Showing papers in "African Journal of Marine Science in 2007"
TL;DR: In this article, the authors assessed whether this shift was environmentally mediated by examining sea surface temperature data from different regions of the Agulhas Bank, which showed that in 1996 the inner shelf became 0.5°C colder than in previous years and has since remained that way.
Abstract: Cape anchovy Engraulis encrasicolus spawners in the southern Benguela showed an eastward shift in their distribution on the Agulhas Bank that occurred abruptly in 1996 and has since persisted. We assessed whether this shift was environmentally mediated by examining sea surface temperature data from different regions of the Agulhas Bank, which showed that in 1996 the inner shelf of the Agulhas Bank to the east of Cape Agulhas abruptly became 0.5°C colder than in previous years and has since remained that way. In addition, signals, coherent with the 1996 shift recorded in sea surface temperatures, were also found in atmospheric surface pressure and zonal wind data for that region; interannual coastal SST variability is also shown to be correlated with zonal wind-stress forcing. As a result, increased wind-induced coastal upwelling east of Cape Agulhas is proposed as the main driver of the observed cooling in the coastal region. The synchrony between the environmental and biological signals suggests that the...
159 citations
TL;DR: In this paper, trends in the population of Cape fur seals Arctocephalus pusillus pusillus were estimated from counts of pups on aerial photographs of colonies taken between 1972 and 2004 to determine trends in overall population and subpopulations.
Abstract: Trends in the population of Cape fur seals Arctocephalus pusillus pusillus were estimated from counts of pups on aerial photographs of colonies taken between 1972 and 2004 to determine trends in the overall population and subpopulations. Incomplete coverage resulted in missing data in some years. Various methods of determining proxy values for missing data were assessed, and it was concluded that different methods were applicable to Namibian and South African colonies. This reflected variation in trends of pup counts between the countries, which was associated with differences in productivity between the southern and northern Benguela ecosystems. In Namibia, temporal changes in pup numbers were non-linear in some years and there was correspondence in fluctuations at most colonies. This appeared to be on account of an effect of periodic, wide-scale prey shortages that reduced birth rates. There was a northward shift in the distribution of seals in the northern Benguela system. In South Africa, pup counts w...
62 citations
TL;DR: In this article, a new method known as the sequential t-test algorithm for analysing regime shifts (STARS) is applied to a set of biological state variables as well as environmental and anthropogenic forcing variables in the southern Benguela.
Abstract: Long-term ecosystem changes, such as regime shifts, have occurred in several marine ecosystems world-wide. Multivariate statistical methods have been used to detect such changes. A new method known as the sequential t-test algorithm for analysing regime shifts (STARS) is applied to a set of biological state variables as well as environmental and anthropogenic forcing variables in the southern Benguela. The method is able to correct for auto-correlation within time-series by a process known as prewhitening. All variables were tested with and without prewhitening. Shifts that were detected with both methods were termed robust. The STARS method detected shifts in relatively short time-series and identified when these shifts occurred without a priori hypotheses. Shifts were generally well detected at the end of time-series, but further development of the method is needed to enhance its performance for auto-correlated time-series. Since 1950, two major long-term ecosystem changes were identified for the southe...
56 citations
TL;DR: The simple Chl Index derived for the Benguela appears to be robust and generic enough to be applied to other upwelling systems as a reasonably good index of variability in phytoplankton biomass for comparative studies.
Abstract: The spatial and temporal variability in phytoplankton biomass in the Benguela ecosystem was investigated over an 8-year period using a chlorophyll index (Chl Index) derived from low resolution SeaWiFS ocean colour data. Monthly composite images revealed that the surface chlorophyll a concentration was high near the coast and generally decreased with distance offshore, allowing observation of the spatial extent of phytoplankton distribution in relation to coastal upwelling. The Chl Index was estimated as the integration of the chlorophyll a concentration from the coast to an offshore minimum level of 1mg m−3. The index captured both the cross-shelf structure and the major part of the chlorophyll-rich areas and was used to examine the average seasonality and annual variability in phytoplankton biomass. The data indicated that the Benguela ecosystem may be divided into two major sectors, north and south, with the divide occurring at the Luderitz upwelling cell where the Chl Index, the chlorophyll a concentra...
