Estimating seasonal Pacific geoduck clam Panopea generosa show-factors from long-term observation

Objective The geoduck show-factor is the probability an individual geoduck is detectable to a diver during a stock assessment survey. It is one of the values used to estimate the abundance of geoducks. This paper presents estimates of show-factor according to data collected during 21 approximately monthly surveys over a 32-month period. The new estimates are compared against other long-term studies of show-factor and against estimates made from shorter term (approximately daily surveys over a period of approximately 1 week) studies.Methods Data were collected from three plots near Marina Island, British Columbia, and three novel methods were used for the analysis. Two of the methods estimate show-factors at the time of data collection, while the third method treats show-factor as a function of the time of year. All three methods generate probabilistic results.Result Each method of show-factor analysis indicated a strong seasonal effect. Show-factors were highest from March to June and lowest from October to December. The difference between the high and low show-factors was at least a factor of two. The three methods generated estimates of show-factor that are generally lower than the values previously used as part of stock assessment on the coast of British Columbia.Conclusion If this difference between long- and short-term show-factor can be generalized to the many commercial beds on the coast of British Columbia, the abundance of geoducks has been underestimated and the fishery is even more cautious than currently believed. As of yet, long-term data have not been collected on a coastwide basis and conclusions cannot be more definitive.