Muskellunge Scale Interpretation: The Question of Aging Accuracy

Abstract

Age and growth data extracted from scales are often used when managing fish populations. However, accurate interpretation of scale annuli in certain species, such as muskellunge Esox masquinongy, has been suspect. The accuracy of age determination was tested with three experienced esocid scale readers who interpreted positive photo images made from acetate slide impressions projected on a microfiche reader. Images were made from scales of 25 known-age muskellunge (3-10 years old) from five Wisconsin lakes. Each scale image was interpreted twice; once with fish length, sex, and date of capture given and once without any information. Of 149 scale interpretations. 32% were assessed with the correct age. 54% underaged. and 14% overaged. The average interpretation deviated from the known age by 1.0 years. Accuracy did not improve when additional fish information was available. General confidence in the interpreter's age assessments was low and possibly related to quality of the photo images. Interpreters were accustomed to using acetate images with several impressions rather than a single photo image. As a follow-up, we selected 12 acetate slides used to make the photo images and allowed each interpreter to determine age from the slides. The percentage correctly aged with the subsample improved from 24% to 47% with the acetate slide impressions. Increasing the magnification of acetate slides from 23 x to 35 x magnification did not affect accuracy. Image quality and multiple impressions can improve accuracy of age determination; however, accuracy was still less than 50%. The low accuracy when interpreting scale annuli of muskellunge 3-10 years of age suggests that managers need to use caution when evaluating data taken from muskellunge scales.

Factors Affecting Recruitment of Age‐0 Muskellunge in Escanaba Lake, Wisconsin, 1987–2006

“…A microfiche reader equipped with a 36-433 objective was used to interpret ages (Hoff and Serns 1986). We recognized the inaccuracies associated with aging muskellunge by scales (i.e., scale-based methods are notorious for underestimating true age; Fitzgerald et al 1997); however, we determined that the bias should be consistent between years and therefore concluded that the ages would be reflective of the true average age for sexually mature muskellunge of any given year. Because the fish were tagged, many individuals were observed multiple times during the study.…”

“…Because the fish were tagged, many individuals were observed multiple times during the study. Therefore, scale interpretation for these fish was unnecessary because an earlier, presumably more accurate age estimate was available (Fitzgerald et al [1997] determined that age was more accurately estimated for younger muskellunge). This increased our confidence in the estimated ages, especially for older fish.…”

Accuracy, precision and quality control in age determination, including a review of the use and abuse of age validation methods

“…Nor can this method be used on species which cannot be reared in captivity prior to release. Nevertheless, this method has been used with success to confirm absolute age and growth increment formation at both the daily (Tsukamoto & Kajihara, 1987;Secor et al, 1995b) and the yearly scale (Fitzgerald et al, 1997;Svedang et al, 1998).…”

“…In practice though, only a subset of the available methods possess the necessary precision. Release of known age or chemically-marked young-of-the-year (YOY) fish is well suited to this type of application (Ferrell et al, 1992;Fitzgerald et al, 1997). For annulus studies, modal progression with age subsampling is also straight forward and accurate; monitoring the modal length of the presumed YOY (for example, through periodic research surveys) confirms their identity as YOY, while inspection of the marginal increment in those same YOY in the season of annulus formation confirms the formation of the presumed first annulus (Ferrell et al, 1992).…”

Ecology and management of northern pike in Wisconsin

“…Growth was also an important parameter but not included in our analysis after considering interpretation errors from scales resulting from irregular growth and resorption or erosion on the midlateral region (Casselman, 1990). Research on aging accuracy with muskellunge scales from Wisconsin (during the pike evaluation period) confirmed that scales were not reliable for accurate age estimation (Fitzgerald et al, 1997). Changes in esocid growth need to be determined using scale samples from younger fish (\8 years) and preferably in combination with the use of cleithra (Casselman, 1979;Casselman, 1996).…”