Dissolved Oxygen and whether to Fish

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 19^o 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.)