54 citations
TL;DR: This paper focuses on the development of simple environmental indices that can serve as a measurement or proxy of the state or intensity of certain important oceanographic processes or variables for temporal comparison.
Abstract: The Namibian Ministry of Fisheries and Marine Resources has the task of collecting and providing relevant oceanographic information for management of their fisheries. However, quality information is difficult to provide because of the complexity of the processes involved in the Namibian marine ecosystem. Nonetheless, there are oceanographic data available that can contribute to fisheries management by improving the general understanding of important oceanographic processes related to fisheries. The focus of this paper is on the development of simple environmental indices that can serve as a measurement or proxy of the state or intensity of certain important oceanographic processes or variables for temporal comparison. These indices relate to oceanographic processes such as upwelling, frontal movements, anoxic bottom conditions and biological indicators, which can be updated on a regular basis to provide a long-term perspective of the particular processes to management and marine scientists.
53 citations
TL;DR: It is concluded that C. maenas is poorly adapted to survive in wave-swept conditions and hence unlikely to displace indigenous crab species along the open wave-exposed coastline of South Africa, however, it may invade other sheltered locations, particularly Saldanha Bay and False Bay.
Abstract: The European green crab Carcinus maenas has established considerable breeding populations in harbours and sheltered bays in the South-Western Cape, South Africa, but appears unable to flourish on the wave-exposed coastline. This study compares the abilities of C. maenas and those of an indigenous rocky-shore crab, Plagusia chabrus, to resist hydrodynamic forces. C. maenas had less than half the vertical tenacity of P. chabrus (371.5g and 780.5g respectively) and was unable to grip against as fast a unidirectional flow (0.23m s−1 vs 0.53m s−1) as P. chabrus. C. maenas also has significantly shorter and lighter limbs than P. chabrus and the dactyls of its walking legs are poorly adapted to grip onto rocky substrata. We conclude that C. maenas is poorly adapted to survive in wave-swept conditions and hence unlikely to displace indigenous crab species along the open wave-exposed coastline of South Africa. However, it may invade other sheltered locations, particularly Saldanha Bay and False Bay.
52 citations
TL;DR: In this paper, the demersal biomass has been reduced by 75% on the Mauritanian continental shelf over the past 25 years, corresponding to a biomass loss of around 20 000t per year.
Abstract: Mauritania is characterised by fast-growing fisheries that have developed over the past decades. Since 1982, scientific trawl surveys have been conducted regularly, allowing assessment of the impact of this increasing fishing pressure on exploited species as well as on demersal communities. Based on 55 bottom trawl surveys and using linear model techniques, the annual abundances were estimated for a selection of 24 fish stocks and for the whole demersal biomass. Changes in the demersal community structure were also investigated, using Biomass Trophic Spectra representations. It is shown that the demersal biomass has been reduced by 75% on the Mauritanian continental shelf over the past 25 years, corresponding to a biomass loss of around 20 000t per year. Top predators abundance has been reduced by 8–10-fold and in some case up to 20-fold. The trophic structure has been significantly modified and the mean trophic level of the catchable biomass decreased from >3.7 to <3.5. The results are discussed at the r...
50 citations
TL;DR: In this paper, a South African kelp inventory has been compiled using all available data for the 900km-long West Coast (Cape Agulhas to the Orange River), although a few gaps remain.
