Identifying migration flexibility and the environmental factors that influence variation in recruitment success in partially migratory Hawaiian fishes
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Behavior flexibility during the larval stage influences differential mortality, recruitment, and population dynamics; recruitment is poorly understood, yet important for understanding population persistence. The purpose of this dissertation was to examine fishes for migration flexibility and to identify factors at different spatiotemporal scales that are influential to differential survival within and across populations. I found that four underexplored native freshwater fishes of Hawai‘i exhibited a larval migratory strategy, but many were flexible. One species (Sicyopterus stimpsoni) showed all individuals made a migration, and the others showed 25 – 40 % did not migrate. Next, I examined if migrant (S. stimpsoni) and flexible species (Awaous stamineus) showed lunar rhythmicity at hatching and settlement. Migrants of the flexible species showed more hatching around the full moon and settlement around the new moon, and residents showed the opposite pattern. The divergence in life-history timing appears to be a balance between ecological costs and benefits. Next, I examined the influence of variation in localized conditions on the ratio of migrant to resident contingents (A. stamineus) in adult populations. Residents were least represented when stream flow and nutrients were low; however, the abundance of a common invasive predator showed a negative relationship with resident abundance. Highly urbanized systems may impose such stressful conditions that resident recruitment is diminished, which underscores the importance of stream management to conserve vulnerable native species. Lastly, I examined the frequency of contingents (A. stamineus), larval duration, and growth rates across ENSO. The proportion of migrants was highest during La Niña, and resident proportions were highest during weak La Niña and strong El Niño; once El Niño became very strong, resident proportions decreased. Migrants had faster growth than residents across all growing stages. Migrant growth rates in the early larval stage were highest during El Niño, and pre-settlement growth was highest during La Niña. Resident growth was fastest during neutral conditions. The duration of the early fast-growing period was shortest during El Niño, and the larval duration was longer. These dissertation findings show the importance of evaluating multiple influential scales to understand life-history strategies of individuals, especially those that inhabit multiple ecotypes.
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