We have completed the prototype for our plastic-free microplastics trawl. We have named it after the basking shark, which uses its large round mouth to filter feed--just as our round trawl design filters microplastics out of the water. In this article, we break down our design and build processes, compare the Following Seas Basking Trawl to the existing manta ray-based trawl designs, and discuss our next steps.
As we discussed in a recent article, studying microplastics necessarily involves the use of plastic. Most significantly--and most unavoidably--the nets used to collect samples are made of plastic, and there are no non-plastic alternatives available.
But, we can all reduce the amount of plastic we use, and this one of our priorities.
The first microplastics trawl we built (and that we are continuing to use) is based on a published design. While it is made primarily of wood, protecting this wood in saltwater environments requires use of a plastic-based sealer. The published design also calls for the use of a PVC-based coupler between the net (which is attached to the trawl) and the collection bag.
Our new design, which we are calling the Following Seas Basking Trawl, is made exclusively out of aluminum, stainless steel, and cork. It mimics the mouth of the basking shark, which filter feeds--in much the same way that our trawl will filter microplastics from the water.
Public domain photo from the National Oceanic & Atmospheric Administration (NOAA)
While the published design we are using alongside the Following Seas Basking Trawl has been scientifically tested, our preliminary testing has revealed what we believe to be opportunities for improvement. This includes opportunities related to cost, construction, and measurement accuracy.
Why We Wanted to Design Our Own Microplastics Trawl
With this in mind, we wanted to develop a plastic-free microplastics trawl that facilitates accurate measurement. We have also wanted to reduce the cost of materials and build time--with the goal of developing a microplastics trawl that is suitable for citizen science.
Here are some of the key benefits we have achieved by designing and building the Following Seas Basking Trawl:
1. It is Plastic-Free
As noted above, the Following Seas Basking Trawl is plastic-free. The trawl itself is constructed using aluminum sheeting and rivets, along with stainless steel clamping materials and cork floats. We have also designed and built a plastic-free coupler of a similar design.
2. It is Easier, Quicker, and Less Costly to Build
Compared to the published design, the Following Seas Basking Trawl has proven, in our experience, to be easier and quicker to build. It can be built using basic hand tools; and, without the need to apply multiple coats of sealer, it can be completed in a single day. The materials to build our plastic-free trawl were also less costly than the materials to build the published design.
3. It is Designed with a Focus on Increased Measurement Accuracy
In our initial testing, one of the challenges we have faced with the published design is maintaining the trawl at a consistent level within the water column. This is significant, because studies have shown that the majority of microplastics float in the top three inches of the water column, and it is important to know where in the water column you are testing. The Following Seas Basking Trawl is designed to consistently sample the top three inches.
Key Characteristics of the Following Seas Basking Trawl
To validate the Following Seas Basking Trawl, we will be comparing its sampling data to the sampling data collected using the published design. We will also be comparing samples from both trawls to the expected microplastics volume based on the volume of water sampled. With this in mind, some of the key characteristics of our plastic-free trawl include:
1. It Measures the Top Three Inches of the Water Column
By floating at the surface of the water, the Following Seas Basking Trawl consistently measures the top three inches of the water column. The horizontal band is located at the top of the three-inch central zone of the circular trawl area, and the cork floats keep this band floating on the surface of the water.
2. It Measures the Same Area of the Top Three Inches as the Previously Published Design
The circumference of the Following Seas Basking Trawl was selected by calculating a three-inch central zone with an area that is equal to a three-inch vertical segment of the published design. This means that we will be sampling the same amount of water in the top three inches of the water column, which will allow for direct comparison of the two trawl designs.
3. It is Scalable for Collecting Larger Samples
The Following Seas Basking Trawl can also be scaled up for collecting larger samples. Going forward, we anticipate that larger models will be used to collect samples using larger vessels, particularly in offshore environments.
Designing and Building Our Plastic-Free Microplastics Trawl
The Following Seas Basking Trawl was designed to be built out of a single 6" sheet of 0.3mm aluminum. We purchased an 11' roll, which is readily available from online retailers, and which was more than enough to build the trawl and coupler.
We began by measuring and cutting a 45" section. This was based on the 43 9/16" circumference needed for a 13 7/8" measurement of the top and bottom of the 3" central zone to ensure an identical collection area to the published trawl design. The additional length was used to create the overlap needed to rivet the two ends together later in the process.
