MINUSCULE larvae seem helpless in the face of massive ocean currents. Yet it turns out some of them don鈥檛 just drift with the currents, but instead stay surprisingly close to home. The finding could help marine biologists persuade local authorities and fishermen that it is in their interests to establish marine reserves.
One of the most important factors for marine management policies is knowing how far the larvae of fish, crustaceans and algae are spread by ocean currents. 鈥淲e don鈥檛 know what the dispersal is for virtually any commercially important fishery species,鈥� says Stephen Palumbi of Stanford University鈥檚 Hopkins Marine Station in California. 鈥淥ur ignorance is actually so vast that we don鈥檛 know the dispersal of these species to within orders of magnitude.鈥�
But a consortium called the Partnership for Interdisciplinary Studies of Coastal Oceans (PISCO) is starting to tackle this problem. As part of the project, Palumbi鈥檚 team has been collecting barnacle larvae along the western coast of the US. The free-swimming larval stage lasts for three weeks, and calculations based on ocean current speeds suggest that the larvae should move hundreds of kilometres in this time.
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To the researchers鈥� surprise, genetic analyses revealed that the larvae do not venture far from home. For instance, in a giant circular current called the Heceta gyre off Oregon, Palumbi found barnacle populations that are genetically distinct from those to the north and south. This means that the dispersal range of larvae must be just tens of kilometres.
Similar results are being obtained by Robert Warner of the University of California, Santa Barbara. His team is studying otoliths, small bones found in the inner ear of fishes and some invertebrates that form layer by layer throughout an organism鈥檚 life. Trace metals such as barium are incorporated into each layer as the otolith grows, reflecting the metal鈥檚 concentration in seawater. By comparing the metal鈥檚 concentrations in the bone layers with those in regions of seawater, the team can track where larvae were born and how far they have migrated.
Their studies of rockfish and molluscs also show that larvae do not travel very far. 鈥淭he rate of spread of most species is surprisingly low, sometimes several orders of magnitude lower than suggested by particle dispersal by currents,鈥� says Warner.
Other PISCO studies are revealing how larvae avoid being swept away by the currents. Margaret McManus鈥檚 team at the University of California, Santa Cruz, has developed an acoustic technique for locating and identifying larvae. A device on the ocean floor sends out sound waves at frequencies ranging from 265 to 3000 kilohertz. The ultrasound waves that bounce off the larvae reveal not only their location but also which species are present.
The technique is being used to study rockfish larvae in Monterey Bay off California, where the coastal waters are characterised by 鈥渦pwellings鈥�, as warm surface waters move away from the shore and cold, nutrient-rich water rises to the surface. The team has found that rockfish larvae accumulate at the boundary layer between the warm and cold currents, where the temperature is around 13 掳C.
鈥淭he larvae are pinpointing this narrow temperature range,鈥� says McManus. 鈥淭heir food source is settling down on these temperature discontinuities, and the larvae are swimming in to feed on them.鈥�
Divers have confirmed that there are nearly five times as many organisms in this boundary layer as there are in the faster-flowing waters above and below it. Such behaviour explains why larvae do not disperse as widely as expected, Palumbi says. 鈥淚t indicates that they are a lot smarter than we give them credit for,鈥� he says. 鈥淭hey are not just passive particles being wafted around but are able to control their tiny little fates.鈥�
These findings suggest that even small marine reserves will be sustainable, as some larvae will manage to stay within the reserve rather than being swept away to unprotected areas. And this knowledge is already influencing policy. Last October, a network of small marine reserves was created within the Channel Islands National Marine Sanctuary off southern California. Researchers expect these reserves to keep themselves replenished as well as occasionally seeding each other with larvae.
More importantly, the discoveries may help conservationists to convince local authorities and fishermen of the benefits of setting aside protected areas. Previously, there was always a worry that local reserves would only benefit the distant areas to which larvae were carried. But it is clear now that local conservation can have local benefits. 鈥淚ndividuals who give up something may be able to get something back,鈥� says Warner.
While all the studies so far have been done off the US west coast, researchers expect the same will apply to many coastal areas around the world. But experts caution against taking the new ideas too far. 鈥淓veryone is going to start thinking that all marine larvae disperse short distances,鈥� says Larry Crowder of the Duke University Marine Laboratory in North Carolina. 鈥淭hat would be wrong. We don鈥檛 know that yet.鈥