A Device to Measure Shell Hardness of Dungeness Crabs and Trial Application in the Kodiak Island, Alaska, Commercial Fishery

Abstract

An objective method of measuring shell hardness of Dungeness crab Cancer magister is necessary because the time elapsed since molting is used in part by fishery managers to set commercial seasons. Limiting the catch of soft-shelled crabs in a fishery is important to decrease mortality from handling and to maintain product quality. We developed a durometer to measure shell hardness of Dungeness crabs. The durometer accurately measured the pressure required to produce shell indentation of a given depth, which indicates hardness of the crab shell in durometer units of 0–100. To calibrate the durometer, we collected 27 recently molted male Dungeness crabs from waters of Kodiak Island, Alaska, and measured the shell hardness monthly for one year in the laboratory. The nonlinear relation between durometer reading (Y) and month (X) was analyzed. Of several models evaluated, the cumulative logistic function Y = β₀/(1 + e^{(X−β₁)/β₂} ) provided the best fit (b₀ = 82.2, b₁ = 2.7 b₂ = −1.5). Estimation of months since molt was obtained from the inverse relation x = b₂log_e(b₀/y − 1) + b₁ and 95% confidence limits from graphic interpolation. We estimated month of molt from survey and commercial catch data, and we discuss the effect of temperature on durometer calibration. Shell condition from survey data from four Kodiak bays was classified by using a probabilistic neural network. Classification of new-shell versus other shell hardness resulted in a 90.8% correct classification at a durometer reading of 66. The use of the durometer as a standardized management tool removes the subjectivity of existing shell hardness methods.

Notes

³ Reference to trade names does not imply endorsement by the Alaska Department of Fish and Game or by the National Oceanic and Atmospheric Administration, National Marine Fisheries Service.