Abstract: Two species of kelp are exploited commercially in South Africa. Use of beach-cast Laminaria pallida is limited, although it comprises the bulk of the biomass on the northern West Coast. Ecklonia maxima dominates the southern West Coast and provides most of the material for the South African kelp industry. Harvests of E. maxima fronds for abalone feed have reached 3 000–5 000 tonnes fresh weight (t f wt) y –1 , whereas beachcast material of both species (about 1 000t dry wt y –1 ) is collected mainly for alginate extraction. For the first time, a South African kelp inventory has been compiled using all available data for the 900km-long West Coast (Cape Agulhas to the Orange River), although a few gaps remain. Beds of E. maxima and L. pallida reaching the surface at low spring tides were mapped using one or several methods: infrared aerial photography, digital multispectral aerial imagery, Landsat satellite imagery, and physical mapping with a hand-held GPS. The data are on a GIS database. Landsat 5 TM imagery could identify the presence of kelp beds, but only infrared aerial imagery and multispectral imaging at low spring tides accurately quantified surface areas of kelp beds. Biomass of the main kelp ( E. maxima ) is variable in space (between 3kg f wt m –2 and 24kg f wt m –2 ) and time (changes of up to 50%), making estimates for management difficult. Using an average biomass value of 12kg m –2 , the total biomass of surface-reaching kelp beds on the West Coast is estimated to exceed 593 000t f wt, but extensive subsurface beds remain unquantified. Results are discussed in relation to sustainable limits to harvesting, gaps in knowledge, and the improved management of kelp resources. Keywords: Aerial infrared, Ecklonia maxima, GIS, kelp mapping, South Africa African Journal of Marine Science 2007, 29(3): 369–378
43 citations
TL;DR: The fact that neither larvae nor adults are strong swimmers has resulted in genetic subdivisons comparable to those of passively dispersing coastal invertebrates in southern Africa, suggests that the amount of gene flow between regions may be too high for evolutionary divergence to have taken place.
Abstract: Recent genetic studies have shown that most widely distributed, passively dispersing invertebrates in southern Africa have regional intraspecific units that are associated with the three main marine biogeographic provinces (cool-temperate, warm-temperate and subtropical). The caridean shrimp Palaemon peringueyi also occurs in all threeprovinces, but the fact that it can disperse both actively and passively (i.e. larval drifting, adult walking/swimming and potential adult rafting by means of floating objects) suggests that the amount of gene flow between regions may be too high for evolutionary divergence to have taken place. Samples of P. peringueyi were collected throughout South Africa and an intraspecific phylogeny was reconstructed using mitochondrial COI and 16S rRNA sequences. Three major clades were recovered, which were broadly associated with the three biogeographic regions. This suggests that, even though P. peringueyi can disperse actively, the fact that neither larvae nor adults are strong swi...
42 citations
TL;DR: A review of the African sea level dataset is provided in this paper, which is limited not only in size, but also in quality, especially given the great length of African coastline, and the conclusions on the need for major new investments in sea level infrastructure are undoubtedly the same as would be arrived at through any other approach.
Abstract: A review is provided of the African sea level dataset, which is limited not only in size, especially given the great length of the African coastline, but also in quality. The review is undertaken primarily from Permanent Service for Mean Sea Level (PSMSL) and Global Sea Level Observing System (GLOSS) perspectives, but the conclusions on the need for major new investments in sea level infrastructure are undoubtedly the same as would be arrived at through any other approach. Stations to be installed as part of the Ocean Data and Information Network for Africa (ODINAfrica) programme are described and a survey of currently existing and planned sea level stations in Africa is presented, together with information on where data for existing stations may be found.
39 citations
TL;DR: In this paper, the movement behavior of adult Roman Chrysoblephus laticeps (Sparidae) was investigated using mark-and-recapture techniques in the Goukamma Marine Protected Area (GMPA) and the Tsitsikamma National Park (TNP) on the South African temperate south coast.
Abstract: The movement behaviour of adult Roman Chrysoblephus laticeps (Sparidae) was investigated using mark-and-recapture techniques in the Goukamma Marine Protected Area (GMPA) and the Tsitsikamma National Park (TNP) on the South African temperate south coast. The study was designed to improve the spatial resolution of previous studies on Roman that suggested residence behaviour, but which could not estimate home range size. A combination of conventional barbed dart tags and visible implant fluorescent elastomer (VIFE) tags were used to tag Roman from a skiboat in the GMPA. Roman were tagged with dart tags from the shore in the TNP. Of the recaptures, 61% were within 50m of the tagging position, confirming that Roman are very resident. A few fish, independent of size and sex, moved distances up to 4km. A model was developed to quantify the degree of residence, by accounting for tag loss, mortality and the distribution of recovery effort. The probability of a Roman remaining within an area of 1ha is 0.91 (GMPA) a...
