Whew Bazza, a world of questions to be answered there, and I'm going to do my best because I think you raise some interesting and important points...
I cut my teeth fishing the Upper Waihou, Waimakariri, and Waiomou streams and some other little nameless gems of that area. Those rivers are where I learnt to fly fish and while I get to fish them maybe once per season these days I see very little change apart from the stinking fricking walkway up the best part of the Waihou and the access track into what was always our "secret Blue Spring" area where you never saw another angler and the fish averaged 3-4 lbs.
OK, I've got that off my chest.
The Waimakariri has a lot of fish in it to around the 3lb mark. Late in the season often more and some that will be up to 5 and bigger. The Waihou has a higher percentage of bigger fish than the Waimak. Just because you don't catch them up there doesn't mean they are not there... I will give you some tips for targeting the bigger fish toward the end of this... One of the main problems is getting your fly to them away from the throngs of smaller fish.
Back in those days... a long time ago, Rowen Strickland was the fish and game officer for that area and he explained the situation with those streams thus:
Both offer miles and miles (In their upper reaches) of perfect spawning water which, due to the spring creek nature of them, rarely floods. Neither have eels as the falls at Okororie on the Waihou prevents their upstream migration. Very few cormorants seem to frequent the area either.
This adds up to the "perfect" reproduction conditions. Lots of spawning, no floods to wreck the reds, few predators to reduce the fry and fingerling's which result.
Trout, like many fish, will if overpopulated, stunt to some degree. A bit like putting a goldfish in a bowl... it's a goldish... put it in a lake and it becomes a bloody great Carp.
Trout will ordinarily want about 7 ft of "personal Territorial space" per foot of it's length so it's easy to see how a stunting mechanism will come into effect with these fish when this is forcibly reduced to only inches per fish. There are too many fish. Many of those fish do drop back and in the less atractive areas around Okororie there ares ome very fine catches to be made. There are monstors all the way down at Paeroa but the river a there is so ugly a 20lber wouldn't persuade me to fish it. Most of the fish in the upper reaches are, I believe, juviniles as evidenced by near all of them still having parr markings. But by the time the whole transient thing has happened, so has the next spawning. There's no shortage of food... the biomass of these creeks is incredible.
Way's to reduce their numbers? Netting has been found to be ineffective in both streams due to the high amount of weeds. If you can get a drag without pulling 500lbs of weed you don't get the fish because they have so many cress and weed areas in which to hide. There are more "big" fish there than most anglers believe so methods of trying to get a "mass kill" are inappropriate. (Anglers have been proven to be the
least most reliable method of guaging a streams fish population or health... I hate that fact
)
I founded the Matamata Freshwater Anglers Club in about 1977 and right from its inception the problem of these streams was a major focus for us. As you correctly say the situation has not improved a huge amount. I think this is because the message has not been brought across strongly enough to many anglers that the wanton act of catching a small fish and throwing it in the blackberry is actually a good thing to do.
I believe anglers may be the only answer to the woes... even organised cull days!!! Maybe some radical stuff like they have with Carp... make it illegal to return any fish under 12 inches?? Blasphemy or inventive fisheries management... I don't know!
OK, so how do you catch these bigger fish I talk about??? Don't fish in the same part of the stream as everyone else. The bigger fish, almost exclusively, live and feed in the eddies that are prevalent all along the edges and entraces to each pool or where the current hits the bank or cress beds on both streams. In many instances you will need to "dap" for them as it is near impossible to get a cast into these places without drag and given the conditions the fanciest mend, roll or curve cast, or stack mend, usually just spooks them.
Sneak up the edges to these back eddies and keep very low (I usually crawl up completely prone), full camo if you can and look into these small eddies, usually at the head of pools. The fish are often quite easy to see as the white sand builds up in the eddies. The fish will be facing into the current so may well be looking directly at you... be prepared to back off and make a huge circular stalk around the other side of the eddy to make a cast... dap......
Nymphs or dries both work well. If you practise this method exclusively your average size will come up to 16 - 18 inches on both of these streams but your catch rate may not be high, but you'll be surprised at how many 3 lb plus fish you start to encounter. By blinding or wading up the middle fishing the seams where you would ordinarily fish you will encounter the plagues of small fish.
One of the reasons these streams offer such a great resource to the competitions is for the high points possible with high numbers of small fish. many of the popularised techniques by competitive anglers on these rivers are specifically designed with those fish in mind... unless you are after a mess of snapper bait forget them....
For introducing kids or newcomers to the sport I believe these streams to be the finest about... Instant action and only 30 minutes from where they can haul (literally) a 20lb brown out of the Ngongataha.
It's back to NZ methodology of softly softly, sneaky, sneaky and staying in cover....