Case Studies in Fisheries Conservation and Management: Applied Critical Thinking and Problem Solving
Case 16: Sampling Gear Biases: Size Structure of Bluegills Collected from the Same Population with Different Gears
Students (and biologists for that matter!) have a tendency to accept sampling data at face value. If a gear type primarily collects small fish, then they assume the population is dominated by small fish. If a gear type captures big fish and lots of them, then they assume the population is dominated by large fish. In reality, many biases are possible and actually are very common. To truly understand sampling data, biologists must first understand the biases associated with each gear, and only then will the true nature of the population’s structure (e.g., size or age structure) and dynamics (i.e., recruitment, growth, and mortality) be revealed.
Various sampling gears may be differentially effective for different species, and even differentially effective for different sizes of the same species. For example, largemouth bass are commonly sampled with electrofishing gear. The numbers and sizes of largemouth bass collected can vary widely across seasons. During the spring and fall, more and larger largemouth bass tend to be nearshore and vulnerable to the electrofishing gear, which is used in that shallow-water habitat. During midsummer, fewer largemouth bass would be sampled at the same locations because many of the larger bass will have moved offshore to deeper water as a result of the warm summer water temperatures.
In this “gear bias” case study, we will explore the differential size structure of bluegills captured by two common sampling gears, electrofishing and trap nets (also known as modified fyke nets). The trap nets had 1.2- X 1.5-m frames, dual throats, and 19-mm bar mesh. Night electrofishing was undertaken with pulsed DC electricity at approximately 250 V and 8 A. Samples were collected in late May at a water temperature of 23°C.