Growth, survival and immune activity of scallops, Chlamys farreri Jones et Preston, compared between suspended and bottom culture in Haizhou Bay, China

We examined the growth, survival and immune response of the scallop, Chlamys farreri, during a 1-year period in deep water of Haizhou Bay. Scallops were cultured using two methods: (1) in lantern nets at a 5 m depth and (2) in a bottom culture system (sleeves) on the seabed at about a 25 m depth. Shell heights, meat dry weight and immune activities in the haemolymph (superoxide dismutase and myeloperoxidase) were measured bimonthly or quarterly from July 2007 to June 2008. Survival was measured at the end of the study and environmental parameters in the experimental layers were monitored during the experiment. The growth and immune activities of scallops were lower when the water temperature was high, which was consistent with the main mortality occurring in summer. The growth and immunity of scallops were higher in the suspended culture than in the bottom culture during the experiment, with the exception of shell growth during the last study period. Survival of scallops in the suspended culture (54.6 +/- 12.3%) was significantly lower than that in the bottom culture (86.8 +/- 3.5%) at the end of this study. We conclude from our results that the high mortality of C. farreri can be prevented by culturing them in a bottom culture system before November of the first year, and then transferring them to a suspended culture to improve scallop production.

Studies on mass summer mortality of cultured zhikong scallops (Chlamys farreri Jones et Preston) in China

Mass mortalities of cultured zhikong scallops (Chlamys farreri) have occurred each summer in most culture areas of northern China since 1996. Among the hypothesized causes are high culture density, infectious disease and genetic inbreeding. To investigate these potential agents, C. farreri were deployed at three densities (low, medium and high) at three sites (Jiaonan, Penglai and Yantai) in the summer of 2000. Scallops were sampled for survival, growth and histopathology before, during and after a mortality episode. Most of the mortality occurred in July and August, during and toward the later part of the spawning season, when water temperature reached 23-26 degrees C. Final cumulative mortalities reached 85% to 90% at all three sites. Scallops in the medium and high densities had higher initial death rates than did those at the low density. High densities also inhibited growth. Ciliates from the genus Trichodina, larvae of various organisms and anomalous secretions were observed in sections of the gill cavity, with highest prevalence during and at the end of the mortality period. Prokaryotic inclusion bodies were found in the soft tissues, but their prevalence was low and apparently without correlation with mortalities. Genetic analysis with random amplified polymorphic DNA markers showed slightly lower heterozygosity in the cultured stocks (0.301) than in the wild stocks (0.331). It is possible that the mortalities are caused by a combination of several factors such as stress associated with reproduction, high temperature, overcrowding and poor circulation in the growout cages, opportunistic invaders or pathogens, and possibly inbreeding. (c) 2005 Elsevier B.V. All rights reserved.

Effect of annual weeds on the growth of perennial grass mixtures in the alpine region of the Qinghai-Tibetan Plateau

In the alpine region of the Tibetan Plateau, five perennial grass cultivars, Bromus inermis (B), Elymus nutans (E), Clinelymus nutans (C), Agropyron cristatum (A), and Poa crymophila (P) were combined into nine communities with different compositions and ratios, B+C, E+A, B+E+A, E+B+C,C+E+A,B+E+C+A,B+C+A+P,B+E+A+P and E+C+A+P. Each combination was sown in six 10 X 10 m plots with three hand-weeded plots and three natural-growing plots in a completely randomised design in 1998. A field experiment studied the performance of these perennial grass combinations under the competitive interference of annual weeds in 3 consecutive years from 1998 to 2000. The results showed that annual weeds occupied more space and suppressed the growth of the grasses due to earlier germination and quicker growth in the establishment year, but this pattern changed in the second and third years. Leaf area indexes (LAIs) of grasses were greatly decreased by the competitive interference of weeds, and the negative effect of weeds on LAIs of grasses declined and stabilised in the second and third years. E+B+C, B+E+C+A, and B+E+A+P possessed relatively higher LAIs (P < 0.05) among all grass combinations and their LAIs were close to five when the competitive interference of weeds was removed. Grasses were competitively inferior to weeds in the establishment year, although their competitive ability (aggressivities) increased throughout the growing season. In the second and third years, grasses were competitively superior to weeds, and their competitive ability decreased from May until August and increased in September. Dry matter (DM) yields of grasses were reduced by 29.8-74.1% in the establishment year, 11.0-64.9% in the second year, and 16.0-55.8% in the third year by the competitive interference of weeds. B+E+C+A and B+E+A+P can produce around 14 t/ha of DM yields, significantly higher (P < 0.05) than the production of the other grass combinations in the second and third years after the competitive interference of weeds was removed. It was preliminarily concluded that removal of competitive interference of weeds increased the LAIs of all grass swards and improved the light interception of grasses, thus promoting the production of perennial grass pastures. The germination stage of the grasses in the establishment year was the critical period for weeding and suppression of weeds should occur at an early stage of plant growth. The grass combinations of B+E+C+A and B+E+A+P were productive and can be extensively established in the alpine regions of the Tibetan Plateau. Two or three growing seasons will be needed before determining success of establishment of grass mixtures under the alpine conditions of the Tibetan Plateau.

