Pacific white shrimp (Litopenaeus vannamei) are reared in a variety of RAS systems at high stocking densities. Shrimp tend to arrive from hatcheries with a high rate of size variability, which is compounded by high-density culture conditions. Left uncorrected, the size variation leads to problems with marketing the shrimp later. The purpose of this study was to determine whether a weight to width ratio of shrimp exists and to use that information to reduce size variability by grading and sorting shrimp.
To establish a weight to width ratio, a wide range of shrimp were weighed and measured. A linear regression and polynomial regression were each applied to the data, resulting in R2= 0.873, P= 0.001 and R2= 0.909, P= 0.000 values, respectively. The polynomial equation was used to decide the width of a grader in an attempt to separate shrimp that were above and below the mean weight. The mean weights of two shrimp populations were measured and the coefficients of variation (CV) were calculated.
The first population of shrimp (N = 228) had a mean weight of 2.7g and a CV of 4.5%. A simple, wooden frame was made with small dowels fixed to it that were 7.8 mm apart, according to the prediction of the polynomial equation. After grading and separating into two sub-populations of shrimp, the larger shrimp (N = 59) had a mean size of 4.2g and a CV of 1.3%. The smaller shrimp (N = 169) had an average size of 2.9g and a CV of 0.8%. These two sub-populations were grown for two months and sampled again. This later sampling revealed that the larger sup-population having a mean size of 19.3g and a CV of 2.0%, while the smaller sub-population had an average size of 12.9g and a CV of 2.6%. The same process of sampling and grading was repeated on a second population of larger shrimp (N =117) with a mean weight of 9.9g and a CV of 9.3%. After grading the population with a grader width of 11.9 mm, the mean weight of the larger shrimp was 11.4g with a CV of 3.2% and the smaller shrimp had a mean weight of 8.8g with a CV of 3.6%.
These results indicate that there was a predictable relationship between shrimp weight and width, and this relationship could be used to decide how to grade shrimp. Simple, in-house made graders can be an effective way to divide shrimp populations and by doing so, reduce the size variability of each subsequent group. Although size variability remained lower than the original values, in this case variation increased again over the course of a two-month growing period. Furthermore, it seems that grading shrimp earlier was more effective at reducing variability even after shrimp grew larger. Future research should examine how frequently grading should be done in shrimp populations. Also, the question of whether variability is similar among genetically different shrimp populations should be addressed. Ultimately, reducing the variability in shrimp size should help farmers market a more consistent crop and generate more consistent revenue as a result.