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

NINA SPECIAL REPORT 34
deleterious effect on <strong>the</strong> local wild salmon populations,
releases were discontinued.
Changes in wild and hatchery proportions <strong>of</strong>
annual Atlantic salmon (Salmo salar) catches
in <strong>the</strong> Baltic Sea
Marja-Liisa Koljonen
DNA level information, an 8-loci microsatellite baseline
database <strong>of</strong> 32 Atlantic salmon (Salmo salar L.) stocks, has
been used toge<strong>the</strong>r with a Bayesian estimation method to
estimate stock and stock group proportions <strong>of</strong> Finnish
catches in <strong>the</strong> Baltic Sea area. The proportions <strong>of</strong> seven
stock groups important in terms <strong>of</strong> fisheries management
were assessed in catch samples taken in four years (2000,
2002, 2003 and 2004). In <strong>the</strong> Gulf <strong>of</strong> Bothnia area <strong>the</strong>
proportion <strong>of</strong> wild fish showed a rising trend until 2003 in
all areas, but was mainly <strong>the</strong> result <strong>of</strong> decreasing total
catches, which was due to relatively higher mortality <strong>of</strong>
hatchery fish than <strong>of</strong> wild fish. In 2004 <strong>the</strong> total numbers
<strong>of</strong> caught wild fish had also increased, indicating an
increase in <strong>the</strong> abundance <strong>of</strong> <strong>the</strong> wild stocks. In catches
from <strong>the</strong> Åland Sea, <strong>the</strong> proportion <strong>of</strong> wild fish increased
from 44% to 70% in 2000-2004, corresponding to catch
numbers from 4,628 to 7,329 individual fish. In <strong>the</strong> Gulf
<strong>of</strong> Finland, <strong>the</strong> largest contribution was made by local
hatchery fish, <strong>the</strong> Neva salmon, which are released by
Estonia, Finland and Russia. In <strong>the</strong> western part <strong>of</strong> <strong>the</strong>
Gulf <strong>of</strong> Finland, fish originating from <strong>the</strong> Baltic Main Basin
made a substantial contribution. The threatened, eastern
Estonian and Russian wild stocks were recorded only in
<strong>the</strong> western part <strong>of</strong> <strong>the</strong> Gulf <strong>of</strong> Finland, with a proportion
from 9% in 2003 to 19% in 2004.
Disease and Parasite Interactions
and Their Management
Parasite interactions between wild and
farmed fish
Ron Stagg, Rob Raynard and Sandy Murray
This paper will consider both micro- and macro-parasite
induced disease <strong>of</strong> fish. Micro-parasites such as bacteria
or viruses are microscopic, <strong>the</strong>y have life cycles that are <strong>of</strong>
very short duration into relation to <strong>the</strong> life expectancy <strong>of</strong>
<strong>the</strong> host (hours versus years) and <strong>the</strong>y tend to elicit a
strong immune response in <strong>the</strong> host. Macro-parasites
tend to be larger and to have life spans which are similar
in magnitude to <strong>the</strong> life expectancy <strong>of</strong> <strong>the</strong> host and <strong>the</strong>y
only have weak interactions with <strong>the</strong> host immune system.
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Epidemics <strong>of</strong> micro-parasitic organisms are associated with
<strong>the</strong> dynamics between susceptible, infected and recovered
(immune) hosts and are <strong>the</strong>refore associated with <strong>the</strong>
number <strong>of</strong> infected hosts. In contrast macro-parasitic
epidemics are associated with both <strong>the</strong> number <strong>of</strong>
infected hosts and <strong>the</strong> intensity <strong>of</strong> infection. In both cases
successful invasion <strong>of</strong> <strong>the</strong> host by <strong>the</strong> parasite and <strong>the</strong>
spread or evolution <strong>of</strong> an epidemic is indicated when <strong>the</strong>
basic reproductive number (Ro) is greater than one. This
occurs when a host infected with a micro-parasite gives
rise to more than one secondary infection or a macroparasite
produces more than one <strong>of</strong>fspring surviving to
reproductive age in <strong>the</strong> course <strong>of</strong> its life. Parasite
epidemics can affect both farmed and wild fish and have
consequences for both: population regulation in wild,
economic damage and welfare effects in farmed fish as a
result <strong>of</strong> losses, decreased growth or trade limitations.
Wild fish are <strong>the</strong> ultimate source <strong>of</strong> parasites and can also
be a reservoir <strong>of</strong> infection which impedes eradication
programmes in farms. However, <strong>the</strong> farming <strong>of</strong> fish can
exacerbate disease problems through <strong>the</strong> promotion <strong>of</strong>
conditions leading to epidemics (such as enhanced
transmission and evolution), long-range transport (through
trade and introductions) and spill-over <strong>of</strong> parasites back
into <strong>the</strong> wild. Normally <strong>the</strong>re is a balance between <strong>the</strong>
ability <strong>of</strong> <strong>the</strong> host to resist invasion and <strong>the</strong> ability <strong>of</strong> <strong>the</strong>
parasite to reproduce successfully. Maintaining fish in
culture can alter this balance such that epidemics are
more likely. The rearing <strong>of</strong> fish under aquaculture
conditions, with good husbandry and management, should
not necessarily cause reduced resistance to parasites
because <strong>of</strong> stress in <strong>the</strong> host. Much more important will
be <strong>the</strong> impact <strong>of</strong> culture conditions on <strong>the</strong> transmission <strong>of</strong>
infection, particularly <strong>the</strong> contact rate between susceptible
and infected hosts and <strong>the</strong> duration <strong>of</strong> infection. Although
originating in wild populations <strong>the</strong> emergence <strong>of</strong> parasitic
infections is an inevitable consequence <strong>of</strong> farming fish and
this is illustrated by an examination <strong>of</strong> some <strong>of</strong> <strong>the</strong> more
important diseases that have emerged in Scottish
aquaculture in <strong>the</strong> past 20-30 years. However,
examination <strong>of</strong> some <strong>of</strong> <strong>the</strong>se case histories also indicates
that epidemics in farmed fish do not necessarily result in
epidemics in wild populations and that management
measures such as good bio-security and husbandry can be
effective risk mitigation measures.
Sea lice biology and genetics
Karin Boxaspen
Studies <strong>of</strong> sea lice biology, including both <strong>the</strong> species