MONS monitoring plan small pelagic fish : food for higher trophic levels

Couperus, Bram; Hal, Ralf van; Ouderaa, Isabelle van der; Volwater, Joey


This report contains an outline of a working plan for the monitoring of small pelagic fish on the Dutch Continental Shelf. Pelagic fish include species that are not connected to the bottom and live large parts of their life in schools in midwater. Stakeholders within the North Sea Counsel (Noordzeeoverleg) discussed what kind of research is required in the context of sustainable economic use of the North Sea. This resulted in the North Sea Agreement (Noordzeeakkoord - NZA). Part of the North Sea Agreement is the Monitoring-Research- Nature Restoration-Species Protection (Monitoring-Onderzoek-Natuurherstel-Soortbescherming - MONS). A commission has defined the research questions that are at stake and the sort of research that is required to answer these questions from 2022 onwards. Part of this research has received priority - labelled “No regret studies” - of which the monitoring of small pelagic fish is one. The MONS commission has asked Wageningen Marine Research (WMR) to make a proposal for the monitoring of small pelagic fish in the next five years. The first year is a pilot year meant for testing (parts) of proposed methods. The key questions of the client are “What is the distribution of small pelagic fish species in Dutch waters, by season and from year to year?” and “How can these geographical and temporal distributions be explained by the known natural history of the species involved, in terms of known behaviour and habitat requirements?”. This information is required to describe the starting situation for future impact assessments of windfarms construction and other offshore infrastructural building. In addition, the client asks that the plan consists of the following components: (1) A year-round beach sampling of the shallow surf zone and (2) A year-round and annual sampling of small pelagic fish. A summary is given of existing and innovative survey methods. Followed by an overview of historic research on small pelagic fish and of coordinated international surveys in the (Dutch) North Sea. Potential monitoring techniques are briefly described, including both standard and innovative techniques. Furthermore, a limited inventory is made of wishes and requirements of the MONS monitoring proposals for other animal groups. This is followed by practical considerations and the actual monitoring proposal. The proposal consists of two hydro acoustic surveys per year along the Dutch coast in a zone of 20 nautical miles, which includes the (proposed) near coast locations of wind farms. Each of these surveys covers approximately 750 nm with a duration of 3 weeks. One survey is proposed to take place in January/February simultaneous with the International Bottom Trawl Survey (IBTS). The other survey is proposed to take place in June immediately preceding the international Hydro Acoustic Survey for Herring and Sprat (HERAS). The proposed surveys will provide a small-scale distribution map of pelagic fish species in the Dutch coastal zone and will thus give relative distributions of all pelagic species in the Dutch coastal zone. The international data from the IBTS and HERAS are publicly available. HERAS is an acoustic survey and will provide biomasses for sprat and herring on predefined areas in the North Sea (strata) for comparison. For the IBTS – a trawl survey - it is proposed to collect acoustic data during the Dutch part of the survey. This requires some additional effort and training of personal involved. The gathering of acoustic data from foreign vessels in the IBTS, requires consultation with international partners and will therefore take longer than one year. After five years comparison of indexes of the acoustic surveys with the IBTS and HERAS indexes will give answer to the questions (1) “Can abundance changes from year to year in the Dutch coastal zone be explained by shifts in distribution or can it be explained by changes in abundance in the North Sea?”, which contributes to the description of the starting situation mentioned above. It is proposed to fill the temporal gaps between these two surveys without running into extensive costs using standalone hydro acoustic stations (Wbat). For this two methods are being tested during the pilot year: (1) two standalone echosounders in a frame on the bottom, pointing upwards to the surface and (2) a drone that sails repeated transects in a windfarm. As a third supportive method (3) qualitative fish information will be collected by means of gill net sampling. Measurements will be carried out simultaneous during the month of June. In order to get additional temporal reference data over time it is proposed to deploy the Wbat and the gill nets at least in the period during and between the acoustic surveys, preferably the whole year. For the sampling of small pelagic fish in the surf zone, a proposal is made, containing a two weekly sampling scheme running for a large part of the year with a hand towed beam trawl from the beach and sampling with a gill net. It is expected that this sampling scheme – although not tailored to sample pelagic fish – will contribute to a better understanding of pelagic fish distribution, by direct comparison of the gill net catches with the offshore sampling and by providing information on the seasonal occurrence of species and their lengths. The second key question “How can these geographical and temporal distributions be explained by the known natural history of the species involved, in terms of known behaviour and habitat requirements?” is harder to be answered by monitoring. The monitoring data might provide some insight into habitat use and possibly even into schooling behaviour. However, additional studies will have to be initiated in the future, focusing on life history questions and accessory behaviour of the small pelagic fish species in the Dutch coastal zone. For these studies and for studies addressing the impact of offshore constructions on the DCP, the proposed monitoring will provide the necessary baseline information.