Why do fish jump out of Mission Bay?

Dear Matt:

My boyfriend and I recently had a post-sunset picnic at Mission Bay. We observed fish jumping out of the water (really!). I thought they were jumping out and flipping down to scratch themselves. The boyfriend thought they might be eating little bugs Who's right? Or were we imagining the whole thing?

-- Something's Fishy, O.B.

From the Hip Matthew:

I have lived in San Diego all my life. I am sitting at the MB Aquatic Center picnic bench. Can you explain why it is that the fish in Mission Bay jump up aerobically and splash/dive back down under? This only occurs at about 4:30 to 6 p.m. Especially when it's quiet and calm, without jet skier activity.

-- Unsigned, Mission Bay

Imagining the whole thing? Heck, no. There's even a famous song about it. "Summertime, and the livin' is easy./ Fish are jumpin', and the cotton is high." Or anyway, down at the beach you can bet something is high. The elves dusted off their accordions and composed an answer. And it goes something like this:

(Plaintive chorus of bewildered, child-like voices)

Mission Bay, and the livin' is groovy.

Fish are jumpin'. Can you please tell us why?

Matthew Alice, we ain't half as smart as you are.

Tell us, and we'll each bake you a peach pie.

(Patronizing baritone)

Mission Bay, and the water is funky.

More beach closures every day, every night.

so maybe fish are just tryin' to jump outta there?

Good guess, but we can't say that's it's right.

Dinnertime, and the air's very buggy

Right at sunset, when the wind doesn't blow.

Gnats can hover; fish can see, jump, and eat 'em.

And we can't believe that you didn't know.

Hey, Mullethead!

Highly Esteemed Matthew:

I was truly disappointed in your answer to the jumping fish of Mission Bay question, but elated that after all these years I finally know something that you do not!! I am a tour guide on the Old Town Trolley SEAL Tour, an amphibious vehicle that tours both San Diego Bay and Mission Bay. During runs on Mission Bay, [I've witnessed] as many as thirty fish jumping out of the water in five minutes time.... The fish are striped mullets. They live in the shallow coastal of the Pacific Ocean and the Gulf of Mexico and are famous for their jumping. They jump over the lines of fishermen, and once caught, they jump out of the net!! No elaborate mating dance, no effect of the moon, no parasite problems, no other fish chasing them, not just catching bugs, or escaping funky water. They are just born to jump!!! They do this everywhere!

-- Whoopsea (aka Carolyn Dougharty), at sea

They certainly are mullets (cousins to grunion). They jump for some mystical mullet reason. For the sheer joy of being a mullet, apparently.