Demonstrating pathogen freedom in open water environments is challenging as agent and host status may abruptly change. The level of confidence, especially over time, in data obtained from open settings drives both surveillance design and interpretation. How long is too long between surveys, and how much time is required at each refresh to remain confident that pathogen status is as previously described? We propose a method to assess pathogen freedom probability based on surveillance data coupled with information about pathogen introduction risk and apply that method to an open water mollusc farming environment on the Pacific Coast of the United States. We estimate assurance in absence of several mollusc pathogens inferred from surveillance data gathered over the course of three years; and we estimate absence of new introductions from a model derived by expert panel elicitation. We then derive risk-revised probabilities of freedom, to consider the joint probability of the two. Results provide strong evidence to suggest continuous absence of the target pathogens - Marteilia refringens , Marteilioides chungmuensis , Ostreid herpesvirus 1, Perkinsus marinus , and Perkinsus olseni - for the host species, region, and time-period studied. Our field example shows the feasibility and utility of considering introduction risk in open water surveillance evaluations. Findings demonstrate (1) the ability to retain science-based confidence in freedom for select pathogens through time in an open system, (2) the limitations of sample volume, and the relative importance of sample frequency, for pathogens that are deemed higher introduction risks, and (3) expert elicitation is a generalizable and valid alternative to formal site-specific risk assessment. Statistical models to formulate and simplify application are under development.