Progress Report for the “Vision in Billfish
Project”, Sept 2002
By Dr Kerstin Fritsches
1.
Research
Visual Abilities of Southern Bluefin Tuna
As
the project progresses we have developed a number of sophisticated experiments
investigating the physiology of billfish and tuna eyes which require either
access to stable, large research ships or land-based operations. For our
research, tuna often have the advantage over billfish of being more easily
accessible such as Southern Bluefin Tuna (SBT, Figure 1) which are kept in
captivity in South Australia. Tuna share a habitat with billfish and are exposed
to similar challenges to their visual systems. Therefore they are ideal
candidates for comparing specialisations in physiology and eye design between
these high performance predators. In January 2002 I organised a research trip
with my colleagues Prof. Eric Warrant, University of Lund, Sweden and Dr.
Richard Brill, National Marine Fisheries Service (NMFS), USA to investigate if
the tuna farming facilities were suitable for our purpose.
The success of this trip far exceeded our expectations
and thanks to the outstanding help of the staff of Kalies Tuna farming operation
and their enthusiastic scientist Kirsten Rough we performed successful
electrophysiological experiments in four tuna. We were interested in the effect
of light-levels and temperatures on vision and these experiments on the living
eye showed excellent results. We also collected tissue for anatomical work as
well preserved samples to test for colour vision in this species of tuna. Dr.
Brill, a world expert in the physiology of tuna and billfish was so impressed
with this research opportunity that he is now planning to involve a number of
different research groups in a future trip to boost much needed research in tuna
biology. This is a nice and unexpected spin-off for research in tuna and
billfish biology, helped by the GFAA R&D Foundation and Tailored Marines
Accessories’ contribution to the “Vision in Billfish” project.
Sampling of billfish tissue at Pt. Stephens
The
tournament at Pt. Stephens is always an excellent opportunity to collect
samples, but also to interact with other billfish scientist, people involved in
the game fishing industry and the interested public. This year’s tournament
was no exception and I had many interesting discussions with a variety of
people. My assistant Lenore Litherland and I collected eyes from 20 animals of a
variety of different billfish and tuna species. While these samples will be used
for a series of different studies we were specifically happy about samples of
striped marlin eyes. As described
in my last report from December 2001 we have succeeded in identifying a basis
for colour vision in the striped marlin and the tissue from Pt. Stephens was
used to investigate where in the world around it the marlin is likely to see
colours.
To do this we identify individual cells in the nervous tissue - the
retina - contained within the eye. Striped marlin have two different looking
types of light-gathering cells in their retina, called single and double cones,
which are not distributed evenly as shown in figure 2.
There are many more single cones per twin cone in the part of the retina
that looks up into the bright light.
From
my recent work outlined in my last report in December we now have the crucial
information that single cones “see” the colour blue-purple best while the
twin cones are most sensitive to blue-green. Placed side-by-side these two cone
types allow the marlin to distinguish colours. So for lures or prey ahead, above
and behind the marlin it is very likely that colour vision does play a role.
Below the fish, objects are seen mainly with the blue-green sensitive cells –
a very sensible arrangement since this is the most dominant colour at depth.
Research Expedition on board NOAA ship “Townsend Cromwell”
After
last years very successful trip on NOAA ship “Townsend Cromwell” around the
Hawaiian Islands, my colleague Eric Warrant and I now have a standing invitation
for longline research trips organised by the National Marine Fisheries Service
(each valued at AU$ 60 000 per researcher!). This year we spent 30 days at sea
with outstanding success in collecting specimens and data. We collected samples
from 57 specimens of 8 different species of billfish, tuna and other blue water
fish, one species of shark and one species of squid, a favourite prey item for
billfish. We also conducted a variety of different experiments observing the
reactions of the living, isolated retina in 28 animals. This resulted in a
mountain of invaluable data on the speed of vision in these fish, how they deal
with changes in water temperature and changing light intensity. All this data
will require some time to analyse and interpret and I will send a report on
these and the experiments on the SBTs as soon as I have finished the analysis.
We have preliminary data on a new set of
experiments which we undertook on this research trip. The experiment tests the
quality of fish lenses, the component in the eye which focuses the light on the
retina. The picture in figure 3 was created by slowly moving a coloured laser
beam across the lens. This technique is the most accurate measurement of the
focal length, a crucial indicator of the quality of the image seen by the fish.
Also, animals with colour vision tend to have lenses corrected for different
colours, hence this is a fairly rapid test to screen if a fish is likely to see
colour. The swordfish does not appear to have a lens corrected for colour, which
strongly indicates that this nocturnal hunter does not use colour vision.
1.
Talks & Publications
-
“What do billfish see?” by K. Fritsches, Blue Water 2002 Feb-March, 104-106
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Talk at Pt. Stephens tournament as part of the Science evening organised by J.
Pepperell
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3 lectures in longline fisheries & aquaculture to undergraduate and
postgraduate students at the University of Queensland.