Samenvatting

This study aimed to get more insight into the habitat use and distribution of three sandeel species (Ammodytes sp. & H. lanceolatus) in the Voordelta and describes the findings regarding the Voordelta as a transitional habitat for sandeel populations.

As compensation for loss of habitat and foraging area of a number of species due to the expansion of the Rotterdam harbour (PMR), a seabed protection area was established in the Voordelta. In the area beam trawl fisheries was excluded in order to improve the quality of the benthic community. During a monitoring program by IMARES sandeel was found to be one of the few species experiencing a negative effect of bottom disturbing fisheries, which alternates between living buried in the sand and swimming in the water column. Furthermore, sandeel was found to be an important food source for Sandwich tern, an important species for the compensation measures. This way, the mode of life of sandeel provided a positive link between the exclusion of bottom disturbing fisheries and food availability for Sandwich terns. This link was not expected beforehand as the importance of sandeel for terns was underestimated. However, the general ecology of sandeel (Ammodytes tobianus, A. marinus and H. lanceolatus) in Dutch waters is still poorly understood since it is of no commercial importance.

In the Voordelta, sandeel was caught in a benthos survey (buried fish) and in a fish survey (swimming fish) allowing to investigate several aspects of sandeel ecology including species composition, length composition, age – length ratio, the spatial distribution of different species and sizes and the proportion of sandeel buried in the sand versus swimming in the water column. In addition, data collected further offshore was analysed allowing to identify possible differences in population structure between shallow coastal and deeper offshore regions. The combination of these data sources provided valuable information regarding the use of the Voordelta by sandeel, the prey availability for seabirds and to estimate the vulnerability of sandeel to bottom disturbing fisheries better.

The main findings of this study suggest the Voordelta as a transitional habitat for the Ammodytes sp. and Hyperoplus lanceolatus. Although both Ammodytes species have a similar mode of life a preference of A. marinus for the deepest areas and of A. tobianus (in particular small <10cm) for the shallow parts was observed without signs of competition. A. tobianus appears to fulfil its lifecycle in the Voordelta, while the A. marinus population might be dependent on larval drift from offshore breeding grounds beside the possibility of local reproduction. Sandeel in the Voordelta were younger (0-2 years old) than offshore (1-7 years old) suggesting migration of older fish or a shorter lifespan. However, there was high uncertainty in the age determinations because the otolith structures were often vague. Therefore the PMR fish could contain older individuals as well. Furthermore, sandeel were found both buried in the sand and in the water column during the day in autumn (Sept. – Oct.) suggesting a transitional period between activity and hibernation. This also suggests high vulnerability to bottom disturbing fisheries during this time of the year. Finally, the data is considered not optimal for the purpose of this study. A large amount of sandeel caught in the bottom dredge was damaged and small fish are likely to escape the fishing net used for the fish survey. Therefore, some recommendations are given to produce more representative and complete data.