Comparative assessment of calcified structures for age estimation in Brook Trout and Brown Trout

Objective Nonlethal age estimation is critical for low-density fish populations, threatened or endangered species, and populations with low abundances of mature fish. Past research has shown that nonlethal aging structures, such as scales and fin rays, have mixed accuracy and precision for age estimation in trout. Our study tested the viability of using the maxilla as a nonlethal age estimation structure for Brook Trout Salvelinus fontinalis and Brown Trout Salmo trutta. Methods We collected 282 Brook Trout and 129 Brown Trout from 11 streams in Wisconsin during autumn from 2020 to 2024. We collected sagittal otoliths and a maxilla from each fish. Three readers estimated the age of each structure individually, and a consensus age was determined if all three readers agreed upon the age of a fish. If a consensus age could not be agreed upon, that fish was removed from the consensus age part of the study. We compared age estimates between otoliths and maxillae for each species using age-bias plots. We assumed that otoliths would provide a more accurate and precise—albeit lethal—age estimation structure to compare with age estimates from nonlethal maxillae. Results Maxilla-based age estimates for Brook Trout indicated that readers tended to overestimate age for age-0 fish and underestimate age for age-3 to age-5 fish. Maxilla- and otolith-based age estimates were similar for age-1 and age-2 Brook Trout. For Brown Trout, reader age estimates using maxillae were similar to age estimates obtained using otoliths for fish of ages 0–5. Age estimates for age-6 to age-9 Brown Trout were similar for both structures, although our sample sizes were small for older fish. Conclusions Our findings indicate that the maxilla is a viable nonlethal aging structure for Brown Trout. For Brook Trout, the uncertainties identified in maxilla-based age estimation likely negate the use of the maxilla as an accurate nonlethal structure. Our observations corroborated other studies that have examined nonlethal age estimation methods in trout, suggesting that nonlethal calcified structures are reasonably accurate for age estimation in Brown Trout and more limited for use in Brook Trout.