Project: Canyon Krill

Study of the krill populations in and around Atlantis and Veatch Canyons along the Slope Waters south of Cape Cod.  Zooplankton samples were collected with MOCNESS nets on th RV/Connecticut in 2010 and with a Tucker Trawl annually on the SVV/Corwith Cramer as part of the MIT/WHOI Joint Program new student cruise. The RV/Conncecticut cruise included broadband acoustics measurements of the water column to be correlated with the net samples.

From poroposal:

OBJECTIVES:
The proposed work seeks to understand how physical forcing interacting with local bathymetry and with euphausiid active behaviors leads to the formation, persistence, and availability to predators of euphausiid aggregations in the deep canyons of the New England continental shelf break, using newly-available broadband acoustic scattering techniques. The specific objectives are:
1. To identify and track a euphausiid aggregation in a shelf break canyon in order to quantify changes in aggregation structure and position associated with variations in environmental conditions. Specifically, we will address the hypothesis that the formation and variability of euphausiid aggregations in such canyons relate to the interaction of the flow field, especially upwelling, with the sloping canyon topography and with the euphausiids’ vertical movements and active aggregative behaviors.
2. To test and demonstrate the power of newly-available broadband acoustic scattering techniques for remotely discriminating euphausiids from other sources of acoustic scattering and making accurate and highresolution estimates of euphausiid abundance and size.

METHODOLOGY:
To achieve these objectives, we propose an innovative and comprehensive field program targeting Atlantis Canyon, southwest of Georges Bank. An initial 24-hour 'canyon' survey will be conducted to characterize the overall distribution of euphausiids, other zooplankton (especially copepods), higher predators, and the physical environment. This will be followed by a 48-hour 'tracking' survey of an individual euphausiid aggregation. Surveys will be conducted in September 2010, timed to occur shortly after the Northeast Fisheries Science Center bottom trawl survey of the greater region. In both survey modes, the primary sampling instrument will be a broadband acoustic scattering system (40-600 kHz). Broadband measurements, supplemented by measurements from a more traditional narrowband multi-frequency acoustic system (43, 120, 200, 420 kHz), will be used to quantify the abundance, size, and aggregation structure of euphausiids, as well as the abundance of fish predators and other zooplankton (e.g., copepods). These acoustic estimates of biological quantities will be validated via direct samples collected concurrently with a depth-stratified net system and an image-forming optical system. Characterization of the physical environment and flow field will be achieved via an Acoustic Doppler Current Profiler (ADCP) and a Conductivity-Temperature-Depth (CTD) system also equipped with a fluorometer to estimate the abundance of phytoplankton. During analysis, measurements of the characteristics, movements, and distribution of euphausiid aggregations will be related to these concurrent measurements of the physical and biological environment.

RATIONALE:
Euphausiids are common crustacean members of North Atlantic pelagic ecosystems, but their ecological role in the waters off New England is only poorly understood. Previous studies in the Gulf of Maine region have suggested that euphausiids (especially Meganyctiphanes norvegica and Thysanoessa spp.) can be locally abundant and a key prey item for a variety of higher predators, including commercially-exploited fish and squid. Euphausiids are also notable among zooplankton for their strong swimming capabilities and occurrence in dense aggregations; the formation of such aggregations is thought to be instrumental in determining the availability of euphausiids to higher predators. Overall, however, relatively little is known about the biological and physical processes leading to the formation and persistence of euphausiid aggregations, the interaction of euphausiids with their predators, and the overall significance of euphausiids in the New England Shelf/Slope ecosystem.

Large and dense aggregations of euphausiids have been observed in the canyons of the New England shelf break. The continental shelf break represents a frontier region in current understanding of the dynamics of the western North Atlantic Ocean and its inhabitants, and this project is motivated by the hypothesis advanced in the literature that these aggregations play an important role in subsidizing the productivity of commercial fish and squid stocks in shallower reaches of the shelf. By addressing the bio-physical factors that lead to the aggregation of euphausiids and that determine their availability to higher predators in these canyon regions, the proposed work will take a necessary first step towards defining the essential habitat of fish predators that target euphausiids and quantifying their interaction for the development of ecosystem-based management models. Beneficiaries of the project thus include scientists and managers with National Marine Fisheries Service, the fishing industry, and society as a whole. The project furthermore will demonstrate the feasibility of using newly-available broadband acoustic scattering techniques for quantifying the abundance and size of euphausiids and other pelagic animals, which we anticipate will be of substantial use in the monitoring of marine pelagic habitats globally.