Biofouling, the unwanted growth of organisms on submerged artificial surfaces, is ubiquitous in the marine environment and a particular problem in the salmon farming industry. In Norway, one of the most common and problematic fouling species is the hydroid Ectopleura larynx. Together with other biofouling organisms such as blue mussels and algae, it may reduce the water flow through the cage nets, increase the weight of equipment and the disease risk, and reduce the performance of cleaner fish. Therefore, biofouling not only affects farm management practices, but may also impact fish health.
This thesis aims to increase the overall understanding of the development, the impacts and the prevention and management of biofouling on salmon farms. Further aims were to extend the knowledge on the hydroid E. larynx in order to improve current farm management practices, provide information for future aquaculture risk assessments and contribute to the development of novel antifouling technologies.
A 1-year field study was conducted at a commercial salmon farm to provide background knowledge about how natural and farm operational factors influence the biomass, species richness and community composition of biofouling on cage nets. The effects of immersion period, sampling time, mesh size and variability between three individual cages were investigated. The biofouling community on the cage nets consisted of up to 90 species and multi-species groups. Among the four tested factors, immersion period and sampling time had the strongest influence on the community, resulting in clear successional and seasonal patterns in biomass, species richness and community composition, while mesh size and variability between cages had only limited influence. By controlling the timing of when nets are introduced into the water and the cleaning schedule, farm management has a large influence on the seasonality and succession within the fouling community.
The potential food sources of hydroids and caprellids living on fish cages were analysed in order to identify a possible link between fish farm wastes and high biofouling abundances. The stable isotope analysis of plankton, particulate organic matter, caprellids (which may be preyed on by hydroids), fish feed and fish faeces showed that hydroids mainly feed on zooplankton and that they are unlikely to include fish feed or fish faeces into their diet. This data was supported by estimations of a maximum hydroid biomass of 6.7 t wet weight on a cage with a daily food intake of 0.2 t C which could be met by the local zooplankton biomass. Consequently, the extensive growth of hydroids on cage nets may be largely independent of fish farm wastes. However, the selective feeding on specific prey species could negatively affect the zooplankton community, a fact that should be taken into account in future aquaculture risk assessments. In contrast, the caprellid data suggested that this species may utilize fish feed and therefore could benefit from living at a fish farm.
In a survey of salmon farmers operating along the Norwegian coast, the temporal and spatial prevalence of the hydroid E. larynx was analysed. In addition, the presence or absence of other problematic fouling species was investigated. E. larynx was reported from salmon farms between the southern tip of Norway to regions north of Tromsø, with a high presence in South-West and Central Norway. On some farms hydroids are found all year round while further north hydroids are not found on all farms or occur only during the main fouling season. Besides hydroids, blue mussels, kelps, caprellid amphipods, small algae and diatoms, and occasionally sea squirts were identified as problematic fouling species.
Hydroid biofouling can affect the oxygen levels in a cage through the reduction of the water exchange across the net. In order to clarify if, in addition, the oxygen consumption of the hydroid population can affect the oxygen budget of a cage, the oxygen consumption of hydroids was measured. At ambient water temperatures of 12, 14 and 16°C, E. larynxhad an oxygen consumption rate of 0.54, 1.22 and 1.09 ml O2 g−1DW h−1, respectively. These values are similar to the respiration rate of Atlantic salmon. However, because the biomass of the cultivated salmon by far exceeded the biofouling biomass on the cage, the oxygen consumption of the hydroid population equated only to 1.1% of the oxygen consumption of salmon. Therefore, the oxygen consumption of the hydroids is unlikely to reduce the oxygen budget of the cage below critical levels that would endanger the health of the cultivated salmon.
Finally, to find a non-toxic alternative to copper-based antifouling coatings, the effects of the physical surface properties wettability and microtopography on the settlement of the hydroid E. larynx were analysed. The settlement preferences of hydroid larvae for materials with wettabilities ranging from hydrophobic to hydrophilic were tested. Although settlement differed between materials, no trend regarding the tested wettabilities could be found and none of the tested materials were able to reduce average settlement below 50%. In a second experiment, surfaces with microtextures between 40–600 µm were analysed, but there was no systematic effect of microtopography on the settlement of E. larynx. Similarly, there were no preferences for any of the examined microtopographies in a 12-day field test at a commercial salmon farm. In conclusion, neither surface wettability nor microtopographies are effective at deterring the settlement of the hydroid E. larynx. The high plasticity of the aboral pole and the hydrorhiza of the hydroids may explain settlement even under unfavourable conditions and suggests that antifouling methods based on the tested physical properties may not provide a solution to the hydroid fouling problem.