Catch and effort of the shore and skiboat linefisheries along the South African Eastern Cape Coast

An assessment of catch and effort in the Eastern Cape shore and skiboat linefisheries was undertaken between 1994 and 1996 by means of roving creel and access point surveys. Catch-and-effort data were obtained from direct observation of 3 273 shore-fishers, 172 recreational and 223 commercial skiboat outings. Total effort in the region was high at 903 186 fisher-days year^?1 in the shore fishery and 64 266 and 24 357 fisher-days year^?1 in the commercial and recreational skiboat sectors respectively. The fisheries are multispecies in nature. The shore fishery consisted of 66 species, the recreational skiboat fishery 44 species and the commercial skiboat fishery 48 species. Just 10 species accounted for 75, 83 and 90% of the catch of the shore, recreational and commercial skiboat fishery respectively. The average catch per unit effort (cpue) was low in all sectors, 1.15 kg fisher^?1 day^?1 in the shorefishery, 9.4 kg fisher^?1 day^?1 in the recreational skiboat fishery and 21.5 kg fisher^?1 day^?1 in the commercial skiboat fishery. Catch data showed that professional and club anglers are more successful fishers. Comparisons with historic records for Port Elizabeth revealed that the cpue in the shore fishery had declined markedly, whereas the total effort increase was negligible (0.1%). In addition, the species composition of the fishery has changed.     

Evaluation of a class of possible simple interim management procedures for the Namibian hake fishery

During 1997, considerable scientific differences arose about the status of the Namibian hake (Merluccius capensis and M. paradoxus) resource, and as to whether the hake Total Allowable Catch (TAC) should be substantially decreased or increased. These differences revolved primarily around whether or not abundance estimates from the swept-area trawl surveys by the Norwegian research vessel Dr Fridtjof Nansen should be considered as reliable measures of biomass in absolute (as distinct from relative) terms. The paper relates the computations underlying the Interim Management Procedure (IMP) approach that was put forward at that time as a basis to resolve this impasse. The anticipated performance, in terms of catch and risk of resource depletion, of a number of simple candidate IMPs for the Namibian hake resource is evaluated. The IMPs depend on two parameters, whose values are to be chosen by decision-makers, and adjust the TAC up or down from one year to the next according to whether trends in recent commercial catch rate and survey indices of abundance are positive or negative. Performances are evaluated across the then current wide range argued for resource abundance and status. Trade-offs in performance across the candidates considered are discussed. One of the candidates was subsequently selected by a joint meeting of scientists, industry and Ministry officials in February 1998 and served as the basis for scientific recommendations for the TAC for the hake resource for the following three years.     

吕宋海峡深层水体体积输运的诊断分析

使用WOD09, WOA05, WOA01和SODA四种水文数据,利用水力学理论对吕宋海峡的深层水体体积输运进行了估计。结果表明：由于忽略了摩擦和地形等因素的影响,水力学理论得到的输运结果是深层体积输运的上限。不同数据以及吕宋海峡东西两侧选取的不同区域,都对水力学理论的体积输运估计存在显著影响。SODA数据在吕宋海峡附近的深层海区存在偏差,得到体积输运估计明显偏大,因而不适用于水力学理论进行吕宋海峡深层水体体积输运的估计。为对于南海的环流、混合及生态环境存在重要影响的吕宋海峡深层水体体积输运进行合理估计,对利用水力学理论估计深层水体体积输运的研究方法做了进一步探讨。     

An assessment of the recreational fishery in the St Lucia estuarine system,KwaZulu-Natal,South Africa

St Lucia is one of the largest estuarine systems in Africa, and attracts thousands of anglers each year. Catchcard data from the National Marine Linefish System for the years 1986–1999 were analysed to determine catch composition, catch per unit effort (cpue) and seasonality of catches by recreational anglers. Because not all anglers completed catch cards, estimates of total catch were made using additional data on the number of private boat outings, the number of boat trailers at boat slipways and the number of boats recorded entering the campsite gates during 1992 and 1993. In all, 27 fish families, constituting 55 species, were recorded by recreational anglers. Dusky kob Argyrosomus japonicus, spotted grunter Pomadasys commersonnii, perch Acanthopagrus berda, Natal stumpnose Rhabdosargus sarba, springer Elops machnata and mini-kob Johnius dorsalis were the most prominent species caught in terms of numbers and mass. Catch rates expressed numerically (fish angler^?1 h^?1) peaked during the summer and early winter. However, in terms of mass, catch rates peaked during late winter and spring, when there were increased landings of large dusky kob. Fluctuations in cpue were linked to salinity and estuary mouth conditions (i.e. mouth closure). Despite annual fluctuations in cpue, regression analysis revealed an overall downward trend for the dominant species (dusky kob and spotted grunter) and a gradual increase for stumpnose, perch and springer, but with the exception of stumpnose, these trends were not significant. Socio-economic aspects of the fishery were also investigated by conducting an independent boat-angler survey. The value of the recreational fishery, in terms of accommodation and direct angler expenditure, was estimated to be in the region of R9 million during 1992. Angler attitudes towards fishing regulations were positive and anglers generally had a good knowledge of the regulations for target species. Based on this assessment, a number of suggestions are made regarding the future management of the recreational fishery at St Lucia.     

