Background

Biofouling on aquaculture nets is a major challenge for the aquaculture industry. Biofouling adds weight to nets and equipment and it changes hydrodynamic properties of fish cage systems. Approaches to battle biofouling include prevention in form of anti-fouling coatings, and removal by underwater or land-based net cleaning, net drying or changing or biological controls.  Underwater high pressure cleaning of nets is common in the Norwegian Salmon culture industry and it has a good immediate washing effect. However, some organisms are not completely removed and it was demonstrated that hydroids can quickly re-grow polyps when these are cut off. Underwater high pressure cleaning can lead to increased release of larvae from hydroids, which might rapidly settle on newly cleaned nets. New washing methods that kill or inactivate remaining fragments of fouling organisms on nets and larvae of fouling organisms during the washing process might, in addition to a good immediate washing effect, slow down the development of macro-fouling after washing events.

Biofouling is not necessarily homogeneously distributed on nets and there may be large variations of biofouling accumulation at a location throughout and in between years. Many fouling organisms have planktonic life stages and larvae can often decide if to settle on an available surface. Knowledge about connections between physical and chemical water parameters, composition of plankton, including planktonic stages of fouling organisms, and the development of a fouling community on suitable substrates can help understand biofouling processes and it might allow a forecast of fouling development on nets by monitoring environmental conditions and plankton.

Several techniques are used to evaluate the amount and composition biofouling on substrates, but not all are well suited for the use on nets. Given good image quality, image analysis allows for a fast and reliable calculation of net solidity and net aperture occlusion, which can be used as a measure for the amount of biofouling on nets. 

Methods

Semi-automated image analysis for the quantification of fouling: Requirements and solutions to obtain high quality images. An algorithm for the calculation of net solidity and net aperture occlusion from images of nets and fouling was developed previously. The technique gives good results for images with high contrast between foreground (net and fouling) and background (net openings). Ambient light conditions change with depth and, close to the surface, with time, thus creating inconsistencies in the image quality of underwater images. Nets and coatings can be differently colored and different lighting might be necessary to gain a high contrast throughout images. Camera and light source often have to be at the same side of a net for practical reasons and a number of light settings were tested using an adjustable camera and lighting rig to find optimal settings as well as limitations for white and black lightly fouled nets.

Effect of heat treatment on hydroids. A conceptual model of a washing rig including a high pressure disc washer and a hot water applicator was tested as a means to deliver heat to previously washed nets to prevent re-growth of hydroids from hydroid remains after high pressure washing. A total of 32 net panels on 8 frames were deployed at a fish farm site. 8 nets served as control (fouling pressure between washing events). These nets were removed and replaced with new nets during the washing events. 12 nets were washed with high pressure and 12 nets were washed with high pressure followed by hot water treatment. The washing was repeated 3 times between August and November 2012 and the washing success and re-growth between washing events was measured based on images of the fouled nets.

Connections between water parameters/plankton composition and fouling on nets. To investigate the species composition and amount of biofouling between net types and locations along a fish cage and whether differences can be correlated (and explained) with other parameters, uncoated and silicon coated net panels were deployed in three locations close to and within a stocked fish cage. Samples were taken weekly for Plankton, dissolved nutrients, particles (for C/N analysis), images were taken for monitoring biofouling growth and species determination and the water temperature was logged. Additionally, 10 samplings were carried out for all above parameters within a 24 hour period to gain information about temporal variability .  

Results

Semi-automated image analysis for the quantification of fouling: Requirements and solutions to obtain high quality images. Good image quality for an image analysis resulting in the calculation of net solidity is defined by a high contrast between net and fouling and background. Strong lighting gives good images for light nets and fouling, even though it might lead to loss of structure within net strands. Strobes can be used to reach a light intensity high enough to compensate for even high levels of ambient light. Even very strong lighting might not result in sufficient contrast, as background and nets are relatively dark. Fouling on dark nets improves the image quality, as it appears much lighter than the net and background when sufficiently illuminated. For very dark nets with little fouling lighting the background by moving strong light sources directly against the net to the sides of the camera, but with the light aimed through the net, gave good results. The results enable a more robust and more accurate evaluation of biofouling on nets through better protocols for taking underwater images. All results are transferable to other areas where underwater images and videos are of interest.

Effect of heat treatment on hydroids. The temperature of water leaving the hot water applicator directly at its outlets was about 75C. Strong heat dissipation led to temperatures on the net being within the range of 55C to 58C, which was slightly below the temperature that was shown to be effective against adult hydroids. There were no significant differences in amount of biofouling on nets washed with different methods 4-5 weeks after washing events. The temperature of water reaching fouling organisms might not have been high enough to inhibit re-growth from hydroid fragments remaining on nets after washing events and the time interval between washing and evaluation of fouling after washing events might have been long enough for new fouling to mask an effect of the heat treatment on re-growth of hydroids. The test setup for future experiments is adjusted based on the experience from these tests: a temperature at the net of over 60C will be ensured and the immediate effect of heat treatment in the field will be investigated on the survival of hydroid fragments remaining on nets after high pressure washing and on the settlement success of larvae after heat treatment. A method for heat application capable of inhibiting re-growth from damaged fragments of hydroids and settlement of larvae being released during the washing process can potentially prolong the time period between washing events and suppress rapid re-colonization of cleaned nets with larvae from fouling organisms originating from within the fish farm.

Connections between water parameters/plankton composition and fouling on nets. Net panels were submerged for 6 weeks in August and September, which, according to a previous study at a Salmon farm in Mid-Norway, falls within the peak period for fouling growth on aquaculture nets. However, there was almost no fouling on the net panels throughout the entire test period (see also Figure 6). This underlines the strong variability of fouling growth in space and time. An analysis of water samples over a period of several weeks and over a diurnal cycle is of interest by itself, as it gives an insight into some dynamics in the nutrient availability and the occurrence and composition of plankton. Concentrations of Nitrate/Nitrite and phosphate in the water coloumn were relatively stable over a 24 hour period with no consistent differences between sampling locations. Other studies report much higher, but also much lower nitrate/nitrite concentrations in Norwegian waters. Nitrate and Nitrite concentrations close to fish cages might therefore be more dependent on the ambient concentrations than on fish farm output. Ammonia concentrations peaked at positions 1 and 3 (panels left and right of the cage in Figure 2) at around midnight and slowly decreased to levels measured during daytime over a few hours (see Figure 6). Excretion from fish has a large impact on ammonia concentrations and there is a distinct diurnal pattern of Ammonia release into the surrounding water, which is likely to be influenced by the feeding regime.