Bellamya aeruginosa (Reeve) is the predominant macrozoobenthic species and main food source in an artificial lake used for rearing transgenic common carp (Cyprinus carpio L.) with the 'all-fish’ growth hormone gene constructs. Knowledge of the life history and production of B.aeruginosa are critical in assessing the predation pressure by transgenic common carp. Between February 2008 and January 2009, the population dynamics and production of B.aeruginosa were studied at monthly intervals at seven stations in the artificial lake situated in Hubei Province, China. The instantaneous growth rate method was applied to determine the production and P/B ratio of B.aeruginosa. Production over the period 2005-2008 was then compared from the calculated P/B ratios. Finally, the important environmental factors influencing production patterns were established via multiple regression analyses.
During 2008-2009, annual average density and biomass (wet weight) were 28.96 ind./m² and 17.33 g/m², respectively B.aeruginosa recruitment occurred mainly between April and July. A major peak in standing crops was observed in April (34.29 ind./m² and 54.51 g/m²), with a minor peak in December (25.1 ind./m² and 36.18 g/m²). Based on the peaks in population density and seasonal variations in size distribution of shell-length, the B.aeruginosa population was composed of four cohorts. Growth of the 2005 cohort had nearly ceased, while individuals of the 2008 cohort grew rapidly, with an instantaneous growth rate of 2.34. The 2008, 2007 and 2006 cohorts accounted for 20.2, 52.7 and 25.8% of total density, respectively.
Regressions between shell-length and wet weight (Ww) was lg Ww =2.8791×lg SL -3.4227 (n =96, P <0.001). To reduce the error, the growth curves of each cohort (P <0.05) were also fitted. Using the instantaneous growth rate method, the annual production of B.aeruginosa was calculated at 12.72 g·m^-2·a^-1 (wet weight) or 0.74 g·m^-2·a^-1(dry weight without shell), with a P/B ratio of 0.42. Annual production (wet weight) for the 2008, 2007 and 2006 cohorts were 6.40 (P/B ratio=2.33), 1.99 (0.13) and 4.33 (0.37) g·m^-2·a^-1, respectively. Annual production of the 2008 cohort was ignored because it was only composed of a few aging individuals.
Usingthe P/B ratios, annual production (dry weight without shell) for the 4-year period (2005-2009) was calculated as 2.24, 2.49, 1.50 and 0.74 g·m^-2·a^-1, respectively, showing a significant steady reduction (F =4.497, P =0.013). Pearson correlation analysis indicated that water transparency (SD), total nitrogen (TN), nitrate nitrogen (NO^-₃-N) were positively correlated with production (dry weight without shell) (P < 0.001) and stepwise regression analysis showed that TN was the key factor influencing production (F =7.632, P =0.009).
Water trophic levels and predation pressure by transgenic common carp appear to be responsible for the fluctuations in annual production of B.aeruginosa in this artificial lake. High-level of nutrients may significantly increase B.aeruginosa production, while selective feeding by transgenic common carp had the opposite effect. However, whether production in the artificial lake will continue to decrease with the growth of transgenic common carp requires further research.