The Mysterious Habits of Lesotho’s Rainbow Trout

I have some very nagging questions that I would love the answers to. They relate to the many times I have fished in Lesotho. I have been fishing up there since the mid 90’s and it seems the more I fish there the more confused I get. Where do the trout go during a drought? Where do they go during the winter when the river levels drop? Do they die out? Do they migrate? Do they hide or hibernate? These questions would also apply to the similar but more regularly fished rivers of Rhodes and Barkley East.
I would like to share a few observations of mine over the last 20 years of fishing in Lesotho. This is a post to share some ideas, ask some questions, and hopefully  receive some feedback. I will share five “case studies” which confuse and amuse me.

Case Study 1: Sani River, April/May 2006

It was the autumn of 2006. I spent several weekends fishing the upper reaches of the Sani River, from the Pitsaneng confluence up towards where the back packers is now (about 3 km downstream of the main road). This was undoubtedly some of the best river fishing I’ve ever had. I went up twice in April and once at the end of May. Here follow my catch records:

April /May 2006. The last entry 28/5 was a non-entry as my mate and I didn’t even see a fish. Notice the two trips in April where we slaughtered the fish, and good fish too. Every run and pocket had a good fish. In May I was fishing the same section as I had fished in the second trip. There was literally not a fish in sight. The temperature was very cold, and the river had ice around the edges all day but I still  expected to find the fish in the pools.
This was a typical sight on the Sani River on the second trip. There were a good number of fish of 1 to 3 lbs cruising this pool. Clear water and lower river levels made for great sight fishing. Each pool had some fish, and some had as many as 15 good fish cruising around. On this particular day I carefully sight fished the best water and caught 6 fish of between 2 and 3 lbs. A memorable day!
A fish taken from the above pool.

6 weeks later I had a spare weekend and I went up and camped the night on the Sani River with a fishing buddy, Mike Avery. It was the 28th May. The temperature was a rediculous -13 deg at night and the river was frozen over in many places. We took a long walk down river to go and search of some fish. There was no ice on the bigger pools, and it was easy to see there were no fish present.

I could swear that there was not a fish in the river! We looked hard in every pool but found nothing. I was very disappointed as I have had two previous positive experiences fishing in winter in Lesotho. The Sani River has very little structure in the upper reaches and so it would be difficult for a fish to hide, so the mystery to me was where had the fish gone?

Johnathan Aldous who owned the Sani Top Chalet at the time had a theory that the fish migrated downstream in low river conditions. What lends weight to this theory is that I know of two very big pools lower down on the Sani River that are literally full of fish in times of low river levels, but not nearly as good during good flows. We once caught 30 fish of about 1 – 1.5 lbs in one of these big pools, in only a few hours. It was like fishing a hatchery pond. I have had similar experience on one other river during the dry season. I call these pools “hotspots”.

The migratory instinct would be very beneficial in the Sani River due to it’s lack of structure in the upper reaches. The fish would be extremely vulnerable in the upper reaches if they stayed for the winter, so migration downstream is a a possible explanation.

Case Study 2: Upper Senqu River, July 1997.

In July 1997 I hiked along the top of the Drakensberg from Mont Aux Sources to Rockeries. We were camping in Mponjwane cave at the top of the Rockeries Pass. On our rest day, we walked down to a waterfall on the Senqu River. It was an incredible sight that greeted us. There were literally fish everywhere! All in the 1 to 3lb range. The deep pools and potholes below the falls were all full of fish. There were so many that I even managed to catch one by hand that got stuck under the ice. So it’s not a true fly fishing experience, more and observation. These fish must have been trying to migrate upstream to spawn? The upper Senqu River has much better structure than the Sani River and so a downstream migratory instinct  wouldn’t be as beneficial as in the Sani River. These fish were however out and about in cold temperatures, a clearly they had a upstream migratory instinct.

Case Study 3: Upper Mokhotlong River, July 1999

My second positive winter fishing experience was in the headwaters of the Mokhotlong River. Again I was on a hike along the escarpment while still at school. It was also in July. This time I had my rod.  On one afternoon I ran off down the Mokhotlong valley to find some holding water. I only had to go a few km untill I found the first pools. Most of the river was frozen, but the pools were clear of ice. It was a really small stream up here, and only just big enough to hold fish. In half an hour of fishing I caught 1 rainbow of a pound and about 5 little brown trout of about 6″! It was easy fishing. Ever little pool that I cast into had a fish that was willing to take a fly. Back then in didn’t even know that brown trout existed up in Lesotho! Sadly I didn’t have the time to explore further downstream, but those fish were out and on the feed in the middle of winter when the water temperature was probably close to zero.