The process of developing a management system for subsistence fisheries in South Africa: recognizing and formalizing a marginalized fishing sector in South Africa

Subsistence fishers were first recognized as a formal fishing sector in South Africa when new fishing legislation, aimed at redressing past inequalities, was enacted in 1998. Little information was available about these fishers, their activities, and the resources upon which they rely. Recognizing the imperative to gain an understanding of the fishers and to consult broadly, the national agency responsible for the management of marine living resources, Marine and Coastal Management (MCM) of the Department of Environmental Affairs and Tourism, appointed a Subsistence Fisheries Task Group (SFTG) in December 1998 to provide advice on the implementation of appropriate management systems for subsistence fisheries. This paper describes the process followed to formulate recommendations that were presented by the SFTG to MCM in February 2000. The activities of the SFTG fell into two categories: research aimed at identifying subsistence fishers and gaining an understanding of their activities and socio-economic profiles; and consultation aimed at ensuring that the needs and aspirations of fishers and the experience of local managers were incorporated. Research included both field-based studies and synthesis of information about comparative fisheries elsewhere. Consultation took the form of local interviews and focus-group discussions, meetings with fishers and a national workshop. A pivotal activity was the development of a clear definition and qualifying criteria for subsistence fishers. A significant outcome was the identification of a separate small-scale commercial sector, previously erroneously lumped with subsistence fishers. Needs of fishers and problems identified during the process provided the basis for recommendations in the following areas: definitions, assessment and categorization of resources, management systems, communication mechanisms, application and allocation procedures, capacity building, compliance, research and monitoring, and funding and staff required for the management of this new sector. An evaluation is made of the opportunities presented by the SFTG process, constraints experienced and lessons learnt, giving important insights that are applicable to other similar processes, yet seldom documented in formal literature.     

The effect of fishing on community structure

Fishing can affect the structure of fish communities, but the size of the effects is difficult to measure and they can easily be confounded with environmental effects. The simplest effect is the increase of prey species when large predators are reduced, but these effects are seldom large. Changes in the balance within a group of ecologically similar species can be much more dramatic, especially in the case of stocks of small pelagic species, and several large fisheries on sardine, herring and anchovy have collapsed, sometimes accompanied by a rise of other species. Part, not all, of these changes can be ascribed to the effects of competition, and of selective fishing directed at one preferred species. Other changes, such as the gadoid outburst in the North Sea in the 1960s, may have involved less obvious mechanisms. Because few of the changes in the community structure can be reliably predicted, although they can have dramatic effects on the success of the fisheries, difficult management problems are raised. Managers have to recognize uncertainty and take a range of possible consequences into account when setting policies.     

The effect of cylindrospermopsin on the bacterioplankton community: a microcosm experiment on water from Dishui Lake,Shanghai, China

Eutrophication and climate warming have intensified the global expansion of invasive cyanobacteria such as Cylindrospermopsis spp.and Chrysosporum spp.Cylindrospermopsin(CYN)produced by species of the latter two genera may harm phytoplankton,zooplankton,and fishes.However,effects of CYN on the bacterioplankton community have not been studied.Based on high-throughput sequencing,we explored the effect of CYN on the structure and function of the bacterioplankton community by adding pure CYN to in-s...     

