Alex De Robertis explains his work on fish acoustics aboard the R/V Ocean Starr. Photo credit: Ed Farley

Alex De Robertis explains his work on fish acoustics aboard the R/V Ocean Starr. Photo credit: Ed Farley

One of the objectives of this cruise is to establish the abundance and distribution of midwater fishes.  In particular, we are interested in the abundance and distribution of Arctic cod, as we have found large numbers of young-of-the-year Arctic cod on the Chukchi shelf in previous surveys, but very few adults.  We are trying to unravel the puzzle of what happens to the young Arctic cod that are present in summer on the Chukchi shelf.  What is their fate?  Does the Chukchi shelf serve as a nursery ground for Arctic cod that move to other areas, or do the young cod hang around on the Chukchi shelf, with very few surviving through their first winter?

We are trying to address this question by using acoustic instruments to detect the Arctic cod from ships and moorings on the seafloor, and by sampling them with trawls. This summer, we are conducting a large-scale acoustic-trawl survey from the R/V Ocean Starr to map the distribution of the Arctic cod.  During this survey, as the vessel cruises along, we send out calibrated pulses of sound from the hull of the ship, and record the echoes from fish in the water column that are reflected back.  When we see significant concentrations of fish, we stop to take a sample with a midwater trawl.  This allows us to determine the species and sizes of fish in the water. We use the trawl and acoustic measurements to estimate abundances of fish by species in the surveyed area.  Many of the fish we catch are kept for lab studies by our colleagues.  This acoustic-trawl method works best in areas dominated by a single abundant species and size class, and the Chukchi Sea fits the bill:  our catches so far have been almost exclusively baby Arctic cod less than two inches long.  So far, we have seen these fish distributed all over the northern Chukchi Sea this year, and at higher abundance and with a broader distribution than in surveys conducted in 2012 and 2013. 

Acoustic image of an Arctic cod aggregation detected along the cruise track.  The school is visible as the green to red color band at ~18 to 30 m depth.  White represents low backscatter (few fish) and red high backscatter (many fish).  The seafloor is visible as a dark red band at 42 m depth.

Acoustic image of an Arctic cod aggregation detected along the cruise track.  The school is visible as the green to red color band at ~18 to 30 m depth.  White represents low backscatter (few fish) and red high backscatter (many fish).  The seafloor is visible as a dark red band at 42 m depth.

This vessel survey will allow us to describe the distribution and abundance of Arctic cod during the summer, but this is just a single snapshot representing only late summer.  What are the cod doing during the rest of the year?  To try to understand what the Arctic cod are doing during other times of the year, we have deployed three battery-powered sonars on the seafloor.  These upward-looking instruments will collect information throughout the entire year and allow us to monitor what the Arctic cod do when the survey wraps up and the ice comes in.  We hope to track individual fish as they move through the beam to understand their migrations.  But, we will have to wait and see.  We will have to wait until next summer to recover the instruments from the sea floor, open the waterproof cases and pull out USB memory sticks. We hope this data will be the key to better understanding what happens to the young Arctic cod over the upcoming winter. 

Deploying an upward-looking sonar to monitor Arctic cod throughout the year.

Deploying an upward-looking sonar to monitor Arctic cod throughout the year.