Estimating size of opalfish from otoliths: implications for analyses of New Zealand sea lion diet

Abstract

Anomalies can arise when species-specific otolith equations are applied to congeneric species in analyses of seal prey remains. Opalfishes, Hemerocoetes, two spp., are the most numerous fish in the diet of New Zealand sea lions, Phocarctos hookeri, at their population base at the Auckland Islands. Opalfish identified from otoliths in sea lion prey remains accounted for one-third of prey items in this and two previous diet studies. This highlighted the need to estimate size of opalfish accurately. Equations to estimate fish size from otolith length differed between the two species because otoliths of H. morelandi were relatively smaller than otoliths of H. artus. However, otoliths of these species were indistinguishable and this precluded definitive estimates of opalfish length and mass from otoliths. In particular, misidentification of otoliths could create untenably large estimates for fish size. A combination of the equations for otolith size to fish size from the two species avoided this anomaly and superseded previously published equations.