Age-Structured Stock Assessment and Life History of Kala (Bluespine Unicornfish, Naso unicornis) Around Maui

Abstract

Kala (Bluespine Unicornfish, Naso unicornis) is an ecologically and culturally significant herbivore in Hawaiian nearshore reef fisheries, yet effective management has been hindered by limited demographic data and growing skepticism from local fishing communities toward existing stock assessments. This study addressed these gaps through a collaborative, fisher-engaged approach, collecting 578 Kala around Maui between 2024-2025 and integrating otolith-derived age estimates, sex-specific growth parameters, reproductive data, and both length- and age-based stock assessment methods. Kala on Maui ranged from 2-38 years of age, with rapid early growth resulting in nearly 80% of linear growth completed within the first 3-4 years of their lifespan. This growth pattern causes multiple age classes to overlap within a narrow size range, fundamentally limiting the reliability of length-frequency analyses for this species. Older individuals were disproportionately represented along the northeastern coast of Maui, consistent with local fisher knowledge of seasonal spawning aggregations at these sites. Preliminary sex-specific growth fits suggested mild sexual dimorphism, with females reaching slightly larger asymptotic lengths (522.2 mm FL) than males (521.6 mm FL). Both length-based (LBSPR) and age-based spawning potential ratio (Chapman-Robson, Spawning Potential Ratio (SPR) approaches indicated that the Maui Kala stock is currently exploited below the SPR₄₀ reference point. The length-based method estimated SPR at 0.66 (95% CI: 0.62-0.73) and the age-based method estimated SPR at 0.55 (95% CI: 0.55-0.56), both exceeding the 40% reference point, with the length-based estimate approximately 20% higher than the age-based estimate. Corresponding F/F_SPR40 ratios were 0.26 and 0.50 respectively, both indicating that current fishing mortality remains below the threshold associated with overfishing. Selectivity parameters indicated that Kala are targeted at or near their asymptotic length on Maui, a substantially more favorable pattern than documented for Oʻahu and Micronesian populations, where smaller and younger individuals dominate the catch. Despite this healthy stock status, the divergence between length- and age-based F/F_SPR40 estimates underscores a critical methodological limitation: the LBSPR model permits more than double the fishing pressure at SPR40 compared to the age-based approach because it is difficult to adequately resolve age structure from length data alone in species with the box-shaped growth curve exhibited by Kala and other surgeonfishes, as well as other reef fish taxas. Current size-based regulations alone may be insufficient to protect the disproportionate reproductive contribution of older, more fecund individuals, nor is it clear whether such regulations ensure do they account for that the functional ecological role of large Kala as macroalgal browsers critical to reef resilience is maintained. Age-informed management strategies such as seasonal closures aligned with spawning periods, spatial protections at known aggregation sites, and catch limits that account for age structure are recommended to sustain both the fishery and the broader ecological function of this species on Maui reefs. Since Kala demographics on Maui differ substantially from those reported for Oʻahu, island-specific assessments are essential, and management strategies developed without local demographic data risk being ecologically inappropriate and losing the community buy-in necessary for effective implementation. This study demonstrates the value of integrating age- and length-based methods with fisher engagement for the sustainable management of ecologically and culturally important reef species in Hawaiʻi.