Snorkelers' In-Water Observations Can Alter Salmonid Behavior

Direct underwater observation techniques (e.g., snorkel surveys) are widely used in fisheries science. Data collected from these surveys are used to estimate species abundance, detect presence and absence, and construct statistical models that predict microhabitat use and nonuse. To produce an unbiased estimate or model, fish should ideally behave as if there were no observer present. We conducted a study using underwater video to test whether snorkeling can elicit a change in fish behavior. Four behavioral metrics were measured: upstream movement, downstream movement, total movement, and number of fish in the field-of-view. Significant differences were detected in upstream, downstream, and total movements as a function of the in-water observer. These results suggest that an in-water observer can disturb fish, resulting in altered behavior, which in turn may bias study results. We suggest researchers use caution in making inferences to an entire population when data-collection methods have potential to bias fish behavior.