Tracking invisible fish

On White Rock beach, it was low tide. Unfortunately, we had not started out early enough, and the water was already racing towards us as we hurried out to the edge. I was hoping to get to the starfish/geoduck/crab grounds, but they were already too deep.

We waded out as far as we could still see the bottom; it was all drifting seaweed and smooth sand. And small, racing sculpins; as we splashed along, they would burst almost from under our feet and dash away, just far enough to vanish into the sand. We walked east, towards the outlet of the Little Campbell River, laughing as we tried to see the fish before they saw us, and always failing.

In shallower water, about ankle deep, the sculpins were joined by small sand soles, maybe 2 or 3 inches long.  We could only see them when they moved. Suddenly, one would appear just ahead, skate quickly along the bottom, and disappear. I tried to follow them with a finger, and point out to Laurie the place where one just had to be, even if neither of us could see it; he would try to get a photo. Difficult, because the camera could never discover what to focus on, and because when we got near enough the fish would be up and away.

We came home with many photos of sand, one photo of sole.

Can you see it?

Cropped, with the colour muted and the contrast jacked up, it's a little clearer:

Laurie caught it just at lift-off.

The first stickleback we saw was floating in the tide, recently dead.

Repeat of yesterday's photo. Three-spine stickleback.

Laurie saw two more dead fish; I wandered around looking for live ones. I think I saw several. I think.

All three species of fish did the same thing; lie invisibly on the sand until we were close, then scoot away. I couldn't always see the shape; just a speeding grey thing gone almost as soon as I'd found it. But I could soon recognize them by their swimming style.

The sculpins swam like tadpoles; the big head followed by a curved, wagging tail. They took a twisty route, usually an inch or so above the sand. A second later, they would sink straight down, wriggling to dig themselves a hole. For a minute or so they could still be seen, but soon the sand covered them completely.

What I thought were probably the sticklebacks zipped away in a straight line, well above the bottom. I never saw any settle; they swam away out of sight.

The sand soles also swam in straight lines. But they stayed right down at the sand surface. The flat tails flip up and down, so that they made a temporary dotted line of holes, each with its tiny puff of thrown sand. Where they stopped, a cloud of sand rose into the water and drifted away. That's where I would point; not at the fish, which was instantly hidden.

We wondered about the dead sticklebacks. The one I picked up didn't seem damaged in any way; what killed it?

A Wikipedia article may offer a clue.

Marine sticklebacks live two or three years, returning to freshwater to breed once only, in the spring, before they die.

...males defend territories where they build nests on the bottom of the pond or other body of water; ... Only the males care for the eggs once they are fertilised. Parental care is intense, involving nest maintenance and fanning of the eggs to ensure a fresh water supply, even at night. Males build the nests from vegetation, sand, pebbles and other debris, adhering the material together with spiggin, a proteinaceous glue-like substance secreted from the kidneys.

These fish were near the outlet of the Little Campbell river; freshwater where they can build nests. Could it be that their care of the young is finished by now, the beginning of our summer?