Munroe, D., Narvaez, D. A., Mann, R., & Klinck, J. (2016). Fishing and Bottom Water Temperature as Drivers of Change in Maximum Shell Length in Atlantic Surfclams ( Spisula solidissima ). Estuarine Coastal and Shelf Science, 170, 112–122. doi:10.1016/j.ecss.2016.01.009
We know that humans really can do a number on the environment. Our love of seafood has caused population crashes and ecosystem degradation. It turns out that it really is true when your grandparents say, “back in my day the seafood really was bigger!” Fish were quite a lot bigger back then (check out some amazing pictures of prize catches over the years in this NPR article). This study focuses on a slightly less charismatic but important marine animal – the Atlantic Surfclam (Spisula solidissima).
Fisheries often have size limits for the animals that can be kept; sometimes fisheries target medium sized fish, other times they target the larger ones. While size limits are intended to preserve the health of the population, they could potentially change the age structure of the population leading to changes in the number of big of small fish. Surfclams are typically harvested above a certain shell length. Size is also influenced by environmental factors that determine growth rates. The Atlantic Surfclam has a known thermal tolerance and outside of this range the clams won’t grow as well. This study examines the size of the Atlantic Surfclam population off the east coast of the U.S. and how fishing pressures or increasing temperature may be playing different roles in the average size of the clams.
The researchers used federal clam survey data to compare the size of surfclams from 1982 until the present. About every three years assessments of the surfclam population are carried out, including size and age measurements. Information about the general biology of the clam was used to model growth and predict changes based on fishing pressures and environmental variables over the years.
The model predicted growth of clams at different bottom water temperatures. Rates of respiration, feeding, growth, and reproduction change with temperature and were accounted for in the model. The predicted size of clams over the years could be compared to the field collected size data.
Results and Significance
In the early 1980s, the average size of surfclam was 18 cm across; now, the surfclams average about 16 cm. This translates to a loss of almost 40% of a large clam’s biomass. The model showed that largest clams were most affected by high temperatures. Clams were predicted to be smaller as water temperature increases. With climate change we can expect an increase in ocean temperature which may further intensify this problem. The model also predicted the size of clams as both the temperature changed and the amount of fishing increased. Fishing was predicted to decrease maximum shell length, so more fishing means smaller clams. The researchers also modelled what would happen if fishing pressure was removed. It took 10 years before a difference was noticeable in the average size of clams.
Based on the empirical data collected over the years we know the size of surfclams is changing. Because bigger clams are likely better at spawning and creating more of the next generation, this could be a problem. The smaller size also means an overall decrease in biomass, which could threaten the stability of the population or be detrimental for the fishery. It is difficult to know what is causing the change in size; this study found that higher temperature and fishing restricted to larger sized individuals can lead to smaller average clam size. The good news is that we have a chance to make a difference by decreasing the fishing pressure.
Modeling studies such as this one are important to show what changes we can make now to solve our problems and to give us some understanding of how long we need to wait to see results. Waiting ten years for discernable recovery is not easy though, and full recovery could take much longer. We can’t control the temperature, but we can change our behavior and we might as well start that 10 year countdown now!
I am a doctoral candidate in Ecology and Evolutionary Biology at Tulane University. My research focuses on the larval dispersal and development of the blue crab in the Gulf of Mexico.
When not concerning myself with the plight of tiny crustaceans I can be found enjoying life in New Orleans with all the costumes, food, and music that entails.