In the same way that we describe distances in terms of how long it takes rather than kilometres, longer time periods are described as winters survived. I have now done twelve consecutive winters in Rhodes. A lot of climatic variation has manifested over this period; from drought to changes that we now attribute to global climate change. I do not subscribe to Donald Trump’s world view, rather, I am relying on personal experience and what the scientists are telling us. I must emphasize that this article based on visceral experience rather than scientifically gathered data.
I have been aware of climate change as a scientific fact since the mid-eighties, at that juncture an American Presidential advisory think tank already viewed climate change as the greatest threat facing the planet. Here in South Africa, we were aware of the encroaching Karoo and drying out of the country from the west. A decade ago, I was informed that the hard science on climate change in South Africa was that seasons had moved on by six weeks. At that stage, change was mostly observable in that our seasonal rainfall was arriving later and later in the year.
The main drivers influencing local climate are the typography and the prevailing North Westerly winds. South westerly winds are funneled along the mountain ranges and have more influence on weather patterns below the escarpment.
In the past, local residents used to tell me that we’ll get no rainfall till the cold fronts stop coming through the Western Cape. These days the fronts seem to move up the coast year-round, often being blown out to sea off our southerly coast. Nowadays we only seem to receive significant rainfall when a cut off low pressure system moves down from Angola.
Locally, we experienced drought conditions over the period 2016-2018 and so a lot of the changes are difficult to ascribe to climate change. What was noticeable over this period was that the average winter daytime temperatures had risen possibly by as much as three degrees. The nights were as cool as ever. There was less winter snowfall over these years as well.
Winter snowmelt charges the ground water and runs off scouring the streambeds.
Our initial rainfall arrives as frontal showers allowing the vegetation to take hold before the more destructive thunder showers and cloud bursts arrive.
Where this all starts to affect fishing is in the stream levels and flow rates; in fact the availability of water at all. The later arrival of our seasonal rainfall and lack of winter snowfall coupled with warmer spring temperatures and attendant winds adversely affect stream levels and concentrate the surviving fish populations in pools where they become easy prey to the predators.
To spawn, trout, require clean scoured out gravel and good flow of oxygenated water. These conditions are often as a result of melting snow. Cold water holds a higher oxygen content than warmer water.
Trout from Northern hemisphere where they originate and generally experience even harsher winters than us spawn in late winter and early Spring (March /April). At this stage snowmelt increases flow and water temperatures are rising. In New Zealand trout spawning seems to be triggered by heavy rainfall increasing flow and dropping water temperatures. Trout spawn in water temperatures of between 10 and 15 degrees centigrade.
South African trout are said to spawn between June and September. Our wild (feral) Rainbow population (the gene pool possibly stretches back to the early twentieth century) should be fairly well adapted to local conditions and may possibly even be able to tolerate higher water temperatures.
What has prompted this rumination has been the occurrence of different sized small fishes about. Traditionally, in year of regular precipitation (rain and snowfall) at the end of summer there are numerous small 8” fish about. I have heard it said that fry grow at approximately an inch per month and this would seem to indicate to me that the little 8” fish encountered in March would be 8 months old, spawned in July or August of the previous year.
In December 2018 rains eventually arrived and at the end of February 2019 I was seeing a lot of small rainbows, sized two to three inches. When did these fish hatch?
We experience heavy rainfall in March 2019 which limited the available water for the festival. Quite a few sixteen- and eighteen-inch fish were taken, there were also many sub-six inch as well as an eight to ten-inch size class caught. The size range seems to indicate different ages to me.
In the Bokspruit, during November 2019. There were definitely two different size classes of small rainbows being caught, sub-six inches and around ten inches. This seems to indicate a possible age difference. There are also little fry of two to three inches about.
October 2019 fish in Basie’s dam still not spawned as the feeder stream had not flowed through winter.
At the end of November 2019, the Smallmouth Yellowfish (Labeobarbus aeneus) had not spawned yet. Low levels have restricted their upstream migration. In years of good flow, I have observed the Yellowfish migrate upstream to spawn in October. The itinerant Yellows populate pools and seem to spend a lot of time swimming around looking for suitable spawning areas. At the stage the larger fish (said to be the females) refuse all flies thrown at them. Sometimes they don’t even spook, they just ignore your offerings: they are not feeding. Time passes and all of a sudden there is feeding frenzy. I believe that this only comes about once they have dropped their first batch of eggs. They are also said to deposit eggs more than once every season. A fisheries scientist told me that Sterkfontein dam Yellowfish spawn when water temperatures reach 22 degrees Celsius. My own local observations seem to indicate that the may well spawn in slightly cooler water here.
The period when the Yellowfish migrate was often accompanied by rainfall and hence murky water, one did not always see the migrating yellows; all of a sudden, they were all over the place in the higher reaches upstream. Fortunately, now in mid-December 2019 after rains and rising levels the Yellowfish migration has taken place. As I type the larger skittish Yellows seem to be searching for areas to spawn and are proving uncatchable. This should change over the next week or two.
The Orange river Mudfish, (Labeo capensis) also migrate upstream to spawn and are to be found in the middle reaches of the Kraai river. Murky water keeps fly fishers from the areas, so we do not often observe the Muddies spawning. In other parts of the Orange/ Vaal system, the Muddies spawn before the Yellowfish.
A perusal of records seems to indicate that years of good rainfall produce more fish and that there are definite size classes. This would seem to indicate different spawning times. Recent conditions i.e. how long has the stream been experiencing good, sustained levels also must have an influence.
The spawning of Trout from the northern hemisphere and New Zealand seems to be triggered by increased water flow and either cooling or warming water. I suspect that flow rate is the most important factor influencing local trout spawning, more so than water temperature, angle of the son or any seasonal manifestation. What if we only get the flow rates when water is warmer?
I can only conclude that when conditions are unsuitable, Trout keep eggs within their bodies and only deposit their eggs when
conditions improve, in other words: spawning only when suitable conditions prevail. Would not decent flow rates with lots of tumbling water provide enough oxygen and scour the stream bed gravel. In addition, warmer water allows the trout eggs to hatch quicker and there should be more food available to growing fry.