Case Study 4: Lesotho River, 2013/14 

This river won’t be named. It’s one of the few of my very special places that I won’t share. It’s a river that has a high waterfall that stops any movement of fish upstream. The top 20 km of this river near the Drakensberg escarpment has a population of trout that are essentially isolated from the rest of Lesotho’s trout population since someone stocked them up there some time ago (no idea when).

My experiences on this river lead me to believe that these trout are very territorial. I first fished the river in April 2013. The fish were very scarce. In the top 10 km of river that’s close to the Drakenbsberg escarpment, we found 11 fish. We caught 5 and saw another 6. They were all around 1.5lbs. It was very exciting sight fishing to them in this small and crystal clear stream. For a small stream it has good holding water, but was very spread out. We would sometimes walk a km between holding water. It’s pretty typical of many of the high altitude Lesotho streams.  We fished for two days and on the second day we saw exactly the same fish in the same places, no surprises there.

In January the following season I hiked up to the stream again. The only access is over the Drakensberg. The amazing thing was that we seemed to find fish in EXACTLY the same places as before, except now we only caught 2 fish and saw 4. There were no fish in new places. I recall two particular fish which were a cock fish and a hen fish in the same places. I don’t have the photographic evidence to compare markings but I am convinced they were the same fish. They were now bigger than the year before but in exactly the same pools.

Why didn’t these fish move around? Maybe the fact that if they venture downstream they can’t return and this has selected for a non-migratory bloodline? In all rivers in Lesotho, the migratory instinct would be very beneficial to a trout’s survival. Maybe that’s why the population is so sparse? They are only just clinging to existence without the ability to be restocked from bigger holding water downstream.

In April 2013 I was broken off under that rock by a good fish which appeared to be a hen fish. In January 2014, Andrew Descroizilles was broken off by what I’m convinced was the exact same fish. On both occasions the fish took a dry fly as it drifted past the rock at the head of a rapid, and on both occasions the fish broke us off under the rock.
Stalking a fish that I saw cruising the pool.
One of the pools that held a solitary fish on both occasions.
A perfect hook up after sighting the fish cruising the pool.
It’s hard to forget a fish with markings like this. I am sure that I have caught this fish twice in the same place, almost a year apart.

Case Study 5: Malibamatso River, May 2017.

My experience on the Malibamatso last week got me thinking about me writing this post. You can read my previous post about the days fishing there. It’s called “Heaven on Earth”. I thought that the river was almost devoid of fish. I had the perfect day for sighting fish. I saw one monster fish that I didn’t manage to catch. I saw one other smaller fish and one tiddler splashed at my dry fly.

I had some theories as to why the river was devoid of fish. These were all blasted out the water after the feedback to my previous blog post. Clearly it’s a commonly fished river and several people who read the post had fished there this season.

I thought the river was in beautiful condition, but all respondents remarked on how low it was. I could see that the river had risen a lot due to the recent snow so it had been lower the previous week. All the people who said they had fished there this season said it was full of fish! That blew me away! Where the hell did they go? I was fishing in perfect conditions for sighting fish and I only saw two fish.

Have they gone upstream to spawn? Had they already spawned and now gone downstream? One fish in particular appears to have been caught before. The large fish that I spent hours trying to catch. I thought it to be around 6lbs.

Alfred Röhrs, who I don’t know, read my blog and recognized the pool where I photographed the fish. He sent me a picture of his fish that he caught in exactly the same place cruising around exactly the same rock. This was sometime this season. He measured the fish at 64 cm long. That fish could be even heavier than 6lbs judging by the condition of the fish. He sent me a picture of the fish he caught. I didn’t want  overestimate it’s size but I could see how fat it was from the shadow on the bottom. Well done Alfred, that’s a fish of a lifetime!

The fish I saw and tried to catch.
The absolute hog of a fish caught by Alfred Röhrs. Unbelievable fish!
I spent hours staring into pools like this. Just watching and hoping for a fish to appear.

Where on earth have all the other fish gone since the summer? Pieter Snyders also reported that there were an almost annoying number of fish when he guided there 5 months ago. Gary Glen-Young also said he caught several fish in a short space of time a few months ago just above Oxbow lodge.

I am well aware of the trout’s ability to hide. They do this almost every day in Lesotho. You hardly ever catch a fish in Lesotho in the early morning, and you don’t see them either. They seem to miraculously appear at around 11 am and come on the feed. I don’t know if it’s a water temperature thing or feed availability?

