A conceptual example of the relationship between the relative NIS risk at each site (numbers within hexagons, assuming an exponential distribution) and the site visit rate assuming random, risk-based and heavy risk-based surveillance, over three site visits (blue outline) during a model run. The highest risk hexagons in this example represent two port sites, one seeded with a NIS at the beginning of the simulation (orange fill). Under random surveillance three sites are visited with no relationship to risk, under risk-based, three high risk sites are visited and under heavy risk-based surveillance, the highest risk site, only, is visited three times.