Junker T.,Institute For Ostseeforschung Warnemunde Iow |
Mohrholz V.,Institute For Ostseeforschung Warnemunde Iow |
Siegfried L.,Institute For Ostseeforschung Warnemunde Iow |
van der Plas A.,National Marine Information and Research Center Nat
Journal of Marine Systems | Year: 2017
We present long-term current meter records from the Benguela system together with salinity and temperature observations gathered by a mooring on the Namibian shelf across 13 years (2002–2015). From this unique data set a climatological mean state is estimated enabling us to investigate seasonal to interannual variations of these variables on the Namibian shelf. The present study highlights the importance of the alongshore advection for the water mass characteristics in the Benguela system on a seasonal time scale. The annual cycle of the alongshore transport is characterized by a biannual flow reversal. Poleward directed currents dominate from October to April, and from May to September equatorward currents prevail. In addition, we present observational evidence for a biannual intrusion of tropical waters into the Benguela system with maxima in October and February. Based on the in situ temperature data, several anomalous events are described that affect the whole water column. During the outstanding warm event in austral fall 2011 the monthly temperature anomaly exceeds one Kelvin for five consecutive months peaking in March (2.4 K) in the upper layer of the water column. Our study suggests, that the occurrence of such extreme temperature events in the Benguela upwelling system is closely related to the strength of the alongshore advection in austral summer. © 2016 Elsevier B.V.
Bode M.,University of Bremen |
Kreiner A.,National Marine Information and Research Center Nat |
Van Der Plas A.K.,National Marine Information and Research Center Nat |
Louw D.C.,National Marine Information and Research Center Nat |
And 3 more authors.
PLoS ONE | Year: 2014
Long-term data sets are essential to understand climate-induced variability in marine ecosystems. This study provides the first comprehensive analysis of longer-term temporal and spatial variations in zooplankton abundance and copepod community structure in the northern Benguela upwelling system from 2005 to 2011. Samples were collected from the upper 200 m along a transect at 20°S perpendicular to the coast of Namibia to 70 nm offshore. Based on seasonal and interannual trends in surface temperature and salinity, three distinct time periods were discernible with stronger upwelling in spring and extensive warm-water intrusions in late summer, thus, high temperature amplitudes, in the years 2005/06 and 2010/11, and less intensive upwelling followed by weaker warm-water intrusions from 2008/09 to 2009/10. Zooplankton abundance reflected these changes with higher numbers in 2005/06 and 2010/11. In contrast, zooplankton density was lower in 2008/09 and 2009/10, when temperature gradients from spring to late summer were less pronounced. Spatially, copepod abundance tended to be highest between 30 and 60 nautical miles off the coast, coinciding with the shelf break and continental slope. The dominant larger calanoid copepods were Calanoides carinatus, Metridia lucens and Nannocalanus minor. On all three scales studied, i.e. spatially from the coast to offshore waters as well as temporally, both seasonally and interannually, maximum zooplankton abundance was not coupled to the coldest temperature regime, and hence strongest upwelling intensity. Pronounced temperature amplitudes, and therefore strong gradients within a year, were apparently important and resulted in higher zooplankton abundance. © 2014 Bode et al.
Paterson B.,Memorial University of Newfoundland |
Paterson B.,University of Cape Town |
Paterson B.,Saint Mary's University |
Kainge P.,National Marine Information and Research Center Nat
Ecology and Society | Year: 2014
One of the most important fisheries in the northern Benguela is the Namibian hake fishery, which targets both Merluccius capensis and Merluccius paradoxus. In spite of attempts to rebuild the hake stocks that were severely depleted by distant-water fleets before Namibia's independence in 1990, stocks have failed to recover. Because the ecological goal of stock rebuilding competes with social and economic objectives on the political stage, the ability to make accurate abundance estimates is important. However, the precision of abundance estimates is impeded by lack of understanding of hake behavior and of the effects of environmental factors. Furthermore, at present both species of hake are assessed and managed as one Namibian stock. We present qualitative information derived from interviews that we conducted with Namibian hake trawl and longline fishers during the 2009 and 2010 fishing seasons, and information gleaned from analyzing logbook data. We contextualize both types of data within the scientific literature on Namibian hakes and the Namibian hake fishery. Fishers monitor sea surface and bottom temperature, water quality, currents, and weather, and they have detailed knowledge about the behavior and habitat of hakes. Fishers differentiate between three different types of M. capensis, which they associate with different fishing areas. They also describe innovations that have taken place over the past 20 years, which are of relevance to the assessment of fishing efficiency and effort, but have not been taken into account in the stock assessments. Our analysis of logbook data supports the increase in efficiency. The results show that closer collaboration between scientists and fishers has the potential to improve the accuracy of survey estimates and stock assessments, and thus is important for rebuilding of hake stocks and the hake fishery. © 2014 by the author(s).
Flynn B.A.,University of the Western Cape |
Richardson A.J.,CSIRO |
Richardson A.J.,University of Queensland |
Brierley A.S.,University of St. Andrews |
And 7 more authors.