Are the fish in the Malibamatso currently all hiding under the rocks for the winter, and was I just there on a bad day? Have they swum upstream to spawn or downstream for better holding water? I wouldn’t think there’s any need for migrating as there are plenty deep pools in this section. Was it the fact that it was such a calm and clear day that they just feel insecure about being out and on the feed? I would generally associate those habits of hiding in bright light conditions with Browns, not Rainbows.

Conclusion

It’s difficult to come to any general conclusion, other than it’s probably multi factorial and river dependant.

I have observed trout moving upstream to spawn in the Senqu River. They were out and about and easy to see in the  500m or so of river that I saw below the Senqu waterfall. I have caught Brown and Rainbow trout in the half frozen headwaters of the Mokhotlong River.

I have observed that the Sani River trout seem to migrate downstream in times of low flow to several “hotspot” pools. The upper reaches of the Sani probably has the least holding water of all the Lesotho rivers that I have fished and so a downstream migration would be hugely beneficial to their survival.

I have observed that the trout of the Malibamatso River near Oxbow seem to migrate either upstream or downstream at this time of year (observations made this year and in 1987 by Doug Kretzman). It’s possible they were hiding, but in water conditions such as those I encountered, Rainbow Trout are generally out and about. In almost every Lesotho trip that I have gone on, I observe that it’s almost a waste of time to fish before 10am. They clearly hide under a rock or a bank until the water warms up a bit and they start to feed. What appeared to a be a river devoid of fish, suddenly comes to life.

I have been trying to answer these questions for 20 years but I keep changing my mind. It’s one of the mysterious and unpredictable aspects of fishing in Lesotho that keeps me coming back for more.

Here follows the link to a recently written account of an adventure to fish the Malibamatso River 30 years ago. Its amazing that 30 years ago the Malibamatso Rainbows exhibited the same winter disappearing habits  as I onserved. Thanks Doug Kietzmann:

http://dkretzmann.blogspot.co.za/2017/05/malibamatso-river-lesotho-1987.html?m=1

After the initial posting of this piece on my own blog, A Stream Beyond, I have chatted to several fisherman who have added there own ideas: 

We all know that fish can hide from our view from quite long periods, and we also know that trout migrate. What if all fish have an upstream migratory gene, a downstream gene and a sedentary gene. Maybe the nature of each river leads to each population of trout being selected for the river.

A population of trout in river with a waterfall downstream will generally be the sedentary type or upstream type. The downstream gene will not proliferate as they can’t come back up. This is all speculation, but it definitely seems that the individual trout populations adapt to suite their home river. 

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7 Comments

  1. I can’t say this for sure Rex, but I’m of the opinion that the trout, and many riverine fishes, migrate to ‘known’ refuges. These refuges may be up or downstream in any respective stream, and are really just deepwater areas that offer more stable temperature conditions during dangerous highs and dangerous lows, and deeper water levels during dangerous low water. The bit that confuses me is that we are not talking about elephants here, we are speaking about very short-lived creatures, so how on earth do they learn about and know where these refuges are? The other thing is that trout have a beyond-imagination ability to hide away, but as you allude to in your article, you already know that part. Something else that you mentioned that could also be very valid, is that in each given stream, fish could be being selected for particularly traits, and this could explain how the refuges are ‘learned’ by a population and become locked into the genetics rather than requiring learning by individuals. For example, assume we have a stream that has excellent upland drought refuges, and that that part of the trout population that originally had the instinct to migrate upstream during low water found those refuges and thus the trout in that population gradually got selected for that trait due their greater survival. Ditto for a river with lowland refuges and those fish in the population that instinctively migrated downstream during droughts. But whatever the answers all are, it keeps us thinking!

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  2. “there were an almost annoying number of fish when he guided there”
    I have been fishing for nearly fifty years now, never yet have found a river with “annoying numbers” of fish.. is Pieter sure that was the Malibamatso ? or was he guiding Mr Theodore Castwell ?
    http://flyanglersonline.com/lighterside/part364.php

    the first time I fished the M. we did quite well, with the fish in superb condition. My brother and I went back later that year in the winter, and found low water and a nearly fishless river – one or two fish in three days. I’m writing up a blog post about those trips and will link here when done..
    My guess is a combination of movement to the deep pools, plus as I noted in last post, have never seen fish hide under rocks the way they do in those high ‘berg streams. Interesting..

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    1. Interesting that you found the same “fishless” river in the winter. Did you fish the same section? Below Oxbow? wouldn’t have thought that the fish end to migrate as there is no shortage of holding water. When did you last fish there?

