Stock assessment: Improving models and methods for sampling and stock assessment
Fish stocks differ. For some stocks, we have more information than for others. We try to adapt fish population models to the different levels of information available for a given stock. The aim is to improve stock assessment outputs and their forecasts.
The status of commercially used fish stocks in the Baltic Sea is estimated using internationally recognized population models. Scientists of the Thünen Institute of Baltic Sea Fisheries (OF) prepare the German catch data. Each spring they calculate the catch options for the coming year together with scientists from other Baltic countries in so-called assessment working groups of the International Council for the Exploration of the Sea (ICES) – for example in the Herring Assessment Working Group (HAWG) or in the Baltic Fisheries Assessment Working Group (WGBFAS). The data basis varies from fish stock to fish stock, for instance when data on removals from the recreational fisheries are available. Changes in neighboring fish stocks can display very different trajectories so that a recovery plan is necessary for one stock while the adjacent stock is in good conditions. The heterogeneity between fish stocks requires appropriate methods for data collection and tailored models to be able to calculate robust stock assessments. This project is part of activities that are conducted within at OF within the scope of the Data Collection Framework (DCF), EU regulation 199/2008.
We collect fisheries-independent data on our research cruises and fisheries-dependent data via the data collection from commercial fisheries. The research cruises are conducted according to international standards – for instance the Baltic International Trawl Survey (BITS) or the Baltic International Acoustic Survey (BIAS). The Ruegen Herring Larvae Survey (RHLS) is our national survey in which we sample the main spawning grounds of Western Baltic herring in the Greifswalder Bodden area according to best scientific and statistical praxis. In comparison to such scientific surveys, the data collection from the commercial fisheries in the Baltic Sea is much more complex. Uncertainties of all kinds are part of the day-to-day routine of the fisheries and the captains and thus, are also our routine, e.g. when we try to sample fishing activities on board of commercial vessels. Under these circumstances, the implementation of a statistically sound sampling scheme is a considerable practical and theoretical challenge.
Progress was achieved in several fields of research. This includes methodological advancement in the fisheries-independent acoustic research surveys that sample the small pelagics herring, sprat and sardines. The bottom trawls surveys (BITS) were also improved, e.g. by establishing additional sampling stations and a trash and litter monitoring system. We have supported the ICES in a central service, i.e. advancing algorithms for automatic analysis of BITS data which are provided publicly in the data base DATRAS.
The cod catches from the western Baltic Sea by the German recreational fisheries were included in the stock assessment. This was a unique event in European waters.
Since 2012 a probability-based sampling scheme is being applied in the data collection of the German commercial fisheries in the Baltic Sea. This resulted in some minor but also some major improvements, especially regarding improvement of the quality of the data submitted to the fish stock assessments of ICES.
We worked up historical data sets on flatfishes which partly go back to GDR times. Improvement of the stock assessment of the flatfish stocks is of high priority in ICES. Our data provide part of the basis for stock assessment calculations and our scientists are involved in central positions (i.e. ICES stock coordinator or stock assessor) in advancing the flatfish stock assessments.
To advance the multi-species approach, information on predator-prey relationships within the food webs are needed. We collect stomachs of cod and whiting on our research surveys and provide them for further analysis and also analyse cod stomachs in our own laboratory.
In different working groups of ICES we attempt to determine reliable reference points for fish stocks. The definition of reference points and their estimation are important to assess the stock status according to given standards.
Permanent task 1.2001
Project status: ongoing