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1.
A population of Donax sordidus (Mollusca, Pelecypoda) has been studied for two years on an open sandy beach. There were summer and winter settlements each year with the former being larger. Growth was rapid, the animals reaching 20 mm length within one year. Few live longer than 17 months. Production averaged 168,7 mg dry matter m-2y-1 and the steady state P¯/B¯ was 1,30y-1. Energy values averaged 17,84 kJg-1dry tissue and had a seasonal cycle with maximum values before spawning.  相似文献   

2.
The redclaw crayfish Cherax quadricarinatus is a recent introduction into Lake Kariba, Zimbabwe where it is rapidly spreading. There are, however, no studies that have investigated the population dynamics and possible ecological impacts of the crayfish in the lake. This study investigated aspects of the population biology of C. quadricarinatus in the lake. These included sex distribution, population growth and mortality parameters, probability of capture, recruitment and length at age of ge at maturity. Crayfish were captured monthly between January and December 2013 from 13 sites using opera house bait traps. Aspects of the species population biology were estimated using a fish stock assessment tool, FISAT II. A total of 3 205 crayfish were captured, of which 44% were male, 49% female and 7% intersex individuals. The asymptotic length (L∞ = 112.88 mm), curvature parameter (k = 0.72), longevity (tmax = 4.17 years), growth performance index (? = 3.96), total mortality (Z = 2.06 y?1), fishing mortality (F = 1.07 y?1), natural mortality (M = 0.99 y?1), rate of exploitation (E = 0.52) and length-at-first capture (Lc = 44.48 mm) were estimated. Fecundity averaged 503 ± 229 eggs female?1 and increased with size (weight and carapace length). Gravid females were found throughout the year but the highest proportion of females with eggs (4–10%) were found in the first quarter, whereas recruitment peaked in May and June. The smallest crayfish with eggs (48.75 mm CL) was estimated to be 1.02 years and the largest (94.82 mm CL) 2.85 years. Cherax quadricarinatus exhibited an r strategy life-history pattern due to continuous reproduction and high fecundity. In Lake Kariba, C. quadricarinatus is now well into the establishment stage of the Introduction–naturalisation–invasion continuum and spreading in the lake. Management options should thus aim at preventing further spread and reducing the population size and the potential negative impacts of this species.  相似文献   

3.
The importance of sexual reproduction in tussock grasses that regenerate through vegetative growth is unclear. Festuca gracillima Hook. f. was studied as a model because it is a perennial tussock-forming grass that produces abundant seed but rarely regenerates through seedlings. The Study area was the Magellanic Steppe, Patagonia, Argentina (182 mm rainfall), managed with sheep-grazing regimes of 0.65 (high), 0.21 (low), and 0 (exclosure) ewe equivalents · ha?1 · yr?1. Tussock size and spikelet production of 358 individuals were recorded over 5 yr. Yearly models of reproductive effort in relation to plant size were tested using a maximum likelihood procedure. Seed was collected and soil cores were tested for germination and viability. Survival and growth of cohorts of seedlings sown in nylon bags were recorded. Eighteen experimental plots were cleared, and seed establishment under protected and grazed conditions was registered. Reproductive effort varied with years and plant size, with a mean of 2.41%. Florets were produced at mean density of 544 ± 217 · m?2. Predispersal losses reduced viable seed production to 187 ± 48 seeds · m?2. Seed weighed 2–2.5 mg, with 65–95% germination. Postdispersal losses reduced the seed bank in spring to 33 ± 1.3 seeds · m?2. Seedling survival curves were negatively exponential, with 95% mortality in the first year. Up to 5% of resources were used for sexual reproduction in favorable years and a recruitment of 1–3 new seedlings · m?2 · yr?1 was expected. These new plants were not observed in undisturbed plots, but established naturally in cleared plots and reached a density of 1 plant · m?2 after 10 yr, together with 44 plants · m?2 of other species. Competition might block the final establishment in these grasslands. Grazing does not appear to interfere in any stage of seed reproduction. Seed production may not maintain population numbers but could enhance genetic variation in these clonal plant populations and enable dispersal and recolonization of disturbed areas.  相似文献   

