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Abstract

Amoebic gill disease (AGD) is the single most significant disease affecting
Tasmanian marine salmonid farming. The current treatment of AGD is freshwater
bathing affected sea-caged fish. However, this current method of treatment is
becoming less effective with an apparent increase in bathing frequency over the years
fuelling a rise in production costs to the Tasmanian Atlantic salmon industry. The
research reported in this thesis primarily aimed to identify an improved method of
treatment that not only has an enhanced efficacy compared to the current treatment,
but also exhibits minimal physiological consequences to treated fish.

Initial experiments aimed at assessing the pathophysiological effects of AGD and
freshwater bathing, with the intention for later comparison to improved and novel
treatments. Freshwater· bathing elicited a minor ionoregulatory, and possibly acid
base, dysfunction that was enhanced in AGD-affected Atlantic salmon (Salmo salar
L.). Significant histochemical changes to gill mucous cell populations were also
documented in response to salinity and AGD. These results highlighted the potential
importance of the mucous layer in differing saline conditions and disease events.

Freshwater bathing was subsequently refined and its effectiveness enhanced with the
use of soft freshwater as opposed to hard freshwater. Commercial salmonid farms
have access to a number of freshwater sources that vary considerably in their total
hardness (mg L-1 CaC03). We found that soft freshwater bathing delayed re-infection
and subsequent pathology in treated Atlantic salmon by 2 weeks, representing a
significant financial saving to the industry. From blood plasma analyses, no
significant effect of soft freshwater bathing was found. It was documented that one of
the therapeutic effects that :freshwater bathing offers is its ability to reduce mucus
viscosity, aiding flushing of gill-associated amoebae and hyperplastic tissue. Soft
freshwater was found to significantly reduce mucus viscosity by 1 h into bathing
(P<0.05) whereas hard freshwater significantly reduced the viscosity by 3 h (P<0.05).

Due to the effective action of freshwater bathing on fish mucus, our research
progressed to the trial of the mucolytic agent L-cysteine ethyl ester (LCEE) as an in
feed additive. Cysteine based mucolytics are widely used in human and domestic '
animal medicines, and LCEE has previously been tested in aquaculture to protect
coho salmon (Oncorhynchus kisutch Walbaum) against a harmful diatom. We tested
LCEE both in vitro and in vivo for its potential benefit at alleviating AGD. As an in
feed additive, LCEE significantly delayed the progression of AGD pathology by 50
57 %, compared to controls, in Atlantic salmon. Prophylactic administration for 2
weeks at 45-50 mg LCEE kg-1 fish dai1 prior to, or at the lowest point of infection,
was found to be the most successful oral delivery method. No detrimental
physiological consequences were found in LCEE-treated fish, even when freshwater
bathing followed prolonged medication. The mechanism of action of LCEE was in the
alteration of the hosts mucous layer, with no effect on isolated gill amoebae.

An underlying theme that required an intimate understanding for the assessment of
improved and novel treatments was that of fish mucus. The limited knowledge in the
literature on fish mucus also paved the way for some interesting and much needed
research. Gill mucous cell histochemistry and cutaneous mucus viscosity and
biochemistry showed significant differences between three salmonid species; Atlantic
salmon (S. salar), brown trout (S. trutta) and rainbow trout (Oncorhynchus mykiss).
Significant differences due to salinity and AGD were also documented. Some
important findings were that mucus viscosity was significantly greater in seawater
compared to freshwater fish for all three species, and significantly reduced due to
AGD in Atlantic salmon and brown trout. Trends for all three salmonids were shifts in
mucous cell glycoproteins from neutral mucins in freshwater to acidic mucins in
seawater, and shifts towards neutral mucins, with an increase in mucous cell numbers,
due to AGD. Such findings not only hold implications for disease management, but
also aid in a better understanding of differing fish physiologies and disease
susceptibilities.

The research conducted for this thesis has identified improvements for the treatment
of AGD and represents new pathophysiological information on a gill disease in three
euryhaline salmonid species. In addition, significant contributions have been made to
the infant field of fish mucology. The documented findings will hopefully pave the
way for future research into fish disease treatments and pathophysiology.

Item Type:	Thesis - PhD
Authors/Creators:	Roberts, Shane(Shane David)
Keywords:	Atlantic salmon, Atlantic salmon, Bacterial diseases in fishes
Copyright Holders:	The Author
Copyright Information:	Copyright 2004 the author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).
Additional Information:	Thesis (Ph.D.)--University of Tasmania, 2004. Includes bibliographical references
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