Estimating Seasonal Geoduck (Panopea generosa) Show‐Factors from Long Term Observation

Abstract

The geoduck show‐factor is the probability an individual geoduck is detectable to a diver during a stock assessment survey. This paper presents three methods to estimate geoduck show‐factor from data collected from long term show‐factor plots near Marina Island, British Columbia. Divers surveyed the plots 21 times over a 32‐month period. 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.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. As a result, there is the possibility that previous estimates of show‐factor contributed a negative bias to estimates of geoduck abundance.As an ancillary objective, this paper presents a strategy for removing outliers from the long term show‐factor data. Outliers did not change the general trends but did affect the quantitative results.