Acoustic and Trawl Observations of Pelagic Swimming Behavior by the Round Goby in Lake Ontario

Since its introduction to the Great Lakes, the ecological role of the abundant round goby (Neogobius melanostomus) has been of wide interest for fishery managers and scientists.  Paired acoustic-trawl samples were collected from repeated, synchronized acoustic and mid-water trawl surveys in nearshore waters (3 to 30 m water depth) of Lake Ontario near Oswego, New York between April and November 2009 and 2010.  A 200- and 70-kHz Simrad EK60 split-beam echosounder collected acoustic data concurrently with tows of a 3.0-m high x 6.1-m wide net (5.1 to 0.6 cm stretch mesh) made at either 1.6 m below the surface or 6.1 m off the bottom, which resulted in 12 tows with round goby.  The mean total length (TL) of round goby was 45.5 mm (n=35, range=25 to 81 mm). The target strength (TS) distribution of single echo detections in the water column for these sampled depth layers was tri-modal.  The two highest TS modes corresponded to the expected TS of co-occurring juvenile alewife and rainbow smelt.  The lowest TS mode was about 10-20 dB less than the expected TS of swim-bladdered fish of observed round goby size.  The TS distributions of three individual round gobies (54, 90, and 156 mm TL) tethered to a monofilament line 2 m above a reference target also resulted in TS of 10-20 dB lower than predicted by TS-TL relations of fish with a swim bladder.  The reduced target strength of echoes and concurrent round goby catch from the mid-water column provide evidence of pelagic swimming behavior by round goby.