Oyster transports are among the leading anthropogenic vectors of coastwise introduction of nonindigenous species. Using the oyster industry of the Netherlands as a model system, we investigated the relationship between vector strength (number of invasions) and vector tempo (magnitude and frequency of transport) in analyzing and predicting invasion patterns. We reviewed literature on oyster-associated species introductions, analyzed the scale of commercial oyster imports, and collected and identified epiflora from Pacific oyster shells. A total of 35 protist, algal, and invertebrate species have been introduced to the Netherlands with oysters, and we found 41 species of macroalgae on transported oysters. However, the number of introductions and quantity of oysters imported are not necessarily positively correlated, particularly in the past 20 yr, when oyster imports decreased but the rate of introductions increased. The discrepancy between vector tempo and strength can be explained by unreported imports and vector characteristics: a single oyster may harbor a large number of species which are introduced with their substrate, thus facilitating establishment. Further, the recently developed extensive Pacific oyster reefs in Dutch waters provide a suitable substrate, enabling establishment even after low propagule pressure introduction events. Assumptions that are made about crucial parameters need to be reconsidered: reported propagule pressure is not the same as actual propagule pressure; per-episode diversity of potential inoculants is not at a fixed level without episodic unpredictable spikes, and the recipient environment is not static. With increasing interest in predicting invasion patterns, caution must be taken in assuming that reduced propagule pressure will lead to reduced invasions.

This paper provides a brief overview of ecological research in the Wadden Sea during the last 50years and its role in the conservation of the area. Conservation of the Wadden Sea would not have been possible without solid scientific information and a good interlink between science and conservation will also be crucial for the future management. Some suggestions for future research topics are made.

The Wadden Sea is one of the largest intertidal areas in the world and has been designated as a UNESCO World Heritage Site in recognition of its unique natural features. Major changes in the morphology and ecology of the Wadden Sea over the past millennium resulted from increasing anthropogenic influences such as coastal protection, land claim from the sea and drainage of wetland for agriculture, exploitation of natural resources from hunting and fishing to the extraction of groundwater, gas and oil, industrialisation at port locations and tourism at the islands. A sustainable future can only be achieved if policy and management are backed by solid science. Many of the anticipated changes result from the upscaling of pressures on the Wadden Sea system. Economic globalization leads to upscaling of fisheries, tourism and industrial activities, and thus to changed pressures on space and nature. Climate change will lead to changes in hydrographic patterns, species distribution and possibly tourism; through sea-level rise it will put pressure on coastal protection and the extent of intertidal areas. Invasions of exotic species will transform the ecosystem. There are three major related challenges to management: 1. Nature conservation in a changing system requires a focus on preservation of the values and not the state of the system; 2. The adaptation of the management structure to the scale increase of the pressures, so that local and regional management becomes better nested in transregional and transnational governance structures; 3. Finally, the management approach needs to deal with increasing uncertainty in external forcing of the system, as well as with nonlinearities in system dynamics when it is pushed outside its normal range of operation. Based on these pressures and management challenges, we advocate an integrated social-ecological systems approach for the scientific study and the science-based management of the Wadden Sea Region. The essential characteristics of this approach are strong interdisciplinarity and a focus on aspects of scale and cumulative processes.

Oyster transports are among the leading anthropogenic vectors of coastwise introduction of non- indigenous species. Using the oyster industry of the Netherlands as a model system, we investigated the rela- tionship between vector strength (number of invasions) and vector tempo (magnitude and frequency of transport) in analyzing and predicting invasion patterns. We re- viewed literature on oyster-associated species introduc- tions, analyzed the scale of commercial oyster imports, and collected and identified epiflora from Pacific oyster shells. A total of 35 protist, algal, and invertebrate species have been introduced to the Netherlands with oysters, and we found 41 species of macroalgae on transported oysters. However, the number of introductions and quan- tity of oysters imported are not necessarily positively cor- related, particularly in the past 20 yr, when oyster imports decreased but the rate of introductions increased. The discrepancy between vector tempo and strength can be explained by unreported imports and vector characteris- tics: a single oyster may harbor a large number of species which are introduced with their substrate, thus facilitat- ing establishment. Further, the recently developed exten- sive Pacific oyster reefs in Dutch waters provide a suit- able substrate, enabling establishment even after low propagule pressure introduction events. Assumptions that are made about crucial parameters need to be recon- sidered: reported propagule pressure is not the same as actual propagule pressure; per-episode diversity of po- tential inoculants is not at a fixed level without episodic unpredictable spikes, and the recipient environment is not static. With increasing interest in predicting invasion patterns, caution must be taken in assuming that reduced propagule pressure will lead to reduced invasions.