4.
N. Hanekom 《African Zoology》2013,48(4):325-330
Regular and quantitative sampling for Dosinia hepatica was performed in the Swartkops estuary over a period of a year. The size of the D. hepatica recorded at LWMST was significantly (P<0,01) larger than that sampled at 0,4 m above LWMST This appeared to be a result of a spatial separation of large and small individuals. Size frequency analyses of the populations sampled and growth ring lengths were used to construct a growth curve for D. hepatica. The calculated growth rate was slow, with D. hepatica reaching a shell length of approximately 16,3 mm and a dry tissue mass of ca. 44 mg after three years. The potential life span of the bivalve was estimated to be 8-9 years. The annual somatic production (per m2) of the population sampled was calculated to be 0,58 g shell free dry mass or 8,63 kJ, while the production biomass (P/B) ratio was 0,3.  相似文献   

5.
1. An equation is derived which relates the maximum force, Pm , exerted on an egg shell to the relative velocity, v0 , of the egg and a body on which it impacts and to their masses, M1 and M2 , and stiffnesses, S1 and S2 ; it is

2. Another equation is derived which relates the maximum force exerted by the egg to the reciprocals, Ra and Re , of its average and first principal curvatures at the point of impact, to the shell thickness, Tε , that is effective in respect of tensile strength, and to the ultimate strength of shell material, Stu ; it is Pm = kStuTεRχ α(Ra/Re)y in which k, χ and y are constants.

3. These equations are shown to be concordant with published data on egg shell fracture under both quasi‐static and dynamic conditions when x and y have the values — 1 and 0 respectively.

4. The equations provide a theoretical structure that can be used to identify the factors likely to affect egg shell fracture under specified conditions.  相似文献   


6.
Populations of large and ‘stunted’ (adult carapace lengths = 17,0–24,0 and 12,0–19,0 mm respectively) Upogebia africana were regularly and quantitatively sampled in the Swartkops estuary. At the site with ‘stunted’ prawns a well-defined spring (July to October) and summer (December to March) breeding cycle was recorded. The ‘stunted’ female prawns produced two broods of eggs per annum and the brood size ranged from about 305 to 2282 eggs depending on the size of the female. At the site with large prawns the spring and summer breeding cycles tended to merge, forming a longer breeding season, with the adult females apparently producing three broods of eggs per annum. The brood size of these females ranged from about 1304 to 4819 eggs. The calculated reproductive output (Pr) and reproductive biomass ratio (Pr/B) of the latter population were much greater than those of the ‘stunted’ prawns (1331 kJ m?2 y? and 0,53 v. 332 kJ m?2 y?1 and 0,26).  相似文献   

7.
《African Zoology》2013,48(2):202-217
The population dynamics of the estuarine isopod, Exosphaeroma hylocoetes, was investigated monthly from February 2006 to August 2007 in three temporarily open/closed Eastern Cape estuaries, the East and West Kleinemonde and Kasouga Estuaries. Mean isopod abundances and biomasses ranged between 0 and 4791 ind/m2 and 0 and 9.65 mg/dwt/m2 in the West Kleinemonde Estuary and between 0 and 108 ind/m2 and 0 and 0.318 mg/dwt/m2 in the nearby East Kleinemonde Estuary. In the Kasouga Estuary, the values ranged between 0 and 3650 ind/m2 and 0 and 5.105 mg/dwt/m2. Temporal changes in the abundance and biomass of E. hylocoetes within the three systems was primarily linked to mouth phase, with populations crashing when the mouth was open and to a lesser extent, seasonality. In all three estuaries maximum abundance and biomass was recorded in the middle reaches, which could be ascribed to the presence of submerged macrophytes. E. hylocoetes demonstrates strong sexual dimorphism, with males being significantly larger than females (P < 0.05). Sex ratios were skewed in favour of females in all three estuaries. Females were encountered carrying young from an overall length of 2.5 mm throughout the year  相似文献   

