Comparison of Vented and Ventless Trap Catches of American Lobster with Scuba Transect Surveys

Ventless trap surveys are becoming an increasingly common method to estimate American lobster (Homarus americanus) population structure and abundance. These surveys typically consist of strings of conventional commercial vented traps set alternately with experimental ventless traps, which lack escape vents and, as a result, capture higher proportions of sublegal-size lobsters. To determine how accurately ventless trap surveys reflect lobster populations on the seafloor, lobster density, size distribution, and sex ratios observed from scuba transect and tagging surveys were compared with ventless trap survey catches at 17 locations that varied by average temperature and dominant substrate. Diver surveys indicated that higher lobster densities were associated with increasing substrate complexity and constant temperature. Trap catch per unit effort increased with substrate complexity and decreased with increasing water temperatures. Comparisons of survey methods suggest ventless trap surveys overestimate average carapace length and proportion of males, and that selectivity and catchability varies with respect to substrate and temperature. To avoid violating the assumption of constant sampling of the population, time-series data based on relative abundance estimates generated from ventless trap surveys should consider stratification by substrate and temperature. To account for potential geographical variability in the relationship between trap catches and lobster density, further consideration and development of spatial and temporally specific calibration studies are required prior to assigning stock assessment parameters confidently based solely on existing trapping techniques.