Feeding habits, daily ration and vertical migration of the cape horse mackerel off South Africa

Crustaceans, principally copepods and euphausiids, were most frequently found in the stomachs of Cape horse mackerel Trachurus trachurus capensis collected during demersal research surveys (1992–1995) in summer on the west coast and in winter on the south coast of South Africa. Fish (mainly pelagic) were infrequent (< 10%) in the diet of fish from both coasts. The feeding periodicity, rate of gut evacuation and vertical migration of horse mackerel were investigated from midwater and bottom trawl collections taken during five diel sampling periods between 1993 and 1995 at fixed positions on the South Coast. Horse mackerel feed only during the day, mostly in late afternoon prior to their ascent into midwater at around sunset. Based on the exponential rate of decline in stomach fullness throughout the night, the rate of gut evacuation by horse mackerel was estimated to be 0.22.h-1. Using the Elliott and Persson method, the daily ration was estimated as 3.8% of wet body mass. Horse mackerel appear to migrate vertically for reasons other than feeding, and selective advantages of this behaviour are discussed

Trends in the abundance and distribution of anchovy and sardine on the South African continental shelf in the 1990s, deduced from acoustic surveys

The South African pelagic resources have been monitored acoustically since 1983. The results of these surveys are currently used to determine catch limits for anchovy Engraulis capensis and sardine Sardinops sagax. Twomain surveys are conducted annually, in winter to determine the strength of the year’s recruitment and in summer to estimate the size of the adult stock. The results of the surveys conducted between 1990 and 1997 are presented and discussed in the context of the time-series of stock abundance. Distribution patterns, size structure and biomass levels are related to general trends in the life-history strategies of anchovy and sardine, as well as the major environmental features in the southern Benguela in the 1990s. The biomass of adult anchovy appears to be driven largely by the strength of the early recruitment, corresponding to fish spawned in the first half of the summer spawning season. The biomass of adult sardine, however, is better explained on the basis of the biomass of 2-year-old and older fish in the previous year’s survey. Anchovy tend to move east with age, whereassardine appear to move both north, along the south African west coast, and east with increasing age

Distribution patterns, stock size and life-history strategies of cape horse mackerel trachurus trachurus capensis, based on bottom trawl and acoustic surveys

Research surveys of Cape horse mackerel Trachurus trachurus capensis abundance on the south coast of South Africa are complicated because changes in the species’ vertical and horizontal distribution limit the valueof stock assessments based a single survey method. Annual bottom trawl surveys conducted in spring provide estimates of the abundance of fish close to the bottom over trawlable grounds. Between 1991 and 1994, hydroacoustic surveys conducted in spring have been used to estimate the pelagic portion of the stock, as well as the portion over untrawlable grounds. These two research datasets, as well as data from purse-seine, midwater and bottom trawl commercial landings, are reviewed to elucidate distribution patterns of horse mackerel and their migratory and spawning strategies. The problems and advantages of bottom trawl and acoustic surveys are discussed in the context of fluctuations in estimates of the size of the stock between 1991 and 1994 and the prevailing environmental conditions. It is concluded that combined acoustic and bottom trawl surveys are the only effective means of surveying horse mackerel, and that effort should be concentrated east of 22°E to assess the spawner stock. It is suggested that research effort directed at improving understanding of exchanges between West Coast (including Namibia) and South Coast population of horse mackerel, as well as of the role of verticalmigrations in modulating these exchanges, would be beneficial

Direct hydroacoustic observations of chokka squid Loligo vulgaris reynaudii spawning activity in deep water