8.
SummaryGrowth rates of thoroughbred horses are not as well defined as those of other farm animals, and only a few articles summarize growth of thoroughbred horses over a prolonged period. Body weight (BW), heart girth (HG), wither height (WH), body length (BL), and hip height (HH) of 128 thoroughbred horses (59 colts and 69 fillies) were recorded from birth to 15 months of age at 14- or 28-day intervals. Data were obtained from consecutive 20 foal crops. At birth (0 day), BW was 53.55 ± 5.20 kg (range, 39.04–67.19), HG was 0.82 ± 0.03 m (range, 0.75–0.90), WH was 1.02 ± 0.03 m (range, 0.93–1.10), BL was 0.74 ± 0.03 m (range, 0.67–0.82), and HH was 1.05 ± 0.03 m (range, 0.93–1.14). At weaning (112 ± 3 days), BW was 199.57 ± 13.58 kg (range, 163.44–234.26), HG was 1.29 ± 0.04 m (range, 1.19–1.37), WH was 1.27 ± 0.03 m (range, 1.21–1.35), BL was 1.17 ± 0.03 m (range, 1.08–1.30), and HH was 1.32 ± 0.03 m (range, 1.24–1.39). At 6 months (181 ± 4 days), BW was 237.16 ± 18.48 kg (range, 186.14–288.74), HG was 1.36 ± 0.04 m (range, 1.24–1.45), WH was 1.33 ± 0.03 m (range, 1.26–1.40), BL was 1.25 ± 0.03 m (range, 1.17–1.33), and HH was 1.38 ± 0.03 m (range, 1.28–1.44). At 12 months (361 ± 8 days), BW was 337.73 ± 26.61 kg (range, 267.86–394.98), HG was 1.56 ± 0.05 m (range, 1.41–1.66), WH was 1.45 ± 0.03 m (range, 1.36–1.55), BL was 1.42 ± 0.04 m (range, 1.31–1.51), and HH was 1.49 ± 0.03 m (range, 1.41–1.57). At 15 months (447 ± 8 days), BW was 392.48 ± 30.61 kg (range, 317.80–457.18), HG was 1.64 ± 0.05 m (range, 1.52–1.76), WH was 1.49 ± 0.03 m (range, 1.42–1.58), BL was 1.48 ± 0.04 m (range, 1.40–1.59), and HH was 1.53 ± 0.03 m (range, 1.46–1.62). Two regression equations (y1 from birth to 112 days of age and y2 from 113 to 450 days of age) were calculated. WTkg is estimated by y1 = 1.28x + 57.82 (R2 = 0.94) and y2 = 0.57x + 133.28 (R2 = 0.86). HGm is estimated by yl = 0.0041x + 0.86 (R2 = 0.90) and y2 = 0.0011x + 1.16 (R2 = 0.84). WHm is estimated by y1 = 0.0022x + 1.03 (R2 = 0.85) and y2 = 0.0006x + 1.22 (R2 = 0.80). BLm is estimated by y1 = 0.0038x + 0.77 (R2 = 0.92) and y2 = 0.0009x + 1.09 (R2 = 0.85). HHm is estimated by y1 = 0.0024x + 1.07 (R2 = 0.87) and y2 = 0.0006x + 1.27 (R2 = 0.78).  相似文献   

