This report presents the activities and results from the ZKO Wadden Sea study “839.08.242 Acoustic surveys and plankton sampling” funded by NWO.This pilot study consisted of two parts: (1) testing the possibility to mount a scientific echosounder on the TESO ferry between Den Helder and Texel to monitor the abundance of pelagic fish in the Marsdiep area and (2) testing the possibility to sample plankton by means of an Autonomous Plankton Sampler on board the ferry MS Vlieland between Harlingen and Vlieland. The tests with the installation of these systems are described. The study included also four reference hydro acoustic surveys, targeted on pelagic fish, in the Marsdiep in May and October 2010 and 2011.Hydro acoustic data collection with an echosounder on the TESO ferry was not possible due to air bubbles causing noise and transmission loss. We expect that this can be solved in the future, by making special adjustments on the hull where the equipment is mounted.The hydro acoustic surveys revealed the presence of high concentrations of clupeids in the Marsdiep, dominated by sprat (Sprattus sprattus). The summed biomass of Clupeids in the Marsdiep was estimated to be 613/369 tonnes in May 2010/2011 and 67/69 tonnes in October 2010/2011. The biomass of pelagic fish per water volume in October is approximately 20 times as high as the biomass of demersal fish per water volume as estimated from the DFS survey in autumn. The majority of the pelagic fish schools are found in the upper layers of the water column. Almost 50% of the fish is distributed in the upper 6 m. As the depth of the hull mounted transducer on the TESO ferry is 5 meter, it is to be expected that most fish will be missed. For future monitoring of pelagic fish it is therefore recommended to install a fixed installation at the bottom heading towards the surface.An Autonomous Plankton Sampler (APS), based on the widely applied Continuous Plankton Recorder, was installed at the MS Vlieland, the ferry sailing from Harlingen to the isle of Terschelling. The efficiency of zooplankton collecting appeared to be sensitive to the water inlet system for which the necessary adjustments could not (for safety reasons) be implemented on the ferry. Different vessels were therefore used to further improve the APS, in particular the inlet system by using a gauze funnel of different sizes and sampling directions. An improved design was tested for robustness under offshore conditions, and a final design was used to collect samples in the Wadden Sea. The final design was able to collect different taxonomical groups of (meso)zooplankton, including copepods and larvae of zoobenthic species, in relevant quantities. In case the system would be implemented in monitoring, it is recommended to install the APS on board of a monitoring vessel. Further research should be dedicated to the catch efficiency of different species and groups of zooplankton. |
