Mean annual Secchi depth measurements in the period starting in 1969 provide a measure of water column transparency in the CalCOFI inshore and offshore areas, 1969 - 2004.

The Secchi depth provides a measure of water turbidity. Measurements are made as part of the California Fisheries Cooperative Investigations (CalCOFI). Secchi depth is measured on each station on each cruise. The mean annual is obtained by averaging stations over time and at stations defined as either inshore or offshore.