Variability in Age Estimation Results in Ambiguity and False Understanding of Population Persistence

Abstract

Mortality, growth, and recruitment are the primary dynamic rate functions that regulate fish populations. Age data obtained from calcified structures can provide direct and indirect information needed for calculations of these metrics; therefore, knowledge of the fish population age structure is often coveted information. Unfortunately, potential sources of error exist in the form of subjectivity in interpretation of growth increments, inconsistent deposition of growth increments, and lack of validation in age-estimation processes. However, many sources of error are either not known or simply ignored, particularly for long-lived fishes. Therefore, we included a level of uncertainty in our age estimates of a periodic life history strategist, the Shovelnose Sturgeon Scaphirhynchus platorynchus, by incorporating variability in reader assignment of age. We used a bootstrapping procedure to generate a matrix of new age distributions and demonstrated how calculations of mortality varied as a result of reader assignment variability. Varying mortality rates of adult Shovelnose Sturgeon in a three-stage, female-only population projection model resulted in a wide range of potential responses. Hypothetical population projections after simulating for 30 years ranged from 2,249 to 10,673 individuals, despite mortality estimates varying by only 15% (28–43%). Age-determination errors are an impediment to understanding fish population dynamics using conventional age-based assessments; therefore, consequences of aging error should be considered when attempting to model population dynamic processes and long-term viability.

Received August 8, 2015; accepted February 12, 2016 Published online May 16, 2016

Acknowledgments

We thank J. Hammen, M. Rugg, T. Anderson, and others who assisted in sampling, preparing, and aging of Shovelnose Sturgeon. We thank K. Pope, R. Holland, and K. Eskridge for reviewing earlier drafts of this manuscript. Funding for this study was provided by the Nebraska Game and Parks Commission, project F-180-R.