Sunday, January 13, 2008

How do you know if fish are enjoying themselves?

I grew up close enough to the Jersey shore to have spent many hours submerged up to my nose in the Atlantic Ocean. If you tilt your head back underwater, you can see your reflection in the underside of the sea; a silvery, shimmery Neptune child gazes back.

I mostly bobbed up and down, nose sometimes in the water, sometimes out, pretending I was a salt-water crocodile. With eyes so close to the surface, the seaweed and broken reeds floating by loomed like large islands. When I turned away from the shore gazing eastward, I was the largest creature in the universe, not quite human anymore.

In New Jersey, what most of the world calls silversides or smelt, we call spearing. Menidia notata. They are mostly transluscent, no bigger than a pinky. Each side has a silver band that looks like smooth tin foil. They have straight jawlines that make them look rather glum close-up, but since we mostly saw them when threading them on a hook as bait, looking glum seemed appropriate.

Spearing travel in huge schools, almost invisible except for the occasional flash as the sun catches the silver. The surface sometimes erupted with them when a predator came underneath the school, but otherwise spearing had no particular reason to jump.

Or so the books will tell you.

One August afternoon, when I was 11 or 12ish, and I was busy conquering the seaworld, a piece of a phragmites reed drifted by. A tiny fish jumped over it. Odd.

I drifted closer to the reed, my eyes inches away. One fish, then another, their bands of tin flashing in the sun. I spotted the school just below the surface. I figured a few got too close to the reed, and jumped over it out of need. I continued to watch.

The fish jumping over the reed appeared to turn back. The school was mulling about in no particular direction. The fish were lining up to jump over the reed.

For the empiricists:

The individual subjects were observed approaching the reed at about 1 to 2 inches below the surface, then leaping about 1/2 inch before the reed, clearing it by no more than a 1/2 inch, then appeared to turn after reentering back into the water. The fish consistently approached the reed from the same side.

For the rest of you:

How do you know the fishes are enjoying themselves? They jump for no apparent reason over a randomly floating object on a lovely day when (for the moment) no predators were interested in them, when the water was not cloudy with the milt of spawn, and when they forgot a crocodile sea god was watching).

I observed this more than once, or so I remembered. I am old enough now confuse imagination and memory. The tao tale above says as much as needs to be said contemplating joy in fish, and the tale reminded me of my jumping spearing.

Then the tao met Google.

Had anybody ever reported seeing fish jump over reeds for no apparent reason (or at least for any reason apparent to humans, who have an unsatiable need for "reasons"). If you throw "fish" and "jumping" and "twig" together, you get a few hits. One of the hits is for an entry in Fish-Sci.

Fish-Sci is a listserv, a "scientific forum on fish and fisheries." On it fish biologists carry out long, serious conversations about, well, fish. You will find discussions on "otoliths in dolphinfish", "iron content in adult eel", and "fish biomass estimates for oligotrophic systems," all within the past 6 months.

The inquiry started innocently--Randy E. Edwards, Ph.D. and principal scientist for the Center for Coastal Geology needed to present a poster to International Symposium on Sturgeon in Oskosh, WI back in 2001. His question was simple: why do Gulf sturgeon jump? In his thoughtful letter, he listed numerous known reasons why fish jump.

A number of hypotheses have been brought forward to explain jumping behavior and include: parasite shedding, startle reflex, behavioral communication (to alert other individuals of their presence), to help shed eggs during spawning, nuptial behavior, and air gulping or swim bladder adjustment. .... Gulf sturgeon jumping is not temporally random, but instead is concentrated in the early morning and late afternoon. Why mullet jump (often in the same habitats as sturgeon) is not known.

The resulting discussion takes on a dance worthy of Albert the Alligator and company in Pogo. Fish apparently jump, at times, for no discernible reason.

Ivor Growns, a scientist with the Australian government, dodged the issue with an anecdote:

On a lighter note, I have heard of a member of the public sending a letter to their local parlimentarian asking why fish jumped. The minister asked for an explaination [sic] from the Fisheries department. The staff member sent back a reply saying "Because they are happy".

Another scientist, Glenn Crossin, a salmon specialist for Centre for Applied Conservation Biology in Vancouver, Canada, notes that sockeye salmon expend tremendous amounts of energy getting to their spawning grounds, yet when they get there, spend two weeks jumping and wasting energy.

Energetically one might think that this would be a risky behavior. Salmon typically expend most of thier [sic] fixed somatic energy reserves (mostly lipid) just reaching the spawning ground. Thus to expend limited energy unnecessarily, particularly when their one and only spawning opportunity lays ahead, seems risky.

When he asked his 9 year old nephew what he thought, the child answered "maybe they are just so happy to have made it there."

Ha-ha, kids are cute, let's get back to science.

Dr. Rodney Rountree is a scientist. He has a Ph.D., he teaches at the University of Massachusetts, he knows fish. He finally said what the others were skirting:

Fish likely jump for a lot of reasons, but I've often observed fishes jumping for no obvious reason (i.e., no predators or feeding behavior). I've often felt that the often cited purpose of jumping as an effort to dislodge external parasites (e.g., ocean sunfish) seemed inadequate. I even admit to thinking that some fish are just playing after on many different occasions watching Atlantic silversides (Menidia menidia) jumping over floating twigs over and over again. It sure seemed like a game.... The jumping fish never made contact with the twig, which might be expected if they were trying to rub off a parasite or scratch an itch.

Spearing like to play. Or at least it's a reasonable hypothesis.

I wonder what else I taught myself to forget.


FISH-SCI archives, June, 2001,
Personal observations and a ragged memory

1 comment:

momomom said...

I once saw squid line up and march in formation...sort of. I was snorkeling and they were watching me. If I moved forward the line moved back to maintain the same distance. OK, that makes sense, BUT if I moved back they also followed me maintaining the same distance as well as maintaining their line formation!