Salmonid Field Protocols Handbook: Techniques for Assessing Status and Trends in Salmon and Trout

Tower Counts

Carol Ann Woody

doi: https://doi.org/10.47886/9781888569926.ch17

Counting towers provide an accurate, low-cost, low-maintenance, low-technology, and easily mobilized escapement estimation program compared to other methods (e.g., weirs, hydroacoustics, mark–recapture, and aerial surveys) (Thompson 1962; Siebel 1967; Cousens et al. 1982; Symons and Waldichuk 1984; Anderson 2000; Alaska Department of Fish and Game 2003). Counting tower data has been found to be consistent with that of digital video counts (Edwards 2005). Counting towers do not interfere with natural fish migration patterns, nor are fish handled or stressed; however, their use is generally limited to clear rivers that meet specific site selection criteria.

The data provided by counting tower sampling allow fishery managers to

• determine reproductive population size,
• estimate total return (escapement + catch) and its uncertainty,
• evaluate population productivity and trends,
• set harvest rates,
• determine spawning escapement goals, and
• forecast future returns (Alaska Department of Fish and Game 1974–2000 and 1975–2004).

The number of spawning fish is determined by subtracting subsistence, sport-caught fish, and prespawn mortality from the total estimated escapement.

The methods outlined in this protocol for tower counts can be used to provide reasonable estimates (± 6%–10%) of reproductive salmon population size and run timing in clear rivers.

Tower counts enable practitioners to systematically sample a selected salmon population to estimate reproductive population size and determine run timing.

Counting towers provide an elevated vantage point for visually sampling Pacific salmon spawning migrations. Aluminum scaffolding is typically used (see Figure 1), but biologists are creative and employ tower surrogates, such as tall trees, bridges, dams (see Figure 2), or high river banks to accomplish their sampling. Since the 1950s, counting towers have played a central role in Pacific salmon management in Alaska and to a lesser extent in Canada and Washington (Rietze 1957; Cousens et al. 1982; Anderson 2000; Kohler and Knuepfer 2002; Fair 2004). Towers are used on both single- and multispecies systems (see Table 1) and on small to large (10–130+ m in width) clear water rivers.