Little Fish with a Big Job
Surf smelt may be small, but it plays a big role in the marine food web in the North Pacific ocean ecosystem. Like herring and sand lance, surf smelt is a type of forage fish – small, schooling fish that eat microscopic organisms and are consumed by larger predators. Surf smelt provides an important food source for sea birds, marine mammals and other fish, including chinook and coho salmon. Many people also eat surf smelt, which supports recreational and commercial fisheries in many areas of Washington.
The early life of surf smelt – specifically how they spend their energy in their larval and embryonic stages – can affect their growth, survival and reproduction. Surf smelt rely on unaltered shorelines, where they deposit their eggs on gravel beaches during high tides. Once fertilized, the eggs are exposed to both air and sea water, experiencing a range of environmental conditions as the tide comes and goes. During this time, surf smelt embryos consume energy from the yolk and lipid droplets in their eggs until they hatch. The tiny, newly hatched smelt larvae, swimming in the nearshore water, continue to rely on internal energy reserves from their yolk sac and oil globule until they begin external feeding. However, physical habitat conditions – which are changing as the climate warms – influence how young surf smelt spend their precious energy reserves and whether they survive their early days.
Surf smelt is an understudied forage fish and very little is known about its response to climate stressors like warming and acidifying ocean waters, despite its critical role in the marine food web. Since reductions in surf smelt populations could cause a cascade of negative effects on the North Pacific marine food web, there is a pressing need to understand how climate change may affect this fish and what managers can do to help it adapt to a warming climate.
Understanding How Climate Stressors Affect Young Surf Smelt
In response to this need, 2018-19 Northwest Climate Adaptation Science Center Research Fellow Megan Russell partnered with the Washington Department of Fish and Wildlife forage fish team to examine the combined effects of warming ocean temperatures and ocean acidification on the energy expenditures of surf smelt during its early life stages. The research team measured the energy spent by surf smelt embryos and larvae under three temperature and two acidity scenarios. They did this by measuring heart rates and egg yolk depletion in smelt embryos and measuring yolk sac and oil globule depletion in smelt larvae.
Megan Russell, with Western Washington University co-authors Dr. Brady Olson and Dr. Brooke Love, describe this study’s findings in a new paper in the journal PLOS ONE. They found that elevated water temperature was a key driver affecting early life stages of surf smelt. Surf smelt embryos and larvae consumed the nutrients from their egg yolks and oil globules more rapidly under warmer water temperature conditions. For the larvae, the oil globule is not only an energy source for their metabolism, but also likely aids their buoyancy as they swim and start to feed externally. This means that in elevated water temperatures, surf smelts’ rapid consumption of the oil globule may lessen their buoyancy, hindering their ability to swim efficiently and thus capture food. For surf smelt embryos and larvae, the accelerated use of internal energy reserves under warmer water conditions could impact their ability to grow, survive and reproduce, potentially leading to declines in surf smelt populations.
Though warmer water temperatures were found to negatively affect surf smelt embryos and larvae, compared with a similar study, this research showed them to be relatively more tolerant to elevated temperature compared with other fish species. This is likely due to the wide-ranging conditions that surf smelt embryos experience during their time in the intertidal zone.
Researchers also found that elevated water acidity had little effect on early life stages of surf smelt. Embryos actually showed a slower metabolic rate in response to more acidic water. It’s likely that surf smelt tolerance to ocean acidity can also be attributed to the highly variable conditions, including PH levels, that they experience during their time as embryos in the intertidal zone.
Altered Shorelines Exacerbate Warming Effects on Surf Smelt
Rising water temperatures aren’t the only thing surf smelt need to worry about. In Puget Sound and beyond, increasing water temperatures from climate change are intensified by coastline development, which reduces shoreline vegetation and the shading it provides. Without shading, altered shorelines experience higher beach gravel temperatures than natural beaches. Shoreline development poses an immediate threat to surf smelt who depend on gravel beaches for spawning, while climate-induced warming will continue to compound this threat into the future.
More Research Needed on Surf Smelt Response to Climate Change
The results of this research help provide clarity to the Washington Department of Fish and Wildlife about the effects of warming and acidifying ocean waters on early life stages of surf smelt and how human activities such as shoreline development can exacerbate these climate stressors. This research also helps identify gaps in our knowledge — including how climate stressors will affect surf smelts’ later life stages and overall population abundance — supporting the need for more research on surf smelts’ role in ecosystem response to climate change.