Hi mate im no expert but your gain is so high that it might be showing a whiting as the size of a snapper.

 Agree your gain is set way to high, on my 587 my gain is around 2-3 for shallow water up to 25m and from there on I adjust accordingly and it never goes past 5. I run a tm260 

 Interesting the comments on gain, some people think run as higher gain as you can before you get clutter or interference.

I normally run 6-7 gain

as for whiting showing up as snapper, not sure how that works as from my understanding it's showing an echo of the fishes swim bladder, here for a whiting won't show up like a monster fish.

My comments on your pic, the blue / green spots could be small snapper but likely shit fish

the bigger orange / red dots could be something worth looking at, potentially bigger demersals or sambos

try changing your background colour to black, I find this a lot easier to read

 The bottom dense red stuff you see on the right of the bottom lock looks like coral or colly weed, but this stuff holds fish, smaller lumps means less likely anyone else knows about it 

 You can have yellow tail and herring showing up like big blobs  if gain  is too high.gain of 1-2.5max for that depth.i run 1 gain looking for pinks in 10-15m

The flare on bottom lock  can also be from a ledge or square edge but cannot see the hf screen on the far right so cant say for sure.

Thats how I used to learn. Start recording showin the sounder for reference, then drop it over, great for sussin out what ur seein on the sounder, ground and fish wise and easy to do in shallow water. Otherwise, its just speculation

 turn gain down  and run unit in manual for a start check out sounder threads on this site for more information 

My experience:

I run my Furuno in full auto "fishing" in anything under 150m. I find the Furuno auto is almost as good as I am with manual control IF I continuously cock about with it.....which I cant be bothered with, hence the auto is much better than manual for me, except in deep water where you really do need to manually tweek it.

 Try  these  settings ssteve

Pic advance  1/1

Tvg medium  for anything under 120m

Colour erase 10%

Clutter 10%

Interference auto 

Run your gain at   1-5. (10m -75m)

I normally have range on 15m

Set  function  key to shift then you can keep the bottom on the picture (if targetting bottom fish)

Once you've  got these setting you only have to shift the bottom in screen as you head deeper and adjust  the gain  slightly (5 seconds to do this)

Hope  it helps mate