Abundance, Biomass, and Production

Fisheries scientists face a challenge in that virtually all methods of fish capture or observation are selective. Further, most fish capture methods can be applied to only a fraction of the entire area of interest. Thus, measures such as catch per unit effort (C/f) or catch per area can only be regarded, at best, as being proportional to the true population abundance (see Chapter 7). The methods presented in this chapter are designed to address these problems and provide estimates of absolute abundance or “true” fish density. In general, these methods require additional sampling beyond that required to estimate relative abundance. As such, careful consideration should be given to whether relative measures of abundance are adequate or if the need for estimates of absolute abundance justifies the additional cost. In many cases, relative abundance is sufficient to answer important research or management questions. One example is when the principal goal is to determine if abundance has changed over time. As long as vulnerability to the gear remains constant over time, trends in C/f can accurately indicate changes in abundance (see Chapter 7). In such cases, the extra effort required to determine absolute abundance is better spent in sampling more sites. In general, estimates of absolute abundance are needed when catchability is likely to vary across time or between sampling sites, confounding comparisons of C/f across space or time. Absolute abundance estimates are also important when harvest quotas are being computed. Whether relative or absolute measures of abundance are desired, it is critical to define the population of interest carefully. In many cases, some part of the population is excluded from consideration because of limitations of the sampling gear. For example, population estimates of yellow perch in midsummer conducted by means of gill nets would likely not include age-0 fish because they would not be