TL;DR: Analysis of movement data using a 95% fixed kernel algorithm suggests that roman occupy small home ranges of between 1 800m2 and 3 000m2, which shows that this species is well suited for protection in small MPAs, which does not seem to be related to fish size or habitat quality.
Abstract: Information on the movement of fish is vital to determine the effectiveness of marine protected areas (MPAs) for fish conservation and fisheries management. This study investigates area utilisation and activity patterns of 13 adult roman Chrysoblephus laticeps (Sparidae) using telemetry and underwater observations. Acoustic transmitters implanted into C. laticeps in tanks had no apparent effects on growth and mobility. Natural behaviour of the treated fish in the field was verified by SCUBA divers. Manual boat- and diver-based tracking was carried out inside the Castle Rock MPA, False Bay, South Africa, over a 17-month period. A radio acoustic positioning system (VRAP, VEMCO Ltd) was used to record automatically fish positions over two 1-month periods during and after the spawning season of roman. Manually recorded fish positions and VRAP positions inside the triangle of buoys within the system were accurate within 10m; deviations increased with increased distance of the fish from the centre of the system...
TL;DR: In this paper, the authors used data from two cross-shelf sediment sampling cruises to explain why sediment biogeochemical variability in respect of carbon, nitrogen and sulphur, and how the cycling of these elements governs the biogeochemistry of the overlying water through their control of the redox conditions.
Abstract: Data from two cross-shelf sediment sampling cruises were used to explain reasons for the sediment biogeochemical variability in respect of carbon, nitrogen and sulphur, and how the cycling of these elements governs the biogeochemistry of the overlying water through their control of the redox conditions. The spatial extent of this benthic–pelagic flux link is limited to the innershelf mud belt system on the Namibian shelf. The inshore mud belt is the primary deposition area of the carbon and nitrogen new production export flux. The offshore organic-rich zones are thought to be relict particulate organic matter originating from the inshore mud belt rather than from an overlying pelagic source. These data were used to set up a multi-layer sediment model that was used through sensitivity analyses to elucidate the input characteristics that result in the most significant feedbacks on hypoxia in the overlying water. The analyses showed that, although the new production flux is a requirement to drive an oxygen demand in the sediments, the onset and persistence of anoxia may depend critically on a low-oxygen boundary condition threshold. This is thought to be a key differentiating factor between systems that, despite comparable carbon export fluxes, are characterised by a persistent hypoxia/anoxia signal and those that are characterised by episodic hypoxia events. It was concluded that sediment oxygen demand and methane and ‘sulphide' emissions from the central Benguela sediments are responses to external hypoxia boundary conditions rather than the local drivers of oxygen variability. Keywords: anoxia, Benguela Current, carbon, hypoxia, modelling, Namibia, sediment biogeochemistry African Journal of Marine Science 2007, 29(1): 37–47
TL;DR: In this paper, the authors investigated the impact of the total extraction of fish from the Red Sea ecosystem, which will help the assessment of the resource and its management, and found that factors that provide incentives to fishers to misreport were obtained by examining the historical development of the fisheries; analysis was based on interpolations, guided by these incentives, between independent quantitative estimates of unreported catch ('anchor points'). Errors were estimated using a Monte Carlo sampling technique.
Abstract: Unreported catches from three major fisheries in the Eritrean Red Sea were investigated in order to estimate the impact of the total extraction of fish from the ecosystem, which will help the assessment of the resource and its management. The fisheries target small pelagics, demersal finfish and shrimps, and were chosen for their major contribution to the total Eritrean catch, economic importance and/or significant contribution to unreported catch. The analysis covers the period from 1950 to 2004, subdivided into five-year blocks. Factors that provide incentives to fishers to misreport were obtained by examining the historical development of the fisheries; analysis was based on interpolations, guided by these incentives, between independent quantitative estimates of unreported catch ('anchor points'). Errors were estimated using a Monte Carlo sampling technique. The fishery industry in Eritrea operated smoothly from the mid-1950s to the end of 1960s, when it was disrupted by political instability. Fishing...
