Improved calculations with fish data

Background and Objective

The status of commercial fish stocks in the Baltic Sea is estimated using internationally recognized population models. Scientists from the Thünen Institute of Baltic Sea Fisheries (OF) process the German catch data and, together with scientists from other Baltic Sea countries, calculate the fishing opportunities for the following year in so-called assessment working groups of the International Council for the Exploration of the Sea (ICES) each spring - for example, in the Herring Assessment Working Group (HAWG) or in the Baltic Fisheries Assessment Working Group (WGBFAS). Data vary from fish stock to fish stock - for example, when data on recreational catches are available. Even fish stocks occurring in close proximity can vary greatly in their development, requiring a recovery plan for one stock while the neighboring stock is in good condition. Heterogeneity among fish stocks requires appropriate data collection methods and tailored models to calculate robust stock predictions. This project is part of the activities we are carrying out at OF under the EU Data Collection Program (DCF; EU Regulation 199/2008).

Approach

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.

We collect fishery-independent data on our research cruises and fishery-dependent data as part of commercial fishery sampling. We conduct the research trips according to internationally agreed standards - for example, the Baltic International Trawl Survey (BITS) or the Baltic International Acoustic Survey (BIAS). Nationally, we conduct the Rügen Herring Larval Survey (RHLS) and the Fehmarn Juvenile Cod Survey (FEJUCS). In latter surveys, we sample the main herring spawning area of the western Baltic Sea according to best scientific and statistically valid practices and collect data on the recruitment strength of western Baltic cod in cooperation with commercial pound net fishers. In contrast, sampling of the Baltic Sea commercial fisheries is much more complex. Uncertainties of all kinds are the daily business of the fisheries and the captains and crews, and thus of us as well, when we attempt to sample fishermen's catch activities. This requires the development and flexible implementation of tailored sampling approaches.

Preliminary Results

We have made a whole series of advances in recent times. These include the methodological development of the acoustic research cruises, which record the schooling fish herring and sprat independently of the fisheries. Improvements have also been introduced in the demersal research cruises (e.g. BITS); for example, additional stations and plastic debris monitoring were introduced. At a central point, we supported ICES with our expertise in the development of algorithms for the automated analysis of BITS data in the publicly accessible DATRAS database. The removals of cod from the western Baltic Sea by German recreational fisheries were included in the stock assessment, a unique feature in Europe in 2015. Random sampling of commercial fisheries in the Baltic Sea has been taking place since 2012. In addition to a whole series of smaller and larger advances, this has significantly improved the quality of the data we provide for the annual ICES stock assessments. We digitalise data from historic commercial samples of flatfish catches. In some cases, they date back to GDR times. The development of the stock assessments of flatfishes of the Baltic Sea has a high priority at ICES. We provide the basis for the calculations with our data and are ourselves involved in the development of the stock models of the flatfish stocks that are important for Germany. In order to further develop the multi-species approach, information on predator-prey relationships within the food webs is required. On our research cruises we collect stomachs of cod, whiting and flatfish and deliver them to our partners for further analysis and also perform our own stomach content analyses. In different working groups we collaborate on identifying reliable reference points for fish stocks. The definition of reference points and their calculation are important to assess the stock situation according to existing standards.