The effects of stocking density on egg production and hatching success, cannibalism rate, sex ratio and population growth of the tropical calanoid copepod Acartia sinjiensis

Abstract

The difficulty in achieving high density copepod cultures is a major bottleneck limiting their utilization in aquaculture hatcheries. The present study evaluated the effects of initial adult stocking density on various productivity-related parameters of the tropical calanoid copepod Acartia sinjiensis, a specie with good potential for mass culture as a live feed for marine larvae. Five stocking densities of 125, 250, 500, 1000 and 2000 adults/L were set-up for all experiments with 5 replicates per treatment. A. sinjiensis used for all trials were originated from plankton tows conducted in October 2006 and have since been cultured under laboratory condition. Daily egg production of A. sinjiensis was monitored over 12 consecutive days, the results showed that while at the lowest stocking density of 125 adults/L, survival of A. sinjiensis over the 12 days period was significantly higher than 250, 500 and 1000 adults/L treatment ( p < 0.05), average egg output per female was not significantly affected by either stocking density or by the age of the females ( p > 0.05). Hatching success within 48 h was significantly higher ( p < 0.001) for eggs produced by females cultured at lower densities. However, when incubation period was increased to 96 h, hatching rates were not significantly different among treatments ( p > 0.05), indicating a possible delayed effect on hatching of eggs produced at high stocking densities. Cannibalism of adults and late copepodites (C-5) toward nauplii increased steadily with predator density; with naupliar mortality at 2000 ind./L significantly higher than that at 125 and 250 ind./L treatments ( p < 0.005). Under optimal culture conditions, population growth of A. sinjiensis at different initial adult stocking density was also assessed over a 12 day culture period. The results showed that the sex ratio of the adults on the final day of the experiment was not significantly affected by the initial stocking density ( p > 0.05). It was also revealed that A. sinjiensis can sustain a high culture density of 2000 ind./L, and likely higher, as increases in A. sinjiensis populations over the 12 day culture period were found at all densities tested. Nevertheless, the rate of population growth of all post-egg-stages of A. sinjiensis was significantly affected by the initial stocking density ( p < 0.005), over 12 days of culture, the final number of all post-egg-stages increased by 470% and 408% respectively for the lowest initial stocking densities of 125 and 250 adults/L while at the highest densities of 1000 and 2000 adults/L, the increases were substantially lower at 244% and 171% respectively.