Pelagic trawls: what happens?
- One of the daily trawls
Until now, we have been following a protocol for acoustic surveys in northwest Africa (If you want to access the Guidelines for acoustic surveys in the northwest African region, the publication is available in English and French here) and so when and where and for how long we trawl are based on registrations from the acoustics as it is not recommended to pre-determine trawling stations.
During the Mauritanian leg of the survey, we had at least one trawl during the day and one at night, and sometimes up to three per day. The size of the catch is highly variable and the video below showed a large day catch of just fish, but often we had very little fish in our catch, sometimes deceptively so! In a photo below you can see what was our largest size of “catch” but there were actually very few fish and the bulk came from the hundreds of jellyfish we ended up catching instead!.
Typically, a trawl begins with the ship slowing down to between 1-3 knots and the pelagic net can be deployed for 30 minutes to an hour. The exact timing of each is normally left up to the Captain to decide based on ocean conditions and how much fish is caught (the net is equipped with a headline sensor that sends signals to the ship on how much fish have entered the net and other information).
- The largest size of “catch”
Pelagic trawling requires a lot of skill from the crew. Luckily, we have on board two experienced groups of deckhands led by the Fishing Master (blue shirt in the video below) who has over 23 years of experience.
The video shows what normally happens when a pelagic trawl is pulled back on board.
The first thing that is pulled back are two large blue “doors” seen on the outside of either side of the back opening, and these are suspended in the water to keep the mouth of the net open.
The winches then start pulling the net slowly back in and the Fishing Master must always watch the chains around the winch in case something is tangled.
The crew then remove the heavy iron chains that weigh the opening of the net down when it’s in the water and remove the orange buoys.
- A bulk with hundreds of jellyfish
The four large orange balls mark the opening of the net and keep it afloat and they are also removed.
As the net starts coming in, you can see lots of small creatures like jellies and juvenile fish caught in the net and fish start becoming visible in the green part of the net. The catch is concentrated at the tail end of the net and the Fishing Master lets some of the water drain out before bringing it onboard.
This is the point when the crew must get to work untying the bottom of the net and making sure no fish are caught in the neck of the net before it gets wrapped around the winch (drying, rotting fish is not a good smell for a net!) and then hooking the net up to a crane that lifts the heavy catch and dumps is out for the scientists to sort.
- Four orange balls mark the opening of the net
- Hooking the net to a crane
- Collecting baskets of fish
- Decapturus rhoncus, or false scad
Once the fish are on board, scientists get to work collecting baskets of fish (3-5 baskets depending on the size of the catch). Once the fish have been sorted into their species groups, they are weighed in bulk and then individual readings of total length are taken with the electronic measuring board.
All of the scientists assist with this part of the job, with one acting as the official recorder and the others helping to sort, weigh, and measure the fish. The data is then entered electronically in NanSis for further analysis by Diana, the IMR technician.
All in all, from the time the net goes out to when the last data entry is saved in NanSis, it can be around two hours of work! Luckily the scientists enjoy their work and everyone helps out where needed: from cleaning the labs to cleaning the protective gear and sorting area.
Until next time!