Wednesday, 8 July 2015

Student from Wageningen University

My name is Alessandro, a master student from Wageningen University. I joined this research cruise last Sunday, in Aberdeen. The aim of my MSc project is real-time fish species and size specific discrimination using acoustic backscattering data, which can be quite challenging among species that show similar acoustic responses. In this respect, the new broadband echosounder EK80 uncovers new possibilities, as we become able to study the fish school’s frequency response (intensity of sound reflected at different frequencies) over the complete spectrum 17-460 kHz. To do so, we need clear and consistent data in order to calibrate and validate an identification algorithm in the next months.

When passing through several fish schools and observing a pure catch of a certain species in the net, it is quite reasonable to think that all the aggregations seen on the screen belong to the very one species. Likewise, it can happen to observe a broad variety of species within the same catch, as it happened on Tuesday, July 6th fishing on several schools around 52 12 N; 00 34 W (Haul 9). 

The catch consisted of mixed Norway Pout, Mackerel, Haddock, Whiting, Grey Gurnard, and Herring, for a total of about one tonne. 

Diverse species on the conveyor belt (Mackerel, Haddock, Norway Pout, Grey Gurnard)

In these circumstances it becomes quite hard to successfully distinguish the different caught species on the echogram!

Which fish school belongs to Herring? Which one to Norway pout?

Thanks to the trawl camera we can attempt to track the history of the fishing session, connecting each school on the echogram to the related species observed with the camera.

A group of 13-16 cm Norway Pout entering the net at GMT 12:19:40 

A mix of Haddock and Whiting caught at GMT 12:30:25

Probable localisation of Norway Pout school after our reconstruction: to do this, we took into account the distance between the transducer and the trawl camera, along with the boat speed

However, for the best performance in species identification we hope for pure catches in the next days. Especially Norway Pout and Herring will be targeted, since it appears to be quite difficult to distinguish them acoustically.

In the meanwhile, I’m helping HendrikJan in sampling catches, taking measurements and collecting otoliths for the Herring survey, to get a more complete insight of the life on a research vessel!

Tuesday, 7 July 2015

Herring researchers: ambassadors of Dutch flavours and taste

One of the advantages of working on a governmental vessel is that you have to spend the weekend in port. Of course it is not very efficient for the execution of the survey, but since you find yourselves a foreign port, you may as well just enjoy your time!
A great way to spend you time is let the local people taste "Maatjesharing" (Dutch, slightly salted and fermented herring) and convince them that is edible and tasty.
This weekend in Aberdeen, on Sunday evening, our team visited Fiona, Berta and Katherine (just back from a day of Munro bagging) and presented to them a portion of homemade Maatjesherring.
After a brief introduction by the guys...

... Berta is the first to try. Fiona can hardly wait. Katherine is less enthusiastic!
Fiona's turn. Berta is going to have a second one!
Katherine has a hard time to swallow a small piece of herring. She actually found a bone!
Her sister Fiona shows how to eat a herring, Dutch style!
Conclusion: Berta and Fiona seemed to enjoy eating Maatjesherring. Katherine still has to become used to the idea of eating (almost) raw herring. May be next year?
We left a portion for Fiona for a tasting session in the Marine Lab in Aberdeen.
Happy, to have found ourselves new ambassadors of Dutch marine quisine, we walked back to Tridens where we welcomed Alessandro, student from Wageningen University, who will join us next week and will contribute to this blog.

Friday, 3 July 2015

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.

Thursday, 2 July 2015

young herring & sprat, fish schools in 3D!!

Yesterday towards the evening whilst approaching Moray Firth we shot the net on some marks along the seabed. The haul contained young herring (around 18 cm mean length) and sprat (mean 11 cm). 
trawl 5 (on 1.7., 16:34UTC @ 58.05N 2.30W)

The multibeam echosounder is running throughout the survey and also during trawling, so we collected data on the schools we targeted. Lining up the multibeam data creates 3D images of the fish schools. Below are several examples of these viewed from different angles.

3D views of fish schools based on the multibeam echosounder data

the same scene of the 2 fish schools as seen in 3D and 2D 
combining different views for better data interpretation

From the combination of the single beam EK60 and the multibeam ME70 we expect to improve school identification. It allows us to characterise the school shape which may be different for different species. The crew on board the Tridens also use the information during catch operations to steer the net towards the schools.
Bram releasing one of the sharks we caught
gulls having a rest on the fore deck while we steam through the Moray Firth

Sascha getting his hands dirty

Haul 5. Mixed catch with 50% herring and 50% sprat

Bram and Sascha processing the catch

Sprat and young herring look very similar to one another. But there are a few tell them apart. The pelvic fins of the herring are behind the start of the back fin, whereas on the sprat they are in front.The herring has smooth gill covers, and moderately blunt keel scales along the edge of the belly, whereas the sprat has pointed keel scales that feel prickly when a finger is run along the belly. 

Sascha explaining the morphological differences between sprat and herring

Juvenile Atlantic herring (Clupea harengus) with blunt keel.

European sprat (Sprattus sprattus) with jagged keel

A beautiful morning in Moray fifth (Thursday 2nd July)

Before every deployment of the net Dirk attaches a GoPro and light source to the top panel of the net. With this setup we aim to identify when a particular species enters the net which can then be linked to acoustic recordings to aid echotrace classification. 

The way every good GoPro video begins

Footage from inside the net. Note the catshark on the left of the image.

A herring school being caught by the pelagic trawl

The net being hauled after an hour and a half in the water