Mark,
I am no scientist, but the generally accepted theory is that the black GTs are spawning males, I would love a scientist to actually confirm this, but I cannot for a fact confirm this is the case, but I believe it to be true based on what we've seen. A lot of the fish in those mobile bommies are quite dark and nearly a full black colour, there's enough of a mixture of black and normal silver ones in those schools to believe that the black ones are males anyway.
The other theory is that the black colouration is just an aggression type thing and they go this colour when excited/feeding. I guess they also get excited when they're spawning, so the black colour is probably the same sort of deal as when a big marlin gets "lit up" behind a teaser. The marlin also get lit up when spawning, so maybe the black colour on GTs is just the standard colouration for when they're excited.
The black ones are always angry and fight harder, that's for sure.
Hey, make sure to put a little lubrication in your cushit before use, no pun intended, and I'm sure they'll go well for you. Good luck when you're out there next.
Damon.
