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Linking hydrographic frontal activity to ecosystem dynamics in the Northe Sea and Skagerrak: Importance to fish stock recruitment



One of the main objectives of the Common fisheries policy as agreed at the intermediate Ministerial Meeting in Bergen in 1997 and subsequently formulated by the General directorate for fisheries (in: ""The Common Fisheries Policy"", Section 8. 1) is to consider environmental issues and interactions between fisheries and the ecosystem in fisheries management decisions. This so called ecosystem approach is based on

- the identification of processes in, and influence on, the ecosystems which are critical in maintaining their characteristic structure and functioning, productivity and biological diversity
- taking into account the interaction among food-webs of the ecosystem
- protecting the chemical, physical and biological environment necessary to the well-being of these ecosystems

The commission highlights the role of research in this context and states that as a first step the knowledge of the ecosystem as a whole will be increased. Scientific projects are expected to produce new insights into the functioning of whole marine ecosystems. The call for more in depth ecosystem oriented research is a consequence of the presently limited understanding of ecosystem functioning. Neither is it possible to explain the reasons for the gadoid outburst in the mid 60s nor is the role of environment (circulation anomalies) in the recruitment failures of the North Sea herring stock fully understood. In other ecosystems dramatic break downs of stocks (Northern cod, West-Greenland cod) have recently occurred, which were believed to be well monitored and managed. Managers and scientists are blamed for being responsible for these developments, but also environmental effects may have contributed to the recruitment failures. The present interpretation of the processes involved in these stock collapses remains speculative as insufficient research effort was directed to the impact of environmental processes on stock development trends and available stock abundance indices. For the North Sea ecosystem recent studies demonstrate the importance of the NAO influence on several populations of zooplankton and fish by means of simple correlation analysis, however, explicit hypotheses about the mechanisms, that translate the climate signal into population fluctuations, still need to be formulated.

The present project is a major step in this direction, taking a holistic approach to the understanding of a key element of the North Sea ecosystem: resolving the importance the frontal regions, a key organising process in the marine environment, for recruitment success of important North Sea fish stocks. The fulfilment of this objective, when combined with spawning stock characteristics, will provide environmentally sensitive tools for the assessment, management and conservation of North Sea commercial fish stocks. In order to achieve this primary objective it is necessary to perform a number of research tasks which will also have significant management and scientific implications.

First we will resolve the influence of intra- and interannual climatic forcing on bottom up processes i.e. limiting food resources for subsequent trophic levels in frontal and stratified regimes. This will involve the development and implementation of a nested small-scale physical/biological model incorporating organism behaviour and nutrient fluxes to simulate phytoplankton and zooplankton production and aggregation relative to frontal processes. This work step will allow us to simulate the horizontal and vertical distribution of potential prey items for larval and juvenile fish as well as the inter- and intrannual variability of these items relative to frontal processes. Simulated fields developed by the model will then be validated with state of the art field and remote sensing techniques. Using this approach and time series of climatic forcing the program will develop a time series of frontal activity, as a proxy variable for larval fish prey availability, for testing of recruitment variability of fisheries stocks. The completion of this research task will also contribute significantly to the understanding of the dynamics of the North Sea ecosystem as it has recently been hypothesised that up to 40 % of the North Seas annual primary production occurs in these regions with climatic processes potentially modifying this dramatically.

The second key mechanism influencing recruitment success of fish stocks in the North Sea are ""top down"" or predation processes. Aggregations of planktivorous and piscivorous fish species in frontal environments are common occurrences in the marine environment. The resolution of the impact of aggregations of predatory fish species on prey availability for and survival success of young of the year fish is at present a black box. A key objective in this program is to resolve the impact of predation on the abundance of zooplankton as well as larval and early juvenile fish by planktivorous and piscivorous predators in frontal regions relative to stratified non-frontal regimes. Estimates of consumption rates and predatory impact will be obtained on basis of a multidisciplinary field program covering different hydrographic regimes via the analysis of stomach contents and the utilisation of food web biomarkers. Estimates of mortality rates will then be developed based on the observed distributions and abundance of different species relative to water column characteristics during cruise programs.
Utilising GIS procedures, the combination of information on;

o the spatial temporal distribution of frontal processes from the hydrodynamic modelling component, remote sensing and the field programs with

o the distributions of young of the year and predatory fish species will result in the development of spatially and temporally explicit maps of the distribution of these organisms based on their preferred habitats.

Coupled with the identification of food web linkages activities this will;

o describe species guilds and community structure relative to hydrographic features and by this identify key regimes influencing ecosystem dynamics and recruitment success for potential utilisation as ecological reserves or for the development of area specific management strategies.

Furthermore, the results of the distributional and food web linkages resulting from this program have ramifications for the assessment of commercial fish stocks in the North Sea as well as other systems:

- The resolution of heterogeneous distributions of fish species due to hydrographic features and/or predator/prey availability will identify if a need exists to adapt surveys assessing recruitment and adult stock abundance to hydrographic structures, an approach, which will effect the sampling strategies, employed in the North Sea and other frontal ecosystems.

- The functional feeding response of key predators as modelled in existing multi-species models (MSVPA, MSFOR) will be examined. At present these models assume a constant suitability coefficient of a specific prey species/age group for a given predator age-group, regardless of changes in prey availability and their characteristics, e.g. growth rates. These assumptions may be invalid, particularly if prey selection varies with aggregations and changes in growth of prey in frontal regions, a process neither resolved by historic stomach content programmes in 1981 and 1991 or by standard trawl surveys. The outputs of this program, i.e. new prey selection and preference models, will enhance the methodology for assessing the historical development and present state of fish stocks in the North Sea, in particular improve recruitment estimates utilised in stock recruitment relationships, but also enhance existing medium- and long-term projection methodology considering species interactions.

Funding Scheme

CSC - Cost-sharing contracts


Danish Institute for Fisheries Research
Jægersborgvej 64-66
2800 Lyngby

Participants (6)

Baltic Sea Research Institute Warnemünde
Seestrasse 15
18119 Rostock
Nordnesgaten 50
Institut für Meereskunde an der Universität Kiel, Anstalt des Öffentlichen Rechts
Düsternbrooker Weg 20
24105 Kiel
United Kingdom
Pakefield Road
Lowestoft, Suffolk
Edmund-siemers-allee 1
University of Bergen
Prof. Keysergt. 8
5020 Bergen