Meredith B. Pfennig

Objective: Age data are widely collected in fisheries science because of the vital information it provides about population characteristics. Moxostoma, a common genus of Catostomidae, are abundant throughout North America, yet there are few studies that focus on methodological procedures to estimate age for these species. Moreover, studies that have concentrated on estimating age of Gray Redhorse M. congestum, a species whose otoliths look very similar to those of other catostomids, are also uncommon. Therefore, we wanted to present insight into the morphological features, processing methodologies, and interpretation of annual increments in Gray Redhorse otoliths to provide guidance and consistency for aging this species and other catostomids. Methods: Gray Redhorse were collected from central Texas rivers quarterly throughout 2021. The lapillus otoliths were removed, morphologically assessed, sectioned, and imaged for annual increment interpretation. Results and conclusion: All of the whole otoliths presented uniform characteristics, including presence of a gibbus maculae (i.e., a hump). In the sectioned otoliths, the annual increments were easily distinguishable for ages 1–4. Otoliths aged ≥5 presented some difficulties due to shading or darkening of increments; therefore, we suggest finding and counting annual increments on the lateral edge of the structure. Furthermore, a completed annulus should be counted as an opaque zone that is characterized by either a thin brown or a distinct thin dark-gray line, paired with a translucent zone. Given that Gray Redhorse otoliths have a morphology and increment patterns that are similar to those of other catostomids, the methods and interpretation of increments in otoliths presented here can likely be applied more broadly to the family.

Objective Age estimation methods have not been validated for many species. Thus, we focused this age estimation study on four species from central Texas: the Guadalupe Bass Micropterus treculii, Channel Catfish Ictalurus punctatus, Gray Redhorse Moxostoma congestum, and Redbreast Sunfish Lepomis auritus. Our objectives for each species were to (1) investigate whether increments form annually in otoliths, fin rays, or spines; (2) determine the seasonal timing of zone deposition in each calcified structure; and (3) compare age estimates between otoliths and fin rays or spines. Methods Fish were sampled from central Texas waters and injected with oxytetracycline (OTC) quarterly in 2021. In July 2022, otoliths and fin rays or spines were collected to determine whether the increments formed annually and to estimate age. Result On average, two readers identified the correct number of annuli after the OTC mark in at least 95% of Gray Redhorse, Guadalupe Bass, and Redbreast Sunfish otoliths but in only 69% of Channel Catfish otoliths. However, the more experienced reader identified the correct number of annuli in at least 95% of otoliths from age-2–4 Channel Catfish. Compared to otoliths, the correct number of annuli after the OTC mark was identified in a smaller percentage of fin rays or spines (47–62%). The probability of identifying the correct number of annuli after the OTC mark differed between the two readers for all spines and fin rays. Age difference plots revealed that age estimates based on fin rays or spines were either overestimated or underestimated compared to otolith-based ages for all species. Conclusion We consider annual increment formation in otoliths to be validated for all age-classes of Gray Redhorse, Guadalupe Bass, and Redbreast Sunfish examined and for ages 2–4 of Channel Catfish. Differences in correctness between readers were most likely due to differences in age estimation experience. Given the level of error observed in our study, caution is advised if using age estimates from unvalidated structures and species.

The antibiotic oxytetracycline (OTC) is a fluorochrome marker, and fluorescence microscopy is used to view OTC marks in fishes' calcified structures. However, OTC marks have been observed in calcified structures using standard light microscopy for multiple species. Therefore, we conducted an experiment to investigate potential factors (i.e., season, total length of fish, growth rate, and sex) influencing the observation of OTC in calcified structures (otoliths and fin rays or spines) from channel catfish Ictalurus punctatus, gray redhorse Moxostoma congestum, Guadalupe bass Mircopterus treculii, and redbreast sunfish Lepomis auritus viewed using standard light and fluorescence microscopy. OTC stains were not observed in any otoliths under standard light; however, OTC marks were commonly observed in I. punctatus spines using standard light microscopy (56.2%). Ninety‐nine percent of otoliths and 88.9% of spines and fin rays had a visible fluorescent OTC mark when viewed using fluorescence microscopy. There was a negative relationship between the observed OTC mark and total length of fish for each season, but fish injected in the summer had the most structures with an observed OTC mark under either light condition. Understanding how OTC marking is affected by biological processes and environmental conditions will assist in future studies that rely on chemical marking of calcified structures by increasing efficacy of OTC marking and interpretation of marks.