Chapter 17: Visual Observation of Fishes and Aquatic Habitat
Russell F. Thurow, C. Andrew Dolloff, and J. Ellen Marsden
Whether accomplished above the water surface or performed underwater by snorkel, scuba, or hookah divers or remotely operated vehicles (ROVs), direct observation techniques are among the most effective means for obtaining accurate and often unique information on aquatic organisms in their natural surroundings. Many types of studies incorporate direct observation methods, including broad-scale inventories of aquatic organism distribution and abundance (Hankin and Reeves 1988), highly specialized observations of behavior (Drew et al. 1976), evaluations of habitat use (Fausch and White 1981), estimates of population size structure (Griffith 1981), assessments of gear performance (High 1971; Jones et al. 2008), and censuses of surrogate structures such as spawning nests or redds to estimate fish abundance (Dauble and Watson 1997). Visual observation methods fall into three broad categories: direct underwater observation by divers (snorkel, scuba, and hookah), surface observation (land-based, aerial, boat-based, and viewing windows), and remote methods (cameras and ROVs).
Advantages of direct observation methods include their conceptual simplicity, versatility, cost-effectiveness, nonintrusiveness, and ability to obtain in situ behavioral information. Most direct observation methods are straightforward and accessible to both professionals and laypersons. Student volunteers or members of clubs and civic organizations can be trained to conduct counts and basic surveys. For example, the popularity of sport diving with snorkel or scuba has made possible broad-scale surveys of fish presence and relative abundance by laypersons (www.reef.org), similar to nationwide bird counts. Direct observation may be most appropriate when effectiveness of other sampling methods, such as capture by electroshocking, seines, gill nets, or trawls, is compromised by environmental conditions such as extreme conductivity (too low or too high for electrofishing), excessive depth (too deep to seine, conduct electrofishing, or capture fish without barotrauma), thick ice cover, or habitat complexity that can foul trawls or seines. The relatively modest gear requirements of most direct observation methods may reduce equipment costs and facilitate use in remote locations (Thurow 1994). The need for fewer personnel as compared with traditional methods results in less sampling time, thereby reducing cost and enhancing sampling efficiency (Hankin and Reeves 1988). For example, snorkeling requires fewer personnel and substantially less time than does depletion electrofishing (Mullner et al. 1998; Thurow et al. 2006). Direct observation methods also avoid the risks of potentially more destructive methods when sampling threatened, endangered, or sensitive species; because fish are not handled, they do not experience physical damage or barotrauma, and stress is usually minimized. The versatility of direct observation methods enables description of habitat structure and assessment of species distributions without the disturbance generated by capture sampling methods; for example, bottom substrate is not damaged by trawls, and fish are not herded by nets.