Water temperature and fishing 2 micro.
Importance of understanding water temperature in fishing from an academicals point of view.
Just as temperature and its fluctuation is important for fish and aquatic invertebrates so is temperature naturally important for the angler. Why ? Well firstly because anything that causes the fish to stop biting concerns us so we need to know at least the basics of thermal stress. Too large fluctuations too soon up or down will immediately cause the fish to stop biting and even cause death as we all know. The fish deaths after big hail storms on very warm water are normally the clearest example of this. The following article (extracts from a scientific paper ) will show how complex water temperature is in the aquatic environment and that the angler should apply his mind accordingly.

Spatial and temporal variation in water temperature in our rivers.
I have briefly touched on the factors influencing water temperatures earlier. (Hydrological, Climatic, Structural and regional.)
One must take into consideration daily and annual or seasonal fluctuations. Temperatures are normally lowest in early morning hours and warmest in the mid to late afternoons. This also shifts with the seasons. From winter to summer naturally. Of course large deep rivers , small heavily canopied streams have the least diel ranges in water temperature while on the other hand shallow streams exposed to direct solar radiation and braided rivers have the largest dial range of water temperature.
Seasonal  temperatures will naturally have warmer temperatures in summer and lowest in winter. Then there is the inter annual variations like wet cold seasons or dry warm seasons etc . So the averages will thus differ. 
“If spatial and temporal variations is combined three broad spatio-temporal scales may be defined (Gunderson et al.1995 cited by Rivers-More et al.2004);namely,
micro-scale  (operating at areas  ‹10m² (site or reach) and over a time period of hours (diel)]; 
meso-scale operating between 10m² to 10km²n (longitudinal) over a time period of days (seasonal)]; and 
macro –scale [operating at  areas ›10km (regional) and over a time period of years (inter –annual)].”Helen Dalas. University of Cape Town.

