Production of Atlantic salmon (Salmo salar) in closed confinement systems (CCS) : salmon lice, growth rates, mortality and fish welfare

Abstract

Farming of Atlantic salmon have been in rapid growth since the 1970's and is now an important industry in many countries around the North Atlantic as well as Chile. Since 2000, all countries with farming of Atlantic salmon and rainbow have faced challenges with the development of drug resistant sea lice. The use of drugs and the cost of production has increased, this has undermined the aquaculture industry's profitability and reputation. In Norway, the rapid growth of salmon farming was arrested from 2012, mainly because of increasing problems with drug resistant salmon lice (Lepeophtheirus salmonis). Norwegian authorities have issued new farming licenses with the purpose of encouraging fish farming companies to solve the most important environmental challenges. Increased production of salmon in land-based facilities, the development of offshore aquaculture and different varieties of closed containment systems (CCS) have been proposed. When this thesis was developed (2012-2015), the knowledge about fish health and welfare in commercial scale CCS was limited. It was important to assess if the use of untreated deep water could provide sufficient protection against sea lice and if introduction of lice could lead to sea lice reproduction and sustained infestations in the closed cages. Furthermore, it was necessary to investigate growth rates, mortality rates, mortality causes and fish welfare in CCS. Our studies showed that CCS with water intake at a 25 m depth provided effective protection against sea lice copepodites (Lepeophtheirus salmonis and Caligus elongatus). Adult Caligus elongatus were observed occasionally and at low abundancies. When sea lice were introduced into CCS, we observed no signs of reproduction or sustained infestations. Without sea lice, there was no need for treatments. This reduced the environmental impact and improved fish welfare with production of salmon in CCS, compared to traditional net-pens. Mean thermal growth coefficient (TGC) for post-smolt in CCS was close to 3.0, ranging between 2.24 and 3.94. The lowest growth rates were caused by low specific flow and suboptimal water quality (early trials). For the majority of cages, increased water velocity could be an important explanation variable for increased growth rates and condition factors in CCS, compared to net-pens. Increased water temperatures during winter (September-May) in CCS compared to net-pens could also be of significance, because most trials were performed with off-season smolt (S0). Results from large scale trials were supported by small scale trials with post-smolt, where moderate water velocities (19-21 cm/s) showed a significant increase of growth rates and condition factor compared to low water velocities (6-8 cm/s). Increased water velocities increased the fillet yield in harvest sized salmon (3000 g), but without increased deposition of body fat. Cumulated mortality rates in CCS were moderate to low, compared to mortality rates in net-pen studies. Ulcers and fin rot caused by bacterial infections was an important health and welfare issue during the post-smolt period and occurred at different fish sizes and different water temperatures. Simultaneous lesions and bacterial infections of the skin on the body and the fins are probably caused by the same bacterial pathogens. These chronic infections caused increased mortality rates, but were also associated to suppressed appetite, reduced growth rates and condition factors. Suboptimal quality of smolt at sea transfer increased the risk of post-sea-transfer mortalities. From our studies, recommended minimum specific water consumption (SWC) during production of post-smolt Atlantic salmon is 0.2 L/kg/min. The recommended maximum feed load is 35-40 g feed/m3. Mean values of oxygen saturation were close to the values described as optimal for the growth performance of farmed Atlantic salmon. The (short-term) extreme variations were above the threshold for severe hypoxia (LOC) and below toxic levels. Carbon dioxide (CO2) concentrations were mostly below the threshold for negative impact on welfare and growth performance (10 mg/L), with a few cases of CO2>15 mg/L, where we also observed a negative impact on appetite and welfare. Harmful levels of ammonia (NH3) were not recorded. All threshold values and indications of low mortality and high growth rates should however be interpreted with caution. Our understanding of the complex interactions between the salmon, the environment and the rearing conditions is still limited. Future research should not aim narrowly at identifying the maximal biological input and minimum standards of life conditions for the farmed fish. It should be equally important to study how farmed salmon respond to more optimized and high-quality environments, in both closed cages and other rearing systems.

Oppdrett av laks har vært i rask vekst siden 1970-tallet og er nå en viktig næring i mange land rundt Nord-Atlanteren samt Chile. Siden 2000 har utvikling av legemiddelresistente lakselus ført til store utfordringer i alle land med oppdrett av Atlantisk laks og regnbueørret. Mengden av legemidler og kostnadene til produksjon har økt og dette har undergravd oppdrettsnæringens lønnsomhet og omdømme. I Norge stagnerte produksjonsveksten i 2012, hovedsakelig på grunn av problemene med lakselus (Lepeophtheirus salmonis). Myndigheter og næringsliv i Norge har satt som mål å utvikle ny oppdrettsteknologi for å løse noen av disse utfordringene. Økt produksjon av laks i land-baserte anlegg, utvikling av offshore merdanlegg og ulike varianter av lukkede merdsystemer i sjø er foreslått. Da arbeidet med denne avhandlingen begynte (2012-2015) var det lite tilgjengelig kunnskap om fiskehelse og velferd ved drift i lukkede merdsystemer. Det var viktig å finne ut om bruk av urenset dypvann kunne beskytte mot lus og om inntak av små mengder lus i lukkede merder kunne føre til oppformering av parasittene og høye lusetall på laksen inne i merdene. Det var også viktig å kartlegge dødelighet og dødelighetsårsaker, veksthastighet og fiskevelferd ved drift av lukkede merder.