Marine populations are not what they once were. Centuries of exploitation by humans have reduced populations of many species to very low levels, and have driven a series of once abundant species to commercial extinction. Management of these species, and management of ocean ecosystems in which these species once played a critical role, requires firm knowledge of past population sizes. But these data are very difficult to obtain.
Historic records, fishing logs, shell middens, fossil beds and environmental correlations have all given us information about particular species when these particular records happen to be available. But there is an emerging scientific tool that has the potential to provide historical information about the population size and sizes changes of virtually any species. Written into the DNA of every population or every species is a veiled record of the past population size of that species, and the kinds of population changes that have occurred in the past. Reading this record for any one species is difficult, but new genomic and analytical tools promise to make this task easier and easier in the future. Most powerfully, these new tools can be compared with data carefully extracted and analyzed from the historical record. Because these sources provide data with different strengths, they can be used in combination to offer the best picture of the past of marine populations.
We have begun applying this tool to the study of past populations of whales in order to answer the question: How many whales were there before whaling? The basic approach is to measure the amount of genetic diversity of current populations and use knowledge of DNA mutation rates to estimate how many individuals a population must sustain over time to accumulate the measured diversity. This method has been used to measure effective population size and population trajectory of terrestrial animals such as bison, penguins and even humans but has not been applied broadly to marine species.
Our recent work has shown that baleen whales in the North Atlantic ocean have an exceedingly large amount of genetic diversity - so much so that genetic estimates of pre-whaling population size are up to 10 times higher than those suggested by analysis of historical documents and current abundance estimates. Discrepancies for North Atlantic minke whales (3 fold) are lower than for fin or humpback whales (10 fold). The differences between genetic and document-current abundance based estimates of pre-whaling abundance for these species have sparked a re-evaluation of both historical whaling documents as a source of historical whale removal information and the interpretation of genetic variation in terms of historical population reconstruction. The discrepancies between genetic and document-current abundance approaches are unsettling, and the International Whaling Commission has raised several concerns about the management implications of genetic variability data, suggesting that the differences between conventional and genetic population sizes need to be reconciled.
To address these questions requires a focus on species for which multiple, powerful data sets on genetics and log-book records can be developed. The most effective way to accomplish this goal is to identify whale populations for which targeted empirical research can finalize both genetic and historical-document data sets. In addition, to assure generality, our choice of research topics needs to cover a range of species and a variety of ocean basins. To this end, we have identified a set of whale species and ocean basins that will provide genetic, historical document, and current abundance estimates. These new data will allow us to complete modeling efforts that will advance the science of historical whale population biology to a great degree. The efforts will include work in the Atlantic, Pacific and Southern Oceans, and include species that are highly threatened, recovered, or currently hunted under scientific whaling. A central collaboration among resource historians, geneticists, and modelers is key to the success of these efforts. We also will include program advisors close to the international management of whales who will help place these new research efforts in context of the continuously evolving efforts of the International Whaling Commission to manage whaling.
We used genetic data from nine nuclear introns and two mitochondrial markers to determine pre-whaling population size in eastern Pacific gray whales, which were previously thought to have recovered to their original population size, and found that the average abundance in the past was 96,000 individuals, approximately 3-5 times that of today's population. Coalescent simulations show that our estimates are a measure of the population of the entire Pacific, including the currently endangered western Pacific population.
This larger population of gray whales would have had important impacts on Arctic and sub-Arctic nearshore ecosystems. Gray whales feed in the benthos and we estimate that the amount of benthic matter resuspended by the larger population would have been twelve times the sedimentary input of the Yukon River. Seabirds are frequently observed foraging on prey brought to the surface by gray whales, and we estimated that the larger population of gray whales would have provided supplemental food for a million seabirds a year. These results suggest that the diminished population of gray whales has had significant implications for Pacific ocean ecosystems. Additional work is badly needed to quantify pre-whaling population ecology in gray whales, and to document the ecosystem impacts of marine mammal declines.
Current work is focused on using ancient DNA samples to estimate pre-whaling population dynamics in the eastern Pacific population. Ancient samples can provide direct evidence of historical population expansions and bottlenecks, and will be useful for understanding the relative impacts of climate and human hunting on whale populations. We are working in collaboration with Tim Smith at the University of New Hampshire and Randy Reeves at Okapi Wildlife Associates who are assisting with analysis of whaling logbooks. In addition Scott Baker's lab at Oregon State University's Hatfield Marine Science Center is working on global humpback genetics and the genetics of southern minke whale populations.