9.
Donax serra ROding (Pelecypoda) isalargesand mussel which forms vast populations on some East Cape shores. It is most abundant on exposed beaches where the sand is not too coarse and phytoplankton production is high. Adultsoccupy 1 zonejust above the mean level of spring tide. Spat settle subtidally and move upshore as they grow. Growth is initially rapid, to about 32 mmatoneyearand48 mmattwoyears. Thereafter growth is very slow and most adult productions goesinto reproduction, with a small summerspawning and a large winterspawning each year. Somatic production is low, as large adults totally dominate the population and have low growth and low mortality. Reproductive production is relatively high and P/B values of 0,2 and 0,5 are suggested for growth and reproduction. D. serra was found to be very tolerant of fluctuating salinitiesforupto four days and should be able to penetrate river mouths, but is limited by restricted swash action. Total dry-tissue mass, energy and biochemical constituents showed some seasonal fluctuations related mainly to the reproductive cycle. Carbohydrate appears to be a moreimportant energy reserve than lipids.  相似文献   

10.
为准确掌握三江源高原鼢鼠密度、雌雄比例、体重、体长、毛色及繁殖性能等特征,我们从2020~2021年对三江源地区,重点进行了野外实地监测和破洞捕捉、解剖观察,结果表明：高原鼢鼠一年四季均有活动,表现为交配、哺乳、分居贮粮、巢内越冬等不同活动时期,活动高峰有两次：春季4~6月为繁殖期,秋季8~10月为贮粮期,11~3月其只限于巢内生活。雄鼠数量占雌鼠40.6%,雄鼠平均体重1 715.42 g,体长313.7 mm;雌鼠平均体重2 205.75 g,体长为206.3 mm,雄鼠占雌鼠体重的77.77%。雌鼠平均产仔4只,高达7只,每只鼢鼠1年平均挖土1 t左右,推出地面的土丘数252.1个·hm-2,仅向地面推出土壤干重就达1 043.8 kg,覆盖草地面积达22.5 hm2,每只鼢鼠日食量为90~120 g,一年7个月每只鼢鼠可食鲜草21 kg,繁殖率高,破坏力强。  相似文献   

11.
1. Oxygen permeabilities (KO2 ) of the shell and shell membranes of fertile and infertile chicken eggs were measured at 37.5 °C and a relative humidity of 0.60 throughout 14 d incubation, with turning. The KO2 of the shell and membranes of infertile eggs was around 1.0 × 10?7 cm3 O2STP sec?1 cm?2 Torr?1 (1 Torr = 133.322 Pa) throughout incubation. With fertile eggs, from which there was a linear loss of water during incubation, the KO2 of the shell and shell membranes was about 1.0 × 10?7 cm3 O2STP sec?1 cm?2 Torr?1 for the first four days of incubation. Thereafter the majority of shells and membranes had a Ko2 of about 1.0 × 10?6 cm3 O2STP sec?1 cm?2 Torr?1.

2. A diminution of the Na+ and K+ content of the shell membranes of fertile eggs was not associated with changes in the dimensions of the glyco‐protein mantle on the cores of the individual fibres of the membranes. There was, however, a progressive deterioration in the limiting membrane of fertile but not of infertile eggs.

3. It was concluded that changes in the O2 resistance of the integument of fertile eggs were not a product of change in either of the shell membranes but of damage caused to the limiting membrane by the chorioallantois.  相似文献   


