Changes in acoustic abundance - a signal indicatior for stock dynamics for cod (Gadus morhua)

To be able to take the proper and timely management actions, it is crucial to reliably defect major changes in trends of fish stock abundance. For the assessment of Northeast Arctic cod, the Norwegian bottom trawl survey is important for tuning the VPA (Virtual Population Analysis). The indices are auto-correlated and the time series show maxima with vaguely defined peaks. This might lead to a delay in detection of the first signs of stock reduction. Norway has conducted combined acoustic and bottom trawl surveys for the main demersal fish stocks in the Barents Sea since 1982. The idea of combining the results from the two surveys into one estimate of abundance has been explored, but a number of confounding factors related to natural and affected fish behaviour has until1 now prevented the establishment of a reliable method for combining the two indices. The acoustic indices receive less weight or power during the tuning process than those based on the bottom trawl survey. There are, however, specific differences in the dynamics of the indices of the two time series that have not yet been exploited. In this paper these differences are analysed with the aim of extracting patterns that can serve a.s a. signal for changes in trends'of stock development. Application of the results for monitoring the stock is discussed