When I was about to start out on this survey, I had nearly no idea what methods were going to be used or how all these species were going to be collected. From a beginner’s perspective, nets are simple tools to catch fish and that’s that. From a scientific researcher’s point of view, these nets are crucial elements in gaining information. On this survey I counted six different nets used for specific purposes. So why are there so many different types of nets with different designs? If there are so many nets that are used to catch different sizes and species of animals, how do we know what is really out there and what hasn’t been caught?
Here in the Arctic Ocean, our fish tend to be considerably smaller except for the jellyfish and marine mammals. These little fish were hard to capture due to the fact that they can outswim smaller nets and they can escape out of bigger nets. Our midwater trawl net has extra nets with smaller mesh sizes in addition to the primary net, nets upon nets, but why would you need nets on nets? Here’s what I have learned.
Alex DeRobertis shows us a pocket net on the midwater trawl net. Photo credit: Alicia Flores
There is a probability of catching each species due to the different size of the nets' diamond-shaped openings. Trial and error from previous surveys have allowed the researchers to find the right type of nets which will work on the specific organisms even for the beam trawl and bongo nets. Our midwater trawl net is usually deployed based on the acoustic trawl survey instruments that record the echo feedback of the species below.
One of the best things about any science experimentation is the fact is there is always room for improvement. On midwater nets, the catch is usually at the back of the net, which is called the codend (shown in blue in figure below). On this midwater trawl net, there are three pocket nets (shown in red) on the forward, middle and aft (back) sections of each side of the net. The meshes in the primary net are bigger than the fish, so that water can flow through the net. However, the fish can escape from these larger meshes, especially if they are small. Since these meshes are covered by a pocket net, the fish that would ordinarily escape are now recaptured.
Illustration of a midwater trawl net. Codend is highlighted in blue and pocket nets are highlighted in red.
When the trawl comes back, we begin collecting the fish from each pocket net as well as those in the codend. We then take them back into the lab to identify, measure, and weigh the catch. We have seen that most of the fish caught are in the middle pockets and on the bottom of the net.
Genevieve Johnson retrieves the fish from one of the pocket nets. Photo credit: Alicia Flores
Tubs of different species taken from a midwater trawl. Photo credit: Alicia Flores
Most of our catch has been small 1-2-inch Arctic Cod. Other types of species that are caught in the midwater trawl are jellyfish, capelin, prickleback, sculpin, and arthropods. I had a great laugh watching our researchers measuring jellyfish from the Arctic Ocean... I played with those same types of jellyfish on the beaches as a child.
The pocket nets allow one to estimate how many fish that entered the net escaped. The area of the pocket nets cover less than 5% of the entire net total. Basically, a probability estimate can be attained of how many escapee fish there were. For example, if 10 fish were caught in pocket nets covering 5% of the trawl, and 100 fish are captured in the codend, we can estimate 200 fish escaped (10/0.05) and that (100/300) one third of the fish entering the net were captured in the condend! These types of studies have led to improvements in the net we are using today.
Scientists crowding around a midwater trawl catch. Photo credit: Alicia Flores