Most of the time the old closed season March 15th to June 16th covered the tench and carp spawning times, although occasionally they would go through to the end of June or early July.
Possibly the only fish it may have missed is pike, which I have seen spawn at the start of March, even the end of February in warm early months.
If you think about it, almost every species of fish in UK, we want to catch them at their heaviest weight, so will be fishing right up to spawning times, including pike. This year I stopped fishing for pike when I noticed I was getting males chase the females in the margin as I was netting or returning them. One of the fish I landed was 2lb heavier than her autumn or winter weight, she had recovered from a cormorant strike, not long before a mate caught her, the scars and markings were a match when we checked pics. She had obviously fed up, enough to heal and build her roe up.
This is where Yonny and I disagree on fishing during spawning times, but I do understand his views. One of my Ardleigh carp I caught while fish were spawning in the reedy inlet to my left, the fish I caught came from the deeper water around 50metres away. Had it spawned, or was it going in there?
It wasn't an empty fish, no 'spawning scars', although long term battle worn.
Again at Taverham I watched fish spawning on The Meadow area, yet there were fish who weren't Meadow fish, at their end of the lake still feeding.
I would think the best choice is close off swims that can reach the spawning areas when the fish are there, but then again, if the lake is fished by numpties at any point, or those who don't know shutting may be the option.
I have seen fish spawning, emptying out, and quite literally within hours are feeding heavily again, at the other end of the lake, or even munching their way through their own eggs.
Some fish are always 'footballs', that is their shape, the strain or mix that they are. Look at Italians, often dark, but massive gut. Get a mix of strains in a water, you get Leney scale pattern for example, but with Italian shape.
Dinks or Dinkelsbuehl carp are also gutty.
I started this reply, but Dayvid made another post in meantime, so I'll do a quick edit.
16.11 in Nazeing normally weighs 45/46lb. A couple of years ago she had dropped to 36 in July, she had emptied out totally, and was caught just after. By September she was back up to 45lb.