Chapter 6: Passive Capture Techniques
Wayne A. Hubert, Kevin L. Pope, and John M. Dettmers
Passive capture techniques involve the capture of fishes or other aquatic animals by entanglement, entrapment, or angling devices that are not actively moved by humans or machines while the organisms are being captured (Lagler 1978). The behavior and movements of the animals themselves result in their capture. The techniques used in passive sampling of fish populations are similar to those used for food gathering over the centuries. Nets and traps have been widely used among various cultures, and many of the currently applied techniques were used by the ancient Egyptians, Greeks, and Romans (Alverson 1963).
Based on their mode of capture, passive sampling devices can be divided into three groups: (1) entanglement, (2) entrapment, and (3) angling gears. Entanglement devices capture fish by holding them ensnared or tangled in webbing or mesh made of natural or artificial materials. Gill nets and trammel nets are examples of entanglement gears (Figure 6.1). Entrapment devices capture organisms that enter an enclosed area through one or more funnel- or V-shaped openings that hinder escape after entrance. Hoop nets, trap nets, and pot devices are examples of entrapment gears (Figures 6.2 and 6.3). Angling devices capture fish with a baited hook and line. Trotlines and longlines are examples of passive angling gears (Figure 6.4).
Gear selectivity and gear efficiency are important considerations with respect to passive sampling devices. Often these terms are used interchangeably, but they have different, specific definitions. Gear selectivity is the bias of a sample obtained with a given gear (Box 6.1). Selectivity for species, sizes, and sexes of fishes occurs in samples taken with specific types of gear. Species selectivity refers to overrepresentation of particular species in samples as compared with the assemblage of species present. Similarly, size or sex selectivity refers to overrepresentation of specific sizes (lengths) or one sex within samples from a fish population. Fisheries scientists may use gear selectivity to their benefit when targeting specific species or sizes of fishes, thereby enhancing their sampling efficiency. The efficiency of a gear refers to the amount of effort expended to capture target organisms (Box 6.2). It is generally desirable to maximize the efficiency of a sampling gear to save time and money in single-species assessments of fisheries. Even with efficient sampling gear, the sampling effort needed to estimate the relative abundance and other descriptive statistics for a given species may be unrealistic (Gerow 2007).