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      1. same section, my brother and I both recognized the pool with the big trout in it.. we spotted a submarine in there one day, it wasn’t feeding and eventually had enough of us pestering it, faded down into to the deep water.

        1987 was the last time.. was working full-time and doing postgrad part-time after that, left SA in 1990 as my annual Army camps sent me to doing police work in the townships, couldn’t stand it anymore. Always wanted to fish the Berg streams once more, still dream about them..

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      2. Fascinating that the fish seemed to exhibit the same “winter disappearing” habits 30 years later. I loved the piece about Mr Theodore Castwell.

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  3. Howsit Rex, I’m not a fisheries scientist but I do spend a lot of time looking after trout health and fishing for them in streams/lakes wherever I travel. In my opinion, trout, like most fish (the ‘most’ part is purely philosophical, because like birds – and most other animals – some fish may have a higher ‘IQ’ than others) act purely on stimuli. The ‘stimuli’ or rather parameters that are most important in aquaculture (for successful farming and from a health perspective) are: oxygen, temperature, pH, toxic ammonia levels, nitrate and nitrite levels, hydrogen sulfide, water hardness (including TDS), turbidity (better defined by suspended solids), dissolved carbon dioxide and the presence of various metal ions (of which the toxicity again depends on pH and water temperature). These are not my own assumptions, they are well-researched and published criteria with recommended minimum and maximum values, i.e., recommended standards. So besides a lack of food, which one would assume is plentiful in the aquaculture environment, suboptimal concentrations or ‘levels’ of the listed parameters will lead to abnormal behaviour or may kill trout – either instantly (acute death – a sudden drop in pH from say pH 7 to pH 3; or a sudden temperature drop from 20 degrees Celsius to 4 degrees Celsius) or slowly over time (constant exposure to water with high suspended solids, which often leads to bacterial gill disease).

    In a wild fishery or the ‘natural’ environment (a river or a lake), oxygen, flow rate (mainly the rate at which fresh oxygen is supplied – more turbulent water also generally contains more dissolved oxygen), temperature and pH (in that order) play the most important daily to seasonal roles and fish can move around ‘towards’ more favourable conditions (of which the movement or migration will be stimulated by the same parameters as listed above – except food of course, I can imagine that a lack of food will also pressure wild fish to move), which is not the case for trout in a cage/net. So when there’s a negative shift in the parameter range, either towards the lower or upper limit of what the fish can tolerate (which may be slightly different in different age groups, different strains or acclimatized populations – again probably via natural selection (?) – in specific areas) there will be a response. It’s frequently reported that ‘older’ fish can tolerate a larger variance in parameter, like a sudden drop or rise in pH and/or temperature below or above the optimal range that could be lethal to juvenile trout. You can make your own assumptions from that re your findings in the Lesotho streams.

    Anyway, oxygen is probably the most influential parameter for fish that don’t have a ‘lung’ or an air sac that they can use in oxygen poor water or even on land. Trout are particularly sensitive to a drop in oxygen and stress easily during a lack of oxygen, so as soon as oxygen levels drop they will try to find ‘more oxygen rich’ water – typically in a deep pool or just below a rapid/water fall (and they can travel kilometers looking for water with more stable oxygen levels). Oxygen levels typically drop with water temperatures greater than 20 degrees Celsius (which is not applicable to your findings), low pressure systems, lack of flow and during the night – I’ve noticed while taking air pressure and oxygen readings on farms in the Cape that the ‘air’ pressure (and with it oxygen saturation) in the water climbs and becomes optimal around 10 am. You can again make your own assumptions here.

    Then, I’ve seen in many places – rainbow trout in the Jan Du Toit’s River and the Kromrivier in the Cederberg, and for brown trout in nearly all the rivers I’ve fished for them (for them it’s more a characteristic in behaviour than abnormal behaviour) – how trout can disappear and appear in pools where it almost becomes inherent behaviour and a characteristic of that stream (which again for rainbows in particular could have to do with parameters of that stream which we can’t see, like lower oxygen levels or a lower/higher water temperature etc. slowing down their metabolism or forcing them to take ‘rests’ – more about this further down). In those streams trout have long ‘rest’ periods in their rituals. I’ve seen them ‘rest’ under rocks, so that only the head is tucked away, or so that the fish is completely hidden, and I’ve seen them rest exposed lying on the bottom often in very shallow water on the edge of the stream – I’ve almost stepped on such fish (and the pools are much smaller than the pools in your photos, some as big as a bath tub – and it was still a challenge to see them)! I’ve fished the JDTs and the Witte where there were no fish hiking upriver in the morning, also in prime sight fishing conditions, and then coming back down in the afternoon I spooked a fish lying dead centre of almost every pool that was devoid. Some of those pools had ‘no’ hideouts to the blatant eye, but clearly they were in there, hiding under a rock or just lying dead still in a strange spot, sometimes even very big fish (20+ inches). I’ve even seen browns behave like this in well flowing rivers in New Zealand as well.

