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Malcolm Thomson and John Leask Electro fish the Burn of Ireland |
Survey of sea trout smolts migrating downstream through the Burn of Ireland in spring 2004
The Bay of Ireland is a popular spot for sea trout anglers and it is thought that sea trout spawn in the Burn of Ireland, which runs into the bay. In order to estimate the number of sea trout smolts leaving the burn for the sea, a fish trap was installed in the burn, approximately 100m upstream of the beach, between the dates of 31st March and the 6th June 2004 (see Figure 1). The trap was set in an existing fish ladder and a screen was placed across the adjacent weir in order to prevent fish bypassing the trap and to get as accurate an estimate as possible. The trap was visited every day to check for fish and to note water temperature and height. The trap was removed for two days during this period and re-installed after some above-water headroom and an escape ramp was added, after the unfortunate death of some mallard ducklings at the start of May.
Malcolm Thomson and John Leask Electro fish the Burn of Ireland
On each day, fish caught in the trap were counted and anaesthetised before
measuring their length and weight. A scale sample was also collected and
each fish was tagged with a florescent red liquid dye, which was injected
into the clear tissue behind the right eye of each fish (see Figure 2).
The dye sets into a soft flexible plastic and should be retained as a visible
mark for the life of the fish, allowing their origin to be known if recaptured
at a later date.
| Malcolm
would like to remind all Sea Trout Fishers to be on the look out for
the tagged fish. If you think you have captured one please contact
Malcolm at these locations and telephone numbers |
Malcolm
Thomson Home tel. 01856 851 650 |
Over the period of the survey, a total of 185 trout were caught in the trap, of which 150 (81%) showed clear signs of smolting (smolts take on a distinctive silver coloration, see Figure 4). This represents a minimum estimate as some trout were probably missed due to several flood episodes when fish would have been able to bypass the trap. The run-timing is shown in Figure 5. There was a distinct period, between the 9th of April and 5th May when most trout descended the burn usually during periods of increased water flow after rain. The timing of this run is similar to sea trout smolt runs in other areas of Scotland. Most smolts ranged in size from 130 – 190mm. From analysis of the scale samples, the majority of smolting fish were two years old, although some were only one year old (see Figure 3).
The survey suggests that the Burn of Ireland supports a fairly healthy brown trout population. Brown trout and sea trout are the same species (Salmo trutta). As is the case elsewhere, the population in the burn seems to consist of two sub-groups: one which stays in freshwater and looks like the typical brown trout, and one which smolts and migrates to sea to become the silver sea trout. The reasons why some brown trout smolt and go to sea are unclear. It may be due to competition for space and food in the burn, i.e. if there are too many trout for the available resources, then some may go to sea. Another theory is that female trout have a greater tendency to go to sea because of the richer feeding. Reproduction is more energy demanding for females as their bodies have to produce hundreds of eggs, and this energy demand is better met at sea. The ratio of males to females in the trout caught this spring is unknown as it is impossible to tell the sex at this stage of development. Mature adults are more easily distinguished so if any spawning trout can be caught this winter then we can compare the sex ratio in migrating fish and freshwater residents.
A number of the tagged fish have been recaptured further downstream in electro fishing surveys conducted during June. This indicates that the tag is being retained and hopefully some will be caught at sea which will let us know how far sea trout move once they leave the burn.
The survey was a tremendous success in terms of getting an impression of how many smolts a burn the size of this one can produce. It will be useful to repeat this survey for the next few years in order to study variations in the number of smolts migrating from the burn.
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| The original trap installed in the fish ladder | and the modified trap
with more headroom and escape ramp. |
All fish trapped in
the Burn of Ireland were tagged with a red dye behind their right
eye, as shown here. |
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| Scale of a two year old smolt from the Burn of Ireland. The two areas where the growth rings are closely bunched correspond to slow winter growth, whereas the faster summer growth rings are more widely spaced. This fish was born in spring 2002 | The difference between
brown trout and sea trout smolts. The top picture shows a brown trout
which has grown up in the Ireland burn and is not showing any signs
of going to sea… |
and two smolts which
have lost their brown/yellow colouration and taken on a characteristic
silvery appearance. These are going to migrate to sea to become sea
trout and probably return to the Burn of Ireland to spawn if they
survive their time at sea. |
| Sea trout smolt tagging Using the Association’s new electrofishing equipment as well as a variety of fish traps, Malcolm Thomson (OTFA environment sub-committee) and Duncan Clarke have been busy looking for sea trout smolts this spring. Neither was quite sure how successful this venture would be but it is fair to say that both have been pleasantly surprised with the number of smolts they have found. This has yielded interesting information on the productivity of some of Orkney’s sea trout burns. In fact it has provided the basic information required to say that a particular burn is even producing sea trout, which alone is tremendously useful. However, it is hoped that some of the most valuable information is yet to come. The several hundred smolts caught so far and all have been released with a tag which identifies the burn of origin. Any recaptures will provide interesting information about the movements and growth rates of sea trout at sea.We would urge everyone catching sea trout from now on to have a quick look to see if they have caught a tagged fish
1. All tagged fish have had their adipose fin clipped; and 2. The tag itself is a coloured fluorescent (red, orange, yellow or green) mark somewhere on the fish’s body. The tag colour combined with it’s
location on the fish relates to the burn of origin. If you find a
tagged fish (you don’t have to kill it), take a note of the
colour of the tag and where it is on the fish. For example, all smolts
from the Burn of Ireland have a red mark to the rear of their right
eye (illustrated). The capture of tagged fish should be reported to
Malcolm on 851 650 (home) or 07761 729 850 (mobile). Thank you and
tight lines! |
Trout sampling - Stenness loch
Sea lice and the damage they cause
to our fish
Karen Bowler, a student from ICIT, carried out a research project on the
occurrence of sea lice in Stenness loch. This came about after concern had
been expressed by many anglers over the appearance of lice on trout in the
loch, reflecting the general situation affecting sea trout around the rest
of the Orkney coastline. After the generous co-operation of anglers in the
two Stenness competitions, a good number of trout were donated for examination.
