Field notes: Are Puget Sound herring limited by loss of eelgrass?

Could recent declines in Puget Sound herring be linked to decreases in native eelgrass? Biologist Tessa Francis reports on a new study that may provide insight into the health of one of the region's most iconic forage fish.

Closeup of herring spawn on kelp.
Closeup of herring spawn on kelp.

On a sunny day in early spring, our research boat slowly approaches a Hood Canal shoreline. A raft of hundreds of seabirds – scoters, gulls, ducks – floats on the water. As our boat nears, the birds take to the air, close enough that we can hear the slapping of webbed feet and the whistling of wings. Dozens of dark, shiny sea lion torsos slide in and out of the shallows. All around the boat are teeming thousands of small, silvery fish, schooling in three feet of clear water. Half-buried in the sand, starry flounders (sandy-grey flatfish) skitter away when disturbed. Rooted to rocks, sand and mud on the shallow bottom, seaweed and seagrass lay bent over from the weight of attached fish eggs. On the seaweed crawl tiny stick-figure caprellid amphipods, picking their way across the egg masses. Gulls pick at herring eggs attached to seaweed on the shore, exposed by the receding tide. Nearly every tree near the water's edge bears a bald eagle on its branches. A pair of deer skid down a cliff to reach the shoreline.

This is a Puget Sound herring ecosystem in the middle of spawning season.

Quilcene Bay shoreline.Pacific herring (Clupea pallasii) were once abundant in Puget Sound, but in recent decades their numbers have been in decline (Essington et al. 2011). The precise causes of this decline are not yet known, but we are here to test a hypothesis. Could it have something to do with the decline of another vital component of the Puget Sound ecosystem— native eelgrass (Zostera marina L.)? Herring are known to use eelgrass as a spawning substrate, and our study at the University of Washington Puget Sound Institute, in conjunction with partners at NOAA’s Northwest Fisheries Science Center and Washington State Department of Fish and Wildlife (WDFW), looks at this possible connection.

Pacific herring are small, schooling fish that belong to the group known as forage fish, which includes other species such as anchovies, sardines, eulachon, surf smelt and sandlance. Birds at Cherry Point.Forage fish are critically important species, providing a number of ecological, cultural, and economic services to their associated ecosystems. They play a key role in marine food webs, linking microscopic zooplankton prey to higher trophic level species. Herring dine primarily on zooplankton, and in turn are a preferred prey for larger marine species, including Pacific salmon, marine birds, harbor seals and sea lions. This link is important because much of the carbon and energy available in marine systems is found in very small organisms, otherwise unavailable to larger predators. Not only are herring themselves a popular prey item, but their eggs are too. Diving marine birds such as scoters eat herring eggs attached to nearshore vegetation, and herring eggs are regularly found in the stomachs of other fishes that use the nearshore habitat. Humans have also come to the table. Before the recent declines, Puget Sound herring supported a large commercial fishery, providing benefit to the regional economy, and herring are of cultural and economic benefit to native tribal communities that harvest them around Puget Sound.

Read more about eelgrass habitat in the Encyclopedia's Science Review.

Because of their critical importance in the coupled human-natural Puget Sound ecosystem, we are interested in understanding what limits herring recovery. Nearshore habitats are particularly sensitive to human activities, such as residential and industrial development of shorelines and connected uplands. It may be that such activity limits the ability of nearshore habitats to support the organisms that rely on these shallow, well-lit, diverse, complex, productive areas fringing Puget Sound. Herring rely upon nearshore habitats during their early life stages: embryos are attached to rooted subtidal vegetation, and larvae are protected from predation by the refuge offered by complex nearshore habitat. Because of this dependence, changes to the nearshore may affect herring populations if they are limited in their early life stages. Therefore, we are investigating whether the loss of eelgrass – one major change to nearshore habitats that has been observed in areas of Puget Sound (Essington et al. 2011) – is a barrier to the recovery of herring.