12.
为建立羊草草地高光谱植被指数(NDVI)与短星翅蝗危害密度之间的关系模型,估计短星翅蝗危害造成的牧草损失,使用短星翅蝗按5,10,20,40和60头/m25个密度梯度在羊草草地进行田间取食危害试验,测定不同危害时长后的NDVI值,最后根据NDVI和生物量的对应关系计算蝗虫危害后的牧草损失量。结果发现短星翅蝗危害羊草草地后,随短星翅蝗密度增加,NDVI值呈现逐渐降低的趋势,但是在密度为10头/m2时,归一化植被指数NDVI值略有上升。模拟短星翅蝗危害不同时间后NDVI与密度之间的关系方程为:Y=0.5932+0.0014x-6.93×10-5x2(5 d),Y=0.5950-4.8500×10-4x-4.01×10-5x2(10 d),Y=0.5848-0.0024x-1.61×10-5x2(15 d),Y=0.6422-0.0031x-2.12×10-5x2(20 d)。其中, y为植被指数NDVI,x为蝗虫密度。同时研究发现,低密度情况下(不大于20头/m2),随危害时间延长短星翅蝗取食对NDVI校正值无显著影响;高密度情况下(大于20头/m2),随时间延长NDVI校正值迅速降低,不同密度间的差异显著。根据草地生物量与NDVI的回归方程(y=614.15x-119.28)将NDVI值转换成牧草损失量,发现随虫口密度增加,牧草损失量呈增加趋势。低密度短星翅蝗(5,10头/m2)危害情况下,羊草草地有超补偿作用,当蝗虫密度超过40头/m2时,生物量降低趋势非常明显。研究结果表明,归一化植被指数NDVI变化与蝗虫危害密度相关关系显著,随着蝗虫密度的增大,NDVI的值先增长后降低。根据蝗虫危害造成的光谱变化,可以估计蝗虫危害密度及造成的损失。本研究为进一步开展蝗灾的大区域遥感监测奠定了基础。  相似文献   

13.
At 18 °C the eggs of the kelp-fly, Fucellia capensis, hatch in 24 – 32 h and the larval and pupal stages last 10 – 12 and 9 – 14 days respectively. The growth curve is discontinuous, with declining relative growth rates in successive larval instars. Food consumption is relatively independent of size, at 0,25 mg dry kelp mg-1 wet larva day-1. Respiration rate is related to larval mass by the equation: Respiration rate (μl O2 h-1) = 2,75 (wet mass larva)0,66. In terms of energy an individual larva consumes 245,5 J over its lifespan, grows to the equivalent of 43,9 J and respires 51,6 J, indicating an assimilation efficiency of 30% and a gross growth efficiency of 17%. Kelp-fly larvae are probably less significant consumers of drift kelp than amphipods or isopods, but they promote the decay of wrack beds and are favoured as food by sea-shore birds.  相似文献   

14.
1. The mean energy, Ef, required for fracture of an egg shell at its equator by a flat plate is known to depend on shell compression speed, vm, for speeds in the range 20 μm/s to 2.lb5 mm/s; analysis of published data shows that Ef bears a linear relationship to log (vm ) throughout the industrially important speed range from 20 μm/s to 1.lb1 m/s, increasing from 1.lb3 to 5.lb0 mJ.

2. At lower speeds down to 2 μm/s Ef is constant.

3. Ef is reduced if the flat plate is replaced by a sphere or cylinder; the greater its curvature, the smaller Ef.

4. Ef for the narrow pole is greater and for the broad pole smaller than that for the equator but these differences may be due in part to . systematic differences in shell thickness as well as curvature.  相似文献   


15.
The invertebrate benthos of South Lake, St Lucia was monitored monthly from August 1981 to July 1982 following a period of stable salinities of approximately 35°/oo. The overall mean biomass for the year (dry mass) was 2,63 g m-2 with mean values for muddy and sandy substrata of 4,19 g m-2 and 1,07 g m-2 respectively. The major contributors to standing stock were the bivalve Solen cytin-draceus and the polychaete Marphysa macintoshi. A total of 37 taxa were recorded compared with 23 in a previous survey in 1972-73. Present results are compared with those of 1972-73 and the differences discussed. The main changes were a decline in Assiminea, Nassarius and Prionospio and an increase in abundance of long-lived bivalves together with larger polychaetes. Species diversity at the sampling sites is analysed and discussed in terms of equitability of distribution.  相似文献   

