Evidence of Diel Vertical Migrations by Bathypelagic Fishes in the Gulf of Mexico: A New Vector of Water-Column Connectivity in the Oceanic Domain

The classical paradigm of the vertical ecology of the pelagic ocean is that the bathypelagic fauna do not vertically migrate on a diel basis, unlike most of the mesopelagic fauna. This paradigm is supported by several factors: the primary driver of diel vertical migration is thought to be sunlight, which is absent at bathypelagic depths; there is a fundamental change in assemblage composition below 1000 m; and the energetic costs of bathypelagic migration are thought to be prohibitive. Nonetheless, there remains a huge gap in our understanding of the bathypelagic realm since the vast majority of deep-pelagic studies do not sample below 1000 m. Two large-scale sampling programs have sampled micronektonic fishes in discrete-depth fashion to 1500 m deep in the Gulf of Mexico since 2010, with one of the goals being to sample above, within, and below the reported hydrocarbon plume centered at c. 1100 m as a result of the Deepwater Horizon oil spill. These data provide an unprecedented amount of information about bathypelagic ecology, and reveal that some bathypelagic fishes do undertake active diel vertical migrations. Potential cues for this migration in the absence of solar light will be discussed. Vertical migration from the bathypelagic realm, combined with bathypelagic foraging of certain marine mammals and large epipelagic fishes, represents a previously unknown vector of connectivity between the epi-, meso-, and bathypelagic food webs. These findings suggest that generalizations about the deep-pelagic fauna may oversimplified, underscoring the urgent need to better understand connectivity through the water column when assessing large and complex ecosystems.

http://www.deependconsortium.org