Event Name/Location

Joint Meetings of Ichthyologists and Herpetologists, Portland, Oregon, July 22-27, 2009

Document Type

Conference Proceeding

Publication Date

7-26-2009

Abstract

The spatial distribution of organisms plays a key role in facilitating biological processes, such as trophic interactions, which influence pelagic ecosystem structure and function. This study combines discrete trawl net sampling with continuous, full water column, acoustic measurements to investigate the distribution of bathypelagic (1000- 3000 m depth) nekton biomass along the Mid-Atlantic Ridge from Iceland to the Azores in the North Atlantic. Two, previously unknown, distinct bathypelagic acoustic scattering layers (ASLs) were observed using 18 kHz echosounder data. One, extending down on average ~200 m from 2000 m depth, appears ubiquitous wherever bottom depth allows, while the second, found within the 1500-2000 m depth stratum, only occurred south of the Sub-Polar Front. Backscatter from the 2000 m ASL was attributed to fish based on net catches, with species drawn from the suite of bathypelagic species observed throughout the study area, rather than any specific group. No general increase in acoustic backscatter, as a proxy for pelagic nekton biomass (primarily fish), was observed in close proximity to the bottom (≤ 200 m), but localized concentrations of backscatter were observed in areas of steep bottom topography at bathypelagic depths. Together these observations demonstrate a previously unreported degree of complexity in the spatial 15 structuring of bathypelagic ecosystems, which is likely to significantly effect the local functioning of those ecosystems.

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