I'm guessing you can tell that it's a fish. For this species, the strong angle in the lateral line (above) is an important identifying character, as are the prominent keeled scales (or scutes) along that lateral line.
The next image was taken in different light. Can you follow the lateral line — starting high on the back and then sloping down to the side? (Hint: It looks like a zipper.)
Those are the dorsal fins (folded downward). Note the wonderful metallic blue and olive green colors along the back, grading to silver and white below.
Are you ready for the entire fish?
Joe caught some Pacific Jack Mackerel (Trachurus symmetricus) in Bodega Harbor today. At first they look similar to true mackerel, but look for two major differences: (1) jack mackerels lack the strong black wavy patterning along their backs that are prominent in true mackerel, and (2) they also lack "finlets" between the dorsal fin and the tail. True mackerel (genus Scomber) are related to tunas, whereas jack mackerel are related to jacks and yellowtails.
I'm not sure how often Pacific Jack Mackerels are seen in the Bodega Bay area. If you've seen them before, I'd love to hear more about how often they're observed here.
P.S. I heard through the grapevine that Yellowtails (Seriola lalandi) — a species more common in southern California — have been caught off Shelter Cove and Tomales Point recently. See one story and photo here.