Choosing the correct raw material for aquaculture cage netting
"THERE IS NO SPECIFIC RAW MATERIAL THAT IS BETTER THAN ANOTHER. EACH RAW MATERIAL HAS ITS ADVANTAGES AND DISADVANTAGES"
by Elihai Radzinski, Fibras Industriales SA, Peru
In the netting industry, FISA, official name Fibras industriales SA, is one of the world"s leading fishing net and cage manufactures with over 70 years" experience". They are a vertically integrated company that starts its process with the extrusion of polypropylene (PP) or polyethylene (PE) followed by twisting and weaving of pp, pe, nylon, polyester and other materials for the twisted knotted netting, braided knotted netting and twisted knotless shogun netting or knitting for the raschel knotless netting.
After finishing our products with pre-shrinking, heat setting and applying the most adequate bonding, we transfer the netting and ropes to our 28,000m2 specialised net loft where the process of rigging nets for purse seiners or cages, for all species of farming, will take place. In our previous article, from July 2017, we covered the importance of abrasion and UV resistance for fish cages and we specified in detail the different benefits of various type of products such as FISA"s Polymax and Polytar ropes.
We emphasised the importance of accelerated abrasion and UV tests, thus reflecting the behaviour of the material over time and the effect these forces have over the products. This effect can possibly lead to a situation where a product with a higher initial breaking strength turns out to have a very low UV and/or abrasion resistance and after a short period of time (12-18 months).
On the other hand, what seemed like a lower resistance product retains more of its characteristics and turns into a longer life product. Following, there will be a short description of the differences and potential advantages or disadvantages of the different raw materials available for manufacturing the netting used in aquaculture fish cages.
The main choices
In a nutshell, our many years of experience have taught us that there is no specific raw material that is better than another. Each raw material has its advantages and disadvantages, and in each case is more or less adequate for one end user than another. For example, when looking at Ultra-High-Molecular-Weight Polyethylene (UHMWPE), products claimed to have the highest initial breaking load and are usually compared to the strength of steel. It is important to notice that they also have extremely low elongation, thus leading to a low working load and less efficiency in cages located in high current zones or even worse in the open sea.
Also, one must consider that UHMWPE loses its tensile strength when twisted, thus the breaking load that must be used when comparing with other raw materials is that of the actual netting and not of the UHMWPE filament. What would seem to be the biggest advantage of UHMWPE is its light weight when compared to the other materials, thus allowing for easer handling when used in extremely large cages.
This article will not go into the details of the potential disadvantages of extremely large cages, but it should be mentioned that larger concentrations of fish in one area/cage lead to easier and faster transfer of disease and there is a higher risk of mass escape of the fish from one single accidental or natural event. As for fouling effect, a factor that contributes to fouling concentration on netting is the number of filaments on the netting. UHMWPE is a multifilament product thus it won"t be the most effective against fouling accumulation as compared to monofilaments product such as FISA"s SUPRA, assuming both products have the same total diameter.
It is important to mention that a good bonding or antifouling agent will help lower the amount of fouling, but this is an additional procedure and the number of filaments in the netting is a more root-based solution. Polyester does not have the highest initial breaking load but it does have relatively good elongation and, more importantly, it is very resistant to abrasion and to UV rays; thus after a relatively short period of use it will be much more work efficient than other materials and thus will have a longer lifespan.
On the other hand, one must take into consideration the fact that polyester is around 16 percent heavier than nylon, making the cage slightly heavier for a similar initial working load—but considering the fact that polyester absorbs less water than nylon, part of this effect is mitigated.
Nylon is probably still the most common material used in aquaculture cages. Nylon has a slightly stronger initial breaking load when compared to polyester and its elongation is also higher, thus leading to a better working force. But over time the effects of UV and abrasion can take a toll, and after a couple years of use it might not have as good characteristics as a polyester cage used under similar conditions.
One of the biggest advantages of nylon over polyester and UHMWPE, when using knotted netting for fish cages, is the loss of memory of the material, thus leading to better adjusted knots for those customers who prefer that material. An important consideration to contemplate when using nylon or polyester netting is to make sure the manufacturer of the netting (not the net loft/cage rigger) has pre-shrunk the material.
If the netting has not been pre-shrunk, a process some manufactures skip in order to cut costs, after a short period of use the shape of the cage will change and will cause what would otherwise be unnecessary labour adjusting the size and shape of the cage.
As a reference, a pre-shrunk nylon cage could shrink an additional four percent to seven percent while a nylon cage that did not go through the pre-shrinking process can shrink between 12 percent and one percent and not necessarily in an even way... especially if the netting used to build the cage is not all from the same production batch.
In comparison, a pre-shrunk polyester material will only shrink an extra two percent to a maximum of four percent, thus what you get in the beginning is basically what you will have after a prolonged period of use. Supra Advanced Fibres is third generation HDPE netting.
Introduced to the aquaculture market roughly four years ago through the help of companies such as Marine Harvest Chile, and Aqua Chile and Cooke Aquaculture, using it for market nets employed in the fattening stage of the fish and predator netting used to protect the cages from various predators such as sea lions, crocodiles, sharks, and piranhas etc. The netting goes through a rigorous procedure of depth ways stretching, in order to adjust the knots, thus helping solve the problem of the material"s natural memory.
At the same time, the fact this is a high-density polyethylene, made of monofilament fibre, means that the fouling has fewer fibres to stick to and it is a much easier product to wash between uses. Supra netting is extremely resistant to UV and has a minimum shrinkage.