Effect of N fertilizer on the productivity and nutritive values of perennial grass mixtures in the alpine region of Qinghai-Tibetan Plateau, China

A study was conducted on grass mixtures that included smooth bromegrass (SB) + drooping wild ryegrass (DW), smooth bromegrass + Siberian wild ryegrass (SW) + crested wheatgrass (CW) and smooth bromegrass + Siberian wild ryegrass + drooping wild ryegrass + crested wheatgrass in the alpine region of Qinghai-Tibetan Plateau. The study was conducted from 1998 to 2000 to investigate the effects of N application rates and growing year on herbage dry matter (DM) yield and nutritive values. Herbage DM production increased linearly with N application rates. The effect of N application on DM yields was greater (P < 0.05) in the 2nd and 3rd production years than in the establishment year. Dry matter yields of SB + SW + CW and SB + SW + DW + CW can reach as high as 15 000 kg ha(-1) at 345 kg ha(-1) N rate in the 3rd growing year. With increased N application rates, crude protein (CP) contents and 48 h in sacco DM degradability of grasses increased (P < 0.05). No effect (P > 0.05) of N application was detected on organic matter (OM) and acid detergent fibre (ADF) concentration. It can be concluded that for increased biomass production in the alpine region of the Qinghai-Tibetan Plateau, a minimum of 345 kg N ha(-1) should be applied to grass stands in three split application of 115 kg N ha(-1), in early June, early July and late July

Productivity and persistence of perennial grass mixtures under competition from annual weeds in the alpine region of the Qinghai-Tibetan Plateau

In the alpine region of the Qinghai-Tibetan Plateau four indigenous perennial grass species Bromus inermis (BI), Elymus sibiricus (ES), Elymus nutans (EN) and Agropyron cristatum (AC) were cultivated as three mixtures with different compositions and seeding rates, BI + EN, BI + ES + AC and BI + ES + EN + AC. From 1998 to 2001 there were three different weeding treatments: never weeded (CK); weeded on three occasions in the first year (1-y) and weeded on three occasions in both the first and second year (2-y) and their effect of grass combination and interactions on sward productivity and persistence was measured. Intense competitive interference by weedy annuals reduced dry matter (DM) yield of the swards. Grass combination significantly affected sward DM yields, leaf area index (LAI) and foliar canopy cover and also species composition DM and LAI, and species plant cover. Interaction between weeding treatments and grass combination was significant for sward DM yield, LAI and canopy cover, but not on species composition for DM, LAI or species plant cover. Grass mixture BI + ES + EN + AC gave the highest sward DM yield and LAI for both weeding and non-weeding treatments. Species ES and EN were competitively superior to the others. Annual weedy forbs must be controlled to obtain productive and stable mixtures of perennial grasses, and germination/emergence is the most important time for removal. Weeding three times (late May, late June and mid-July) in the establishment year is enough to maintain the production and persistence of perennial grass mixtures in the following growing seasons. Extra weeding three times in the second growing year makes only a slight improvement in productivity.

Influence of grazing intensity on performance of perennial grass mixtures in the alpine region of the Tibetan Plateau

Effects of grazing intensity on leaf photosynthetic rate (Pn), specific leaf area (SLA), individual tiller density, sward leaf area index (LAI), harvested herbage DM, and species composition in grass mixtures (Clinelymus nutans + Bromus inermis, Elymus nutans + Bromus inermis + Agropyron cristatum and Elymus nutans + Clinelymus nutans + Bromus inermis + Agropyron cristatum) were studied in the alpine region of the Tibetan Plateau. Four grazing intensities (GI), expressed as feed utilisation rates (UR) by Tibetan lambs were imposed as follows: (1) no grazing; (2) 30% UR as light grazing; (3) 50% UR as medium grazing; and (4) 70% UR as high grazing. Leaf Pn rate and tiller density of grasses increased (P < 0.05), while sward LAI and harvested herbage DM declined (P < 0.05) with the increments of GI, although no effect of GI on SLA was observed. With increasing GI, Elymus nutans and Clinelymus nutans increased but Bromus inermis and Agropyron cristatum decreased in swards, LAI and DM contribution. Whether being grazed or not, Elymus nutans + Clinelymus nutans + Bromus inermis + Agropyron cristatum was the most productive sward among the grass mixtures. Thus, two well-performed grass species (Elymus nutans and Clinelymus nutans) and the most productive mixture of four species should be investigated further as the new feed resources in the alpine grazing system of the Tibetan Plateau. Light grazing intensity of 30% UR was recommended for these grass mixtures when swards, LAI, herbage DM harvested, and species compatibility were taken into account.