The next step was to fold over both sides of the 45" section to strengthen what will become the outer ring of the trawl. This was done using a 6' level and a hammer to create a straight and consistent crease.
After creating the initial crease on both sides, the folded sides were hammered flat to create a reinforced but flexible 45" section that could be gradually bent into a circle.
After gradually bending the 45" section into a circle, being careful not to create creases in the folded edges, the next step was to measure and drill holes for 1/8" aluminum rivets. The rivets connect the two ends of the 45" section together with a 1 7/16" overlap.
Here is a photo of the completed outer ring of the trawl after the rivets were installed:
The nets used to collect microplastics samples will be attached to the trawl using stainless steel hose clamps. We purchased a kit that can be used to make custom-length clamps, and the kit came with more than enough materials for both the trawl and coupler.
After measuring the correct length, we used a battery-powered grinder to cut the stainless steel clamp material. However, the clamp material can also be cut with hand tools.
After cutting the clamp material, we attached the screw mechanism according to the instructions using a pair of pliers.
Here is a photo of the first clamp on the outer ring of the trawl. We build a second identical clamp for redundancy.
Following the same basic steps, we also built a 4" coupler (this, too, was based on the published design).
The coupler uses two clamps, one on each side, to attach the net and collection bag. The photo below also shows the ring that was added so that we can attach a cork float. This will help with keeping the net flowing horizontally in the water, and will also ensure that we do not lose the coupler or collection bag should the net become disconnected for any reason.
For the trawl, the next step was to add the horizontal band that sits at the top of the 3" central zone. This was made using another section of aluminum sheeting folded into thirds, then folded up on each end for riveting onto the outer ring. We began by locating the center of the trawl and identifying the 3" central zone.
The 3" central zone was then marked on the outer ring of the trawl to determine the placement of the horizontal band.
Here is a photo of the horizontal band installed. In this photo, we have also added the attachment points for the tow ropes that will be used to pull the trawl through the water. These are 3/4" strips of aluminum sheeting bent to shape and then riveted onto the outer ring.
Here is a closer look at one of the attachment points for the tow ropes:
Here is a photo of the nearly completed trawl. At this stage, the final step is to determine the correct placement of the cork floats. These will be attached using additional custom-made hose clamps that are slipped into sleeves riveted onto the outer ring.
Here is another photo of the nearly completed trawl with a view of the two clamps that will be used to attach the net:
Here is a view of the nearly completed trawl from directly in front:
We purchased a block of cork that we cut and shaped into two 3" cylinders. It is possible to purchase pre-made cylinders, but the block of cork was less expensive and allowed us to conduct testing to determine the size of floats needed to keep the horizontal aluminum band floating at the water line.
Here are the two 3" cylinders created from the block of cork we purchased. First, we cut 3" square sections out of the horizontal length of the block, then we cut, shaved, and sanded these into a cylindrical shape.
We then tested the buoyancy of the cork cylinders. After this test, we then calculated the location at which we would need to attach the cylinders to the trawl. While we anticipated needing to make some adjustments, our original placement turned out to be correct.
We added sleeves made out of 3/4" strips of aluminum sheet to the trawl at our measured location, using a single rivet for each. These sleeves allow custom-made hose clamps to be slid in and out, and these hose clamps are used to attach the cork floats. As you can see, the floats keep the horizontal band at the top of the 3" central zone floating at the water line, which is exactly what we need.
We have tested the Following Seas Basking Trawl with the net, coupler, and collection bag attached, and we are pleased to say that it works as intended. The aluminum band at the top of 3" central zone of the trawl skims across the surface of the water, allowing for accurate measurement of the volume of water sampled in this key segment of the water column.
Our Next Steps: Completing Our Summer 2026 Pilot Study with the Following Seas Basking Trawl
So, what's next? Our next priority is to use both the Following Seas Basking Trawl and our trawl built to the previous published design to complete our Summer 2026 Pilot Study. Along with conducting on-the-water sampling, this will involve using our other resources to clean and examine samples, and then analyzing and interpreting the data for publication.
About Following Seas Marine Impact Research
At Following Seas Marine Impact Research, we are studying the global microplastics problem through on-the-water sampling and data analysis. Our goal is to provide other organizations with the data and science they need to advocate for change and implement clean-up solutions. We are also developing standards for citizen-scientists who are interested in gathering data where they live. Join our mailing list to follow along.