Shark nursery?

[vjongene]

I was diving last summer on Rongelap Atoll (Marshall Islands) and witnessed a most amazing sight: about 30 juvenile grey reef sharks swimming about, with a few adults making circles around the group. This was in a shallow pass (about 10 m) between the ocean and the lagoon. We were told that this group is always there.

It looked for the life of me like a "shark nursery", i.e. a protected place for juveniles to feed under the supervision of adults. I had never heard of such a thing.

I have posted a short video showing the behavior here. I would very much appreciate any information about observations of similar shark groups elsewhere, and a possible explanation for this behavior.

Victor J.

 

[alcina]

Sounds like it to me. We get the same thing in a shallow bay here on the Ningaloo Reef - usually November through January. Grey reefs and tigers mostly.

 

[archman]

Cool video. And while my ichthyological knowledge often falls short, I've never heard or read about sharks conferring parental care to hatched offspring. You should submit a query to an expert, and see what he/she says.

Those juveniles are pretty big. If it had been me observing this, the "parental supervision" theory would never have occurred to me. I'd have chalked it up as schooling behavior of same-sized juveniles (which happens a lot in immature species that don't display much schooling as adults), with the bigger animals hanging around on the periphery as a instinctual relic.

Other theories for the adults floating nearby would be:
1. They want to eat the younger animals
2. They're exhibiting a derivation of "shadow (or nuclear) feeding"

Nothing much eats sharks when they get as big as those juveniles, 'cept other sharks and very big sea bass. Thus there's no benefit to protecting them. Sharks don't display social altruism anyhow.

 

[vjongene]

Cool video. And while my ichthyological knowledge often falls short, I've never heard or read about sharks conferring parental care to hatched offspring. You should submit a query to an expert, and see what he/she says.

Those juveniles are pretty big. If it had been me observing this, the "parental supervision" theory would never have occurred to me. I'd have chalked it up as schooling behavior of same-sized juveniles (which happens a lot in immature species that don't display much schooling as adults), with the bigger animals hanging around on the periphery as a instinctual relic.

Other theories for the adults floating nearby would be:
1. They want to eat the younger animals
2. They're exhibiting a derivation of "shadow (or nuclear) feeding"

Nothing much eats sharks when they get as big as those juveniles, 'cept other sharks and very big sea bass. Thus there's no benefit to protecting them. Sharks don't display social altruism anyhow.

Thanks for sharing, archman. I didn't see the adults attacking the juveniles over about 15 min of observation, but this doesn't mean they don't occasionally try. Your explanations make perfect sense. I don't know any real experts in ichtyology, and such people as I consider knowledgeable had never heard of this kind of behavior. It is kind of hard to see on the video, but the juveniles were anywhere between about 1 and 3 feet long.

The shark density on Rongelap is such that one often sees them in sizable groups, although the adults don't really show schooling behavior. Maybe the juveniles do. This is the only place we saw them, though.

 

[archman]

Hey, why don't you email Sam Gruber or one of his staff? They're experts in this area, and have their own facility in Bimini. Who knows, maybe they'll be fascinated.

http://www.miami.edu/sharklab/contactus.html

 

[mano kihikihi]

Sounds very much like you have come across a pupping ground for that species. We have a few of these in Maui. Areas that adult sharks frequent ( here Grey and White tip) and very often give birth to their young. Quite often you can witness multiple generations in one area and watch them over the years as they grow, some stay, some leave. The adults I feel fairly confident in saying are not protecting or nurturing the young. The adults will usually stay some what deeper and I have often seen them go after the young and eat the young on a few occasions. The area I speak of in Maui the young are in the 80-100ft depth while the adults are 130+ and the area is a current swept drop off allowing the sharks to lounge in the oncoming water. Definitely a place to visit overand over and seeif you can recognise individuals thru their scarring and hopefully catch a mating, it is an amazingly violent but beautiful thing!