Unusual and distinct hydroacoustic targets were observed in November 1996, May 1998 and November 1998 during routine pelagic biomass surveys off the south coast of South Africa. During the November 1996 survey, seven such targets were observed near the bottom at depths of 115–125 m, directly south of the traditional inshore spawning grounds of chokka squid Loligo vulgaris reynaudii at Cape St Francis. The targets were close to prominent seabed ridges and extended 30–40 m off the bottom. In May 1998, three similar targets were observed at depths 55–80 m off Plettenberg Bay, another well-known squid spawning site. The shallowest target was identified, by means of a midwater trawl, as a mixture of mature male and female chokka. During the November 1998 survey, nine similar targets were again observed on the squid spawning grounds at Cape St Francis, also adjacent to seabed ridges. Drawing on fisheries hydroacoustic experience and knowledge of chokka squid spawning behaviour, the targets are believed to be aggregations of spawning squid. Keywords: chokka squid, hydroacoustic targets, spawning aggregationsAfrican Journal of Marine Science 2002, 24: 387–39

Task-Oriented Conversational Behavior of Agents for Collaboration in Human-Agent Teamwork

International audienceCoordination is an essential ingredient for human-agent teamwork. It requires team members to share knowledge to establish common grounding and mutual awareness among them. This paper proposes a be-havioral architecture C 2 BDI that enhances the knowledge sharing using natural language communication between team members. Collaborative conversation protocols and resource allocation mechanism have been defined that provide proactive behavior to agents for coordination. This architecture has been applied to a real scenario in a collaborative virtual environment for learning. The solution enables users to coordinate with other team members

Importance of fisheries for food security across three climate change vulnerable deltas

Deltas are home to a large and growing proportion of the world's population, often living in conditions of extreme poverty. Deltaic ecosystems are ecologically significant as they support high biodiversity and a variety of fisheries, however these coastal environments are extremely vulnerable to climate change. The Ganges-Brahmaputra-Meghna (Bangladesh/India), the Mahanadi (India), and the Volta (Ghana) are among the most important and populous delta regions in the world and they are all considered at risk of food insecurity and climate change. The fisheries sector is vital for populations that live in the three deltas, as a source of animal protein (in Bangladesh and Ghana around 50–60% of animal protein is supplied by fish while in India this is about 12%) through subsistence fishing, as a source of employment and for the wider economy. The aquaculture sector shows a rapid growth in Bangladesh and India while in Ghana this is just starting to expand. The main exported species differ across countries with Ghana and India dominated by marine fish species, whereas Bangladesh exports shrimps and prawns. Fisheries play a more important part in the economy of Bangladesh and Ghana than for India, both men and women work in fisheries, with a higher proportion of women in the Volta then in the Asian deltas. Economic and integrated modelling using future scenarios suggest that changes in temperature and primary production could reduce fish productivity and fisheries income especially in the Volta and Bangladesh deltas, however these losses could be mitigated by reducing overfishing and improving management. The analysis provided in this paper highlights the importance of applying plans for fisheries management at regional level. Minimizing the impacts of climate change while increasing marine ecosystems resilience must be a priority for scientists and governments before these have dramatic impacts on millions of people's lives

The environmental impact of climate change adaptation on land use and water quality

Encouraging adaptation is an essential aspect of the policy response to climate change1. Adaptation seeks to reduce the harmful consequences and harness any beneficial opportunities arising from the changing climate. However, given that human activities are the main cause of environmental transformations worldwide2, it follows that adaptation itself also has the potential to generate further pressures, creating new threats for both local and global ecosystems. From this perspective, policies designed to encourage adaptation may conflict with regulation aimed at preserving or enhancing environmental quality. This aspect of adaptation has received relatively little consideration in either policy design or academic debate. To highlight this issue, we analyse the trade-offs between two fundamental ecosystem services that will be impacted by climate change: provisioning services derived from agriculture and regulating services in the form of freshwater quality. Results indicate that climate adaptation in the farming sector will generate fundamental changes in river water quality. In some areas, policies that encourage adaptation are expected to be in conflict with existing regulations aimed at improving freshwater ecosystems. These findings illustrate the importance of anticipating the wider impacts of human adaptation to climate change when designing environmental policies