Of course studies recently ranged from thermal pollution by dams and powers stations to global warming . Detailed studies on the thermal characteristics of all our rivers are not well documented or shall I say water temperatures are not routinely monitored in South Africa.
When studying water temperatures they normally record either at different period of the day or measure hourly temperatures. Ecologically it makes better sense (latter) due to the fact that it allows for measurements of extremes that will have the greater significance.
Modelling water temperature 
Water temperature model Studies are categorised so far into three broad categories namely, regression, stochastic and deterministic models .(Cassie 2006)
Regression models include linear (Stefan and Preud’homme 1993), multiple or logistic models (Mohseni 1998).
Simple linear models predict water temperatures asa function  of air temperature (weekly or monthly) where multiple regression models include other factors such as discharge time lag etc, while logistic models account for ground water and evaporative cooling (Mohseni and Stefan 1999)
Stohastic and deterministic models are said to be more realistic as they require only air temperature as the input where deterministic models use all relevant meteorological data to calculate energy components. (Stefan and Sinokrot 1993).S o duration curves and degree curves are common outputs to compare cumulative warmth at a site within a season (Essig 1998, Rivers –Moore 2005).
Anthropogenic factors modifying water temperature.
Direct effects of change in water temperatures will direct and include thermal discharge, or indirect including land use charge, agricultural irrigation return flow, flow modification, (river  regulation), inter-basin water transfer, modification to riparian vegetation, and global warming. The effect these have on South African rivers are not known as far as I’m concerned.
Effects of temperature changes on physical and chemical characteristics of water.
Temperatures have great effect on water including solubility, oxygen and other gasses, chemical reaction rates and toxicity and microbial activity (Dallas and Day 2004)
High temperatures reduce solubility of dissolved oxygen decreasing its concentration and thus its availability to aquatic organisms and when temperatures increase the vulnerability to toxins intensifies (cyanide, zinc, phenol, and xylene.)
The effect then on aquatic organisms and ecosystems.
Temperatures outside of an organisms optimal temperature  range affects growth rate, behaviour, metabolism,  reproduction, feeding habits, geographical distribution, community structure, movements and migrations, diseases and pollution. (Vannote and Sweeney, 1980 ) called it the “optimal thermal regime”. Quite logical stuff well defined you might say. Some organisms have a good tolerance level in extreme cold temperatures and some in extreme hot temperatures but within these two broad examples there are thousands of organisms living in a narrower temperature range within.
Physiological and behavioural effects 
A 10° C increase in temperature level results in a doubling of organism’s metabolic rate (Hellawell1986) and causes stress due to increased respiration and oxygen demand. Metabolic rate increases drastically. Parasites increase drastically and predator prey relationships are altered radically. Hus fish have temperature thresholds at below which they stop feeding or slow feeding. Fish will move to cooler water if the water temperature increases to close to their threshold and visa versa.
Reproduction, development.
It is common sense that water temperature affects breeding, egg incubation periods, hatching success and duration and it is frequently during reproductive stages that organisms are very sensitive to temperature fluctuations. Fish spawning are known to be triggered by temperature, photoperiod, water level and flooding, intra-specific interactions and the presence of spawning substrates . The spawning periods are almost always during the warmer months when water temperatures are warm. One of the main reasons is that the food is most abundant. In South Africa the water temperatures of 18-19° C seems to be the most favourable for our indigenous fish species. It is also when the growth of aquatic insects are at its peak. Water temperature is also the most important factor in the growth rate of our fish species.
Some temperature/spawn studies on some of our species are;
The Clanwilliam yellowfish (Barbus Capensis) spawn in gravel beds in relative shallow water after upstream migrations between Sep and Dec. Temperatures at spawning beds were between 19.1° and 23.1° Celsius. Lower temperatures the next year however between 16.6° and 17.5° was the water temperature at the spawning sites due to late cold weather. No spawning thus occurred. 19° was thus proposed as a thermal threshold.
Labeo umbratus migrate upstream and spawn on flood plains even in heavy flood summer months such as the previous flood season in the Vaal Orange catchment. Larvae hatched at 22° C (Mulder 1971). 
Oreochromus Mossambicus spawn in late November when minimum temperatures get to 18 ° and raising. (James & Bruton1992) 
Barbus trevelyani (border barb) also will spawn at 18° celcius. (Bok and Heart 1982)
Both labeobarbus kimberleyensis (largemouth yellowfish, labeobarbus aeneus (smallmouth yellowfish), labeo umbratus (moggel), labeo Capensis (Orange River mudfish) and barbus natalensis spawn between 18 and 23° Celcius. Most of them when fresh water enters the system or the water rise they migrate to suitable habitat. The Labeo species prefer flood plains or the gravel that are exposed during the dry months. Thus very critically influenced by the water level control by dams.

Algae.
Water temperature greatly influence the geographical distribution of a specie. Algal species diversity increases from 0°-25°  and decreases › 30°, while biomass increases at 0°-30° and decrease  ›30°. Algal classes shift in dominance from diatoms(‹ 20°) to green algae 15°-30°) to blue green-algae (›30°) (DeNicola 1996). So the greater changes in communities occurs ›25 and less below 25°.  
Lethal temperature effects on aquatic organisms.
Lethal effects on aquatic organisms are affected by.
1-Rate of temperature change ( up or down)
2- Acclimatisation. (Previous temperature history of organism)
3-Duration of exposure (acute vs. chronic)
4-Life history stage of the organisms
5-Physiological state of the organism (effects of other stresses.
6- Adaptive strategies.
There are also sub lethal effects like for instance on reproduction, spawning, egg incubation and hatchability, development and growth, survival of fry etc.
Ref; Helen Dalas (University of Cape town)

Summary.
The fishermen should thus not only keep track of the water temperature over a week period but for the duration of the season/s, and try and obtain a average, minimum and maximum during winter, summer, the minimum daily and daily maximum as well as the temperature range in the area he/she is fishing in. We should also know the threshold of our species and the organisms they feed on and have a good knowledge of the structure of the waters we fish in. A sort of mental underwater picture of the habitat and most favourable spots at various thermal regimes including a good knowledge of the waters above and below in rivers we catch in.