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Visibility changes what can be detected
Fog can reduce the distance over which observers see shore features, surface drifters, or equipment markers. It can also change contrast and make familiar colors or outlines harder to distinguish. A low encounter count during fog may therefore reflect reduced detection rather than fewer organisms. Seasonal differences in fog frequency can create an apparent ecological pattern if visibility is not recorded alongside each survey.
At the start of a visit, note whether fog is absent, patchy, or persistent, and record an approximate viewing range using known landmarks where safe and practical. State which part of the site was obscured. Visibility over water may differ from visibility inland, and fog can lift or thicken during a survey, so include start and end conditions when they change.
Keep effort and method consistent
Use a fixed route, duration, or observation area and record any portion that could not be surveyed. If fog hides the far end of a transect, do not quietly shorten the route and compare the count with a full clear-weather survey. Mark the reduced coverage or postpone the observation if the project’s method requires complete visibility. Safety and access come first; a scheduled visit does not justify entering hazardous conditions.
For example, a clear-day survey covers a 100-metre shoreline section and records several floating organisms. During a later foggy visit, only the first 40 metres can be seen. A lower count across the visible portion is not directly comparable to the earlier full survey. Record the covered distance and visibility so the data can be interpreted or excluded from a particular comparison without losing the observation.
Distinguish detection from occurrence
An organism not seen is not necessarily absent. It may be beyond the visible range, below the surface, obscured by glare or fog, or simply missed during the observation period. Use “not detected during this survey” when that is what the method establishes. Reserve “absent” for cases where the method and sampling coverage support that stronger conclusion.
Images can help document visibility, but camera exposure and autofocus may behave differently in fog. Keep the original image and note camera settings or difficulties if photographs are part of the comparison. A pale horizon or washed-out frame may show conditions, yet it should not be treated as a standardized visibility measurement unless the imaging method is controlled.
Build visibility into seasonal summaries
At the end of a season, compare counts only across visits with sufficiently similar coverage, or show visibility and effort beside the observations. If foggy visits consistently have fewer detections, this may indicate a sampling limitation rather than an ecological decline. A focused follow-up could compare paired observers or use a method less dependent on long viewing distances, if suitable to the study question.
Clear notes make the limitation useful. They show when conditions constrained detection and which parts of the seasonal record remain comparable. Over time, this helps distinguish a change in what was visible from a change in what was present, while preserving field visits that still contain valuable contextual information.
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