Comparison of age-length and growth-increment general growth models of the Schnute type in the Pacific Blue Mussel, Mytilus trossulus Gould

Abstract

Models of Mytilus growth, based mostly on length-at-age data, have typically taken the form exemplified by the von Bertalanffy or Gompertz formulations. These models require assumptions about the form of the growth curve. We used the Schnute general growth model to examine growth in the Pacific Blue Mussel, Mytilus trossulus, in Prince William Sound, AK. The Schnute model provides a convenient analytical method for selecting among all previously published growth models. Mussels were tagged with individually numbered tags at 13 sites in July 1997 and were collected in July 1998. Age was determined from surface growth rings on the shell, and shell length at maximum annulus was measured. Annual deposition of the growth rings was verified through radial sections of mussel valves, aided by acetate peels, in conjunction with in situ annual growth measurements. Growth was modeled with the Schnute general growth model for age-length data or with an analog of the Schnute model for growth-increment data. Bootstrap confidence intervals were obtained for all parameters of the model and for model predicted lengths at each annulus. Confidence intervals of the between-annuli growth-increment model overlapped those of the age-length model at all annuli when growth over the entire range of ages in the population was estimated. Differences in growth model parameters between the age-length model and the mark–recapture analog could be accounted for solely by inherent differences in age-based versus length-based models. Growth estimates generated from between-annuli measurements were equivalent to growth estimates obtained from mark–recapture measurements of annual growth. In general, mussel growth at our sites was best described by the von Bertalanffy submodel.