16.
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17.
Pectoral spines from 750 Clarias gariepinus (Pisces:Clariidae) from Lake Sibaya were used for age and growth determinations. The spines have clear rings in the bone matrix. Rings formed on otoliths, vertebrae and opercula were less useful. A close correlation between ages determined from length frequency data and ages obtained by back-calculation of lengths for different ring classes exists. Spine rings are laid down annually, except in the first year. C. gariepinus in Lake Sibaya grow rapidly and remain in good condition to a length of about 500 mm (age 3+ years) but of the larger catfish, few exceed 650 mm (7+ years). Males grow slightly faster than females, and modal sizes are 580 - 590 mm TL, and 540 - 550 mm TL. Over one third of the adult population measures between 500 and 600 mm TL due to the sharp deceleration of growth rate after 500 mm TL. Modal length and growth rate of Sibaya catfish is equal to that of other C. gariepinus populations, but the number and condition of larger individuals is lower, probably because of inadequate food resources in deep water.  相似文献   

18.
The intertidal distribution and abundance of sand and mud-flat meiofauna at the mouth and the middle reaches of Mngazana estuary were monitored over a period of 15 months from April 1977 to July 1978 and the data compared with those from other South African estuaries. The meiofauna exhibited an early winter maximum with numbers ranging ranging from 2,74 x 106 to 7,27 x 106m-2 in sand and from 2,59 x 105 to 4,15 x 105m-2 in mud, both to a depth of 20 cm. The fluctuations correlated with variations in Eh and temperature. Annual production in the upper 20 cm is estimated at 8,0 gCm-2h-1 and 0,6 gCm-2y-1 in sand and mud respectively.  相似文献   

19.
The shrimp Palaemon pacificus was sampled monthly from six tidal pools for two years. The shrimp populations varied seasonally with peak numbers and biomass found in summer (December to March) following a major influx of juveniles in November. These recruits were composed of discrete modal groups (bimodal) which spread and became less obvious by April (polymodal). Females had a greater mean size range than males. The sex ratio varied temporally and overall male:female ratios varied from 1:1,66 to 1:2,96. Average shrimp total length was significantly correlated to the average and maximum pool depth and to log pool volume while the average number of cohorts per year was found to be negatively correlated to these parameters. In smaller pools the shrimp disappeared above 20-30 mm but remained up to reproductive age in the deeper pools. Average residence time was positively correlated to pool depth, but not to log volume. Mean summer growth rates were almost double those in winter and were combined in the von Bertalanffy growth equation L, = 64,51 (1-e-0,08902(t-0,1001)). P. pacificus reaches first maturity from egg at six months for males and 12 months 2 for females. Mean annual production varied between pools from 0,22 to 2,0 g m -2 and biomass from 0,06 to 0,5 gm-2. The highest P/B ratio of 4,24 was obtained from the smallest pool which had a low biomass but high productivity and shrimp turnover.  相似文献   

20.
The euryhaline component of the Zooplankton in the Sundays River estuary was sampled monthly at 10 stations from August 1979 to April 1981. In deeper regions surface and near-bottom samples were taken (Stations 1-7) using WP2 nets. Shallow (< 2,0 m) stations were sampled in surface waters only. Water temperature ranged between 14-27 °C. A full salinity gradient was present and salinity stratification was well developed. Acartia longipatella was most abundant during winter and spring (maximum 73 300 m-3) in the lower estuary. The interaction of temperature and salinity as factors regulating temporal and spatial distribution was evident, these factors also interacting in the water column owing to stratification. The pioneer copepod species, seudodiaptomus hessei, attained high abundance following floods or increased river inflow. Three species of mysid shrimps were common in the plankton in summer, each species showing clear zones of maximum distribution. Zooplankton standing stock (dry mass) ranged from < 10 mg m-3 to 1 450 mg m-3 In surface samples and < 10 mg m -3 to 8 275 mg m -3 in bottom samples. Contribution of mysids to standing stock was rarely less than 70% and often exceeded 90%. Differences in standing stock between surface and bottom samples were owing to behavioural differences between the Zooplankton species, particularly the mysids. Behavioural adaptations play an important role in the retention of the indigenous Zooplankton in the estuary. Aspects of trophic relationships are discussed and demonstrate the significance of the Zooplankton in the transfer of energy to higher trophic levels In this estuary.  相似文献   

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