Here we go, a bit of a novel:
I've encountered my share of fish with this milky/mushy flesh. One thing I know is that the worst ones are easy to spot when caught. They are skinny. Without any actual data what I have noticed over my years of fishing out west is that by far the most common fish in this condition are kahawai and gurnard. I have caught literally thousands of kahawai from the Manukau harbour and Auckland's west coast. For around 25 years I've actively fished for them shore based. For 15 years or so I'd fish exclusively for kahawai at night over winter. Sometimes 3 to 5 times a week. I used bait, swf, flies, livies, jigs, home made lures, bare hooks, novelty rigs such as beer can tabs etc etc.
My point is that in all that time with a conservative estimate of 1000 kahawai from two land based locations over winter, I don't think I ever caught a skinny or milky fleshed kahawai. I would only keep no more than maybe 10% of those I caught but they were all fat little (mostly 1.5 to 3kg) chubby fullas.
On the boat however I would catch them all year round, sometimes in huge numbers. Over summer through to late autumn skinny kahawai were not really uncommon, especially from the harbour but the longest kahawai I ever caught was 72cm long and it weighed 3.1kg. It was in 8m off of Hamilton's Gap on the west coast and it made a baracoutta look muscle bound. I weighed it for a comp and won that category but it was only good for gurnard bait. To be fair it put up a good tussle considering its poor condition. To put that in perspective my heaviest kahawai was landbased in winter and weighed 3.6kg. It was around 63cm from memory. Having said that we got a very skinny kahawai out of the harbour two weeks ago.
Don't get me started on gurnard. OK I'll tell you anyway. The longest gurnard I've caught was 52cm long. It was from the Manukau harbour in February and weighed something like 1.1kg. That fish would go 1.6kg plus depending on condition. That like many others caught over summer in the harbour was skinny and inedible. These days I don't keep any skinny fish of any species, even for bait.
So west coast snapper Smudge? Ohhh yeah: Over spring and summer we get lots of snapper. None have looked skinny from off the coast but about 10% of those fish will have whiter looking flesh than the others which have that lovely translucent 'colour'. The whitish ones aren't bad enough to be mushy and they taste fine.
We iki all our fish, we take two 90 litre bins, one full of ice and the other for the fish. The ice goes in the fish bin straight away.
What all that means I have no idea!