During the mariculture of fish stock the ability to control the
water quality is severely diminished so there for there are several
environmental factors that pose a risk to those fish stocks such as but not exclusively
toxic algal blooms. These alone when they reach concentrations high enough are
toxic in their own right but little is known about the affect they have on the susceptibility
of fish stock to diseases at lower algal concentrations. One of the most
significant diseases this could have an effect on is Viral hemorrhagic
septicaemia virus (VHSV) and the most susceptible farmed fish species to this
disease is Oncorhynchus mykiss
(Rainbow Trout).
P. parvum was
cultured in the lab using 10L Pyrex bottles and exposed to a diurnal light
regime of 12hrs light and 12hrs dark. The algal colonies where grown to a
concentration of around 1.0 x 106 cells ml-1 before being
used. The algal cells where used for the experiments whilst they were in the
exponential growth phase. The VHSV isolate used originated from a clinical
outbreak in a Danish rainbow trout farm. 2 different exposures to both the algae
and VHSV were carried out. Experiment 1 was the fish were first exposed to the
algae for 12h before a complete water change before a 2h exposure to the VHSV
before the water was again renewed the fish where then kept for 32 days before
the experiment was terminated. In experiments 2 and 3 the fish where exposed to
the algae and VHSV at the same time for 24hrs before the water was renewed. 3
different viral doses where tested. These tests where terminated after 22 days.
There was no effect on the infectivity of the virus from the
algae regards less of the concentration. The Sublethal levels of P parvum where found to be in the range
of 50 x 103 to 100 x 103 cells ml-1. High
aeration removed the ichthyotoxicity so aeration water kept to the minimum
level to be able to achieve 80% oxygen saturation.
The mortality rate of fish in experiment 1 was not
significantly different between the control and algae exposure although the
fish exposed to algae before VHSV did show a tendency towards a lower mortality
that to that of just the VHSV exposed fish. There was no observed fish death in
either group after 32 days.
In experiment 2 the group that was exposed to the highest
concentration of VHSV showed a 52% cumulative mortality rate without algae
exposure. Whilst the cumulative mortality with exposure increased significantly
to 87%. The groups with lower VHSV exposure showed no mortality with the
exception of 2 individuals.
I think this is a really interesting paper as not only does
it investigate the levels of both VHSV and P.
parvum are lethal to fish it also investigates to some extent the
interaction between the two factors with could lead to new advances in
mariculture and the development of better water management and inoculation
techniques.
Hi James,
ReplyDeleteI was wondering if the authors explained why high aeration removed the ichthyotoxicity? Also, what do you think the implications for mariculture of the trout may be?
Thanks,
Chloe
Hi Chloe,
ReplyDeleteThanks for the question. The authors have only stated that in some of their preliminary experiments they found that it was degraded by high aeration but have not published the data nor have they discussed any causes of this effect. I can only assume the toxin is not stable in contact with oxygen at high saturation so will get broken down. The implications to trout mariculture are the possibility of developing a vaccine that will prevent the fish from being affected by the P.parvum and also help reduce the chance of being infected which exposed to the ichthyotoxin.
Cheers
James