So far there has been no feedback to the questions posed by the fish images among the petroglyphs at the Three Fish Site, the destination of the December 2019 GCAS field trip. Therefore your faithful webmaster will present her own suppositions.
Let's begin by assuming that each of the three fish petroglyphs are factual representations of three certain fish species, made approximately to scale. Secondly, let's suppose that whoever created the fish petroglyphs may not necessarily have recorded a fish that had been caught locally, but that the artist(s) had at one time or another seen such a fish somewhere in their travels and was recording the fish from memory.
The largest of the three fish petroglyphs is that one up top of this post on the far left. The petroglyph measures about 5 feet long and about 18-20 inches high at its highest point. Consider the fish's blunt head, big eye, and the positions of the gills and the set-back dorsal fin and pectoral fins. And then, compare it to the photo next to it of a modern-day dorado (aka mahi-mahi, aka dolphinfish). The dorado in the first photo up there was caught offshore of Cabo Falso, just barely on the Pacific side of the southernmost tip of the Baja peninsula in Mexico. However, dorado seasonally migrate in and our of the entire length of the Gulf of California. They also get much bigger, as shown in the photo on the right here, via the San Diego Reader. The size of that one looks to be close to the actual size of the image of the San Diego Mountain petroglyph.
I propose that an ancient puebloan trader to the Gulf of California could have seen such a fish, and the mental image of such a beast would not have grown dim or doubtful. Perhaps upon their return home to the desert Southwest they told a story of a giant fish that no one believed, so they carved a petroglyph to illustrate their tale.