Publication Date


Date of Final Oral Examination (Defense)


Type of Culminating Activity


Degree Title

Master of Science in Biology



Major Advisor

Peter Koetsier, Ph.D.


Daniel Schill, Ph.D.


Julie Heath, Ph.D.


Redband Trout (Oncorhynchus mykiss gairdneri), a native subspecies of Rainbow Trout residing east of the Cascade Mountains, USA, are a popular sport fish in much of its range. Bluegills (Lepomis macrochirus) are one the most important recreational fishes in North America, and are also sought by anglers in many Idaho waters. There is extensive confusion surrounding the timing and interpretation of otolith zones, specifically, which zone represents fast growth and which represents slow growth. Further, otoliths are a lethal sampling method and regardless of population status, many fisheries biologists prefer to use no lethal sampling methods. To determine if the zonation confusion is a species-related difference, I calculated monthly growth rates and investigated otolith zonation for a cold-water (Redband Trout) and a warm-water (Bluegill) species, in two Southern Idaho streams and three ponds. I also compared the assigned age and precision of sagittal otoliths, pectoral fin rays, and scales for these two species. Redband Trout showed their fastest growth during the month of June, with continuation of growth through September. The opaque zone started to form in March; by June 100% of the Redband had formed an opaque outer edge on the otolith. As with Redband Trout, the fastest growth rates for Bluegill were during late spring and early summer. This fast growth coincided with the formation of the translucent zone, which was observable in 95% of Bluegill by the month of May. Similar to Redband Trout, 100% of Bluegill had begun forming their fast growth zone by June. Based on edge analysis the otolith zonation pattern for Bluegill is translucent-forming during summer months and opaque-forming in fall to spring. Conversely, the otolith zonation pattern for Redband Trout is opaque-forming during summer months and translucent-forming in winter. Our findings suggest that Redband Trout and Bluegill do indeed form opposite appearing otolith zones during their respective periods of fast somatic growth. Although we have limited data for other warm and cold-water species in our study waters, we observed similar patterns for other species in these two groups. In addition, the otoliths for the first two age classes of both species were validated as forming one annulus per year. These findings have implications for both experienced and novice biologists conducting ageing studies. Lacking water-specific validation, the annulus for temperate warm-water centrarchids should be considered the opaque zone. Conversely, the annulus for temperate cold-water trout should be considered the translucent zone. Otoliths of Redband Trout were the most precise at both locations, followed by fin rays with scales being the least precise. I found no difference in the assigned age of fin rays and otoliths at Mores Creek. However, I found a statistical difference between assigned ages of otoliths and fin rays, with fin rays producing lower age estimates, specifically on older fish at Harris Creek. Scales ages were less precise and had lower age estimates than that of otoliths or fin rays at both locations. Though, our findings showed a difference in assigned age, at Harris Creek, and precision at both locations. I feel that fin rays produced an acceptable age estimate for montane Redband Trout. These findings, along with those that have shown that fin ray removal did not affect growth and survival, leads us to suggest that fin rays may be an acceptable, non-lethal, ageing structure for Redband Trout in montane streams. We suggest this with caution and suggest further research be completed. Conversely, we do not recommend the use of scales, given the fact that scales are less precise and produced lower age estimates. Otoliths of Bluegill were found to be the most precise at both water bodies. Scale and fin ray age estimates differed in precision depending on water body. Scale age estimates were more precise at Atwood’s Pond while fin ray age estimates were more precise at Bruneau Dunes Pond. Pairwise regression comparisons showed that scale age estimates significantly underestimate the age of fish when compared to that of otoliths, at both locations. There was not a significant difference between the assigned age of otoliths and fin rays at either location. We do not recommend the use of scales or fin rays as the primary aging structure for Bluegill. Although, we did not find assigned ages of otoliths and fin rays to differ, estimates of the latter demonstrated far less precision. The difference in precision concerns us. We suggest a study be undertaken to validate fin rays prior to them being used as a primary ageing structure for Bluegill in Idaho waters.