TL;DR: Gadget is illustrated using a single-species case study with two stock components on one area and a range of different assumptions and estimation methods are evaluated including the effect of reduced data availability on parameter estimation by excluding age data from the likelihood components.
Abstract: Gadget is a statistical modelling framework that can be used to assess individual fish stocks and to create multispecies, multi-fleet and multi-area models. The development of a complex multispecies model requires understanding of the modelled single-species population. Simple single-species models are essential in evaluating whether more complex models increase understanding of the population dynamics. In this paper, Gadget is illustrated using a single-species case study with two stock components on one area. Features of the implementation include immature and mature stock components, a maturation process, along with commercial and survey fleets. Parameter estimation is done using maximum likelihood based on a variety of datasets. The estimated parameters relate to growth, maturation, fleet selection, recruitment and the initial population. The data types used to calculate the likelihood include survey indices and biological samples from the commercial catch and surveys. Optimisation of the model, along...
TL;DR: It is concluded that M. paradoxus do not appear to spawn in Namibian waters, and this new concept, designated here as a 'maturity reference line', could enhance understanding of the spawning biology of other species with a similarly complex, indeterminate spawning strategy.
Abstract: Spawning time and areas, and the length of the spawning season of shallow-water (Merluccius capensis) and deep-water (M. paradoxus) hake, were investigated from bottom trawl collections taken in Namibian waters between September 1998 and October 2000 and from August to November 2001. The gonadosomatic index (GSI) and the incidence of advanced maturity stages of M. paradoxus were low along the entire coast throughout the year, but with a few exceptions in the south (between Luderitz and Orange River). By contrast, M. capensis spawn in the area throughout the year, but mainly between July and October. Evaluating the accuracy of visual maturity staging by comparing results with those of image and histological analyses revealed few errors in classifications of maturity stage in the field, with the exception of Stage 5 (spent and resting) in M. paradoxus. Specific GSI values were intercalibrated with the appearance of developing oocytes. As the GSI is quickly estimated, this new concept, designated here as a '...
TL;DR: Treatment measures should be implemented throughout the year but with increased effort when water temperature increases, and growth rate, size at maturity, maximum size, infestation intensity, recruitment, percentage of the population brooding and mortality appear to be affected by abalone feeding regime and water temperature, and these factors need to be considered in controlling infestation.
Abstract: Polydorid polychaetes can infest cultured abalone thereby reducing productivity. In order to effectively control these pests, their reproductive biology must be understood. The population dynamics and reproduction of polydorids infesting abalone Haliotis midae from two farms in South Africa is described using a length-based, age-structured model. Shells were infested mainly by introduced Boccardia proboscidea. Polydora hoplura and Dipolydora capensis were also present but in numbers too few to identify factors influencing infestation. At both farms, B. proboscidea lived for a minimum of 12 months. Growth rate, size at maturity, maximum size, infestation intensity, recruitment, percentage of the population brooding and mortality appear to be affected by abalone feeding regime and water temperature, and these factors need to be considered in controlling infestation. Brooders and recruits were present throughout the year, but increased significantly during mid- to late winter/early spring when water temperat...
TL;DR: Management intervention is required to reduce the negative impacts of disturbance due to kelp gull Lasus dominicanus predation on other breeding seabirds, primarily the African penguin Spheniscus demersus, during the carcass collection process.
Abstract: In 2002 there was a widespread epizootic involving seabirds on five of the offshore islands of the Western Cape, South Africa Since then, avian cholera Pasteurella multocida outbreaks have been occurring annually on one of these islands, Dyer Island This paper reports on the three subsequent summers, 2003/04, 2004/05 and 2005/06, during which further avian cholera outbreaks were recorded It focuses on the outbreak in 2004/05, which was the largest in extent and the most closely monitored The mortalities during 2005/06 were not as extensive as expected The management measures used to bring these outbreaks under control are described Removal of all the carcasses from the entire island in one day is important in reducing mortality Management intervention is required to reduce the negative impacts of disturbance due to kelp gull Lasus dominicanus predation on other breeding seabirds, primarily the African penguin Spheniscus demersus, during the carcass collection process
TL;DR: Mismatch distributions, a haplotype network, and Fu's Fs test, all suggest that the J. tristani population underwent population expansion between 12 000 and 99 000 years ago, possibly mediated by larval dispersal in the South Atlantic gyre system.
Abstract: The commercially exploited spiny lobster Jasus tristani has a disjunct distribution in the southern Atlantic Ocean, with populations occurring at the Tristan da Cunha Archipelago, Gough Island and on Vema Seamount. A distance of 2 000km separates Vema and the Tristan Archipelago. In order to determine genetic connectivity of lobster between these locations, a region of the cytochrome oxidase II (COII) gene was sequenced for 193 individuals of J. tristani from five sampling sites (Tristan da Cunha, Nightingale, Inaccessible, Gough islands and Vema Seamount). Our results show that J. tristani in the southern Atlantic share a most recent common ancestry that dates back at least one million years. Analyses of molecular variance and pairwise Φ st analyses reveal shallow but significant genetic partitioning between Vema Seamount and all other locations. No population differentiation was detected among any of the remaining sampling sites. Coalescent analyses show limited gene flow between Vema and the archipelago (including Gough Island), possibly mediated by larval dispersal in the South Atlantic gyre system. Population divergence between Vema and the other populations, using MDIV, was estimated to be approx. 270 000 years ago. Mismatch distributions, a haplotype network, and Fu's F s test, all suggest that the J. tristani population underwent population expansion between 12 000 and 99 000 years ago. Keywords: Jasus tristani , larval dispersal, MDIV, mitochondrial DNA, population genetic structure, seamount, southern Atlantic Ocean African Journal of Marine Science 2007, 29(3): 491–497
TL;DR: The dietary trends of lancetfish are consistent with opportunistic feeding on the most abundant prey, and play an important role in the pelagic trophic functioning of the western Indian Ocean.
Abstract: The role of pelagic crustaceans in the diet of the longnose lancetfish Alepisaurus ferox was investigated from stomach content analysis of fish collected in the waters surrounding the Seychelles Archipelago in the western Indian Ocean. Crustaceans accounted for 88.4% by prey number and 73.7% by reconstituted mass during the South-West monsoon season. During the North-East monsoon season, crustaceans remained the main prey group and accounted for 63.7% by prey number and 46% by reconstituted mass. There was a clear seasonal pattern with the portunid crab Charybdis smithii predominating during the South-West monsoon season and the pelagic stomatopod Natosquilla investigatoris during the North-East monsoon season. This pattern likely reflects variations in prey availability linked to current movements around the Seychelles Archipelago. Most of the prey consisted of slow-swimming and small individuals, which occur in dense swarms during the pelagic phase of their life history. The dietary trends of lancetfish...
TL;DR: In this paper, the recruitment of distinct year-class cohorts in two sparid species, Rhabdosargus holubi and Lithognathus lithognathan, were linked to records of daily mouth state in the intermittently open East Kleinemonde Estuary, South Africa, between 1995 and 2006.
Abstract: The recruitment of distinct year-class cohorts in two sparid species, Rhabdosargus holubi and Lithognathus lithognathus, were linked to records of daily mouth state in the intermittently open East Kleinemonde Estuary, South Africa, between 1995 and 2006. L. lithognathus only recruited into the estuary in years when the mouth opened between late August and January. This was attributed to a limited spawning season and inability to recruit during wave overwash events. In contrast, R. holubi recruitment was uninterrupted and not influenced by seasonality of estuary access opportunities (mouthopening and overwash events). Estuarine-dependent residency periods ranged from 27 months to 48 months for L. lithognathus and 12 months to 23 months for R. holubi. The interannual abundances of these estuarydependent sparids were determined by reproductive seasonality, recruitment strategy and seasonal timing of estuarine access opportunities.
TL;DR: The habitat use by nearly 3, 000 reef fish recruits, comprising 56 taxa, at seven sites in Mafia Island Marine Park, Tanzania, was examined in this article, following the 1998 global cora...
Abstract: The habitat use by nearly 3 000 reef fish recruits, comprising 56 taxa, at seven sites in Mafia Island Marine Park, Tanzania, were examined. The study was carried out following the 1998 global cora ...
TL;DR: The effects of hook size, bait type, water depth and fishing effort on the selectivity and capture mortality of reef fish were investigated in a standardised fishing experiment on the temperate south coast of South Africa, useful for planning fisheries independent catch per unit effort surveys and for developing rules for catch-and-release competitions.
Abstract: The effects of hook size, bait type, water depth and fishing effort on the selectivity and capture mortality of reef fish were investigated in a standardised fishing experiment on the temperate south coast of South Africa. A total of 2 406 fish was caught at 273 stations, distributed according to a random-stratified design. Generalised linear models separated the influences of environmental factors such as depth, temperature and season from the effects of fishing methods on catch rates and capture mortality. Dominant reef predators and larger individuals within species were more frequently caught on large hooks and sardine Sardinops sagax bait. The main cause for capture mortality was gut-hooking, aggravated by increased capture depth. Large roman Chrysoblephus laticeps were more frequently gut-hooked than small individuals. More roman were caught early in the fishing session and the average size of captured roman increased slightly during fishing sessions. The selection of legal-sized target fish can be ...
TL;DR: The diet of Chrysaora hysoscella was investigated from 55 specimens caught near the surface throughout 24h in Walvis Bay Lagoon, Namibia, during September 2003, and there was no clear overall relationship between medusa size and either the number or type of prey ingested.
Abstract: The diet of Chrysaora hysoscella was investigated from 55 specimens caught near the surface throughout 24h in Walvis Bay Lagoon, Namibia, during September 2003. The diet was diverse, ranging from dinoflagellates to carideans, and included abundant benthic species (adults and larvae). There was no clear overall relationship between medusa size and either the number or type of prey ingested, although aspects of both measures were significantly related to size when analyses were confined to the nocturnally collected data. Whereas there was a significant difference in the diet of medusae collected by day and night, principally because of an increase in the abundance of benthic prey items, it was not possible to partition this between the vertical migration of predator (downwards) or prey (upwards) owing to a lack of information on the ambient prey environment. Regardless, the abundance of benthic prey in the diet could help explain the predominance of medusae off Namibia, where there has been an increased flo...
TL;DR: In a controlled aquarium experiment, it is found that VEMCO V8 transmitters could be implanted into white stumpnose without expulsion or any measurable effects on survival and growth.
Abstract: There is a growing recreational fishery for white stumpnose Rhabdosargus globiceps (Sparidae) in Langebaan Lagoon, part of South Africa's West Coast National Park. The upper reaches of the lagoon are protected from fishing, but the extent to which the closure protects the white stumpnose population is uncertain. In a controlled aquarium experiment we found that VEMCO V8 transmitters could be implanted into white stumpnose without expulsion or any measurable effects on survival and growth. Four fish captured in the protected zone were fitted with transmitters and tracked over a 12-day period in the field. The fish moved along deep channels, occasionally penetrating the fishing zone. Net distances covered were between 9km and 15km per fish during the study period. Movement was mostly at night and at twilight.
TL;DR: The Pondoland region has a rich marine biodiversity and is situated within a unique transition zone between subtropical and warm temperate waters, and it is imperative that this rich biodiversity, coupled with the aesthetic beauty of the Pondoland coastline, be adequately zoned for protection within the proclaimed marine protected area.
Abstract: A subtidal marine biodiversity survey was carried out on shallow reefs (–1m to –30m) in the proclaimed Pondoland Marine Protected Area between Port Edward and Port St Johns, South Africa. A total of 26 benthic reef transects was undertaken involving the capture and processing of 1 042 photographic images of the reef benthos. Results of the benthic survey showed a shift from algal-dominated reefs in the north to suspensionfeeder-dominated reefs in the south, probably on account of turbidity (reduced sunlight penetration) and high nutrient levels from riverine input. A similar shift was found with increasing reef depth with algae dominating shallower reefs and suspension-feeding communities dominating deeper reefs. Non-exhaustive inventories were compiled of dominant organisms, including algae, sponges, other invertebrates and fish. The results of this survey confirm that the Pondoland region has a rich marine biodiversity and is situated within a unique transition zone between subtropical and warm temperate waters. It is imperative that this rich biodiversity, coupled with the aesthetic beauty of the Pondoland coastline, be adequately zoned for protection within the proclaimed marine protected area. Keywords: benthic fauna and flora; biodiversity survey; marine geographic information system (GIS); marine protected area (MPA); Pondoland coast; subtropical reef communities African Journal of Marine Science 2007, 29(1): 65–77
TL;DR: The final temperature preference of cultured juvenile dusky kob is determined to provide an indicator of their optimum temperature for growth and has important implications for the optimal location of kob farms in southern Africa.
Abstract: The final temperature preference of cultured juvenile dusky kob is determined to provide an indicator of their optimum temperature for growth. Trials were conducted over a five-day period in horizontal temperature gradients ranging from 21°C to 27°C. Control trials were run for the same duration, in isothermal conditions. The final temperature preference of the juvenile fish was 25°–26.4°C. This falls within the upper end of the temperature range of 12°–28°C that juvenile kob experience in the wild and has important implications for the optimal location of kob farms in southern Africa.
TL;DR: The results indicate that there is scope for the wider use of BRDs on prawn trawlers in the Western Indian Ocean region, although this would require increased support from management agencies and industry.
Abstract: Bycatch reduction devices (BRDs) are increasingly being used in prawn trawl fisheries worldwide. This paper describes an experiment with a Nordm⊘re grid and a square-mesh panel on a prawn trawler off Mocambique. Although numbers of hauls that caught elasmobranchs were low, 75% of hauls with grids caught fewer large rays than those without grids; hauls using grids caught no large sharks at all. In its final configuration, the grid respectively reduced prawn, discard and retained fish catches by 5%, 25% and 32% relative to control hauls; the square-mesh panel increased prawn catches by 3% and respectively reduced discard and retained fish catches by 23% and 18% relative to control hauls. Changes in prawn catches were not statistically significant; however, when the grid and the square-mesh panel were used together, they reduced prawn, discard and retained fish catches by 25%, 47% and 26% respectively relative to control hauls (all significant except for retained fish catches). Overall, the Nordm⊘re grid suc...
TL;DR: In this paper, the authors describe visual and acoustic surveys for porpoises in Atlantic waters of North-West Africa in spring 2005 between latitudes of 20°N and 37°N.
Abstract: The distribution of harbour porpoises Phocoena phocoena in southern Europe and along the Atlantic African coast is poorly understood, with tentative assessments suggesting a discrete West African population with a northern limit around the Straits of Gibraltar. This study describes visual and acoustic surveys for porpoises in Atlantic waters of North-West Africa in spring 2005 between latitudes of 20°N and 37°N. During this period, 276h of acoustic data were collected in all sea states (from 3 125km of trackline) and 60h (824km) of visual effort was conducted in Beaufort sea states of three or less. Harbour porpoises were seen three times on the survey trackline and on four additional occasions off the trackline. The four individuals seen in Agadir Bay (30°N) represent the northernmost living porpoises reported from the Atlantic African coasts and the first sightings of porpoises reported in Moroccan waters. In addition, 31 acoustic detections were made on the survey trackline, of which 23 were made betwe...
TL;DR: The results indicate that larvae are negatively buoyant and sink during the night, even with filled swimbladders, similar to other anchovy species studied in the northern Benguela.
Abstract: The diel vertical migration of anchovy Engraulis encrasicolus larvae was studied in the northern Benguela during a survey in April 2001 The diel pattern is described in relation to the degree of swimbladder inflation and the vertical structure of temperature and dissolved oxygen During the survey, samples were also collected over a 24h cycle at a fixed station at position 20°S, 12°30'E The survey data indicated that, on average, 80% of the larvae were distributed below the offshore-moving Ekman layer (characteristically 25m deep in the study area) A similar pattern was observed at the 24h station, at which larvae were confined to the upper 40m during the day, but they were more widely distributed down to 60m during the night There were clear differences between the daytime and night-time proportion of larvae with inflated swimbladders — which ranged between 50% and 100% during the night and between 0% and 60% during the day The results indicate that larvae are negatively buoyant and sink during the