gigas, but oriented horizontally instead of vertically as in the other species. Despite these significant differences and apparent morphological differences between the three species, absolute differences in feeding current characteristics were small and are not expected to lead to significant differences in feeding efficiency.

The exploitation of living resources in the Dutch Wadden Sea: a historical overview

An overview, based on written sources and personal observations, is presented of exploitation of living resources in and around the Dutch Wadden Sea during the past few centuries. It is concluded that before about 1900 exploitation was almost unrestricted. Exploitation of plants has been documented for saltmarshes and eelgrass beds. Fisheries have occurred for two species of hydroids, two species of polychaetes, one echinoderm species, at least seven species of molluscs, three species of crustaceans, and tens of species of fish. Hunting and egg collecting targeted almost all species of birds. Finally, two species of seals were exploited: information on exploitation of cetaceans is not available. Hence, it is likely that overexploitation may have been involved in the extirpation of several species. This supports an earlier suggestion that overexploitation played a part in the disappearance of at least 17 species from the Dutch Wadden Sea. This conclusion is confirmed by the observation that several extirpated species have returned after protective measures were introduced for the Dutch Wadden Sea area.