An Australian zoologist now at the University of Melbourne, along with colleagues from the United Kingdom and New Zealand, was the first to record a decline in the genetic diversity of a commercially exploited marine species. Their findings, published in the latest volume of the Proceedings of the National Academy of Sciences, shout a warning that could force a rethink to current fisheries management and the research focus into sustainable fishing.
Dr Greg Adcock analysed the DNA found in scales preserved from two populations of New Zealand snapper collected from the 1950s to 1998. One population had been commercially fished since the late 1800s. The other was a 'virgin' population, being subjected to subsistence and recreational fishing only until the scale collection began.
Adcock and colleagues found that the 'virgin' population from Tasman Bay on New Zealand's South Island had suffered an unexpected decline in genetic diversity, starting from the time it began to be commercially exploited in the 1950s.
The other population, from the North Island's Hauraki Bay, showed no decline in genetic diversity in the nearly 50 years to 1998.
The paper reports that the Tasman Bay's effective population size (the number of fish in the population capable of breeding) is100,000 times fewer than its total number, and several orders of magnitude lower than expected.
"In Tasman Bay, commercial fishing has often reduced total numbers to as low as about one million. This leaves only a few hundred fish to contribute to the next generation, a dangerously low genetic base from which to sustain a population," says Adcock.
"With a high effective population you can retain a large amount of rare genetic variation. Such variation is lost as numbers decline. A rare variant may not play a significant role in the current environment, but if a fish population loses a large number of these genes, such as happened in Tasman Bay, they risk losing the ability to adapt to changes such as global warming, pollution and human induced changes to predator and prey populations," he says.
Adcock points to recent assertions that ocean warming is suspected of causing recruitment failure of cold-adapted North Sea cod.
"Until now nobody suspected that any loss of diversity was happening as it was thought that even in over-fished populations where their numbers are still be in the millions, that there would still be a sufficiently large effective population to prevent declines in genetic diversity," says Adcock.
"A population of several million may actually be in danger of losing genetic variability, which may have long-term consequences," he says.
"Genetic diversity should become a management consideration in many exploited marine species. Many fully exploited or over-fished stocks may be already suffering loss of diversity.
"We don't know yet the minimal level of genetic diversity required to sustain a commercial fishery long-term, but there is enough evidence now to suggest we need to be cautious and begin to reassess our understanding of fishery management and the sustainability of the industry."
To assess the loss of genetic diversity, Adcock and his colleagues studied seven regions of the snapper's chromosomes, known as microsatellite loci, which are highly variable and mutate at high rates.
The high rates of mutation in microsatellites produce the levels of variation required for researchers to work out how long ago two or more populations or species diverged from a common population or ancestor. In this case, Adcock and colleagues used this variation to assess the changes in genetic diversity over time.
The Tasman Bay population showed a significant decline in diversity in six of the seven loci.
To explain why Hauraki Gulf failed to show any loss of genetic diversity, Adcock contends that the genetic variation had already been lost in the early years of intensive fishing, prior to 1950.
"Hauraki Gulf is a larger population than Tasman Bay and should naturally retain more genetic variation. When the study began, however, its variation was lower than Tasman Bay's," he says.
Adcock believes the findings open up exciting possibilities of further research and collaboration with the various fishing industry bodies.
"A close collaboration between fishery biologists, geneticists and the fishing industry would be required to carry out research into the biology and behaviour of marine species and their possible implications for fisheries management and conservation," he says.
Adcock's colleagues from Hull University (UK) were Lorenz Hauser (now at the University of Washington), Julio Bernal Ramirez and Gary Carvalho, and from New Zealand, Peter Smith of the National Institute of Water and Atmospheric Research.