Not a yellowfin Matt... well that's my call. You'll find plenty of tuna species with yellow tinges. I'd have to do my research... but the juvenile yellowfins I've caught - are bright yellow/gold with vibrant bars. This looks like a cold water tuna... but being a QLD'er... not quite sure. Stand to be corrected though.

As Daz said, Jnr yellowfin have distinct vertical markings throughout the whole body.

My call would be a bigeye or SBT!

Heres a YFT from Exmouth Matt

Seems to have yellower fins and the vertical markings along the body that yours doesnt (from that pic anyway).

Whatever it is, it's still a nice bonus for the metro.

The camera hasnt really captured the colours properly. Prob about 6-7kg, might look a little bigger in the pic. This picture here I found looks pretty similar. Of the pics of juvenile YFT, not all have had very bright colour yellow that you're talking about Daz (I know the colouration you mean). Perhaps depending on the temperature they're caught in, it could be different levels of markings. It might not be a YFT tuna though, but the blues i've caught have looked a little different. Is there any physiological things I can use to tell them apart (apart from the colour)? The yellow has unfortunately not come through in any of the pics (it wasnt a 'yeah, thats kinda yellow', more 'see the yellow in the fins'). Tinge isnt quite the right word. There was a yellowy stripe underneath the dark blue bit as well (not really bright, but you could see it depending on the light). Possible even the level of yellow we saw could be another species though?

A few more yellowfin here, here, here. Not all have the really bright colouring. The last looks the size and colouring that we caught (again, none of the yellow came up in the pics for some reason).

Found some pics of tuna with yellow fins here that most people are calling bluefin. Maybe bluefin can have yellowfins :p Though, the linked pics might be northern bluefin, not southern bluefin.

Has anyone got any pics of small YFT caught in cooler waters (maybe from that recent PGFC comp)? I wonder if you can differentiate them by dorsal count? 

Juveniles have yellow in the scond dorsal and the tail is yellowish. They turn black as the fish matures

Dunno about YFT...but here's my SBT for comparo.....

I reckon what you got there is either SBT or Bigeye. 

something different for a change

at the risk of sounding like an idiot what does SBT stand for?

Southern Bluefin Tuna

cheers Buz

Yep - after a bit more research - it's a SBT. Are you eating it Matt? Like to hear how it goes. Perhaps a little bit of sashimi for you tonight!

To easy wicked game. Aussies just LOVE making acronyms for everything. TLA (three letter acronyms). Bit like mobile phone texting i reckon. Find lots of them in fishing.

Couple of others.

YFT- Yellowfin Tuna
YTK- Yellowtail Kings

My thoughts are Southern Blue as well, have been a few around lately..

Slice it up,Soy an Wasabi yummmmmmm....

def a sbt. we have caught a few of them , they are nice eating when you steak them up length ways, cut out all the dark meat and bake in the oven topped with a mixture of cucumber, tomato, lemon and lime. good to see them around

Will confirm what the others have said, definately not a yellowfin.

If the yellow colour actually came through in the pics, it'd probably make sense why we were thinking yellowfin, and from the pics I linked, not all YFT have the bright colours like Huggys one (only seen those colours on YFT caught in the tropics, but maybe thats a coincidence, I dont know?). Seeing as none actually came through in the photo, it makes it look like I have no clue, haha. Is there some reliable method of telling SBT and bigeye apart? (dorsal count, disection, etc?). I hear telling juvie YFT and bigeye is a bit of black art too.

A bigeye wont have that. The bigger fin especially the temperate water type with shorter sickles are hard to diferenciate with bigeye.