Chapter 7: Active Fish Capture Methods
Daniel B. Hayes, C. Paola Ferreri, and William W. Taylor
Active fish capture methods use moving nets or gears to collect finfishes as well as shellfishes and other macroinvertebrates. This is in contrast to passive capture methods that rely on fish movement into a stationary device (Chapter 6). The distinction between the two is not always clear-cut. Some active fishing methods (e.g., electrofishing; Chapter 8) are described in other chapters in this book and should be considered when selecting methods to use. In this chapter, we focus on three major types of active fish capture methods: trawls, dredges, and surrounding nets. Other active methods, such as angling and using cast nets, are covered in less detail.
The major active gears enclose or “sweep” a specified geometric space (Figure 7.1) and operate over a specified time, thus allowing an accurately defined unit of effort. Accurately defined sampling effort is particularly important when an index of abundance is calculated (Box 7.1). Other active fish capture methods such as angling, and most passive capture techniques, do not enclose a specified area and as such do not allow such a precise determination of effective effort. Although passive methods are often used to compute indices of abundance (Box 6.1), catches are influenced greatly by fish behavior and may not accurately reflect actual abundance.
Another advantage of active gears is that they are mobile in space and time; samples collected with active gears can typically be obtained in a time span of minutes to hours. This mobility comes at a cost; compared with most passive gears, a larger vessel that may require two or more people for safe operation is typically needed to sample large volumes of water with most active gears. Passive gears, in contrast, effectively sample relatively large volumes of water by remaining in place for hours to days and can often be set quickly from a small vessel with a minimum of labor. Deployment of active gears for short periods of time is advantageous for two reasons. First, shorter sampling periods allow for a larger sample size (i.e., more hauls) to be taken per time spent sampling. Larger sample sizes increase the statistical precision (Chapter 2) of indices of abundance as well as allow sampling to be conducted over a wider area or to cover more completely the area being sampled. Thus, a more precise understanding of fish spatial distribution and habitat use can often be obtained by using active fishing gears. However, active gears may catch fewer fish per time spent sampling than might passive gears. This can be a disadvantage if large numbers of fish are needed for a study. The second advantage of short sampling times is that the time of capture can be determined more precisely. Knowing the time of capture is particularly important in studies of fish diet, feeding rate, behavior, and movement.