Spatio-temporal simulations for assessing the optimality of the Heras survey design and calculation methods

The survey results inevitably depends on the design and selected methods. While it is very important to maintain consistency over the long time series  it is also necessary to revise these factors periodically and ensure that the most relevant properties of the fish stocks are well represented in the survey design and calculation methods. This project will focus on the HERAS, the North Sea Herring survey, for improvements in the accuracy and precision. With respect to the calculation methods, the assumed length-target strength relationship is one of the factors affecting the accuracy and representativeness of the abundance estimates from the acoustic surveys. Target strength is the length-dependent acoustic value specific to the type of the observed target group such as clupeid fish. This value typically comes from models derived from empirical information. For most of the pelagic fish species, the swim bladder is one of the main contributor of the target strength and the size of the swim bladder is known  to be linearly correlated with the fish length. However, there are certain conditions that can introduce nonlinearities to such relationship. One of the well-known effects as such is the depth-dependent compression of the swim bladder. As a result of the hydrostatic pressure, the gas filled swim bladder shrinks in volume and this causes changes in the acoustic returns. This effect has been studied earlier by in-situ measurements and laboratory experiments for different species including herring and some gadoid species. Even though the results of such studies indicate some significant changes, this effect generally disregarded within the routine survey calculations to avoid complexity. One of the species this applies to is North Sea herring. Normally, during the HERAS survey, the main aggregations of the North Sea herring are found more or less around the same depth contours and expected to have relatively homogeneous size distribution.

Uit onderzoek is bekend dat haring de akoestische Target Strength van haring veranderd met de diepte. De verspreiding van haring in het internationale HERAS onderzoeksgebied verschil per jaar, met name doordat groter haring zich verplaatst naar het noorden van het gebied, waar het aanzienlijk dieper is. Aangezien de haringscholen tijdens HERAS zich meestal vlak boven de bodem ophouden, is het aannemelijk dat dat dit een effect heeft op de bestandsschatting. De omvang van dit effect is nog niet onderzocht.

Wij stellen voor om dit effect te onderzoek door middel van een simulatiestudie, waarbij we gebruik maken van de historisch beschikbare biologische en akoestische data van de 25 jaar HERAS data.

Deze studie zal tot een beter begrip van de fouten en variabiliteit van de bestandsschatting leiden en - op termijn - mogelijk tot aanpassingen van (analyse van de) survey HERAS.