short story about taking a catch sample

While on the 4th transect covering our survey area on the way south, we were approaching the north-eastern Scottish coast. Some small schools appeared along the seabed and we took the chance to take another trawl sample.
These catches are not only used to identify our acoustic estimates of fish-density-per-sea-surface-area, but also to get an idea of the population structure: what is the abundance per age and length; how much do they weigh; what proportions are male/female; and at what maturity stage are they.

Below, the various steps of collecting the trawl sample are outlined:
During trawling (haul 8: 3.7., 16:17UTC @ 57.35N 0.40W) we first saw a few small schools on the echogram but suddenly a bigger one (about 20-25m high) appeared:

 

Soon after the trawl sonar indicated that the school entered the net:

trawl sonar screen shot capturing the moment the school previously observed on the echogram entering the net. The square shape represents the net while the pink coloured bar below is the seabed.

The camera footage collected later on also showed the moment the herring went through the net:

From the multibeam echosounder data, we could recreate the shape of the school in 3D:

Soon after that, the net was hauled on board:

the net coming back on board. Sea mist in the background

trawl camera attached to the net

camera and light source

eventually, the catch is lifted on board...

...and emptied over the hatch...

...and down to the fish lab...

...further over the conveyor belt to be sorted 

the catch turned out to be clean herring

HendrikJan getting ready to sample the catch: measuring length, weight, age, sex and maturity

lining up the fish by 0.5cm length classes to recreate the length-frequency distribution

the herring had a mean size of about 22cm

The combination of the different data sources will allow us to recreate the size, volume and fish abundance of the sampled school. These results can then be compared to the acoustic biomass estimate! But that's for later...
Only a few more miles to go to the end of the transect. In the coming week we will continue with the next part of the survey, which includes the Devil's Holes, an area a bit further south where bigger herring aggregations can be expected.