The Puzzle of Puget Sound Herring
Washington Sea Grant-funded Scientists Examine the Genetic Relationships
Among Regional Herring Stocks
By Jeff Bowman, Washington Sea Grant Science Writing Fellow
When most people picture herring, they see jars of small, pickled tidbits sold in the grocery store. But when University of Washington scientist Lorenz Hauser and graduate student Danielle Mitchell think of these silvery schooling creatures, they see a living model of how fish populations are structured in Puget Sound.
With support from Washington Sea Grant, Hauser and Mitchell have developed a way to identify unique stocks of herring in Puget Sound. With this information, they hope to answer some important questions. Perhaps the most puzzling of these is why a distinct stock of herring at Cherry Point, north of Bellingham, is in decline while other stocks at other locales are increasing.
Like other estuaries, Puget Sound supports a vast and complex ecosystem. From tiny phytoplankton at the base of the food web to large predators such as salmon and orcas at the web’s top, many species are uniquely linked by food preferences and habitat needs. Scientists have long thought this kind of interdependence represented a natural system of checks and balances. For example, if any one species is pressured through overfishing, water pollution or natural events, other species are present to take its place. However, within the Puget Sound ecosystem, there are several keystone species — fish, birds or marine mammals that occupy unique niches between the bottom of the food pyramid and its top.
Puget Sound’s herring are among these keystone species. They feed on copepods and other small marine invertebrates and, in turn, are preyed on by salmon, seabirds, spiny dogfish, seals and other large predators. No other species in Puget Sound occupies the exact same position in the food web. As a result, the entire Puget Sound ecosystem is dependent on the health of local herring.
It stands to reason that scientists and fisheries managers would have an interest in maintaining healthy herring populations. However, because of the way that herring and other fish populations are structured, this has not been an easy task. Puget Sound herring are divided among 18 distinct stocks, with each stock spawning in the nearshore environment of a specific locale. The rest of the year, the stocks mingle freely throughout Puget Sound and beyond, making it difficult to monitor the health of any single stock of herring.
Take the example of the Sound’s Cherry Point herring stock. Cherry Point herring were once Puget Sound’s most numerous stock. However, this population has declined by as much as 90 percent in recent years. These fish spawn near the mouth of the Fraser River, at a later time in the year than any other stock in Puget Sound.
Did the fish really disappear or did “missing” Cherry Point herring simply join stocks elsewhere in the Sound? Scientists had long noted that decreasing numbers of herring at one spawning site are often matched by increasing numbers at other sites. This suggested that migration, not a low survival rate, might account for the Cherry Point herring’s decline. To confirm this theory, scientists needed a way to determine the rate of migration among Puget Sound’s herring stocks.
For Hauser and Mitchell, the best way to track herring migrations was by comparing herring microsatellite DNA markers — a class of noncoding “junk” DNA, which consists of short, repetitive DNA sequences that vary in length because of transcription errors during cell division. The microsatellites perform no real function, neither helping nor hindering an organism. As such, they are largely unaffected by selection pressures. Geographically isolated subpopulations develop different microsatellite frequency distributions — a fact that enables scientists to determine who’s who among fish populations. The more isolated a stock of herring is, the more distinct its members’ microsatellite markers will be. Herring stocks that recruit members from neighboring stocks have less distinct markers.
Working with the Washington Department of Fish and Wildlife’s Kurt Stick and Greg Bargmann, Hauser and Mitchell captured herring from places in the Sound the fish are known to congregate prior to spawning. While studying these fish’s microsatellite DNA, the two researchers learned that most Puget Sound herring stocks were indistinguishable, indicating that plenty of migration occurs among the members of individual stocks. However, the Cherry Point herring stocks were genetically divergent — a finding supported by an earlier study conducted by Maureen Small and others at WDFW.
Hauser and Mitchell also report that a small, genetically distinct population identified by Small at Squaxin Pass could not be found. Instead, the fish they collected at Squaxin Pass appeared to originate from stocks further north. This finding supports the idea that some of the fluctuations in herring numbers in southern Puget Sound may be due to migration. The Squaxin Pass stock may have completely disappeared between 2002 and 2005, only to be “reborn” through migration from neighboring stocks. Hauser and Mitchell are currently analyzing new samples to test this theory.
Evidence from Hauser’s and Mitchell’s studies suggests that herring in Puget Sound exist as a complex of ever-changing stocks. Some of these stocks, such as the one at Cherry Point, are isolated. Others, such as the Squaxin Pass stock, may decline or disappear only to be replaced by migrants from other herring stocks.
This system appears to be working for herring populations as a whole. When one stock is in trouble, migrants from another stock may reinvigorate it. Still, the fish’s tendency to migrate makes management of herring and their spawning habitats difficult, because migration masks potential environmental problems within a single stock. And while migrants from other stocks can make up for losses of herring from a declining stock, this strategy does not compensate for any loss in genetic diversity.
“Estuaries are not static environments,” Hauser explains. “It may be that each stock has slightly different adaptations to its environment. As conditions change, regional herring populations can make use of this stockpile of adaptive strategies to survive, and currently less important stocks may become major components in the stock complex. This ‘biocomplexity’ has been shown previously in Alaskan sockeye salmon, but is undervalued in marine fishes. Support by Washington Sea Grant is crucial to improve our understanding of such interactions in marine ecosystems.”
Hauser plans to continue working to solve the herring population riddle. Currently, he and Mitchell are analyzing the data from the 2007 season and intend to publish the findings.
Contact David G. Gordon, Science Writer for Washington Sea Grant, for further information.