Marginal Increment Analysis of Northern Snakehead Otoliths
Quinton E. Phelps, Hae H. Kim, Catherine Lim and John S. Odenkirk
Abstract.—Potomac River Northern Snakehead Channa argus has been the subject of considerable study since discovery in 2004. As part of that study, they were sampled in Virginia Potomac River tributaries monthly from March through October, and otoliths were removed. Age and growth information indicated highly variable length at age and rapid growth in the first three years. Some individuals attained 381 mm total length by the end of the first year (age-0) and 559 mm by age-1. Although Northern Snakehead otoliths viewed in the transverse plane appeared to have distinct, narrow bands interpreted as annuli; it is important to have validation confirming the periodicity of the putative annuli, especially given the high variability in growth rates. Marginal increment analysis is one method that has traditionally been employed to confirm this assumption. Thus, a subset of Northern Snakehead otoliths (n = 200) were subjected to marginal increment analyses (ages 2–10). Differences in index of completion scores occurred among months (F7,192 = 18.26, P < 0.001) but were lowest in July. We demonstrated Northern Snakehead produced a single annulus on sagittal otoliths that is first detectable during early summer in the Potomac River. As such, we recommend the use of otoliths for aging Northern Snakehead accurately to determine critical population characteristics such as recruitment, growth, and mortality.