Report of the 2005 ICES/NASCO Symposium on Interactions ...

NINA SPECIAL REPORT 34
Annex 4: Disease and Parasite
Iinteractions and <strong>the</strong>ir
Management - Session
Chairmen’s Summary
Malcolm Beveridge and Chris Poupard
Fifteen papers on <strong>the</strong> two broad <strong>the</strong>mes <strong>of</strong> science and
management <strong>of</strong> disease and parasite interactions were
presented, none <strong>of</strong> <strong>the</strong>m considering species o<strong>the</strong>r than
Atlantic salmon and sea trout.
Ron Stagg opened <strong>the</strong> session with a broad-ranging
presentation that dealt with micro- and macro-parasites, in
essence covering <strong>the</strong> entire gamut <strong>of</strong> Atlantic salmon
disease agents, from viruses and bacteria to ectoparasites.
The paper examined some <strong>of</strong> <strong>the</strong> more important
diseases that have appeared in salmon farms in Scotland
since <strong>the</strong> inception <strong>of</strong> <strong>the</strong> industry and in so doing
concluded that while originating in wild populations, <strong>the</strong>
emergence <strong>of</strong> parasitic infestations among farmed fish,
sometimes with increasing associated virulence, is an
inevitable consequence <strong>of</strong> farming. Much <strong>of</strong> <strong>the</strong>
presentation focused on epidemics, exploring <strong>the</strong> insights
being gained through <strong>the</strong> use <strong>of</strong> new types <strong>of</strong>
ma<strong>the</strong>matical modelling tools. The paper pointed out <strong>the</strong>
key differences between micro- and macro-parasite
epidemics. It concluded that epidemics among farmed
populations do not necessarily result in epidemics among
wild fish populations, stressing <strong>the</strong> importance <strong>of</strong> good
bio-security and husbandry in mitigating risk.
Sea lice, and Lepeoph<strong>the</strong>irus salmonis ra<strong>the</strong>r than Caligus
elongatus, were mentioned in Ron Stagg’s presentation and
indeed dominated much <strong>of</strong> <strong>the</strong> rest <strong>of</strong> <strong>the</strong> day’s
presentations and discussions. Karin Boxaspen reviewed
what we know about <strong>the</strong> biology and genetics <strong>of</strong> <strong>the</strong>
parasite, focusing on recent research on life history,
especially <strong>the</strong> impacts <strong>of</strong> temperature and salinity on
developmental rates, on behaviour and dispersion. She
reviewed new information on natural distribution and
advances in <strong>the</strong> application <strong>of</strong> genetic techniques to
understand parasite population structure and genetic
diversity. The new discoveries are being applied to refine
pest management strategies and to assess risks to wild fish
populations. Molecular genetics work raises <strong>the</strong> possibility
<strong>of</strong> a vaccine being developed against <strong>the</strong> parasite.
A series <strong>of</strong> presentations explored <strong>the</strong> <strong>the</strong>me <strong>of</strong> wildfarmed
fish pathogen interactions. Neil Hazon reported
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on how sea lice infestation affected <strong>the</strong> physiology <strong>of</strong> wild
sea trout. In a series <strong>of</strong> laboratory trials, <strong>the</strong> threshold
levels <strong>of</strong> infestation at which physiological stress and
osmoregulatory problems occur were determined. A
threshold level <strong>of</strong> infestation at which mortality among sea
trout smolts was likely to occur was determined to be 13
lice per fish. The susceptibility <strong>of</strong> wild, farmed and wildfarmed
hybrid Atlantic salmon to sea lice and to Infectious
Salmon Anaemia (ISA) and furunculosis was explored by
Kevin Glover. No differences among fish treatment
groups were found for furunculosis or ISA, and <strong>the</strong>
differences apparent in exposure to sea lice disappeared
when differences in body size among treatment groups
were taken into account. In conclusion, while
domestication to date does not seem to have led to any
systematic divergence in disease resistance from wild
salmon, future selection programmes may.
The paper presented by Bengt Finstad summarized
present knowledge <strong>of</strong> interactions between lice on
farmed and wild salmonids, drawing on much monitoring
data from Scotland, Ireland and Norway. Key points from
<strong>the</strong> presentation included <strong>the</strong> high infection pressure
imposed by farms on wild stocks in many areas and <strong>the</strong>
inverse relationships between incidence <strong>of</strong> lice on wild sea
trout and distance from fish farms.
In determining <strong>the</strong> infection pressures that sea lice on
farmed fish pose for wild salmonids it is important to
understand sea lice dispersal and <strong>the</strong> behaviour <strong>of</strong> wild
salmonids that makes <strong>the</strong>m susceptible to infestation. Two
papers provided us with insights. Michael Penston
presented two years <strong>of</strong> plankton survey data from Loch
Shieldaig in Northwest Scotland. Higher nauplii densities
were found at sample sites close to farms while
copepodid levels were highest at sites away from farms.
Dispersal <strong>of</strong> copepodids was shown to be largely
explained by wind-driven currents; by contrast, nauplii
were found throughout <strong>the</strong> top 5 m. Generalised
Additive Models were used to analyse <strong>the</strong> pattern <strong>of</strong>
planktonic lice abundance, which showed an increase from
stocking <strong>of</strong> farmed fish throughout <strong>the</strong> production cycle,
followed by a marked decrease when fallowing began
again.
Peter Andreas Heuch looked at <strong>the</strong> effects <strong>of</strong> both
hydrography and infection pressure on infection rate <strong>of</strong>
Atlantic salmon and sea trout smolts emigrating from <strong>the</strong>
two contrasting fjord systems. These studies illustrated
not only differences between areas in terms <strong>of</strong> infection
pressure, but also that within a system year-to-year