Given the unseasonal British weather
I’m yet to venture out onto the bank this season. Between that and refurbishing a new house,
opportunities to kick-off the blog have been few and far between. I thought I’d fill the void by sharing my
thoughts on the discussion around dissolved oxygen and the debate centered
around targeting barbel in such conditions.
As with all things fish-welfare there is a range of strongly held opinions,
many of them conflicting and a lot of misinformation. I always encourage people to heed information
from reputable sources. With a degree in
Marine and Freshwater Biology, I would like to think my own opinions carry at
least a little validity, but I should stress they are just that – my personal
opinions.
For anyone somehow still unaware, the
months of hot weather and lack of rainfall have caused a sharp reduction in
water volume in all of our rivers and a spike in water temperature upwards of 25
degrees Celsius.
The Wye at Kerne
Bridge – Dry as a bone (Photos courtesy of Worcester Angling Society)
The availability of respirable oxygen
for fish in rivers is a complex relationship between saturation, mixing,
temperature, pressure, photosynthesis and salinity. For those wanting a more detailed explanation,
may I recommend https://www.fondriest.com/environmental-measurements/parameters/water-quality/dissolved-oxygen/. My belief, in current conditions ,is that the
discussion should focus largely on temperature.
The remaining factors show a degree of consistency or fluctuate
routinely, sufficient for fish to be appropriately adapted to these changes. Water temperature by comparison is seeing exceptional
increases as a result of, at 35, the most prolonged heatwave in my living
memory.
That said, UK temperature variation
itself has very little direct effect on fish mortality. It is the indirect effects that cause issues,
with the main culprit being reduced carrying capacity for dissolved oxygen
which comes with increasing water temperature; or in extreme cases of course a
complete absence of water when rivers dry up.
The
relationship between Dissolved Oxygen and temperature (Photos courtesy of the
RACG)
For fish mortality, oxygen is everything. When you see large fish-kills due to
pollution incidents, it is not always the pollutant itself which is directly
responsible, rather the effect it has of stripping oxygen out of the water –
either as a result of high chemical oxygen (COD) or biological oxygen demand
(BOD). Substances such as milk for
example aren’t harmful in themselves, but are devastatingly effective at
stripping oxygen from a watercourse. The
availability of dissolved oxygen in watercourses is influenced by a variety of
factors, not just water temperature and one of the main factors to consider is
the effect of daily fluctuations in oxygen levels as a result of photosynthesis
in aquatic plants. These plants include
visible vegetation such as streamer weed, algae and also microscopic phytoplankton
which is abundant in river water at this time of year. The saturation of water with oxygen is achieved
through diffusion, mixing and through photosynthesis, the latter making a
considerable contribution. There is a
clear correlation between dips in dissolved oxygen levels at night-time when
photosynthesis ceases.
The
relationship between Dissolved Oxygen and daylight hours as a result of
photosynthesis (Photos courtesy of the RACG)
So, we know that the capacity of a
river to carry dissolved oxygen decreases with increases in temperature and
that it dips further at night when photosynthesis ceases.
Another factor to consider is that, importantly,
every species of fish is different and adapted to suit a specific survival
strategy. This survival strategy may
relate to the fishes environment, behavior or otherwise.
Pike for example have evolved as
ambush predators, with a physiology designed to use energy and oxygen reserves
within blood and tissues in short rapid bursts when lunging at prey and use
very little energy in between. This is
also part of the reason why pike can be found skulking in slack water, they’re
just not a long-distance runner. As a
result, pike are particularly vulnerable in low oxygen conditions and many
avoid targeting them in the summer months altogether.
Salmonids are different again, being
more sensitive to low DO even in resting conditions. They have evolved to strictly manage energy
budgets to prioritize returning to spawning grounds. Adaptation is generally about compromise and
oxygen sensitivity has found itself demoted in this instance. It’s especially important to never remove a
salmonid from the water once landed, unless you’re planning on dispatching it.
Carp meanwhile have evolved to live
in weed-choked, water-scarce puddles and can cope more than adequately in low
oxygen environments. That said the
impact of reduced oxygen environments can be significantly worsened by
overstocking, hence a number of commercial fisheries need aerators to preserve
fish stocks. Catfish are perhaps an even
better example. In south-east Asia it’s
possible to walk around fish markets amongst live catfish displayed on market
tables, hours after being caught.
So where do barbel sit in all of
this? Similar to salmonids, they exist comfortably in the tooth of the flow and
have no difficulty exerting energy over long periods of time or coping with
sudden bursts of speed. Adaptations are
focused on sensory detection of food sources and being streamlined to exist and
feed in fast flowing water. They are
less sensitive to oxygen availability than salmonids and typically recover
better. However, the very reason anglers
love to pursue them is what makes them especially vulnerable. No matter how you approach landing a barbel,
they are a powerful fish and will always need to be played to a degree of
tiredness. Even under the rod tip when
you think they’re subdued, they always somehow seem to find another burst of
energy such is their fervor for escape.