Aim The tunicate Molgula manhattensis has a disjunct amphi-Atlantic distribution and a recent history of world-wide introductions. Its distribution could be the result of regional extinctions followed by post-glacial recolonization, or anthropogenic dispersal. To determine whether the North Atlantic distribution of M. manhattensis is natural or human-mediated, we analysed mtDNA cytochrome c oxidase subunit I (COI) sequence variation in individuals from cryptogenic and introduced ranges. Location North Atlantic Europe and America; Black Sea; San Francisco Bay; Osaka Bay. Methods Nuclear 18S rDNA sequences were used to resolve phylogenetic relationships and mtDNA COI sequences for phylogeographic analyses. Results Phylogenetic analyses confirmed that M. manhattensis and M. socialis, which are frequently confused, are distinct species. MtDNA haplotype diversity was nearly three times higher with deeper relationships among haplotypes on the North-east American coast than in Europe. Diversity declined from south to north in America but not in Europe. In areas of known introductions (Black Sea, Japan, San Francisco Bay), M. manhattensis showed variable levels of haplotype diversity. Medium-to-high-frequency haplotypes originating from the North-west Atlantic were present in two locations of known introductions, but not in Europe. Private haplotypes were found on both sides of the Atlantic and in introduced populations. The mismatch distribution for the North-east Atlantic coast indicates a recent expansion. Main conclusions Molgula manhattensis is native in North-east America. However, whether it was introduced or is native to Europe remains equivocal. Additional sampling might or might not reveal the presence of putative private European haplotypes in America. The low European diversity may be explained by low effective population size and a recent expansion, or by low propagule pressure of anthropogenic introduction. Absence of medium-to-high-frequency American haplotypes in Europe may be the result of exclusive transport from southern ports, or long-term residence. These arguments are ambiguous, and M. manhattensis remains cryptogenic in Europe.

Behavioral responses of Risso's dolphins (Grampus griseus) to whale watching vessels were studied off Pico Island, Azores. Dolphin behavior was studied from a land-based lookout, enabling observations of groups in the absence and presence of vessels. The number of whale watching vessels showed a clear seasonal pattern, dividing the whale watching period into a low season and a high season. During the low season, Risso's dolphins rested mainly in the morning and afternoon. During the high season, Risso's dolphins rested less and did so mainly at noon, when the number of active vessels was lowest. Data analysis using a generalized additive mixed model indicated that this change in resting behavior was associated with vessel abundance. When more than five vessels were present, Risso's dolphins spent significantly less time resting and socializing. During the high season, this vessel abundance was exceeded during 20% of observation days. While we cannot be sure that the observed changes in behavior have fitness consequences for Risso's dolphins, reduced resting and socializing rates can have negative impacts on the build-up of energy reserves and on reproductive success. We suggest the adoption of precautionary management measures to regulate the timing and intensity of whale watching activities.

The West African lucinid bivalve Keletistes rhizoecus (Oliver, Basteria 50:47–64, 1986 ) is only known from the Niger Delta in Nigeria. Due to inaccessibility of its habitat population biology, growth parameters, biomass, and annual secondary production are unknown. The danger of oil pollution threatens the localities where this species occurs. Hence, ecological characteristics of the species were investigated quantitatively from May 2007 to April 2008 at Sivibilagbara, a protected mangrove swamp at Bodo Creek in the lower Niger Delta. Density of this chemosymbiotic lucinid was significantly higher than data previously reported. Temporal size distribution of the population showed minor changes due chiefly to recruitment and growth increments. Recruits peaked in February and September. The species lifespan is estimated to be 1.2 years. The biomass and production values are relatively high, but comparable to those of other bivalve species, especially those from nearby Andoni intertidal flats.

2 Samenvatting 2

Fisheries management in European waters is gradually moving from a single-species perspective towards a more holistic ecosystem approach to management (EAM), acknowledging the need to take all ecosystem components into account. Prerequisite within an EAM is the need for management processes that directly influence the ecological effects of fishing, such as the mortality of target and non-target species. Up until recently, placing limits on the quantities of fish that can be landed, through the imposition of annual total allowable catches (TACs) for the target species, has been the principal management mechanism employed. However, pressure on non-target components of marine ecosystems is more closely linked to prevailing levels of fishing activity, so only if TACs are closely related to subsequent fishing effort will TAC management serve to control the broader ecosystem impacts of fishing. We show that in the mixed fisheries that characterise the North Sea, the linkage between variation in TAC and the resulting fishing effort is in fact generally weak. Reliance solely on TACs to regulate fishing activity is therefore unlikely to mitigate the impacts of fishing on non-target species. Consequently, we conclude that the relationship between TACs and effort is insufficient for TACs to be used as the principal management tool within an EAM. The implications, and some alternatives, for fisheries management are discussed.

