共查询到20条相似文献，搜索用时 15 毫秒
1.
2.
3.
R. I. C. C. Francis 《新西兰海洋与淡水研究杂志》2013,47(3):375-389
The design and execution of two acoustic surveys of Hawke Bay in February and September/October 1980 are described. Though the resulting acoustic data were, for technical reasons, of limited value, they illustrate the information which may be drawn from acoustic surveys. Fish distributions may be rapidly mapped, not only by area, but throughout most of the water column and over time. Qualitative observations on fish distribution may be quantified. The estimation of variance in acoustic surveys is discussed in detail with particular emphasis on the problems of systematic surveys. The role of replicate observations is stressed. Coefficients of variation for relative fish density estimates from the two surveys probably lie between 5 and 30%. 相似文献
4.
5.
6.
7.
8.
Fisheries development in northeastern Asia in conjunction with changes in climate and social systems
The status and prospects of fisheries in northeastern Asia are reviewed in conjunction with climate change and social issues. This region has the biggest yield, consumption, and international trade of fisheries products in the world. Due to increasing industrialization and greenhouse gas emissions, higher rates of warming have occurred. The impacts of climate change on fisheries could be detrimental because the main fishery products are small pelagic fish whose biomass fluctuations are sensitive to climate changes. Improvements in the economies of the nations in this region offset the demand for fish products in human diets, and employment in fisheries. It is recommended that fisheries management plans consider both changes in climate and in social systems. Stocks which migrate between national jurisdictions are in need of particular attention. 相似文献
9.
《Marine Policy》2017
Small pelagics are the main fish resource in North West Africa. In Senegal, these are mainly sardinellas (Sardinella aurita and S. maderensis) and bonga shad (Ethmalosa fimbriata). The fisheries, mainly encircling gillnets and purse seines, are predominantly performed by artisanal fishers and are of great importance for the Senegalese economy and for food security in the region. However, in recent years, the main conditions for these fisheries have changed and recent observations have shown strong declines in profit. An analysis over the last twenty years (1993–2013) show that the fisheries lost profit between 65% and 100% while operating costs increased by 25% and 90%, for encircling gillnet and purse seine, respectively. While the fuel price dominates as determining factor during the survey period, important other drivers during the last five years were a decrease in fish biomass and an increase in fishing effort. 相似文献
10.
Modelling effects of fishing in the Southern Benguela ecosystem 总被引：2，自引：0，他引：2
11.
The spring-spawning Baltic Sea herring spawn in coastal areas that also serve as nursery areas for the young fish during their first summer. In a bay known as a herring spawning and nursery area, the pelagic fish abundance was quantified using hydroacoustics every second week from late spring to autumn in 2000 and 2001. A dense system of survey transects allowed determination of the acoustic index (the nautical area scattering coefficient) for fish abundance with high precision. The variation, expressed as the geostatistical coefficient of variation, was on average 5% both years and ranged from 3–11% (2000) and 3–8% (2001). Through the hydroacoustic data intra-annual dynamics in acoustic fish abundance, densities and size composition could be followed, which showed similar trends in both years. In spring and early summer acoustic fish densities were low, followed by a drastic, 20-fold increase in late summer. Hydroacoustic data and biological samples suggest that the increase was caused mainly by the recruitment of young-of-the-year herring to the acoustically assessable pelagic fish community. This age class is commonly not well represented in catches when using traditional sampling gears such as gill nets and trawls, and hydroacoustics may help to improve quantitative estimates of small juvenile fish in order to increase the understanding of biological processes in coastal nursery areas. 相似文献
12.
S Swart JJ Zietsman JC Coetzee DG Goslett A Hoek D Needham 《African Journal of Marine Science》2016,38(4):525-538
South Africa’s small-pelagic fishery is a socio-economically important component of the country’s commercial fisheries sector, second in value only to the demersal trawl fishery. Management of this sector relies on infrequent hydro-acoustic surveys, which provide measures of anchovy Engraulis encrasicolus and sardine Sardinops sagax biomass used in the assessments of stock status and in the development of management plans for the sustainable utilisation of these resources. We demonstrate how technological capabilities in ocean robotics at the Council for Scientific and Industrial Research (CSIR) could augment the current resource-intensive hydro-acoustic ship-based survey programme and create opportunities for expanding its spatial and temporal resolution. We successfully implement and demonstrate an autonomous wave glider, fitted with a hydro-acoustic sensor and compare the data to a collocated ‘traditional’ ship-based acoustics survey. In the future these autonomous systems approaches could be seen as a means to lessen the cost burden of the ship-based survey, while at the same time with the added advantage of continuous collection over much wider spatial and temporal domains. This could enable a more reflexive stock management approach taking into account the seasonal characteristics of the fishery and its ecosystem. Gliders thus have potential to increase dramatically the quantity of information available to fisheries managers, thereby reducing uncertainty and contributing to improved management of valuable fish resources. They are likely to contribute to improved knowledge of the ecology of small pelagic fish species off the coast of South Africa in a changing climate and should potentially also permit the collection of biomass data for other marine resources currently not routinely monitored. 相似文献
13.
14.
15.