From the discussions I’ve seen and
been involved in, I don’t think many dispute that barbel are vulnerable in
these conditions. What does appear to
divide opinion is whether a rested and returned barbel will recover fully or go
on to deteriorate further. Many are
skeptical of this. All I will say is
that I’ve seen countless trout removed from the water for a photo or unhooking
only to be returned, and that I have watched these same fish struggle in the
margins for hours before being found floating dead the following morning. This paired with reports (and photographs)
from fellow anglers and kayakers of barbel being spotted floating down the
river is enough, for me, to be convinced.
Belly
up - a dead barbel after a sewage spill
I’ve been especially surprised to see
matches continuing on rivers in these conditions, with recent reports from a
well-known angling writer of 40+ angler festivals and a match where 150lb of
fish were caught, and presumably retained in a keepnet. Fisheries have been too slow to react in my
opinion, dragging their feet on closing their banks and reluctant to postpone
money-spinning matches. I appreciate a
lot of work goes into the organization of these, but if organizers can deal
with postponing for flood events, they should too be able to postpone for fish
welfare reasons. One of the main
arguments for changes to the closed season is that it would be better
controlled by the fisheries. However, I
feel we’ve seen little evidence from a number of these that welfare features sufficiently
on their agenda to place our trust in their governance. It’s intriguing to note that the Wye and Usk
Foundation (WUF) continues to offer day tickets on the Wye when many fisheries
have now finally closed their banks. The
WUFs objectives include to conserve,
protect, rehabilitate and improve the salmon and other indigenous species of
animal and plant life… and to advance
the education of the public in the conservation of rivers, river corridors and
their animal and plant life and the need for conservation, protection,
rehabilitation and improvement of such environments. A charitable foundation set-up with the above
mandate should be leading the way in prioritizing fish welfare and be actively involved
in providing appropriate guidance to the angling community.
Probably the biggest impact which
must be considered before wholesale decisions would be taken to close rivers
would be the impact on local businesses reliant on angling tourism. However, a commitment to book an angling
holiday, months and even a year in advance has always carried a degree of weather
related risk and I believe it is up to the angler to have accept this and have
a Plan B for when they arrive, be it commercial fisheries or otherwise.
Clearly there is a divergence in
opinion across the angling landscape. Some
feel that the risk is too great and that it is especially unclear whether fish
recover from the ordeal of capture in the immediate or longer-term future. For some, an element of doubt is enough and a
prudent approach is adopted. For others,
photos of dead barbel or a capture of their first ‘belly-up’ barbel is enough
to keep them from wetting a line. Others
are firmly of the opinion that if there was that much risk then fishing would
have been prohibited by fisheries or the relevant agencies and trusts. There is a lamentable absence of clear
guidance on how warm is too warm and how low is too low with regard to
available dissolved oxygen. With one eye
on climate change resilience, my hope is that lessons will be learnt from what
may or may not prove to be an exceptional summer and research undertaken to
provide a guideline value, be it temperature or oxygen level based, either for
the closure of fisheries or too at least allow anglers to make an informed decision.
Equivalent guidance exists for
salmonids with fishery closures triggered around 19o and a number of
fisheries already prohibit the targeting of pike during summer months. Perhaps equivalents should be applied to targeting
barbel or other sensitive coarse species in certain conditions.
If you must fish, please follow
sensible precautions. Upscale your
tackle (if necessary) to allow you to land fish as quickly as possible. Use a large specimen carp net that gives
maximum room to rest the fish. Rest fish
in the net before unhooking, and unhook the fish in the water. Photograph fish in the water whilst it is
resting, using the opportunity to hone and perhaps take some artistic photos of
your quarry whilst they rest. Ask
yourself if you really need to weigh the fish, if it isn’t a candidate for a
personal best I would suggest the answer is no.
If you must weigh it, have everything ready by the water’s edge, use a
wet sling and complete the elements where the fish is removed from the water
with absolute haste and once the fish is rested. After each stage of handling (landing,
unhooking, weighing) rest the fish. Ensure
the fish is fighting fit, and then give it another 10 minutes for good measure. If the
fish has gone belly up at any point, consider resting it for longer. Target your fish in environments where dissolved
oxygen is at its highest – downstream of weirs or agitated water; and be
mindful that dissolved oxygen drops sharply at night. If you see evidence of fish populations in
distress, call the Environment Agency – the earlier they have time to act the more
chance they have of making an effective response.
The EA – (left)
aerating a watercourse to increase oxygen levels; and (right)
rescuing fish from the Teme. (Photos courtesy of the EA)
For the time-being then it is a
matter of personal opinion and perhaps one of conscience. As discussed, I would like to think that
conditions this summer will prompt further research and a greater degree of
preparedness. Misinformation thrives on
a lack of clear guidance and I would like the Environment Agency to step up as
they continue to plan for climate resilience going forwards. For me, I’ll not wet a line again this season until I see peak daily water temperatures down to at least
20 degrees on the rivers I target. (Note - for the 2019 season with higher flow volumes pushing through giving greater saturation and high water temperatures not yet being prolonged, I'll be using 25 degrees Celsius as my go/no-go limit for fishing the rivers.)
