Potential highly variable catch efficiency estimates complicate estimation of abundance

Abstract

Stock abundance can be estimated directly from multiple vessel survey catches if variations in catch efficiency are understood. Catch efficiencies and scallop densities were estimated from a depletion experiment using five commercial dredge vessels, to assess how much the catch efficiency of multiple vessels differed across a small area. Each vessel sampled pre-defined lanes across a gradient of intensities within a closed commercial scallop ground. Catch efficiency for commercially sized scallops varied considerably among vessels, ranging from 0.12 to 0.62, and catch efficiency was also variable over time for individual vessels. However, although two vessels produced statistically different catch efficiency estimates, the remaining vessels had overlapping confidence intervals. The density of scallops above the minimum landing size in each lane ranged from 25 to 62 per 100 m2. Simulations confirmed that model spatial resolution, potential positional error and the size of the lanes did not significantly influence the results, and instead confirmed that catch efficiency variance was influenced by the pattern of the haul paths implemented in each lane. We were unable to identify further drivers of the potential variation in efficiencies and densities; however, this potentially high variation over small spatial scales indicates the potential to produce unreliable abundance estimates by scaling survey catches from multiple vessels without greater understanding of catch efficiency variation. Future research should build on the initial work here by obtaining better estimates of catch efficiencies. Simulations indicate this would involve employing more effort and a more even and systematic distribution of hauls in areas selected for depletion trials.