The orange roughy fishery of Namibia: lessons to be learned about managing a developing fishery

Exploration for orange roughy Hoplostethus atlanticus in Namibia started in 1994 and within 12 months several aggregations had been discovered, suggesting the existence of a biomass sufficient to support a viable fishery. At that early stage it was realized that few, if any, recognized management procedures existed for newly developing fisheries, especially with the paucity of data such as existed on Namibian orange roughy. The development of the Namibian orange roughy fishery is reviewed to document the management strategies implemented and how the management of the fishery evolved. The first six years of the fishery are described, including the three-year exploration phase, several years of profitable exploitation, and the severe decline in catch rates. Whether the decline is attributable to fishing mortality or to change in the aggregating behaviour of orange roughy, or both, is not clear. Although many aspects of the precautionary approach were followed, a risk analysis applied and a number of innovative management methods implemented (e.g. incentives to promote exploratory fishing, use of Bayesian statistical methods, implementation of a management plan for long-term total allowable catches), the aggregating biomass declined to between 10 and 50% of virgin levels within the six years. The management methods applied are evaluated in the light of the severe decline in catch rate experienced in 1998 and 1999, so that others may learn from the experience.     

The effect of oscillations on the flow patterns near a simulated bed

This work investigates the effect of oscillations on the flow near a simulated bed and their impact on erosion. Those oscillations were generated by using wavy lids at the top of a small toroidal channel. Particular attention is dedicated to their influence on the velocity field and turbulence near the bed. The experiments were made in a mini circular flume using a 2D LDA system. It was concluded that such oscillations induce non negligible shear stresses in the flow main stream, becoming more important to the dispersion of entrained matter into the flow than to the initiation of the erosion process.     

Downward flow of dense water leaning on a submarine ridge

Large-scale dense bottom currents are geostrophic to leading order, with the main flow direction along the continental slope. Bottom friction makes the water descend to greater depths, but only at a small angle to the horizontal. Here the effect of a submarine ridge that intersects the slope is considered. It is shown that the presence of a submarine ridge greatly enhances the downward transport. By leaning against the ridge it is possible for the dense water to flow downhill, perpendicular to the depth contours, even though the first-order dynamics are geostrophic. The requirement for downward flow next to the ridge is that the frictional transport that it induces is sufficiently large to counteract geostrophic advection along the isobaths and out of the ridge region. The dynamics are similar to those of downward flow in submarine canyons, but ridges appear to be more effective in channeling the dense water downhill, in particular for narrow ridges/canyons with small seaward slope of the ridge/canyon axis. The downward flow is analyzed using a simplified analytical model and the results are compared to data from the Filchner Overflow, which agree qualitatively with the model.     

Effect of observation method on the perception of community structure and water quality in a brackish water ecosystem

The EU Water Framework Directive is a Community legislative instrument in the field of environmental protection that establishes a common framework for keeping water quality at a favourable level. To implement the directive, classification systems need to be established that allow detection of human impacts at early stages and, thus, more effective management of coastal communities. Due to the spatial variability of communities, however, the results of any assessment are highly dependent on the selection of data. In this study we identified local spatial scales in which variability of macrophyte communities was maximised, quantified links between observed patterns of sediment types and communities and estimated how selection criteria impacted the outcome of the assessment of indicator class value in four different communities of the Northern Baltic Sea. The main findings of the study were that: (i) there were no clear local spatial scales in which the variability of benthic communities was maximised; (ii) hard-bottom communities were better predicted by the spatial arrangement of sediment characteristics than soft-bottom communities; (iii) the selection of method had no effect on the estimates of macrophyte cover and indicator class; but (iv) method impacted independently of habitat type on error estimates of macrophyte cover and indicator class. To conclude, in such homogeneous and low diversity macrophyte communities it is preferable to use methods that result in lower error estimates of algal coverage and, thus, result in lower uncertainties of estimates in the water quality class.     

Combined tidal velocity and duration asymmetries as a determinant of water transport and residual flow in Paranaguá Bay estuary

Net transports of water, salt and suspended particulate matter (SPM) for a cross-section in front of the Paranaguá Harbour (Paranaguá Bay, Brazil) are presented for eight distinct tidal cycles. Data include measurements over single spring and neap tidal cycles, during both wet and dry seasons. The main drive forces of circulation and SPM dynamics are identified. Advective transport dominated under moderate to high vertical salinity stratification and weak currents, while mixing processes dominated under well-mixed conditions generated by high currents and low freshwater input. Under partially mixed conditions, both advective and mixing processes were important. The tide-induced residual circulation dictated the magnitude and direction of residual currents and net transports of water and salt, but not of SPM transport. The SPM dynamics was intrinsically related to cyclical processes of erosion, resuspension and deposition driven by tidal currents. The turbulent mixing intensity conditioned the vertical mixing of SPM. Resuspension and vertical mixing were conspicuous in spring cycles, while the horizontal advection preponderated in the neaps. Lags between maximum currents and SPM peak concentrations occurred, with more pronounced hysteresis during ebb periods.     

