Bureau Waardenburg
Varkensmarkt 9
4101 CK Culemborg
the Netherlands
T: +31 (0) 345 512710
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Pelagisch visonderzoek onder sterns

Pelagic fish and terns

As part of the monitoring programme in the Dutch Voordelta Natura-2000 site and the compensation for the extension of Rotterdam port research has been carried out into the food availability for breeding terns. Bureau Waardenburg developed new sampling methods to record the species and sizes of fish available to foraging terns during the breeding season. A net pulled between two small boats is used to sample the uppermost water layer in areas where the terns are feeding.

This research was featured during the Dutch radio programme VARA's Vroege Vogels.

Breeding success and food availability

The availability of herring, sprat and sandeels have a major impact on the breeding success of both common and Sandwich terns in the Dutch Delta. The research into the availability of these prey species provides further insights into the potential success of the colonies in the Delta.

Little is known about the presence and distribution of these important prey species during the terns' breeding season, as well as during the rest of the year. Current fish monitoring programmes in the Dutch coastal zone overlook potential prey species it is of little commercial interest. Monitoring focuses instead on demersal fish and is carried out during trawling for shrimp.

Sampling

In order to find out more about the fish available to terns sampling needed to be carried out in the part of the water column available to terns; this is typically the top 1.5 m. In addition, terns frequently exploit the shallow waters around sand banks in the Delta. Finally, it is important that the sampling is carried out during the most relevant periods for the birds. For the terns this is early summer when their young are at a critical stage in their development.

Sampling in shallow waters is carried out with the aid of a floating net that is pulled between two small boats separated by a rope of standard length (see diagram). The net was designed to sample the upper 1.5 m of the water column and the boats used were manoeuvrable and had a very shallow draft.

Diagram of the sampling method for pelagic fish.

In addition to the fish sampling data is collected on the salinity and turbidity of the water as well as the numbers of foraging birds. During the past three years this research has revealed a strong correlation between the numbers of pelagic fish at the water surface and foraging terns and gulls.  

As well as the use of GPS to measure the area sampled flow meters are used to correct for the currents and water flow through the sampling area so that the amount of water sampled can be calculated. 

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