It has been suggested that two main factors limit geoduck growth- food density and temperature (Strom, 2003). It is still unknown how geoduck's food source will fare with rising ocean temperatures. Robinson and Wane (1999) found in their study that even with a 30% reduction in diatom abundance in the Stait of Juan de Fuca the local geoduck population experienced the highest rates of shell accretion in 155 years (Strom et. al., 2004). There have not been many other studies analyzing the impacts of food abundance on geoduck populations so Robinson and Wane's results should be used with caution. On the other hand, several studies have demonstrated that geoducks will actually benefit from warmer ocean temperatures.
The map to the left (A) illustrates gridded average sea surface temperatures increases during the 1950-1997 time period (higher numbers denoted in red represent warmer temperatures). The bottom diagram (B) illustrates geoduck growth indexes. This figure demonstrates that increased sea surface temperatures over the documented 47 year study are correlated with gains in geoduck growth. The variation in geoduck growth rates in the bottom figure is due to various environmental parameters. Increased temperatures are also accompanied by greater stratification in the water column, reduced upwelling, and lower nutrient content, which can create fluctuations in geoduck growth during a short time period (Strom et. al., 2004).
http://www.agu.org/journals/gl/gl0406/2004GL019440/
These increasing sea surface temperatures could extend the geoduck's habitat range farther north into waters that were previously too cold to support these organisms (Brickey et. al., 2012). This could be a result of geoduck recruitment patterns being correlated with environmental parameters, such as sea surface temperature (Straus et. al., 2008). During years with higher sea surface temperatures geoduck populations had greater recruitment success and therefore were able to establish habitat in higher latitudes (Valero et. al., 2004). Projected warmer ocean waters in the future could therefore be beneficial for geoduck populations because they could have higher rates of recruitment success in previous unsuitable habitat. This remains a viable hypothesis but more research needs to be conducted to further determine this.
http://www.funnytimes.com/playground/gallery.php?tag=ice#.T7rzD9XYjw0
Disease also plays a factor in the geoduck's future success (or lack thereof). High host density
increases contact rates between individuals, increasing the ability of disease to spread quickly (Straus, 2008).
Higher oceanic temperatures also facilitates the spread of shellfish
disease outbreaks, such as the Delaware Bay Disease (Straus, 2008). It is still unclear
if local pathogens and parasites will have a large impact on geoduck
populations with further climate change but remains a concern among scientists and therefore should continually be monitored.
Extensive research has been done to determine the biological impacts of climate change on geoduck populations. While trends thus far demonstrate that geoduck fitness has not declined (but may have actually increased) much is still left unknown about how geoducks will physiologically respond to warmer and more acidic oceans in the future. Will their food source decrease due to greater physiological heat stress with rising ocean temperatures? Will their predators, such as sea otters, thrive with a changing climate and therefore reduce geoduck populations? More research needs to be conducted to further develop our understanding of geoduck fitness in changing oceans.
What is fairly certain is that any shift in geoduck population size, albeit from disease or a change in food source stability, could have large impacts on Washington state's economy. Click on the next page to learn about the economic significance of geoduck harvesting.
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