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Biology

You Are What Your Fish Eats: how an invasive seaweed is contributing to the decline in nutritional value of commercial fish

 

 

Article:  Felline, S., et al. (2014). “Can a marine pest reduce the nutritional value of Mediterranean fish flesh?” Marine Biology: 1-9.

DOI: 10.1007/s00227-014-2417-7

Background:

Even if you aren’t the kind of person who likes fish, you’ve probably heard of the benefits of eating fish; salmon is considered a brain food, right? It’s true; fish meat contains important compounds for human nutrition as well as cellular structure and function. Fatty acids found in fish meat can help prevent cardiovascular disease and diabetes. These compounds also play a role in supplying energy for growth and creating cellular membranes. Seafood is a staple in Mediterranean cuisine as this region is home to a large number of fish species. However, this region is also home to many invasive species, some of which may be affecting the quality of the seafood reaching the consumer.

Caulerpa racemosa (Fig 1) is a green algae that invaded the Mediterranean in the early 1990s. Like many invasive species, C. racemosa began to outcompete local algal species and eel grass and spread like wildfire. This species has been shown to negatively effect food webs and communities.

Fig 1: The green algae Caulerpa racemosa.

Fig 1: The green algae Caulerpa racemosa.

Due to its low nutritional value and toxic compounds, marine animals don’t usually consume it. But as C. racemosa began to take over, some fish species switched from being omnivorous (consuming a mix of plant, algal, and animal tissue) to a diet comprised mostly of C. racemosa. This was observed in the native fish Diplodus sargus, or white sea bream (Fig 2, Fig 3), an important species in Mediterranean fisheries. This was most likely done out of necessity as this seaweed is often the only thing around to eat. C. racemosa seems to function as a food source for the white sea bream, it even contains fatty acids, but it lacks the ones important in human nutrition. Therefore, if fish are consuming this alga at high rates, they are likely not gaining important fatty acids. This could affect the overall health of the fish but also the nutritional value to humans.

Researchers set out to determine how a C. racemosa heavy diet impacts the fatty acid composition, thus nutritional value, of the white sea bream.

Fig 3: The sea bream ready for consumption.

Fig 2: The sea bream ready for consumption.

Fig 2: Diplodus sargus, the white sea bream.

Fig 3: Diplodus sargus, the white sea bream.

 

 

 

 

 

 

 

The Study:

C. racemosa is an ephemeral species, it blooms in the late summer through the fall and declines in the winter and spring. Knowing this, researchers decided to sample for white sea bream from 3 locations in the Mediterranean and during 2 seasons. The three sites were all very similar in structure but two of them contained high densities of C. racemosa, while the third site had significantly lower density of the invasive alga. They sampled in October, during peak bloom, and in June, prior to the summer bloom. Fish were caught by spearing and immediately preserved. Fish tissue was analyzed in the lab for the concentrations of different types of fatty acids and the presence of caulerpin, a molecule found in C. racemosa.

Overall, researchers found significant differences in poly-unsaturated fatty acid concentrations in fish tissue based on when the fish were caught, where they were caught, and the level of caulerpin in their tissue. Researchers found lower concentrations when fish had high concentrations of caulerpin, when fish were located in areas of higher C. racemosa density, and when the C. racemosa had bloomed (Fig 4). These results indicate that when the invasive seaweed is relied on as a food source, fish lose these important fatty acids. Researchers further determined that fish eating mostly C. racemosa contained smaller proportions of the nutritionally valuable poly-unsaturated fatty acids.

Fig 4: This graph shows the concentrations of poly-unsaturated fatty acids in sea bream tissue. Bars are broken up by caulerpin concentration (green = high, gray = low), month captured, and site.

Fig 4: This graph shows the concentrations of poly-unsaturated fatty acids in sea bream tissue. Bars are broken up by caulerpin concentration (green = high, gray = low), month captured, and site.

Significance:

The invasion of C. racemosa appears to have had a larger effect on Mediterranean ecosystems and function than originally thought. Fish that began to consume more of this invader have shown a decline in important fatty acids. Fish that have a mixed diet are able to obtain natural levels of these molecules and can pass the nutritional value along to a human consumer. But when fish consume mostly a diet of C. racemosa they lose that nutritional value. However, during the seasonal decline of C. racemosa, fish diet becomes broader and more balanced. This aspect will be increasingly important as the world’s oceans are warming. Warmer waters could lead to the year-round persistence of algal species like C. racemosa and fish will no longer find temporal refuge from poor diet.

Humans continue to introduce species around the globe, whether intentionally or unintentionally. It is often hard to relate the impact of invasive species to the public, but having a study like this helps. Relating invasive species to the quality of food on one’s plate is a great way of bringing attention to this global problem.

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