young gadoids in the Moray Firth

On our second transect going back towards the west, we had another clean herring haul yesterday morning. The marks were continuous along the seabed: 

haul 4: 02/07/2014 @ 5:43 UTC (58º09'N 0º07'W)

Hendrik Jan starting to measure the catch

While progressing further west we approached the entry of the Moray Firth. Just after lunch, an interesting looking echogram presented itself to us. There were scattered schools spread throughout the water column. As similar looking echotraces turned out to be sprat in the past, we shot the net to verify that.

haul 5: 02/07/2014 @ 11:14 UTC (58º08'N 1º17'W)

The catch however consisted of young Norway pout and whiting.
Another similar looking scene could be seen this morning over one of the deep trenches in the south-eastern Moray Firth:

haul 6: 03/07/2014 @ 5:44 UTC (57º45'N 2º04'W)

The catch again contained young Norway pout and haddock:

We also examined the sound scattering properties of these species and compared them to the ones of herring. When looking at how strong the young gadoids reflect sound at the different acoustic frequencies used, one can see a distinct pattern, as indicated below.

example echogram of a school of young Norway Pout at 4 frequencies (38, 120, 200, and 333 kHz; top panel) and the corresponding typical frequency response (backscatter at a given frequency in relation to the backscatter at 38 kHz). It is evident that the reflection are progressively weaker (more blue colours) at higher frequencies.

example echogram of a herring school at 4 frequencies (38, 120, 200, and 333 kHz; top panel) and the corresponding typical frequency response (backscatter at a given frequency in relation to the backscatter at 38 kHz). The decrease in sound reflection at higher frequencies is less severe when compared to young Norway Pout.