Combining passive and active underwater acoustics with video and laser optics to assess fish stocks

Today, fisheries research and management rely heavily upon the use of age-structured models to make deterministic estimates of the abundance of fish stocks. Scientists freely admit that these estimates are highly uncertain because they lack the means to measure the stock with precision to verify the predictions. Assessment of fish stocks with active acoustics has been practiced for nearly four decades but the method is still limited due to the difficulty in repeating the survey estimates and obtaining biological information on the acoustic targets. In Prince William Sound, Alaska, we have been making precise measures of the Pacific herring stock using quasi-continuous, underwater acoustics and discrete purse seines samples for over a decade to assess the level of uncertainty in the model predictions. Commercial purse seines are used to collect information on the identification and size of the individual fish targets, which are needed to convert the echo integration values into fish biomass. However, the net sampling for biological information is logistically difficult, expensive, and is discrete so it reduces the amount of survey coverage by the more continuous acoustic surveying. For these reasons, it is difficult to conduct net sampling on a level that provides a representative sample of the acoustic targets. We have had some success using underwater video cameras to identify the species of acoustic targets and others have combined of lasers with underwater video to measure the size of individual fish using SCUBA. We are now combining lasers with the video and testing the use of downriggers as stealth, means of towing the sensors during acoustic surveys. If successful, this will eliminate the need for discrete sampling with nets on our acoustic surveys of Pacific herring