On dark nights the schools are likely to be betrayed by the “firing” of the water, caused by the luminescence of the tiny organisms that they disturb in their progress. The trail of bluish light left behind by individual fish as they dart to one side or the other, while one rows or sails through a school on a moonless, overcast night when the water is firing, is the most beautiful spectacle that our coastal waters afford.
From Fishes of the Gulf of Maine by Bigelow and Schroeder, 1953
When bluefish invade mackerel schools at night the fireworks on display below are spectacular. The larger streaks tearing through a mackerel school tell the story.
Al Bezanson
Good Morning Gloucester 
Quite a design on the top of these MACKEREL too thanks for the wisdom sharing here! 🙂 Dave & Kim 🙂
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