Ingestion rates and grazing impact of the brackwater mussel Brachidontes virgiliae in Lake St Lucia,iSimangaliso Wetland Park,South Africa

Bivalves feed on a combination of phytoplankton and zooplankton and have the potential to impact considerably the planktonic biomass, especially when they occur in high densities, such as in oyster and mussel beds. The brackwater mussel Brachidontes virgiliae is numerically dominant during wet phases within Africa’s largest estuarine lake, St Lucia, in the iSimangaliso Wetland Park on the east coast of South Africa. The ingestion rates and potential grazing impact of this small mussel (maximum shell length = 2.5 cm) were estimated for both the wet and dry seasons using an in situ gut fluorescence technique. Ingestion rates were higher during the wet season (5.78 µg pigment ind.^?1 d^?1) than during the dry season (4.44 µg pigment ind.^?1 d^?1). This might be explained by the increased water temperature and food availability during the wet season. Because of the patchy distribution of mussel populations, there could be higher localised grazing impact near mussel aggregations. Results showed a potential grazing impact of up to 20 times the available phytoplankton biomass at specific sites. These high grazing impacts have the potential to deplete phytoplankton stocks in the lake, especially during wet phases in the northern reaches, where mussel densities are highest. This needs to be factored into ecological models of Lake St Lucia, because the system might function differently during increased flood events.     

Diel Changes in Vertical Distribution and Feeding Conditions of the Chaetognath Parasagitta elegans (Verill) in the Subarctic Pacific in Summer

Diel changes in vertical distribution and feeding conditions of the chaetognath Parasagitta elegans (Verill) were observed in three regions of the subarctic North Pacific in the summer of 1997. Samples were collected by repeated vertical hauls with a Vertical Multiple Plankton Sampler (VMPS) for 15–45 hours by demarcating the 0–500 m water column into four sampling layers. Integrated abundance through the entire water column and the proportion of juveniles were higher in the Bering Sea than the western and eastern subarctic Pacific. Juveniles always inhabited the surface layer in the western subarctic Pacific and Bering Sea, but they inhabited the underlying layer in the eastern subarctic Pacific. Stages I–III concentrated into the upper 150 m in the western subarctic Pacific but were distributed widely from 20–300 m in the Bering Sea. Among them, Stages II and III migrated rather synchronously over a wide vertical range in the eastern subarctic Pacific. The feeding rate of P. elegans was calculated to be 0.18 prey/chaetognath/day in the western subarctic Pacific, 0.27 prey/chaetognath/day in the Bering Sea and 0.07 prey/chaetognath/day in the eastern subarctic Pacific.     

缢蛏滤除率与颗粒选择性的实验研究

利用4种不同大小的单细胞藻类作为饵料研究缢蛏（Sinonovacula constricta）的滤除率以及缢蛏对不同大小料颗粒的选择性。实验结果表明：3种规格缢蛏对4种单细胞藻类的滤除率表现出相同的变化规律，即随藻类规格的增大，缢蛏的滤除率亦逐渐变大，并且大规格缢蛏的滤除率变化最显著；在饵料颗粒选择性的实验中，小规格缢蛏对大规格（ESD＝8.83），藻类具有较高的选择性，中规格缢蛏对藻类的选择性不明显，而大规格缢蛏则对小规格（ESD＝4.46）藻类具较高的选择性。     

"合信"牌欧鳗配合饲料精养缓长型欧鳗的试验

本文报告了"合信"牌欧鳗配合饲料精养缓长型欧鳗的试验.结果表明，试验组的平均饲料效率为48.11％，比对照组提高了10.71％；欧鳗的存活率达99.9％，且有较好的肉质，符合出口烤鳗的要求.     

俄罗斯鲟鱼配合饲料主要营养指标的试验

用蛋白梯度及脂肪梯度进行工厂化养殖俄罗斯鲟鱼，经过78d养殖试验，认为适宜俄罗斯鲟的蛋白质含量为41％左右，脂肪含量为9％左右.     

Filter-feeding ascidians (Ciona intestinalis) in a shallow cove: Implications of hydrodynamics for grazing impact

The grazing impact by a dense population of filter-feeding ascidians Ciona intestinalis on horizontally flowing water (driven by density circulation) in a shallow cove (Kertinge Nor, Denmark) has been described and quantified by means of a simple one-dimensional numerical model. The agreement between observations and modelled predictions was satisfactory. The applied numerical model has the following analytical solution in the idealized case: Cx = C₀e^−(fx/Y₂), where C_x = algal concentration at a downstream distance x, C₀ = initial concentration, f = F/v_c; F = area specific population filtration rate; v_c = current velocity; Y₂ = depth of mixed layer below halocline. The numerical model quantifies the actual grazing impact while the analytical model illustrates the governing physics in well-known terms. To describe situations with no current (i.e. stagnant water), we performed simulation studies in the laboratory and measured vertical profiles of algal cells over filter-feeding C. intestinalis. The results showed that phytoplankton became reduced in a near-bottom water layer of 20–30 cm thickness. Such water layers may develop in stagnant water (calm days and no advective currents), thus uncoupling the pelagic food and the filter feeders which within a short time will experience extremely meagre food conditions.     

中国鲎幼体阶段(黄皮鲎)饵料的初步研究

用杂鱼、星虫、牡蛎、乌贼、虾进行中国鲎幼体饵料种类、粒度以及白天和夜间摄食量差异等试验。结果表明：中国鲎幼体阶段(黄皮鲎)对乌贼的相对摄食量最大(2.1％)，而对小杂鱼的相对摄食量最少(0．2％)。中国鲎幼体阶段(黄皮鲎)均对粒度为0.3～0．5cm的乌贼相对摄食量最大(2．0％)，对粒度为2cm的乌贼相对摄食量最小(0．1％)。中国鲎幼体阶段(黄皮鲎)夜晚的摄食量明显大于白天：平均体重为147．4g组的昼夜相对摄食量的差异最大(白天为0．8％，夜间为1．7％)，平均体重为258．7g组的差异最小(白天为ll％，夜间为l4％)。     

Dispersion and feeding of larval herring (Clupea harengus L.) in the Moray Firth during September 1985

A plume of herring larvae dispersing from a spawning site at Clythness in the Moray Firth (northern Scotland) was surveyed during early September 1985. Several cohorts of larvae were evident from the length distributions, and these were arranged in order of increasing length (age) towards the south-west. The spacing of cohort centres indicated a drift rate of 1–2 km day⁻¹.Calanoid copepod nauplii constituted the major proportion of the diet of larvae <10 mm sampled during the study. Cyclopoid copepod nauplii and gastropod veligers were not found in the diet although they were present in the water. The distribution of nauplii in the region was inversely correlated with the concentration of phytoplankton chlorophyll, and nauplii concentrations were above average in the vicinity of the herring spawning site. The drift trajectory of the herring larvae took them towards an area of high copepodite and adult copepod concentration—items which formed an increasing part of the diet of larger (older) larvae.