Addressing Confusion Surrounding Otolith Zonation- a Comparison of Redband Trout and Bluegill Growth and Otolith Edge Appearance

There is a confusion surrounding the timing and interpretation of otolith zones.  We compared  growth rate  and concomitant otolith edge appearance for a coldwater trout species (Redband trout) and a warmwater species (Bluegill) from five Idaho waters.  Monthly instantaneous growth rate was calculated and compared to otolith edge condition to determine which zone (opaque versus translucent) was formed during periods of fast and slow growth.   Rainbow Trout (RBT) showed their fastest growth during the month of June, with continuing but declining growth  observed through September. The opaque zone began forming in March with 37.5% of RBT having an observable opaque edge.  By June, 100%  had formed a complete opaque zone. Bluegill also showed their fastest growth during the month of June with growth declining markedly by July. The fastest growth rates observed for BG were during late spring and early summer, these coinciding with translucent zone formation. Similar to RBT, 100% of BG had formed an observable fast growth zone by June.  However during fast growth periods, RBT otoliths have an opaque edge while BG otoliths have translucent edges.   Our results support studies suggesting some warmwater and coldwater species in north America form opposite-appearing fast and slow growth zones.