Dispersal corridors for marine non-indigenous species in Danish waters – analysing modelled settling areas and observed occurrences

Abstract

In the marine environment, we find a wide range of non-indigenous species. Some of these have stages of pelagic drift and the dispersal of such “introduced non-indigenous species (NIS) with pelagic stages” is dependent on both species characteristics, habitat availability and hydrographic conditions. The NIS may be highly concentrated in some marine areas while their presence is diminished or completely absent in others. Their spread between areas is determined by the corridors through which their pelagic stages are dispersed. There is a need for further understanding of these corridors in order to implement protective actions against the spread of NIS in Danish marine waters. This is specifically relevant for ballast water management with regard to ship traffic. In this respect, the Danish Environmental Protection Agency (DEPA) is assessing exemption applications under the Ballast Water Management Convention's (BWMC) exemptions rules. Information on the potential occurrences of NIS as well as on the transport corridors of their pelagic stages is thus important in support of the risk assessment. The estimation of potential primary dispersal corridors for marine NIS may be used in an evaluation of existing national monitoring stations in Danish waters, and whether these are located in areas that are relevant for monitoring of changes in the overall NIS abundances. If the monitoring stations are outside areas of major NIS presence, the monitoring is not adequate to effectively be used for early warnings on changes in abundances, thereby having consequences for the treatment of exemption applications. Finally, the modelled dispersal corridors may contribute to the understanding of the presence/absence of NIS in the national monitoring program by an analysis of distances between national monitoring stations and main shipping routes through Danish marine waters. In the present study, we aim at identifying the potential dispersal corridors for selected marine non-indigenous species (NIS) in the Danish marine waters, subsequently examining whether these dispersal corridors coincide with areas where NIS observations are conducted during the national monitoring programme, NOVANA or where the main shipping routes are positioned. We specifically investigate whether dispersal corridors and simulated dispersal- and settling densities of NIS (estimated from our drift model simulations) can explain the observed presence or absence of 19 selected species recorded across the NOVANA stations. The results show clear differences in the overall mean probability of presence between species as well as between decades of analyses, where estimates for the recent decades indicate an overall increased probability of presence of NIS over time. This in turn indicates a geographical spread of NIS across the area in recent decades. Our analyses also show that the presence of NIS in Danish waters is affecting a range of factors regarding both their colonization and their establishment. The colonization process is related to the drift and settlement of larvae, described by our drift model simulations and/or the density of vessels acting as potential mediators of dispersal. The establishment process is determined primarily by the environmental conditions and the sea bed habitat at each site, depending on its capacity to allow for survival and reproduction. The modelled settling density is related to the observed NIS occurrences, thus strengthening the potential information value by combining drift modelling and actual observations in future risk assessments. In conclusion, the presented model studies of NIS dispersal have shown that there is a large potential for improving the general understanding of NIS presence and dispersal in Danish waters, demonstrated by combining the analysis of model output and different environmental variables and comparing them to observed NIS occurrences. We recommend continuing the analysis on these data, specifically with regard to the assessment of environmental linkages when identifying sites of primary importance for future NIS monitoring.