Abundance, Distribution, and Swimming Behavior of Under Ice Larval Krill on

NBP02-02 and NBP02-04

Scott Gallager, Philip Alatalo, Alec Scott, Andy Girard, and Gaelin Rosenwaks

The objective of the ROV studies is to observe and quantify the distribution, abundance, behavior and size distribution of larval krill in association with the underside ice surface and sea surface hydrography. The WHOI SeaRover is equipped with a variety of physical and biological sensors including a stereo camera system with a field of view of 0.5 m³, a synchronized strobe, a CTD, an Imagenix 881a 630 kHz-1 Mhz sector scanning sonar, an uplooking DVL Navigator 1200 kHz ADCP, and the standard forward looking pan and tilt color camera. A trackline was established radially away from the ship out to a distance of approximately 100 m. As the ROV traveled the trackline at a speed of about 2-10 cm/s, the stereo camera was used to image the under ice surface and associated organisms. Precise positioning and sizing of the target within the 0.5 m³ is established through post-processing using a sterogrammetry algorithm. The forward speed of the ROV was measured with data from the ADCP and used in conjunction with the image volume to calculate volume sampled per unit time. On the fall cruise, eight deployments were conducted at stations where ice was either forming as grease or pancake ice (northern stations) or year-old ice in the southern stations. Furcilia were either absent or few to the north of Marguerite Bay, but present in dense patches to the east and south of Charcot Island. Long video sequences were obtained allowing swimming behavior to be analyzed. On NBP02-04, we completed 26 successful deployments of SeaRover. In general, larval krill were uncommon in the southern sector of the grid, and common in the northern sector along the under-ice surface in association with crevasses and cracks in the ice. Only a few stations had sufficient numbers where swarms or aggregations formed. At one station, the ROV watched a swarm of furcilia under a layer of ice for more than 10 minutes, continuously allowing swarm kinetics to be analyzed. The last two deployments were part of a 24 hour process station where net hauls and CTD casts were made around the clock. The purpose of multiple ROV deployments was to determine if diel vertical migration was important to furcilia. The general impression is that the furcilia concentrations were far lower than found last year on NBP01-04. Interestingly, where furcilia were abundant, microplankton were scarce. These observations will be discussed in the microplankton report.