Making sense of materials used for manufacturing cage netting
by Fibras Industriales SA, Peru
When deciding what type of cages to install in a fish farm, various factors must be taken into consideration. These factors may include water depth, currents present in the body of water, fouling accumulation, oxygen saturation, water temperature, potential storm risks and more.
After considering all these factors, the next step is to decide the dimensions of the cages required and how many to install in a given area. Some companies prefer spreading labour over a larger number of smaller cages, whilst other companies may prefer farming in less cages which are larger in size, thus reducing operational costs.
It will be part of the farm managers or operators' job to decide the type of netting to use for the cage(s). Different types of netting may include twisted knotted, knotless Raschel, braided knotted or Muketsu twisted knotless netting. Deciding which type of netting to use will be influenced by the role each cage will play in the production process, such as the stage of growth of the fish, whether predator or bird protection is necessary etc. Some of FISA's customers have opted for braided knotted polyester or HDPE braided Supra® netting for cages, thus avoiding the necessity of using both an anti-predator net and cage. This provides an innovative, two-in-one solution for many fish farmers. Each type of netting has its own advantages and disadvantages and is suitable for a variety of farms, locations, designs and operational procedures.
A second factor that must be considered when choosing cage netting is the raw material used in the manufacture of the netting. This issue can be very tricky as there are a number of various materials available and not all manufacturers or net lofts offer a full range of materials, thus their customers don't always have access to the full range of available options.
FISA's many years of experience have led the company to learn that there is no specific type of netting or raw material that is 'better' than another. Each raw material has its own unique advantages and disadvantages and the suitability of each for any individual customer/farm depends on a number of factors.
As an example, ultra-high-molecular-weight polyethylene (UHMWPE) is a raw material that has the highest initial breaking load and is often compared to having the strength of steel. However, it is important to notice that this material also has extremely low elasticity, leading to a lower working load than would be expected. This issue is crucial to consider and can mean this type of netting is unsuitable for some types of fishing where elasticity is critical, but it could also prove a useful feature when used in cages located in high current zones or at open sea.
Also, one must consider that UHMWPE loses its tensile strength when twisted, thus the breaking load that must be used when comparing it with other raw materials is that of the actual netting and not of the UHMWPE filament.
Despite these challenges, many advantages also come with using UHMWPE. What might seem to be the biggest advantage of UHMWPE is its lightweight form, when compared to the other materials, allowing for easier handling, especially so for larger cages.
When considering the potential accumulation of fouling, a factor that contributes to fouling concentration on netting is the number of filaments present within the netting. UHMWPE has a relatively low number of filaments when compared to nylon or polyester, but is still a multifilament product and, as a result, won't be the most effective solution against fouling accumulation, compared to monofilaments products such as FISA's SUPRA Advanced Fibers® netting. It is important to mention that a good bonding or antifouling agent will help lower fouling accumulation, but this is an additional procedure that costs time and money.
Polyester does not have the highest initial breaking load, but it does offer good elasticity and, more importantly, it is very resistant to abrasion and UV rays. Thus, after a relatively short period of time, polyester netting will prove to be much more efficient than other materials thanks to its longer lifespan.
Despite this, one must consider that polyester is around 16 percent heavier than nylon, making the cage slightly heavier for a similar initial working load. However, when one considers that polyester absorbs less water than nylon, part of this effect is mitigated.
Nylon is probably still the most common material used in aquaculture netting. This material has a slightly stronger initial breaking load when compared to polyester and has better elasticity. However, over time the effects of UV and abrasion can take its toll and, after a couple years of use, nylong netting might not hold up as well as a polyester net under similar conditions.
If a company is considering using twisted knotted netting for its cages, it must take into account that one of the biggest advantages of nylon over polyester and UHMWPE is better adjusted knots that are more resilient to loosening over time.
Another important consideration to take into account when using nylon or polyester netting is to make sure the manufacturer of the netting has pre-shrunk the material. If the netting has not been pre-shrunk, a process some manufactures skip in order to cut costs and accelerate the production speed, after a short period of usage the shape of the cage will change and will cause what would otherwise be unnecessary labour, adjusting the size and shape of the cage.
For reference, a pre-shrunk nylon net in a sea cage could shrink by an additional four-to-seven percent, whilst a nylon cage that did not go through pre-shrinking process can shrink between 12-to-15 percent.
The above-mentioned issues are quite complex and require a great deal of thought. For that reason, it is highly recommended to purchase cages from a net loft that provides direct and fluid communication with its netting supplier, thus taking advantage of the full knowledge and experience of both the net loft and the netting manufacturer.
Additionally, it is best to have a netting manufacturer that offers a relatively full range of options for each type of netting and the raw materials used, and won't limit the customer's options to a specific type of raw material or netting.