Seasonal Marginal Growth on Otoliths of Seven Alaska Groundfish Species Support the Existence of Annual Patterns

The Alaska Fisheries Science Center’s Age and Growth Program has been collecting qualitative otolith edge growth data for many of the principal species that it ages. The edge-type data collected are coded on a scale from 0 to 5, in an attempt to categorize the characteristics of the outermost growth zone of the otolith for the month of capture. Edge growth classification data can be subject to biases when age readers are looking for a particular pattern of growth. However, analysis of edge-type data can be used to determine the apparent strength and timing of seasonal growth patterns, and thus provides a weak form of age validation. We develop a simple model that allows us to estimate the signal strength and timing of otolith edge growth using standard nonlinear least squares. We do this by combining edge codes so that they are concentrated into two distinct categories: category 1 which represents an otolith with a full increment of opaque growth or a translucent zone on the edge (with perhaps a hint of new opaque growth), or category 3 which represents an otolith which has substantial growth, 1⁄4 to 1⁄2 of the previous year’s otolith growth, on its edge. Because the remaining categories (2 and 4) contained relatively few observations, categories 1 and 3 can be treated using a logit-like transformation, and then modeled with a cosine function. This model appeared to capture a strong seasonal trend in edge-type growth in 7 species for which reasonable amounts of data were available. These results support that the growth rings found in many Alaska groundfish are generally annual marks, but do not validate that conventional ages from otolith ring counts for any of these species are accurate. Authors: DAniel K. KimurA, DelsA m. AnDerl, and betty J. Goetz are with the Alaska Fisheries Science Center, National Marine Fisheries Service, NOAA, 7600 Sand Point Way NE, Seattle, WA 98115-6349. Email: Dan.Kimura@noaa.gov Acknowledgements: We thank Beth Matta, Dan Cooper, and two anonymous reviewers for many constructive comments. IntroDuctIon The Alaska Fisheries Science Center (AFSC) has a disciplined method of age determination (Kimura and Anderl 2005) that includes lead age readers who resolve questions of ageing criteria, precision testing to ensure repeatability of ageing criteria, and a radiometric age validation specialist who provides radiometric and C-14 age validations when feasible. Age data and edge-type data generated since about 1990 are readily available for study and examination. Conventional age determination from otoliths is based on the assumption that a pattern of annual growth increments is present. In theory, opaque zones represent periods of rapid growth and higher calcification, whereas translucent zones represent periods of slower growth and low calcification (Pannella 1974; Chilton and Beamish 1982). Opaque and translucent bands appear respectively as dark and light zones under transmitted light, or as light and dark zones under reflected light. A seasonal pattern in otolith edge types can aid in the verification of the hypothesis that an annual ring (opaque/translucent zone pair) is laid down once per year. The determination of edge type, as described in this paper, is subjective. Therefore, Campana (2001) considered this “marginal increment analysis” to be one of the weakest forms of age validation. The timing of the translucent zone deposition is of particular interest. By international convention, marine fish species are assumed to have a birth date of January 1, regardless of the actual hatch date (Chilton and Beamish 1982; Panfili et al. 2002). An age reader must therefore interpret edge growth as it relates to this January 1 birth date. When assigning an age, an age reader must attribute any apparent opaque edge growth to either the current growth year (not counted) or the past growth year (counted). This classification takes into account a number of other variables which affect interpretation, such as the date of collection, the general age range (old versus young), and the width of the growth increment in comparison to previous increments. For example, in an otolith collected in July, the reader may observe an opaque band on the edge equal in width to the previous year’s opaque growth. The reader must judge whether this growth is attributable to the current growth (not counted) or whether this growth is attributable to the past year’s growth cycle (counted). In most cases, this edge growth would not be counted. However, if the same pattern was observed