《African Journal of Marine Science》2013,35(4):451-460
Status assessment of exploited fish populations is a general requirement to ensure sustainability of fisheries in both global and local contexts. The blue jack mackerel Trachurus picturatus (Bowdich, 1825) remains unassessed in a data-limited fishery in the eastern central Atlantic, within the waters of the Canary Islands (Spain) off the coast of northwest Africa. This is the most southwestern targeted fishery in the species’ distributional range. To obtain basic life-history information, a total of 2 472 specimens were analysed, ranging from 10.2 to 31.9 cm total length, from samples collected monthly between March 2005 and March 2006. Length–weight relationships (a = 0.0023; b = 3.418; R 2 = 0.97) showed no significant difference between sexes. Deposition periodicity and coherence of the ageing criteria were verified by analysis of the monthly evolution of the otolith edge. Ages were estimated by counting annual growth rings on whole otoliths, with these fish ranging from age 0 to 6 years. The von Bertalanffy growth model, which was used to estimate growth parameters from both direct readings and back-calculated length-at-age data, gave good statistical fits. The parameters from the direct readings were: L∞ = 34.9 cm; k = 0.214 y–1; t 0 = ?2.545 y (R 2 = 0.84), and the natural mortality rates estimated from empirical methods varied between 0.20 and 0.55 y–1. 相似文献
16.
17.
浮游软体动物是海洋中各种终生营浮游生活的软体动物的概称,为海洋浮游动物的一大类群,一般属于腹足纲(Gastropoda)。如前鳃亚纲(Prosobranchia)中的异足亚目(Heteropoda)及海蜗牛科(Janthiniidae),后鳃亚纲(Opisthobranchia)中的翼足目(Pteropoda)、腹翼螺科(Gastropteridae)、波叶海牛科(Phyllirrhoidae )及海神鳃科(Glaucidae)等。其中以翼足目、异足亚目及海蜗牛科为三个主要类群,其余三科无论种类或数量都非常少。
浮游软体动物大多数为暖水性、营大洋上层生活的种类,仅有极少数冷水种及深水种。有的种类在极海出现数量很大,为鲱鱼、鲸及某些海鸟的普通饵料;某些种类在暖海有时出现数量也相当可观,可能也会有一定的饵科意义(如Okutani, 1960,发现Argonauta boettgeri捕食龟螺;著者1960年在广东水产研究所采集的大眼鲷的胃含物中发现明螺。它们的死壳沉积于海底,为生物性沉积的一个组成部分,在热带海形成有名的翼足类软泥,在海洋学上,有的种类已被取用为水团或海流指标。在海洋动物地理学上,也已根据它们的分布特点,对大洋上层浮游动物区系进行过区划。
我国海浮游软体动物的研究工作,迄今还比较零散。 本文整理了中国科学院海洋研究所历年采集的标本、全国近海调查(1958-1960)的标本、中苏东海渔场调查(1958)的沉积样品及浙江近渔场调查(1960)的部分活体标本,共记录了我国近海浮游软体动物11科、28属、59种及6个变种,其中有1个新属、3个新种及1个新变种。以我国近海来说,其中除15种外,其他全为新记录;就南海与东海的全范围而论,约半数的种类为新记录。在本文记录中,翼足类为7科、19 属、35种及6个变种(Agadina属除外),约占翼足类总种数的1/3;异足类3科、8 属、20种,包括异足类所有的科、所有的属及绝大部分的种;海蜗牛类为2种。
另外的几科浮游软体动物,在我国海也有代表,将在以后另文报告。各种浮游软体动物在我国近海的分布状况,亦将另文论述。文中各种类的描述及插图,都是根据著者的实际观察结果。 相似文献
18.
《African Journal of Marine Science》2013,35(2):233-240
Once one of the most numerous seabirds of the Benguela upwelling system, the population of Cape cormorants Phalacrocorax capensis has decreased by 60% in the past three decades and the species is listed as Near Threatened. Declines in prey availability and/or abundance brought about by recent changes in the distribution of pelagic fish stocks and industrial purse-seine fishing are hypothesised to be a key driver of seabird population decreases in the southern Benguela. We investigated the foraging behaviour of breeding Cape cormorants by deploying GPS and temperature–depth recorders on 24 breeding adults from three islands off the coast of South Africa, two of them to the north of Cape Point and a third farther south on the western Agulhas Bank. This provided the first measures of foraging dispersal by a cormorant in the Benguela system, and enabled a comparison of foraging behaviour between birds from these islands. Foraging trips of Cape cormorants lasted between 17 min and >7 h, at a maximum distance of between 2 and 58 km away from their colony. Foraging effort was significantly greater for birds from farther north off the West Coast in terms of trip duration, distance travelled, number of dives and time spent flying compared to those from the southernmost island (Dyer), which is probably a response to low prey availability in the north. Coastal reserves that exclude pelagic fishing from inshore feeding grounds around Cape cormorant breeding colonies may result in increased local prey availability, which would benefit Cape cormorant populations. 相似文献
19.
20.
F Forget L Dagorn B Mérigot JC Gaertner J Robinson PD Cowley 《African Journal of Marine Science》2020,42(2):247-254
Owing to difficulties in accessing the vast open ocean, the beta (β) diversity of pelagic fish assemblages remains poorly studied. We investigated the relationship between assemblage similarity and geographical distance between anchored fish aggregating devices (FADs), sampled by standardised underwater visual censuses in three anchored FAD arrays in the Indian Ocean—at the Maldives, the Seychelles and Mauritius. The use of two complementary indices of β-diversity, based on presence/absence data (Jaccard similarity coefficient) and abundance data (Bray–Curtis index), revealed that geographical distance between sampling sites (from 4 to 257 km) appeared to have no effect on the similarity of fish assemblages associated with FADs within each array. The results of this preliminary study question the generalisation of the paradigm of an increase in β-diversity with geographic distance to the open-ocean fish community. Large-scale studies using a variety of datasets should be conducted to further investigate patterns of β-diversity in the open ocean. 相似文献