Case Studies in Fisheries Conservation and Management: Applied Critical Thinking and Problem Solving
Case 14: Interpreting the Size Structure of a Fish Population Sample: What Can We Infer about the Dynamic Rate Functions?
Three dynamic rate functions, recruitment, growth, and mortality, interact to determine fish population size and age structure. Recruitment adds new individuals to a population, which is balanced by mortality. Mortality can be due to natural causes such as predation or disease, or due to harvest by human anglers. If you are not comfortable with the definitions for recruitment, growth, and mortality, please refer to Box 1.1 in Willis and Murphy (1996). If you would like more information, but still in a general format, review Chapter 3 in Willis et al. (2009).
One of the first things that most biologists do after completing a sampling trip is to construct a length-frequency histogram for fish species of interest. Simply by assessing this histogram, some initial suppositions (i.e., educated guesses) can be made concerning the population. Is the size structure dominated by small fish? If so, is there high mortality, either from natural causes or from angling, that is causing a loss of large fish? Alternatively, could this be a high density, slow growing population that is sometimes referred to as “stunted?” What other sampling will I need to do to determine the true recruitment, growth, and mortality patterns for this population? In this exercise, we would like you to start asking such questions.
You actually need very little information to begin asking questions about the size structure of a population, and the reasons for that size structure. Here, we provide you with a length-frequency histogram for yellow perch sampled from a northern lake using an experimental (i.e., multiple-mesh) gill net (Figure 14.1). If you are unfamiliar with this sampling gear, please see Hubert (1996) for background information. This was a spring sample, so no age-0 fish will be included on this histogram; all fish are age-1 and older.