Abstract
Understanding the spatio-temporal positioning of hosts and their parasites in free-living states is useful in devising methods to diminish parasite encounters in animal production systems. We explored the potential for depth-based control methods of the amoebic gill disease (AGD) agent, Neoparamoeba perurans, in salmon mariculture systems by conducting: (1) depth-stratified N. perurans water sampling surveys in 2 years, and (2) a mensurative experiment comparing depth distributions of N. perurans and salmon hosts in commercial salmon sea-cages. From water sampling mostly at marine salinities, N. perurans abundance (quantitative PCR-derived cells l-1) varied among years, but overall, neither depth, time since freshwater bathing, temperature and salinity were predictors of N. perurans abundance. However, at 1 survey time, depth patterns in N. perurans abundance appeared during strong vertical salinity gradients following rainfall (at 1 site, salinity ranged between 14 and 35 g l-1), with greater numbers of cells below a less saline surface layer. This suggested that salinity mediates N. perurans depth distribution during intermittent halocline development. Fish depth distribution monitoring revealed intense fish crowding, with local swimming densities up to 5 times stocking densities, typically at the surface at night. Simultaneously collected daytime water samples during low levels of fish crowding, with stock scattered amongst upper and lower cage sections, revealed no relationship between N. perurans and fish depth distributions. If intense fish crowding in narrow depth bands leads to high concentrations of N. perurans in cage environments and increased AGD risk, behavioural manipulations that vertically spread fish could be a successful AGD mitigation strategy.
Highlights
The marine amoeba Neoparamoeba perurans, which causes amoebic gill disease (AGD), is one of the major parasites of concern in salmon sea-cagePublisher: Inter-Research · www.int-res.comAquacult Environ Interact 9: 269–279, 2017(Munday et al 1990, 2001, Oldham et al 2016)
Many new preventive methods have surfaced for another important parasite, the salmon louse Lepeophtheirus salmonis (Dempster et al 2009, Korsøen et al 2012, Stien et al 2012, 2016, Nilsen et al 2017), after understanding the spatio-temporal depth distribution of its free-living infective larvae and salmon hosts in sea-cage environments (Heuch et al 1995, Oppedal et al 2011)
Mean abundance of N. perurans cells was low at times 1 (0.04 ± 0.02), 2 (0.10 ± 0.07), and 3 (0.02 ± 0.02), but increased at time 4 (0.35 ± 0.24; Fig. 1)
Summary
The marine amoeba Neoparamoeba perurans, which causes amoebic gill disease (AGD), is one of the major parasites of concern in salmon sea-cagePublisher: Inter-Research · www.int-res.comAquacult Environ Interact 9: 269–279, 2017(Munday et al 1990, 2001, Oldham et al 2016). Many new preventive methods have surfaced for another important parasite, the salmon louse Lepeophtheirus salmonis (Dempster et al 2009, Korsøen et al 2012, Stien et al 2012, 2016, Nilsen et al 2017), after understanding the spatio-temporal depth distribution of its free-living infective larvae and salmon hosts in sea-cage environments (Heuch et al 1995, Oppedal et al 2011). When fish feed and are hungry, they position themselves in the area that feed is dispensed, usually in surface waters (Oppedal et al 2011) This information has been used to model and predict Atlantic salmon swimming depths and densities within cages in Norway (Føre et al 2009, 2013). In SE Tasmania, Australia, caged Atlantic salmon swimming behaviour is yet to be examined, and differences may be expected due to disparate environmental conditions (e.g. strong tidal currents, warm, oxygen-deficient waters and higher light irradiance levels) and inner motivations of fish (e.g. seal presence, increased risk of oxygen and temperature stress and different feeding regimes)
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