Using a hydrographic echo sounder, we encountered an animal aggregation on a sea mount summit (350 m deep) at the West Mariana Ridge (14°07’ N, 142°48′ E). The aggregation suddenly ascended from the summit at civil dawn, and we were able to capture the ascending aggregation at the next dawn using a large midwater trawl net. Hundreds of fishes captured were determined to be Sphyraenops bairdianus (Poey, 1861). Hydrated eggs in the ovary were histologically confirmed, indicating that this species performed a massive spawning associated with an offshore sea mount and sunrise.
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