The effect of water depth on the performance of a small surging wave energy converter

The effect of water depth on the performance of a small surging wave energy converter (WEC) is investigated analytically, numerically and experimentally. It is shown that although the average annual incident wave power is significantly reduced by water depth, a large proportion of this reduction is due to the dissipation of highly energetic, but largely unexploitable seas. It is also shown that the power capture is related more closely to incident wave force than incident wave power. Experimental results demonstrate that both the surge wave force and power capture of a flap-type WEC increase in shallow water.     

The effect of Stokes drift on Ekman transport in the open sea

By introducing the wave-induced Coriolis-Stokes forcing into ageostrophic motion equation,the Eulerian transport is modified by the wave-induced Stokes drift.The long-term mean contributions of the Stokes transport with remotely generated swells being included to the ageostrophic transport are analyzed using the ECMWF(European Centre for Medium-Range Weather Forecasts) reanalysis data.The ratio of Stokes transport to Ekman transport in north-south(N-S) direction can reach a maximum of over 50% in the subtropical region.The preliminary influence of the Stokes transport on the North Pacific gyre is all year persistent,while the effect on the North Atlantic gyre is only obvious in boreal winter and early spring.     

水体溶解无机氮对角毛藻胞内活性氮组分的影响

本文研究了水体中不同形式溶解无机氮(NH_4~+,NO_3~-,NH_4~++NO_3~-)及其含量对牟氏角毛藻和钙质角毛藻胞内活性氮组分(NH_4~+,NO_3~-,AA,Pr,RNA及DNA)的影响。结果表明:角毛藻胞内无机氮储量的大小体现了水体中无机氮的营养水平;培养角毛藻采用混合无机氮比之单一无机氮,其胞内AA和Pr含量较高,AA/Pr值可定量标志水体无机氮的营养水平及藻类本身氮的营养状况;RNA/DNA值是藻类生长增殖状况的指标之一。     

The effect of horizontal Coriolis force on internal wave-mean flow interaction

-In this paper, an improvement and revision of the theory of Miiller (1974, 1976), has been made under two conditions: (1) the horizontal component of the Coriolis force has been taken into account in the equations of motion for the internal wave field; and (2) the role of internal waves with frequencies close to the inertial frequency has been considered. The values of the viscosity coefficients and the diffusivity coefficients obtained in this paper are: Theappearance of the cross-diffusion (vc) of momentum is a natural result from the effect of the horizontal Coriolis force. Therefore the role of the horizontal shear of the mean flow cannot be absolutely separated from that of the vertical shear. So far in testing Muller's theory, the approximation of effective wave stress obtained by Ruddick and Joyce (1979) was extensively used, but it has to be revised under the conditions of this paper. The revised approximation shows that the internal waves with low frequencies close to the inertial frequency pl     

Role of large particles in the transport of elements and organic compounds through the oceanic water column

During the past decade data from a variety of sources have been obtained which show conclusively that the relatively rare, large particles sinking through the water column are responsible for the majority of the downward vertical mass flux in the sea. This finding has important implications for understanding the transfer, distribution and fate of elements and organic compounds in marine waters. The “large” (> 100 μm) detrital particles responsible for vertical flux are primarily biogenic and range in size and composition from small, discrete fecal pellets and plankton hard parts to large, amorphous aggregates or “snow” which contain both organic and inorganic constituents. Depending on size, shape and density, these particles sink at rates ranging from <1 to >1000 m day⁻¹. Several methods have been developed for sampling these particles of which in situ sediment trapping has probably furnished the most comprehensive qualitative and quantitative information on the role large particles play in material transport. Flux studies have highlighted the importance of marine heterotrophs in packaging fine, suspended particulate matter into large rapidly sinking particles which accelerate the movement of incorporated materials to depth. Large particle production via biological packaging is not restricted to the euphotic zone but can occur at all depths and information is now accruing on rates of production of large particles in the water column. Chemical analyses of sedimenting particles collected in sediment traps and those sampled by other means have allowed quantifying vertical fluxes and residence times of elements, radionuclides and organic compounds (natural and anthropogenic) in various oceanic regimes. Pertinent studies dealing with the above aspects are reviewed and several areas for future research are suggested.