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Australia's prawn population rolls with the tide
By Environmental News Network staff
Researchers in Australia, knee-deep in the study of ocean currents and tidal movements, are turning up clues that promise to help sustain the country's $111 million northern prawn fishery.
"We are now a large step closer to locating the prime spawning grounds and predicting which part of the [prawn] stock is contributing to next year's harvest," said Scott Condie, a marine researcher at CSIRO, Australia's national science agency. "Management strategies can then be developed to protect this part of the stock and help ensure the sustainability of the fishery."
The northern prawn industry is Australia's second most lucrative fishery and one of the country's most valuable exports.
The industry has been carefully managed in recent years, but scientists still haven't been able to map the movement of prawns through their life cycle of spawning, hatching and migration to nursery grounds for maturation.
According to the CSIRO study, prawn larvae exploit the most predictable part of their environment — the tides— as a survival mechanism.
"Tiger prawns spawn in waters 20 to 30 meters (65 to 98 feet) deep, some distance from the coast. The young prawns then complete their early development in shallow, coastal seagrass beds that are only found in certain defined areas. Only those that reach the nurseries will survive to the following year," Condie explained.
The researchers noted behavioral changes in the prawn larvae with each incoming tide.
"We have shown that only prawn larvae that are in shallow enough water to detect the incoming tide will make it into the coast, and of these, only those that were spawned close to the nursery areas will survive to become a part of the harvest," Condie said. "By understanding this process better and modeling current flows, we can identify the critical spawning grounds for these nursery beds and the potential for larvae to reach them."
Condie said the research will also show which areas are unlikely to contribute to future stocks because they are too far from nursery beds or exposed to currents and tidal flows that carry larvae away from the beds.
"Prawn larvae recognize and respond to the change in pressure associated with incoming tides," said CSIRO researcher Neil Loneragan. "We are using seawater pressure chambers to study larval behavior and identify the critical water depth associated with behavioral changes."
"From the current flow models," Loneragan added, "we will be able to see which spawning grounds will produce larvae that are likely to be carried in ocean currents into shallow-enough water to detect the beginning of the incoming tide."
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