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Menteith Fishery



Upwelling or spring Turnover Explained 1st June 2009

As some of you will know we experience a fish loss in the Lake 6 weeks ago. The Lake has made a good recovery as the effected area was primarily the deeps north of Inchmhome and most of the fish were able to swim away from the problem. Many anglers are still unsure of the cause – so here is the explanation to “Upwelling or Spring Turnover” that was referred to in an earlier press release.

During the winter the depths of lakes are kept oxygenated by natural circulation. Water is chilled at the surface and this denser water sinks to the bottom replacing warmer less dense water.  In this way, during winter, the sediments and benthos (sediment life) are kept supplied with fully oxygenated fresh water. These organisms (bacteria, fungi etc) flourish, and a dense mat of living material is formed – especially in lowland fertile lochs containing an abundance of nutrients. As spring approaches and heat is supplied to the system the surface waters are heated and become less dense and stay where they are. When this happens the loch is said to be stratified and this stratification (cold at the base, warm at the top) becomes stronger and more stable as summer progresses.

The deeper layers are no longer receiving freshly oxygenated water from the surface and the organisms at the bottom can quickly use up the available oxygen resulting in a die off of those organisms (bacteria, fungi etc) that require oxygen (aerobic).  The breakdown products of these organisms include ammonia, hydrogen sulphide (both of which are toxic to fish at very low levels 1ppm and 8ppb respectively) and other organic compounds, slowly dissolve and dissipate into the water during the summer stratification but are kept away from the fish due to this same stratification.

In a spring turnover event, the loch has developed stratification (typically due to an early heat wave) and the deeps have become de-oxygenated (anaerobic is easier to say) with a kill off of benthic organisms. The weather then changes back to winter with cooling of the surface layers which sink and circulation re-establishes. The harmful breakdown products are brought into contact with the fish. Anaerobic breakdown of organic material during the summer also releases harmful gases and organic compounds but these are not brought into contact with the fish as autumnal destratification is generally more gradual and there has been no prior build up of benthic life associated with aerobic winter conditions.

The fish response to ammonia or hydrogen sulphide is gill hyperplasia, which means the gill filaments or lamellae swell effectively reducing the area of gill in contact with the irritant (there are similarities with athsma). The fish chooses to suffocate rather than be damaged by the irritant. Once the irritant is removed there is usually very rapid gill recovery – and this was found in survivors at the Lake with survivors gills in the pens at Gateside looking normal 2 weeks later.


Spring turnover resulting in fish kills is an infrequent event because the correct destratifying weather has to coincide with the end of a period of early stratification that has been prolonged enough to cause anaerobic conditions to establish in the depths. It does happen, and during my conversations with aquatic scientists I heard of two other similar events occurring recently in Scotland and one 10 years ago and another about 20 years ago. There have also been incidents in England.  The Loch needs also to have deep areas where cold water collects. Perhaps other long term meteorological conditions are required. The Lake had its driest winter (low flushing) since local records began 20 years ago.

Ammonia or Hydrogen sulphide poisoning is difficult to test for because unless some one is there to smell it (detectable by nose at 8ppb) – special sampling is required to pick up the low levels and usually the water has settled down by the time the sampling gear arrives.

We (and SEPA) obviously will be monitoring the loch closely over the next few years. The deeps (between Inchmahome and the Point of the rookery (Coldon) are now stratified and becoming anaerobic. Gateside Bay is still well oxygenated. We will be taking samples and monitoring the various water parameters – initially looking mainly at oxygen, temperature, turbidity (suspended solids) at the various depths but looking at other parameters as our knowledge of the Lake system increases. QG