The Blackwater Estuary from http://www.onlyimage.com
The water temperature in the Munster Blackwater on opening day (1st February) can be and often is above the commonly accepted value of 42 degrees Fahrenheit (6 degrees Centigrade) below which salmon tend not to ascend significant river obstacles. Therefore, in warm winters it is possible to encounter fresh-run salmon high up the river early in the season, and certainly fishing high up the river can be worthwhile then. Nevertheless, most spring salmon caught in February and March will usually be taken in the lower river (below Fermoy).
Moving into April, the middle river (Mallow to Fermoy) will become more productive. And moving on into May, the upper river (above Mallow) will start to fish. Then, the upper and especially the middle beats will usually be more productive than the lower beats depending on exactly which lower beat one is fishing.
Moving into June and July, the grilse run should be underway even if the river is fairly low. Indeed, the lower beats will generally fish well if the water is on the low side, whereas the grilse and summer salmon will run fast to the middle and upper beats if the water is high.
In August, one is mainly dependent on water. If the water is at a reasonable height, most of the river will fish well and the fishing can be excellent, with grilse of above average weight and large salmon. On the other hand, in low water the lower beats will likely fish best. Similarly, in September in reasonable water conditions more or less all beats will fish well, and in low water the lower beats will again be best.
Finally, from May onwards, or even from April onwards in warm weather, fly fishers can find good fishing in most sections of the river, but they should avoid the deep, slow beats such as are found in the vicinity of Fermoy, for example, and one or two other parts of the river.