Aquatic “Crop Rotation” in Shrimp Farming
As the grandson of Ohio farmers, I grew up occasionally overhearing–and sometimes learning–about the business of agriculture. I can still recall conversations between my grandparents discussing the previous season’s surpluses or shortages and how this would effect their choice of which crops to grow in following seasons. But little did I expect decades later to find such an uncanny parallel between their agriculture and the aquaculture TransGenada will support. When I learned about shrimp farmers in the Ben Tre province of Vietnam and how they utilized their own type of “crop rotation” my curiosity was piqued.
As detailed in the September/October 2011 Edition of Aqua Culture Asia Pacific magazine, these shrimp farmers seasonally rotate the species of shrimp they stock. One farmer details how he stocks giant tiger shrimp (P. monodon) for the first two thirds of the season and then Pacific White Shrimp (L. vannamei) during the remaining 75 days of the season. He explains that he knows the chances of a Whitespot (WSSV) outbreak increase from about 20-30% early on in the season in April to about 70-80% later in the season in September and during the dry season (December through February). In fact, the chances for disease are so probable that the authorities have made it illegal for farmers to grow shrimp and for hatcheries to produce postlarvae during the dry season.
What I wasn’t able to discern exactly is why they rotate their types of shrimp like this. Agriculture farmers have several reasons for this ranging from increasing the quality of their soil to helping prevent crop-specific pests from feeling like they’ve found a good consistent field to retire to. But why do the aquaculture farmers of Ben Tre rotate species? My original belief was this was implemented because L. vannamei were less susceptible than P. monodon to WSSV. This made sense afterall because I knew after L. vannamei became more popular after large WSSV and TSV outbreaks. But, I also know WSSV does indeed impact L. vannamei, so perhaps there is another reason, like L. Vannamei have some characteristic, like being cheaper or faster to grow, that naturally makes them more advantageous to grow as farmers approach the end of their season. Or, as is often the way of things, I wondered if there was perhaps a combination of reasons why shrimp farmers would rotate their species of shrimp. I decided to ask TransGenada’s chief scientist, Dr. Jeremy Ellis and what he shared with me was very insightful. He said, “There are a variety of forces that could dictate when crop rotation is an advantageous strategy. Market preference and prices in some regions make P. monodon a more attractive crop, while L. vannamei has been selected for disease resistance for many years. By splitting time between these two they are tapping into the strengths of each species.”
While I know farmers, both agriculturists and aquaculturists, are always aware of the impact of disease, I hope sharing this example will help outsiders realize the depth this problem poses for the pressures farmers face. The industry plays a constant game of tug of war. On the one hand we are pushed annually for greater production to supply an ever-growing populace and to compensate for less and less abundant natural resources. But, on the other hand, they have to take steps like the ones mentioned in this post to keep from being pulled back by outbreaks and the like. Just today I was reading about how China, Japan and South Korea’s demand for Vietnamese shrimp is expected to increase this year (despite last year’s 13.7% increase) and about how Sonora, Mexico’s production was halved from 90,000 tonnes to 41,000 tonnes due to WSSV. TransGenada aims to innovate aquaculture and provide better solutions for its farmers and in so doing for our future generations’ welfare. A goal I’m certain my grandparents could appreciate.