Not the easiest to tell from the pic, but agree with you on looking more like a black.

Heres an obvious blue though: http://fishwrecked.com/node/53778

and hires: http://fishwrecked.com/files/images/bluebaby%20(Large).jpg

Considering the hundreds of fish on fishwreckapedia that are spot on, its pretty damn great overall though!

Called it for a blue with overall colouring with stripes, blotches, finer bill, higher first dorsal fin, (black marlin's dorsal fin is proportionately the lowest of any billfish) and the shape of the second dorsal fin. The pectoral fins are an unknown.

Marlin (Blue) Makaira mazara

Characteristics - Blue marlin are a member of the family Istiophoridae. They possess a long, pointed bill and a non-rigid pectoral fin that can be folded against the body. The back of the fish is cobalt-blue in colour, breaking into silvery white on the lower half of the body. About 15 pale blue bars can be seen along the body, which fade after death.

Confusing species - Blue marlin are distinguished from black marlin (Makaira indica) by the non-rigid pectoral fin and the presence of blue bars displayed along the body. 

Marlin (Black) Makaira indica

Black marlin Makaira indica

Characteristics - Black marlin are a member of the family Istiophoridae. They possess a long, pointed bill, a retractable dorsal fin and a rigid pectoral fin, which cannot be pressed against the side of the body. Black marlin have two keels on either side of the tail base. Their name is derived from the colouration they take on when dead. Unlike blue (Makaira mazara) and striped marlin (Tetrapturus audax), black marlin do not usually have any blotches or stripes along the body.

Confusing species - Black marlin are sometimes confused with blue marlin, however black marlin have comparatively low and rounded dorsal and anal fins. The pectoral fins of blue marlin can be folded against the body.

http://www.bluemarlin3.com/tbf/anatomy-bluemarlin.php

The following two photos show a primary distinguishing characteristic between black marlin and blue marlin.  The flap of skin covering the throat and below the gills and the branchiostegal bones supporting it of a blue marlin are about half the length of the similar flap of skin and branchiostegals of a black marlin.  In a black marlin this flap of skin extends back almost as far as the rear edge of the operculum or main gill cover.

Top pic is a Black and the bottom a Blue.  Applying this to the pic of the fish in question imo makes it a Black.

I would consider this one pretty difficult.  I would call it black generally but mainly because of the side pec fins.  Then again the density of the fish and elongated dorsal fin make me think blue.  I'd like to hear from Glenn on this one to get a definitive answer.

I was thinking blue when I originally saw the picture on the wreckopaedia. Still leaning that way, because of the stripes being distinct (more so than black IMO), but also the bill is longer than a black compared with the size of fish, and the adipose and anal fin (thats the ones dorsally and ventrally {top and bottom} just before the tail). It is sharp and well defined in blues, and more elongated in blacks. It is hard to see here on this pic.

I agree, help Glenn.

Have a look at the pic below.  You can see that the slab of skin I referred to in my post above extends to the point marked 'x' consistent with a black.  If the fish in the pic was a blue the slab of skin would extend approximately to the area marked 'y'.  

I will add that the pics in my post above showing the differences between blacks and blues were contributed by a taxidermist on another fishing website during a discussion about how to correctly ID marlin.  The anglers and weighmasters were offering advice with respect to fin sizes, bill lengths, stripes, bulk etc on how to distinguish one species of marlin from another.  The taxidermist responded, look at these pics, when you understand them it's a no brainer. 

Adam asked me to have a look at this and comment. I saw it was marlin and felt a frown develop - these are tough. It has taken me a while to reply, but I wanted to make sure I could offer an informed response.  I will admit to having limited experience with marlin, but I do have access to a lot of litereature, hundreds of photos and colleagues! The first thing I'll say is that these are really difficult to tell with 100% confidence from a photo. I probably sound like a broken record because I seem to say it about all the tough IDs on here, but I guess that is why they are tough to ID!

Alan, thanks for clarifying where you got the info on the branchiostegal membrane, and I have since found the website you got it from. I have never heard of that character before and not one of the species descriptions, books, etc mention it, so I needed to investigate. None of the fish biologists with expertise in marlin use it as a diagnostic character…yet. I say ‘yet’ because there is currently a research program (overseas) looking at whether the branchiostegal membrane can be used as a diagnostic tool.  At this stage of the research, the short answer is maybe, but there is some variation, so this character should only be used in conjunction with other characters.  To quote my colleague, “it is not going to be the silver bullet for ID”.

So, onto the fish in question - I initially thought Black Marlin.  I asked my colleagues at the other Australian museums, who all agreed.  I also asked the taxonomist who is doing the marlin research (they take a range of measurements of competition-landed marlin and then confirm the species ID with genetics) and he agreed – Black Marlin.  We mostly based our ID on the angle and shape of the pectoral fins, which protrudes from the body at a sharper angle and is more sickle-shaped in Black Marlin than in Blues.  This fish is a youngster, so other characters are a bit harder – like the dorsal fin, which is not typical of a Black Marlin.  Other characters, like the position of fins, are hard to assess because the fish is not perfectly side on.  However, the pectoral fins, along with the position of the branchiostegal frill and some other measurements make this most likely to be a Black Marlin.  I can’t give all the details, because the research is not finished yet.  In saying all this though, the only way to be certain is to take a bit of tissue and get the DNA sequence.  Now, no more marlin pics please :-)

.

So its a "black and blue marlin". A confused teenager.

Interesting to read how difficult it can be to ID these, even from expert.

 Being a billy virgin i learnt alot from this, Must say its very handy having you on here Glenn. 

Thanks