Understanding the stock-recruitment relationship of fish stocks is crucial for generating biologically sound advice for the balanced, sustainable exploitation of fisheries resources. At present the ICES Advisory Committee on Fishery Management uses the concept of safe biological limits (SBL) as a criterion to define exploitation and stock levels. This concept is based mainly on the assumption that below a certain spawning stock size, recruitment is negatively affected as a result of low egg production. When a stock is considered to be inside SBL, recruitment variability is expected not to depend on parental stock size, but rather be related to environmental and ecological factors influencing the survival of the early life history stages. So far, however, stock-recruitment relationships and critical stock levels identifying recruitment-limiting stock sizes are poorly defined for cod and sprat in the Baltic. The sensitivity of SBL's and reference points to environmental conditions, species interactions and parental stock structures for these stocks are uncertain and need clarification. The objectives of the research project are to:
1. Determine stock-recruitment relationships for Baltic cod and sprat in relation to key environmental factors influencing the production of viable spawn and the survival of early life history stages.
2. Improve short-term predictions of stock development by integrating recruitment estimates based on the present status of the stock and its biotic and abiotic environment.
3. Develop predictive recruitment models for medium- to long-term forecasts of stock development under different environmental and fishery scenarios.
4. Estimate biological management reference points, critical stock limits and target spawning stock sizes based on stock-recruitment relationships and stock development simulation models, and considering the precautionary approach for fisheries management.
2. Methodology and research tasks
The key questions to be answered by the proposed project are: how do environmental factors influence the stock-recruitment relationship for cod and sprat stocks in the Baltic and what are the implications of variations in these factors for the use of biological reference points and critical stock limits in the management of the fisheries.
These questions will be addressed by following tasks:
1. Evaluating the accuracy of the spawning stock biomass as a measure of viable egg production by: a) determining the reproduction potential of the stocks in relation to sizes and structures of the spawning stocks, sex ratios, maturation processes and spatial distribution of the populations; b) determining the viability of the produced eggs in relation to parental growth conditions and contamination with toxic substances.
2. Resolving the direct impact of hydrographic factors on the fertilization and developmental success of cod and sprat eggs and early larvae by field and laboratory experiments.
3. Identifying and describing the hydrographic, oceanographic and behavioural processes influencing growth, survival and distribution of young of the year cod and sprat and determining the component of the spawning stock contributing to recruitment through the examination of survivor characteristics.
4. Developing and employing combined drift and feeding models to ascertain the potential role that interannual hydrographic variability has on survival and growth of eggs and larvae.
5. Determining the impact of predation on early life stages of cod and sprat caused by clupeids. Extending the time series of stock and recruitment estimates of cod and sprat utilizing MSVPA. This includes a disaggregating of stock and recruitment estimates into different spawning areas characterized by specific environmental conditions.
6. Integrating the findings of the previous tasks into new recruitment models and thereby assess the utility of such models in management applications, e.g. simulation of medium- to long-term stock development scenarios under different environmental conditions and fishing activities, forecasts of annual recruitment ranges to be used as input in short-term stock predictions, determining SBL's and biological reference points and their sensitivity to environmental perturbations.
Funding SchemeCSC - Cost-sharing contracts
620 35 Farosund
18119 Rostock Warnemunde
106 91 Stockholm