When plenty is not enough: an assessment of the white stumpnose (Rhabdosargus globiceps) fishery of Saldanha Bay,South Africa

White stumpnose Rhabdosargus globiceps is the main target of the linefishery in Saldanha Bay. Increased fishing pressure over the last three decades, particularly by the recreational sector, has led to concerns regarding sustainability of the local white stumpnose stock. The fishery was exceptionally productive between 2006 and 2008, with an estimated annual catch of 141.2 tonnes (t). Only 3% of boat outings surveyed were commercial boats targeting white stumpnose, yet this sector accounted for 39.3 t (31%) of the average annual catch. The recreational boat sector accounted for most of the catch (70.0 t), and the recreational shore sector the least (31.9 t). Commercial boat catch per unit effort (CPUE; 3.7 fish angler^–1 h^–1) was more than 10 times that of recreational boats (0.3 fish angler^–1 h^–1). White stumpnose catch length-frequency differed significantly (p < 0.01) between the fishing sectors, with the commercial sector retaining larger fish (34.7 cm [SD 5.9]) than the recreational boat (33.9 cm [SD 5.9]) and shore (30.4 cm [SD 5.8]) sectors. A decline in commercial CPUE (2000–2015) of approximately 40% and a concomitant severe decline (>95%) in survey data for juvenile white stumpnose CPUE (2007–2016) indicate that the current rate of exploitation is not sustainable. Recovery of the white stumpnose stock will require a decrease in fishing mortality. Possible management regulations include sector-specific effort limitations, extending the ‘no take’ marine protected area, reducing the recreational-sector bag limit to 5 fish person^–1 day^–1, implementing a commercial-sector bag limit, and increasing the minimum size limit to 30 cm TL.     

The Ekman boundary-layer stress due to flow over a regular array of hills

The flow of water over a regular array of hills in a rotating laboratory experiment is studied as an analogue of planetary boundary layers. Gaussian-shaped hills of heighth = 1 cm andh = 1/3 cm covered the floor of a 228 cm diam rotating tank. The characteristic depth of the Ekman layer varied fromD = 0.1 cm toD = 0.3 cm and the Reynolds number for the Ekman layer varied up toRe = 150. The integrated boundary-layer stress as a function ofRe andD/h was determined by the rate of spin-up of the water after a small increase of the rotation rate.Selected functional relations were fitted to the empirical data by the method of least squares for the laminar and turbulent regimes of flow. These results indicate maximum non-linear effects of the hills and a minimum transition Reynolds number nearD/h = 0.2. The patterns of flow as indicated by dye tracers are briefly discussed. Peculiar circular bands of dye appear to be caused by the selective ejection of dye from the boundary layer when the circular flow is at a particular angle of incidence with respect to the triangular pattern of hills.The research reported herein was supported in part by the National Science Foundation under Grant NSF GA 4388.     

徐家围子断陷构造地质特征研究新进展

在徐家围子断陷深层连片三维地震精细解释的基础上,结合前人的研究成果,系统的刻画了深大断裂的性质、产状和时空分布规律。从区域构造应力场着手,详细剖析了徐家围子断陷的构造地质特征,重新构建了深大断裂体系。依据剖面特征和断陷的地质结构,首次在徐家围子断陷内解释出两条深大走滑断裂带,发现深大走滑断裂控制了营城组火山岩的形成和分布。明确了徐家围子断陷"两凹夹一隆、东西分带、南北分块"构造格局的动态演化过程及其控制因素。详细剖析了断陷期地层的沉积发育过程、后期改造过程以及现今赋存特征。为庆深气田火山岩储层分布规律预测奠定了基础。     

塔里木西克尔基性麻粒岩捕虏体的发现及其地质意义

在塔里木西部新生代碧玄岩中新发现了基性麻粒岩捕虏体。其原生矿物组合为Cpx+Opx+Pl (An=30~50)+Sp+Ilm+Tim,次生矿物组合为Ol+Pl (An>50)+alkaline feldspar+glass。原生矿物组合的温度和压力分别为1005~1059℃ 和 0.6~1.1GPa; 次生矿物组合形成于近等温减压过程和固相线上熔融以及之后的近地表快速冷却。这些麻粒岩捕虏体代表了早期底侵到塔里木板块下地壳的幔源岩浆。     

Disagreements between gradient-diffusion and Lagrangian stochastic dispersion models,even for sources near the ground

It is well known that if turbulent mass convection is modelled as diffusion, errors result unless trajectories from the source (ath) to the point of observation (z _p ) comprise many statistically-independent segments (Taylor, 1921). We show that this is not guaranteed merely by the Lagrangian timescale () at the source being small (e.g., source at ground), but that a better criterion istmax[(h), (z _p )], wheret is a typical travel time toz _p .     

