What you have been catching there, Riccard, are white-dotted grouper (Epinephelus polystigma). Also known as white-spotted rock cod, this species occurs in estuaries and rivers from the Philippines south through eastern Indonesia, PNG and the Solomon Islands to northern Queensland, Australia. It appears to have two main forms, either with white dots or a dark-brown or black blotchy color pattern. In some parts of their range, white-dotted grouper are known by natives as "lazy fish," easy to spear when they aggregate near mangroves in shallow water at night, resting on the bottom with their backs out of the water. Because of this behavior, the species quickly becomes rare around areas inhabited by humans. E. polystigma is a reasonably small grouper, growing only to 20 inches or so; thus, the biggest ones you have been catching at 24 inches approximate their maximum size, indicating you must be fishing in some remote areas to catch them that big. Scientists know very little about this species, though they're understood to be solitary usually — the shallow-water aggregations are probably spawning behavior, as they occur mainly on new-moon phases and the fish taken during this time are running ripe. White-dotted grouper sampled between 8 and 15 inches long proved to be mature females, and all fish above 18 inches long were mature males, suggesting that these fish may, like other serranids (groupers), mature first as females before changing sex into males.