Validity of movie recording for quantitative abundance estimation and body size measurement of Macrobrachium shrimps

Abstract

Collecting Macrobrachium shrimps for abundance estimation is challenging, especially in high gradient, boulder dominated streams. The recent advent of digital cameras could change the situation by enabling previously under-used snorkel counting to yield verifiable data. The validity and efficiency of underwater movie recording for shrimp census were tested and compared with those of underwater shrimp net collection and sweeping in a field experiment (1–4 adult shrimps/m²). Total length estimates from the video footage were compared with caliper measurements. Detection was not affected by species, sex, or shrimp density. Movie recording and shrimp net collection yielded high and constant detection probability of adult shrimps (0.955 and 0.874), in contrast to sweeping (0.097). The movie recording had higher efficiency than the other two methods. For juveniles, only shrimp net collection was valid as an abundance indicator, with a detection probability of 0.338. Accuracy of total length estimates made from the video footage did not differ between species or sex and was highly correlated with the actual measurements by digital caliper. These results confirm that in adult Macrobrachium shrimps, the movie recording is valid for body size and quantitative abundance estimation, but not in juveniles.