For carp the minimum acceptable temperature for succesful spawning is around 18c which usually will occur during May or early June at the latest.
I keep reading these sort of "facts" about spawning and they are simply not true.I will give a simplified (but acurate) guide to what triggers the spawning of coarse fish here in the UK.
Most indigenous cyprinids such as Roach,Chub,Bream,Dace,Perch etc require a water temperature of between 15 and 18 degrees.The temps vary slightly to enable a slight spread of the spawning from species to species.Once the water has reached the required minimum temperatures the actual spawning is triggered by the amount of avaiable daylight (ie length of day) This relatively low water temperature is always (other than in truely exceptional cases) reached during the period of the old traditional closed season. This has come about through natural evolution over the millions of years these species have been present.
Carp require a different set of stimulus.The easiest way of putting this across is to explain the "Thousand degree days" principle-
The very first day of the year that the water level reachs 20 degrees note the figure of 20 down.Each day if the temperature is 20 or over add it or the 20+ figure to the preceeding figure or total.If the temperature drops below 20 deg in this time you must stop and start again. When the total reachs a thousand degrees the carp will spawn.As carp are not idigenous they have evolved to the ambient temperatures in the countries of their origin and havnt been in the UK long enough to adapt to ours. In this country a water temperature of 20 degrees is relatively high during the time of year in question in fact many waters (due to their geographical location and other factors) never reach a "thousand degree days" on a regular basis.Hence forth whilst carp quite often spawn late or not at all in this country.
Most indigenous predators have evolved to spawn well before other species as this enables them to be in a position to feed on the newly hatched fry of their prey.Once again their is a slight variation in water temps and available daylight requirments from water to water but this is just a reflection (in most cases) of the variation of spwaning in the prey fish.For example Pike normally spawn between Febuary and March.
People seem to forget that carp havnt been in the UK for very long! well not long in the scale of evolution any way!