Work2fish might be right,
but, it is really difficult to tell. Even in broad daylight.
In the upper peninsula of Michigan we have several subspecies of Lake Trout as well as splake. We have "Fatties" or Siscoits which are really really fat, we don't like to eat - (its a different fish that ussually lives deep- 150-300 feet), Mackinaw which are thinner and good eating live 10-150+ feet, and we have what is called redfins which are best - really red meat and inhabit the same water as Macks. Siscoits have rough skin texture and are fat in the belly! Macks and Red Fin have smoother skin and are thinner in the belly.
In upper Michigan one can look at the color of the pectoral fins to determin Macks or Redfin - if they are red with a light leading edge (like a brookie) it's a Red Fin - really a good eat

, if it is a skiny fish with smoother skin and ligher coloring on the pectorial fins it is a Mackinaw-good eats. Of course none of this is absolute. If it has a wormy pattern (the dark part or the pattern - not the light dots) on its back, it is probabaly a splake. The tail being V shaped or more squared off is even more judgemental - IMO.
For me it is the dark part of the pattern on the back of the fish (which we can't see very well in this photo) is what I use to decide if I call a fish a laker or splake... A dark wormy pattern and it a splake, no dark wormy pattern... it a laker,,,, my boat log comes with a eraser and I reserve the right to change my mind.
Terre