An essential prerequisite of a sustainable fisheries management is the matching of biologically relevant processes and management action. In fisheries management and assessment, fish stocks are the fundamental biological unit, but the reasoning for the operational management unit is often indistinct and mismatches between the biology and the management action frequently occur. Despite the plethora of population genetic data on marine fishes, to date little or no use is made of the information, despite the fact that the detection of genetic differentiation may indicate reproductively distinct populations. Here, we discuss key aspects of genetic population differentiation in the context of their importance for fisheries management. Furthermore, we evaluate the population structure of all 32 managed marine fish species in the north-east Atlantic and relate this structure to current management units and practice. Although a large number of studies on genetic population structure have been published in the last decades, data are still rare for most exploited species. The mismatch between genetic population structure and the current management units found for six species (Gadus morhua, Melanogrammus aeglefinus, Merlangius merlangus, Micromesistius poutassou, Merluccius merluccius and Clupea harengus), emphasizes the need for a revision of these units and questions the appropriateness of current management measures. The implementation of complex and dynamic population structures into novel and less static management procedures should be a primary task for future fisheries management approaches.

Predation by adult bivalves oil bivalve larvae has been suggested to reduce larval abundance in areas with high bivalve filter-feeder biomass. Although the occurrence of larviphagy is well-studied in the laboratory, its effects in the field have scarcely been studied. We studied larviphagy at different spatial scales in the Oosterschelde estuary. On the scale of individuals, we confirmed that larviphagy occurs in Crassostrea gigas and Mytilus edulis in the Oosterschelde estuary, by examining stomach contents of adult bivalves. On a local scale, we studied effects of larviphagy by a Pacific oyster (C gigas) bed on presence of larvae in the overlying water column by sampling larvae with fixed plankton nets. Abundance of blue mussel (M. edulis) larvae was significantly reduced by the oyster. Abundance of C gigas larvae did not seem to be reduced by the oyster bed, but spawning by the adult oysters during the sampling period may have affected the results. On estuary-scale, the effect of larviphagy on larval abundance of C gigas and M. edulis was studied using existing monitoring data over 6 years for M. edulis and 13 years for C. gigas. Numbers of M. edulis larvae showed no significant trend over the 6 years studied. Abundance of C gigas larvae declined with an increasing filter feeder stock (that was mainly caused by an increase in C gigas stock). This decline may be due to direct effects of larviphagy or indirect effects such as lowered food levels, and was not compensated by an increased larval production. All results combined, complemented with a theoretical estimate of the effect of larviphagy on estuary-scale, strongly suggest that larviphagy is major source of mortality for bivalve larvae in the Oosterschelde estuary. (C) 2008 Elsevier B.V. All rights reserved.

Demersal fishing alters seabed habitats and affects the structure and functioning of benthic invertebrate communities. At a critical level of disturbance, such communities may approach an equilibrium disturbed state in which a further increase in disturbance has little additional impact. Such arguments have been used to suggest that an ecosystem approach to fisheries management (EAFM) should protect lightly fished areas and deflect fishing activity into areas that are already intensively fished. In this study, the effects of variation in fishing disturbance on the secondary production, species diversity, abundance, biomass, and community structure of benthic infauna were examined in a region of the German Bight (North Sea) that has been intensively trawled for decades. Variation in fishing disturbance across the study area was determined using automated position registration and vessel monitoring through satellite. Even in such a heavily fished area, linear regression analyses revealed that biomass, species richness, and production decreased significantly with increasing fishing intensity. Although redundancy analyses (RDA) showed that sediment characteristics were influential in determining the structure of the infauna community, partial RDA revealed that fishing continued to have an impact on community structure in terms of biomass. These results suggest that, in implementing an EAFM, managers will need to consider the possibility that, even in areas with high chronic fishing disturbance, further increases in fishing activity may still cause additional damage to benthic invertebrate communities.

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 sys tems (Andréfouët 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 com munities 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 invaded than a site nearer the head of an estuary in Central California, despite the harbors in this estuary being closer to the mouth.

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: