131 research outputs found
Choix d'une unité d'effort de pêche pour les flottilles sardinières sémi-industrielles et artisanales au Sénégal
Different catches per unit of effort available for industrial and artisanal sardinella fisheries of Senegal have been analysed and compared in order to determine whether they are acceptable indices of abundance. Among the four units of fishing effort studied (total number of sets, number of trips, time spent on fishing ground, searching time), the first and the second seem inadequate in the studied fleets. The two other units, particularly the searching time, allow the calculation of catches per unit of effort which best reflect variations in abundance, although they are not completely free of usual deficiencies
A comparison of condition factor and gona dosomatic index of sardine Sardinops sagax stocks in the northern and southern Benguela upwelling ecosystems, 1984–1999
Time-series of condition factor (CF) and gonadosomatic index (GSI) were generated using general linear models (GLM) for sardine Sardinops sagax stocks in the northern and southern Benguela ecosystems over the period 1984–1999. During this period the biomass of sardine in the northern Benguela remained at relatively low levels of African Journal of Marine Science 2001, 23: 123–13
QUANTIFICATION AND REPRESENTATION OF POTENTIAL SPATIAL INTERACTIONS IN THE SOUTHERN BENGUELA ECOSYSTEM
This work explores the potential spatial interactions between 13 key commercial species of the southern Benguela ecosystem: sardine Sardinops sagax, anchovy Engraulis encrasicolus, round herring Etrumeus whiteheadi, horse mackerel Trachurus trachurus capensis, chub mackerel Scomber japonicus, chokka squid Loligo vulgaris reynaudii, kingklip Genypterus capensis, Cape hake Merluccius spp., silver kob Argyrosomus inodorus, snoek Thyrsites atun, albacore Thunnus alalunga, bigeye tuna Thunnus obesus and yellowfin tuna Thunnus albacares. It is based on distribution maps per species after combining available commercial and research databases. The resulting 78 pairs of potential interactions are quantified using three indices: the overlap in area, the overlap in biomass and the weighted kappa index. From additional information on the diet of the different species and trophic models, the main trophic interactions (predation or competition) were identified and mapped. The results are discussed with regard to methodological limitations, habitat selection, fish assemblages, the need for spatial resolution of trophic models and the ecosystem approach to fishery management.Afr. J. mar. Sci. 26: 141–15
The heterogeneous spatial and temporal patterns of behavior of small pelagic fish in an array of Fish Aggregating Devices (FADs)
Identifying spatial and temporal patterns can reveal the driving factors that
govern the behavior of fish in their environment. In this study, we
characterized the spatial and temporal occupation of 37 acoustically tagged
bigeye scads (Selar Crumenophthalmus) in an array of shallow Fish Aggregating
Devices (FADs) to clarify the mechanism that leads fish to associate with FADs.
A comparison of the number of visits and residence times exhibited by the fish
at different FADs revealed a strong variability over the array of FADS, with
the emergence of a leading FAD, which recorded the majority of visits and
retained the fish for a longer period of time. We found diel variability in the
residence times, with fish associated at daytime and exploring the array of
FADs at nighttime. We demonstrated that this diel temporal pattern was
amplified in the leading FAD. We identified a 24-hour periodicity for a subset
of individuals aggregated to the leading FAD, thus suggesting that those fish
were able to find this FAD after night excursions. The modeling of fish
movements based on a Monte Carlo sampling of inter-FAD transitions revealed
that the observed spatial heterogeneity in the number of visits could not be
explained through simple array-connectivity arguments. Similarly, we
demonstrated that the high residence times recorded at the leading FAD were not
due to the spatial arrangement of individual fish having different associative
characters. We discussed the relationships between these patterns of
association with the FADs, the exploration of the FAD array and the possible
effects of social interactions and environmental factors.Comment: 21 pages, 9 figures + 2 supplementary figure
ECOSYSTEM APPROACH TO FISHERIES MANAGEMENT IN THE SOUTHERN BENGUELA: A WORKSHOP OVERVIEW
A workshop was held in Cape Town in December 2002 to introduce the concept of an ecosystem approach to
fisheries (EAF) management in the southern Benguela, and to examine the options for implementing an EAF in
South Africa. The workshop considered alternative modelling approaches that may have potential for an ecosystem
approach to fisheries. Consensus was that an EAF should be implemented in South Africa through an incremental
process, starting immediately. Ecosystem models can be used to provide guidance on reference points and
broader management objectives still currently set on the basis of single-species assessments. Such additional information
would be incorporated into the decision-making process, and comments received at a management
level would also feed back to the modelling process. It was suggested that, at the scientific level, an ecosystem
modelling perspective could be incorporated into existing single-species management recommendations by testing
them with ecosystem models. Compilation of an “ecosystem considerations” document was recommended to
initiate the process. It was proposed that a dedicated EAF working group be established in South Africa to advise
on the process of implementing an EAF in the various fisheries, and to provide overarching guidance and to ensure
consistency in integrating existing data and information for informing the management process
VPA estimates of fishing mortality and exploited biomass from Sardinella aurita catch-at-length data in eastern Venezuela
In this study, length-frequency data on Spanish sardine (Sardinella aurita) from northeastern Venezuela were analyzed for the period 1967-1989. Average growth parameters for the von Bertalanffy equation were established as L sub( infinity )= 26.6 cm (TL) and K = 1.26 year super(-1). The number of recruits to the fishing area, estimated from length-structured Virtual Population Analysis, varied from 10 super(9) at the end of the 1980s. Exploited biomass estimates for the same period varied from less than 20,000 t in the first year to more than 100,000 in 1989. Both recruitment and exploited biomass showed different seasonal patterns between 1976-1983 and 1984-1988. Despite some uncertainty regarding these estimates, it is considered that major population tendencies are adequately represented by this analysi
DISTRIBUTION PATTERNS OF KEY FISH SPECIES OF THE SOUTHERN BENGUELA ECOSYSTEM: AN APPROACH COMBINING FISHERY-DEPENDENT AND FISHERY-INDEPENDENT DATA
Within the context of an ecosystem approach for fisheries, there is a need for quantitative information on distributions of key marine species. This information is valuable input for modelling species interactions in the southern Benguela ecosystem. In the present study, a method is described for mapping the density distribution of 15 key species: anchovy Engraulis encrasicolus, sardine Sardinops sagax, round herring Etrumeus whiteheadi, chub mackerel Scomber japonicus, horse mackerel Trachurus trachurus capensis, lanternfish Lampanyctodes hectoris, lightfish Maurolicus muelleri, albacore Thunnus alalunga, bigeye tuna Thunnus obesus, yellowfin tuna Thunnus albacares, silver kob Argyrosomus inodorus, snoek Thyrsites atun, Cape hake Merluccius spp., kingklip Genypterus capensis and chokka squid Loligo vulgaris reynaudi. The purpose was to make use of all available sources of data to extend the spatial and temporal coverage of the southern Benguela. Six sources of data were combined on a 10´ × 10´ cell grid in a Geographical Information System: acoustic and demersal surveys conducted by Marine and Coastal Management (MCM), and pelagic, demersal (including midwater trawl), hake-directed and tuna-directed longline commercial landings data collected by MCM. Comparisons of distributions between two periods (1980s and 1990s) and between two semesters (April –September and October–March) were conducted, but biases as a result of major differences in sampling strategy prevented detailed analysis for certain species. Maps of density distributions are nevertheless presented here and the method to determine them is discussed.Afr. J. mar. Sci. 26: 115–13
VPA estimates of fishing mortality and exploited biomass from Sardinella aurita catch-at-length data in eastern Venezuela
Fishing mortality, Exploitation, Growth, Frequency analysis, Population dynamics, Biomass Sardinella aurita
SCHOOLING BEHAVIOUR OF SARDINE SARDINOPS SAGAX IN FALSE BAY, SOUTH AFRICA
The schooling behaviour of sardine Sardinops sagax in False Bay on the south coast of South Africa was studied in October 1995 using high-resolution sonar and a hull-mounted, echo-integration unit. School formation and disintegration were studied, and school shape, packing density, swimming behaviour and inter- and intra-school event rates were quantified. Mean fish density was 29.5 fish m-3 (SD 46 fish m-3), but it varied between schools by a factor of about 100 (from 2 to 233 fish m-3). Tracked schools moved at average speeds of 0.67–1.59 m s-3. Schools changed shape on average every 2.08 minutes, and underwent either splits or merges with other schools on average every 5 minutes. Relationships between the geometric dimensions and biomass of the schools were established.Afr. J. mar. Sci. 25: 185–19
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