Growth and production of a tropical predatory shrimp, Macrobrachium hainanense (Palaemonidae), in two Hong Kong streams

Abstract

1. Macrobrachium hainanense is a predatory palaemonid shrimp (total length >7 cm) that can be abundant [density 3-5 m-2; biomass 484-606 mg ash-free dry mass (AFDM) m-2] in forest streams in Hong Kong, China. This study investigated the growth and production of M. hainanense during 2001 and 2002 in pools of two forested streams (one third- and one fourth-order). 2. The growth of tagged individuals was recorded in situ and compared with that of tagged and untagged shrimps in laboratory tanks. Field and laboratory estimates yielded similar growth rates of 0.7 mm carapace length (CL) per month, and instantaneous growth rate was 0.004 g AFDM g-1 day-1. Tagging did not affect growth in the laboratory. Cohort analysis of field populations produced similar estimates of growth to that of tagged individuals, and the growth of M. hainanense was generally slower than has been reported for other Macrobrachium species. Mass-specific growth rate of M. hainanense in the field varied with size and was two to five times higher in small individuals (<10 mm CL). In addition, growth rate varied with season and was 40% lower in the dry season when temperature was at the annual minimum. 3. Males grew bigger than females (36 versus 25 mm CL). The minimum lifespan of M. hainanense in the field, calculated from size-specific growth rates, ranged from 29.3 months (females) to 47.6 months (males). Male lifespan derived from cohort analysis was estimated as 48 and 46 months in the two streams. Females reached maturity in 17-18 months (at 15-17 mm CL) while males matured at 24-26 months (at 18-22 mm CL). Females bred twice (at 2 and 3 years of age) while males probably bred three times (at 2, 3 and 4 years) in both streams. 4. Macrobrachium hainanense production in the fourth-order stream, calculated by the size-frequency method, was 900 and 1096 mg AFDM m-2 year-1 (for 2001 and 2002, respectively) with a production/biomass (P/B) of 2.1-2.3 year-1. In the third-order stream, production was 987 and 1304 mg AFDM m-2 year-1 (for 2001 and 2002, respectively) with a P/B of 1.7-2.1 year-1. Production estimates based on the instantaneous growth method were half of those obtained by the size-frequency method. 5. Although M. hainanense production at the third-order stream exceeded that in the fourth-order, growth rates showed the opposite pattern and were 0.31-0.43 mm CL month-1 and 0.56-0.65 mm CL month-1 in the third- and fourth-order streams, respectively. Greater mortality in the latter may account for low production at a site where growth rate was high. 6. Production of M. hainanense in both streams was lower during 2001 when rainfall was higher. This may reflect the influence of spates associated with monsoonal rains, which could have reduced M. hainanense production through spate-induced mortality or by reducing the abundance of prey. This study provides the first in situ estimate of secondary production by a non-commercial Macrobrachium species in Asia or elsewhere. It involved a whole-pool approach to sampling that allowed the estimation of production and population parameters on a realistic scale.