And Mother Nature looks like she's read the script, with plunging temperatures, rain and even snow forecast for the mountains this weekend.

These conditions are likely to trigger the first serious waves of fish to enter the rivers on their annual spawning migration. Waves of keen trout anglers are certain to follow.

The tried and trusted method of fishing glo bugs and nymphs on either fly or conventional spin gear will be the most popular approach. Spin fishers can also throw hard-bodied minnows in the 50-100mm range.

From May 1, a number of rule changes came into effect to ensure the two major spawn run fisheries - the Eucumbene and Thredbo rivers - are protected from overfishing during this period.

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Until the season closes at the end of the June long weekend, there's a daily bag limit of one fish, a minimum size of 50cm, and a total of two fish in possession.

These rules apply to the Thredbo River and its tributaries, as well as the Eucumbene River and its tributaries.

Fisheries inspectors are expected to be active on both rivers over the coming weeks so ensure you're abiding by the rules. Visit www.dpi.nsw.gov.au for more information.

Estuary fishing traditionally slows down in May but it's really come alive of late thanks to clearing conditions and loads of bait.

Flathead, bream, trevally, tailor and salmon are biting extremely well in many systems, with anglers enjoying the best action on the making tide.

Several species of tuna are creating a buzz inshore and in deeper water. Bonito continue to bite from the stones and the Tathra and Merimbula wharves.

Tathra Wharf also dished up a big surprise last weekend in the form of a solid northern bluefin tuna, caught on light spin gear. Further out, the first yellowfin tuna of the season are showing up on the Shelf.