No lice were found on trout from the first competition in April. However,
about 90% of trout from the second competition in June carried lice. Similarly,
most trout in a sample of 16 collected in October were infected. Lice numbers
per fish tended to be low although some smaller trout probably carried a
fatal level, i.e. more that 15. Karen also undertook extensive plankton
sampling in Stenness but found none of the infectious lice larvae floating
in the water column. It remains unclear if larvae are coming into the loch
with the tide or are spreading from adult lice already in the loch. As the
salinity in Stenness drops in winter we expect that sea lice would disappear
from the loch, as the results from the first competition suggest. However,
the situation requires continued monitoring.
Almost all trout were clear of the flesh worms observed in Stenness trout in 2001 (about 5 worms were found in a total sample of 50 fish
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Sea Trout
The health of Orkney's wild sea trout population has been in the spotlight
over the last few years, following concern over the observed increase in
sea lice infestation. Salmon farms are thought to contribute to these elevated
lice numbers and the OTFA has been working hard to try and guide the industry
away from our most valuable sea trout spawning and nursery areas.
The protection of our sea trout stocks is an ongoing commitment and there
is always a need for more information. Gathering this information in the
past has been difficult, given the open access nature of the sea trout fishery.
However, through the efforts of many anglers, we have been able to collect
numerous scale and lice samples which are in the process of being analysed.
Unfortunately, results are slow in coming, but hopefully this information
will help to strengthen the case for protecting Orkney's sea trout
Once more the 2002 sea trout season is upon us and once more I would like to take this opportunity to ask all anglers in Orkney to help collect more information sea trout stocks. You can do this in several ways.
· Lice collection
We hope to send more lice samples to researchers at Heriot Watt University
in Edinburgh who are looking at lice genetics, to see if there is variation
between lice from different regions, or between lice collected from different
hosts (e.g. wild sea trout and farmed salmon). This is especially the case
for lice found on trout caught in the Loch of Stenness. The presence of
lice on the resident Stenness trout is a totally new phenomenon and an extremely
worrying one. We hope to be able to sample the catch in the two Stenness
competitions this year - details to follow.
So next time you catch a trout from the sea or from Stenness, take a careful
look. Although adult lice are obvious, juvenile lice can be very small and
only apparent on close examination. Get in touch with a member of the Environment
sub-committee and contribute to science!! Contact details below.
· Scale samples
Scale samples are still useful as it allows us to build up a more comprehensive
picture of the sea trout in Orkney, their growth rates and movements. In
a survey done by Nall in the 1930s, Orkney sea trout were judged to exhibit
the second fastest growth rates in Scotland, only behind the Tweed population.
Take a small sample from the side of the fish, about 2/3 of the way back
from the head, just above the lateral line and put the scale in paper packets
(remember to complete the details on the packet, e.g. date, size, location,
etc). You can get packets from members of the Environment sub-committee.
· Catch and release
Finally, please show restraint in your fishing. Sea liced fish may be better
off dead and contributing to science, but there are still some superbly
conditioned fish out there (we hope). It is clear that Orkney sea trout
are under threat from various factors and if it comes down to the wire,
we'll need every healthy spawning adult to sustain the population for the
future.
Contact details:
Malcolm Thomson Tel. (work) 01856 850 605
(home) 01856 851 650
Feel free to drop any fish/samples by Malcolm at his work, at ICIT, Old
Academy, Back Road, Stromness.
Colin Kirkpatrick Tel. (work) 01856 850 234
Again, you can drop fish/samples by Colin's work at Wilsons, Garson Industrial
Estate, Stromness.
Thanks!!!
Magnified scale sample showing growth rings