Understanding amphidromy in Hawaiʻi: ʻOʻopu nākea (Awaous stamineus)

Abstract

Hawaiʻi is home to ʻoʻopu nākea (Awaous stamineus), a culturally significant, endemic, goby that exhibits an amphidromous life cycle characterized by a marine larval stage followed by post‐larval recruitment to streams, where they live to become reproductive adults. However, it was recently suggested that their migration to the ocean might not be obligatory, as originally thought. Despite their importance in Hawaiian traditions and the ecology of Hawaiian freshwater ecosystems, we still lack a full understanding of their migratory patterns and life history due to the difficulties in determining the environmental migratory cues that set the timing and location of their migratory paths. This study examined environmental factors, such as mean annual rainfall, streamflow, and water chemistry, to determine if they play a role in whether A. stamineus spend their larval period in the ocean or their entire life cycle in freshwater streams. We sampled A. stamineus (n = 90) from three streams (Kahana, Kahaluʻu, and Waimānalo) on the island of Oʻahu, Hawaiʻi that represented the range of hydroclimatic gradient in wet‐habitat conditions on the windward side of the island and characterized their migratory pattern using elemental analysis of sagittae, the largest pair of otoliths (calcareous ear structures). Based on otolith strontium:calcium and barium:calcium ratios, we determined if individuals spent their larval period in the ocean or the stream. We found that 100% of individuals displayed clear evidence of marine residence during their larval phase, regardless of the environmental factors the fish experienced. This study highlights the necessity of stream–ocean connectivity for the survival of A. stamineus and emphasizes the importance of stream‐mouth conservation and management as it is a critical transition zone in stream–ocean–stream migratory pathways.

Phenological variation in the life histories of amphidromous gobies endemic to the Hawaiian islands

“…In this study, we tested for lunar periodicity and phenological synchronicity between alternative migratory forms of Awaous stamineus , a facultative amphidromous goby endemic to the Hawaiian Islands (Hogan et al., 2014; Heim‐Ballew et al., 2020; Lisi et al., 2022; though see Ching et al., 2023). For context, we drew comparisons to Sicyopterus stimpsoni , which exhibits obligate amphidromy (Heim‐Ballew et al., 2020).…”

Optimizing invader suppression to restore native species: Leveraging species traits to overcome collateral damage

“…Third, a single-species model does not consider life history differences between native and invasive species, which can sometimes be exploited to improve restoration outcomes. For example, in streams on the Hawaiian archipelago, native 'o'opu (freshwater gobies) are partially or fully amphidromous, with some or all of the larvae spending several months at sea before returning to the same or a different stream (Ching et al, 2023;Heim-Ballew et al, 2020;Hogan et al, 2014). In contrast, the invasive fishes that threaten 'o'opu are largely restricted to freshwater (Yamamoto & Tagawa, 2000).…”