Changes in some key biological parameters of the northern Benguela sardine stock

During the past 50 years, more than 15 million tons of sardine have been harvested from the northern Benguela ecosystem. The sardine population has undergone changes in abundance estimated to range over more than two orders of magnitude, but for the past two decades it is estimated to have remained at less than 10% of former levels of abundance. Some key changes in biological parameters, notably age composition and rates of natural mortality, are discussed in relation to the changes in abundance. Literature values for the period before the 1990s are compared to values calculated from acoustic survey population estimates thereafter. Although these recent data are potentially biased, and often lack estimates of precision, it is argued that they are sufficient to demonstrate general trends. The likelihood of the sardine stock recovering is seemingly reduced owing to the recent high rate of adult natural mortality.Keywords: biological parameters, exploitation, mortality, northern Benguela, sardine, Sardinops sagaxAfrican Journal of Marine Science 2001, 23: 111–12

Dynamic Model for Thrust Generation of Marine Propellers

: Mathematical models of propeller thrust and torque are traditionally based on steady state thrust and torque characteristics obtained in model basin or cavitation tunnel tests. Experimental results showed that these quasi steady state models do not accurately describe the transient phenomena in a thruster. A recently published dynamic model was based on the experimental observations. Describing zero advance speed conditions accurately, this model, however, does not work for a vessel at nonzero relative water speed. This paper derives a large signal dynamic model of propeller that includes the eects of transients in the ow over a wide range of operation. The results are essential for accurate thrust control in dynamic positioning and in underwater robotics. Keywords: propellers, thrusters, dynamic positioning, underwater robotics, thrust control 1. INTRODUCTION Underwater vehicle (UUV) speed and position control systems are subject to an increased focus with respect to ..

Changes in abundance of the northern Benguela sardine stock during the decade 1990 &#150 2000, with comments on the relative importance of fishing and the environment

The northern Benguela stock of sardine Sardinops sagax used to be considered one of the major clupeoid stocks of the world; it supported an average annual catch of >700 000 tons throughout the 1960s. The stock has been in a depressed state for more than two decades, as demonstrated by annual catches that averaged around 50 000 tons between 1978 and 1989 and only slightly more in the 1990s. It has experienced fluctuations in abundance of several orders of magnitude during the most recent decade. Population size increased until 1992, when the acoustic estimate of biomass was about 750 000 tons. Catches increased accordingly, averaging 100 000 tons between 1992 and 1995, but from 1992 to 1996 the stock was in decline and the lowest annual catch in the history of the fishery was taken in 1996. Although there was a small increase during the last three years of the decade, the stock remains seriously depleted. Survey-based recruitment indices suggest that the changes in the 1990s were initiated by fluctuations in recruitment, but the decline was almost certainly exacerbated by continued fishing. Poor recruitment and decreasing catch rates between 1993 and 1996 in a number of other key resources suggest that system-wide environmental changes were an important factor in the decline of the sardine stock at that time. Anomalous oceanographic conditions, such as extensive hypoxic shelf waters in 1993/94 and a Benguela Niño in 1995, support this conclusion.Keywords: environmental effects, fishing effects, northern Benguela, recruitment, sardineAfrican Journal of Marine Science 2001, 23: 67–8

Design and Optimization of Composite Gyroscope Momentum Wheel Rings

Stress analysis and preliminary design/optimization procedures are presented for gyroscope momentum wheel rings composed of metallic, metal matrix composite, and polymer matrix composite materials. The design of these components involves simultaneously minimizing both true part volume and mass, while maximizing angular momentum. The stress analysis results are combined with an anisotropic failure criterion to formulate a new sizing procedure that provides considerable insight into the design of gyroscope momentum wheel ring components. Results compare the performance of two optimized metallic designs, an optimized SiC/Ti composite design, and an optimized graphite/epoxy composite design. The graphite/epoxy design appears to be far superior to the competitors considered unless a much greater premium is placed on volume efficiency compared to mass efficiency