Using a Discrete Fracture Network and Spatial Point Processes to Populate Veins and Model Grade in a Coarse Gold Deposit

Vein-hosted gold deposits are characterized by mineralization, which is spatially restricted to narrow vein structures. Drillholes intersecting a mineralized vein can lead to unreliable and biased assay values compared to selective mining unit scale block grades. In this work, a discrete fracture network is simulated and adapted to model gold mineralization within the veins. Veins are assumed planar and the required inputs are distributions of vein orientation, vein length, and vein intensity (i.e., density). These inputs are collected from drillhole data, geological mapping, and expert knowledge of the deposit. A spatial point process is then applied to model gold grade as discrete events or “nuggets,” which are spatially restricted to the simulated quartz veins for the case of incomplete mineralization of the veins; when the vein is completely mineralized, a vein thickness distribution is required. The methodology is applied to an epithermal gold deposit in northwestern British Columbia, Canada and shows improvement in restricting the influence of the high-grade gold samples without resorting to ad-hoc manipulation of input assays through capping or cutting. The final output of this methodology is a block model of gold grade, which better honors the spatial structure of the veins in the deposit and is suitable for use in mine planning or resource estimation.     

川滇地区地壳上地幔三维速度结构研究

根据云南和四川地震台网174个台站记录的4625个区域地震初至Ｐ波和Ｓ波走时资料，并结合其它深部地球物理资料，确定了川滇地区地壳上地幔三维速度结构．在上地壳速度异常分布中，四川盆地为正异常，川西高原为负异常，龙门山断裂带为正、负异常的边界．龙门山断裂、鲜水河断裂以及红河断裂等，在下地壳和上地幔的速度异常中仍显示出构造分界特征，说明它们可能穿透了莫霍界面．腾冲火山区和攀西构造带在50km深度上呈现负速度异常，与上地幔温度和物质组成的差异相联系．川滇地区地壳结构的总体特征是:地壳和上地幔的低平均速度，地壳厚度变化剧烈，地壳和（或）上地幔存在高导层、高热流值．这些同印度板块与欧亚板块碰撞的构造背景有关．川滇菱形块体在地壳内总体上为正常或正异常速度，而其边界的深大走滑断裂存在负速度异常，它有助于地壳块体沿断裂的侧向挤出．在主要的地震带上，中下地壳的负速度异常与地震活动性相关．多数强烈地震发生在具有正速度异常或正常速度分布的上中地壳深度上，而其下方则通常是负速度异常带．      

徐家围子断陷深层结构形成与演化的探讨

徐家围子断陷是松辽盆地北部深层最重要的含气断陷。本文根据最新地震和钻井资料综合研究成果,结合前人研究认识,探讨了徐家围子断陷的形成与演化,认为徐家围子断陷在区域构造位置上处于东北地区松辽盆地的北部,处于松辽盆地区域莫霍面隆起的西斜坡上,特定的位置决定了其形成演化的各个阶段受东北地区区域板块构造运动的控制,与松辽盆地的形成与演化具有一定的关系。徐家围子断陷形成与演化可以划分为5个阶段,是太平洋构造域板块间的相互作用和深部热力作用发育演化的结果,这种过程造就了现今南北分块、东西分带、凹隆相间、构造复杂和沉积岩与火山岩并存的地质结构。徐家围子断陷深层现今构造格局受控于近NW向的徐中断裂、近SN向的徐西断裂、近NW向的徐东断裂带和四组近NE向的断裂。本次研究,对以往无法解释的很多地质现象给予了很好的解释,得出了一些崭新的认识,也从宏观上为寻找天然气资源给予了有力指导。