| Popis: |
Accurate estimation of fish stock abundance by acoustic surveying methods is now essential for long‐term assessment of many of the world’s largest fish stocks and the status of their ecosystems. Significant improvements of the acoustic oceanic surveying methodology have been achieved over the last decade, in particular in calibration stability, receiver design, post‐processing systems and transducer installations for oceanic, bad‐weather operation. Experiences with introducing these new elements on four ocean‐going research vessels, with examples, are presented and discussed. Since many of the potential survey errors related to the technical instrumentation seem to be overcome or minimized, the present focus for improvements must now be on biotic factors such as target strength variability, vessel avoidance and vertical distribution of the fish relative to the acoustic sampling volumes. The effect of significantly reducing vessel‐emitted noise and improved observation methods in the echo sounder dead zone... |