We are measuring egg loss rates – how many eggs are left at the time of hatching compared to how many were initially spawned – on eelgrass and other spawning substrates at multiple herring spawning sites around Puget Sound. Egg loss can be caused by a number of factors, including predation, exposure, wave action, and natural mortality. We expect to learn whether egg survival rates are higher on eelgrass as compared to other spawning substrates. In other words, we will determine whether eelgrass is the highest quality spawning habitat for herring, or whether there are acceptable substitutes that can take the place of eelgrass that is being lost from areas of Puget Sound. One example is the invasive seaweed species Sargassum muticum (“Japanese waterweed”), a very commonly-used surface for herring eggs, and we will determine the extent to which this exotic species is providing a key ecosystem service.


Herring spawn on kelp. Herring spawn on red algae. Herring spawn on red algae and Sargassum muticum. Herring spawn on red algae. Herring spawn on Sargassum muticum. Herring spawn on eelgrass.

(Photos by Tessa Francis.)

Simultaneously, we are assessing the availability of eelgrass around Puget Sound to determine whether, if eelgrass is indeed the highest-quality habitat for herring, there is enough of this habitat available for spawning herring. Is eelgrass at spawning sites used in the same proportion that it occurs, or is there eelgrass that goes unused by spawning herring? Together, these two pieces of information will tell us whether indeed herring are limited by the availability of eelgrass to use as spawning substrate. We are also measuring natural mortality rates of herring spawn at each of our study sites, so that we can account for intrinsic variation in egg mortality in comparing egg loss rates between sites  (see figure below).

Preliminary results showing hatch rates of Puget Sound Pacific herring embryos, across a gradient of urbanization. Embryos were collected from herring spawning sites in Puget Sound, then incubated in the laboratory. Error bars represent 2 S.E.s of the mean of hatch rates across 8 incubation tanks for each site.

Next steps

After we determine whether egg loss rates vary among different spawning substrates, our next step is to understand the sources of egg loss, and how they vary among sites and substrates. We will ask questions about the sources of herring egg predation, and whether herring eggs are more or less vulnerable to predation by birds, fish, or invertebrates on eelgrass versus other type of vegetation. We are also working with our partners at WDFW and NOAA to access a historical data set describing habitat characteristics at herring spawning sites, to determine whether large-scale changes in habitat over time are associated with declines in herring.

Connections to management

The Puget Sound Partnership has adopted a performance management framework (Neuman et al. 2009) to monitor implementation and effectiveness of their Action Agenda. A key aspect of this framework is the identification of specific targets for ecosystem components (Levin et al. 2010). Pacific herring and eelgrass are two ecosystem indicators identified by the Puget Sound Partnership, for which recovery targets have been developed. Developing strategies to achieve the herring target is hampered by a lack of specific information about what is limiting Pacific herring populations in Puget Sound. This study directly addresses this critical need.  View Puget Sound Vital Signs: Pacific Herring

Video: scenes from the herring study

Video by Paul Hillman (NOAA)


  • Essington, T., T. Klinger, T. Conway-Cranos, J. Buchanan, A. James, J. Kershner, I. Logan and J. West. April 2011. The Biophysical Condition of Puget Sound in Puget Sound Science Update, April 2011 version. Accessed from Puget Sound Partnership. Tacoma, Washington.
  • Levin, P.S., A. James, J. Kershner, S. O’Neill, T. Francis, J. Samhouri, C. Harvey, M.T. Brett and D. Schindler, 2010. Chapter 1A, Understanding Future and Desired System States in Puget Sound Science Update, April 2011 version. Accessed from Puget Sound Partnership.
About the Author: 
Tessa Francis is an aquatic ecologist at the University of Washington Puget Sound Institute, and her research is related to aquatic food webs, and the impacts of climate and other environmental variables on food-web dynamics. She is interested in the important associations between terrestrial and aquatic habitats, and how watershed and shoreline dynamics impact aquatic food webs and populations. She is engaged in projects related to forage fish and food webs in Puget Sound, including addressing key scientific gaps in the ecosystem-based management of Puget Sound forage fish, assessing the trade-offs associated with simultaneous recovery targets assigned to interacting marine species, modeling the movement of contaminants through pelagic food webs. Tessa holds a PhD in Biology from the University of Washington, a BS in Wildlife Science from the University of Washington and a BA in Political Science from UC Berkeley.