    Some of these parameters affect trout at the molecular level – so their behaviour is controlled by physical or chemical factors at a cellular level and the fish are not ‘thinking’ of what they should do when the pH, temperature or oxygen levels drop, for instance. In short, read the abstract from a research paper I downloaded many years ago – I’ll email you this paper to read in full, it is very interesting:

    PHYSIOLOGICAL RESPONSE TO
    ACID WATER IN BROWN TROUT (SALMO TRUTTA L.):
    CELL VOLUME REGULATION IN HEART
    VENTRICLE TISSUE*
    BY KJELL FUGELLF AND TONE VISLIE*-3

    In brown trout surviving in acidified brook water, the plasma osmolality is
    reduced 15-25%. The decrease is much less than that expected from the
    measured reduction in the plasma concentration of Na+ and Cl~. This
    discrepancy cannot quantitatively be explained by the increase in plasma
    concentration of K+ (100%) and free amino compounds (mainly taurine;
    280%) but appears mainly attributable to a reduction in plasma volume.
    The osmolality of heart ventricle cells is also reduced. Water content of
    the cells is unchanged, and there is a significant decrease in the intracellular
    concentration of K+, taurine and to a lesser extent glutamic acid, accounting
    for 25, 45 and 8%, respectively, of the osmolality reduction. ‘.These
    findings indicate the existence of a cell volume regulation mechanism in
    the fresh water brown trout which counteracts osmotic swelling of tissue
    cells during periods of salt loss in acidified water. The significance of this
    mechanism for survival under such conditions is discussed.

    As mentioned previously, eggs, fry and fingerlings are much more susceptible to pH change than adults.

    Then re your questions around migration: i.e., trout migrating upstream/downstream and to places they seem to already ‘know’ about. Bigger, older fish tend to inhabit a specific stretch of river – not necessarily one big pool. Mature fish that have lived in such an area for a while also seem to ‘know’ the topography of that stretch of river well – like a big fish swimming straight to the best hideout in a pool when spooked. It will always head for that one good hideout when it sees you. These fish also don’t seem to venture far from a big pool or the biggest pool in such an area, so they have easy access to deep water when/if necessary. I’ve seen this in the JDTs, and two brown trout rivers I frequently fish in the Western Cape. I also saw it in two NZ rivers I returned to a few times in a five month period, simply because I was after a particular ‘trophy’ fish in them. Those trophy fish were always near a big pool or in a section with a few big pools, but they were not necessarily in one specific pool, they rather moved around in that area and I even found them feeding in very shallow runs well above those pools. How do I know it was the same fish? Hell I knew ’cause there just weren’t that many 8 – 10 lb fish around in those NZ streams! Besides having a distinct mouth or old injury I could recognise. One of the big browns I caught in the Cape moved about a kilometer upstream overnight where it sat in another very big pool, after it obviously felt threatened. So they travel quickly and far.

    In NZ I saw huge chinook salmon swim to almost the limit of a catchment area, from the sea, to spawn. They were 10+ lb fish in spring creeks that were maybe three foot wide and a foot deep – government protected streams for good reason. Same with the browns – I found browns in literally the first head-water pool of springs, where the water flowed out of the ground. Billy de Jong, and Stanton Hector and I found browns way off the main stream in tributaries that even had dry lower sections in summer (over 20 km of dry river bed) in the Cape; and it’s not every trib, just a select one or two of main stems in which the upper reaches have trout (it’s another approximate 51 km to the nearest source where the mature trout could have travelled from to populate a section of the trib, another 22 km above the main stem). Those streams had also never been stocked (confirmed by the farmers). So those brown’s ‘fore fathers’ migrated up there to spawn due to oxygen/pH/temperature/TSS/TDS and-who-knows-whatever-else being just right and they sensed it as soon as they passed the junctions of the tribs with the main stem. One of these tribs have a 1.5 m waterfall which Billy and I had to climb up and there were the odd big fish above that barrier – the barrier looked impassable, but the browns got up there! Fish can also migrate in surprisingly shallow water and through thin rapids at pace – when you think it’s not possible, it is possible for them. So we often misjudge their capability. They may in 24-48 hrs swim 20 km upriver and then back down 40 km to find suitable water.

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