African Journal of Marine Science | Year: 2012
There has been debate in the literature about whether jellyfish abundance has increased in the northern Benguela upwelling system, or not, over the past five decades and what impact they are having on pelagic fish. Here we review old expedition literature as well as more recent spatial and temporal patterns in distribution of jellyfish off Namibia at a number of different scales, using both published and previously unpublished data. Specifically, we have used data from fishery-dependent sources of both the demersal (359 638 trawls) and pelagic fisheries (11 324 purse-seine sets) that cover the period 1992-2006, supported by data from fishery-independent demersal (6 109 trawls) and pelagic trawls (1 817 trawls) from 1996 to 2006. Using frequency of capture as an index of abundance, it is clear that jellyfish are not randomly distributed within the northern Benguela ecosystem, but show specific areas of concentration that broadly reflect regional oceanography and the distribution of other zooplankton. Although jellyfish are present throughout the year, peaks in abundance are shown that often coincide with peaks in the spawning activity of fish of commercial importance. Interannual changes in jellyfish abundance observed from all sources do not agree, with some showing increases, others declines, and still others showing no change, which suggests caution should be exercised in their interpretation. Based on the multiple lines of evidence synthesised here, we conclude that jellyfish abundance has increased concomitant with a decline of pelagic fish stocks. We conclude that future recovery of the pelagic fishery off Namibia is likely to be considerably challenged because of significant overlaps in space and time between fish and jellyfish, and through the effects of competition and predation effects of jellyfish on fish. © 2012 Copyright NISC (Pty) Ltd.
Jarre A.,University of Cape Town |
Hutchings L.,University of Cape Town |
Kirkman S.P.,University of Cape Town |
Kreiner A.,National Marine Information and Research Center Nat |
And 15 more authors.
Fisheries Oceanography | Year: 2015
The NansClim project (2010-2013) represented a regional collaboration to assess the effects of climate on Benguela dynamics. Based on in situ (since the 1960s in Namibia and South Africa and 1985 in Angola) and satellite (since the 1980s) observations, the project focussed on four subsystems, namely the Angola subtropical, northern Benguela upwelling, southern Benguela upwelling and Agulhas Bank. This contribution summarizes the findings for selected key questions, ranging from changes in the physico-chemical habitats, plankton, pelagic and demersal fish communities, to cross-cutting evaluation at subsystem and regional scales. The results underline the overriding importance to of considering the combined effects of climate and fishing as drivers of the dynamics of the ecosystem components. Each subsystem currently continues to function largely as a separate entity as described in earlier reviews. However, some changes have been observed across several subsystems, e.g., a coherent shift from one relatively stable period to another occurred in the northern and southern Benguela in the mid-1990s. Future climate change could weaken the boundaries between the four subystems. The findings underline the need for continued regional research collaboration and regional surveys focussed at ecosystem, rather than resource, assessment. Our conclusions include implications for ecosystem-based fisheries management, and recommendations for future regional research. © 2015 John Wiley & Sons Ltd.
Iitembu J.A.,University of Namibia |
Iitembu J.A.,Rhodes University |
Iitembu J.A.,National Marine Information and Research Center Nat |
Richoux N.B.,Rhodes University
African Journal of Marine Science | Year: 2016
The two sympatric species of Cape hake, Merluccius capensis and M. paradoxus, have been the main targets of bottom-trawl fisheries off Namibia for several decades. The feeding ecology of these hakes has been studied mainly using stomach content analyses and thus there remain some gaps in our knowledge about food assimilated over the longer term. In this study, we used fatty acid (FA) profiles to characterise the dietary relationships of M. capensis and M. paradoxus. Muscle samples from hake (n=110) and their known prey (n=68) were collected during trawl surveys off Namibia during 2011. Significant differences between the neutral FA profiles of the hake populations were detected in December 2011 but not in January 2011, an indication of temporal variations in diet and resource partitioning. Comparisons of the neutral FAs in hake and the total FAs of potential prey showed no clear trophic connections, with the exception of flying squid Todarodes sagittatus, which had FA profiles very similar to those of M. paradoxus in December 2011. Our results highlight the complex and temporally shifting relationships that exist between hake and the large pool of prey available to them, and between the two hake species that overlap in their feeding habits and distribution within the highly productive Benguela Current region. © 2016 Taylor & Francis
Salvanes A.G.V.,University of Bergen |
Utne-Palm A.C.,University of Bergen |
Currie B.,National Marine Information and Research Center Nat |
Braithwaite V.A.,University of Bergen |
Braithwaite V.A.,Pennsylvania State University
Marine Ecology Progress Series | Year: 2011
Nutrient-rich, upwelling marine areas with high productivity often produce sediments dominated by organic-rich mud. Here, intense decay processes create hypoxic conditions with high concentrations of hydrogen sulphide and methane in the muddy surface layers. Such environments are inhospitable to most forms of life and those organisms that can survive in these areas tend to be specialists that cope with anoxic or hypoxic conditions, e.g. sulphide-oxidising bacteria and chemolithotrophic bacteria. Surprisingly, during recent acoustic and survey work in the northern Benguela region off the coast of Namibia, it was observed that the bearded goby Sufflogobius bibarbatus spends much of the day on the seabed interacting with the hypoxic and sulphidic mud, making a diel vertical migration (DVM) to spend the night in more oxygenated, but jellyfish-rich waters. We describe a series of experiments that demonstrate physiological and behavioural adaptations that enable the gobies to cope with hypoxia, anoxia and exposure to sulphide for prolonged periods of time. We also observed that the fish burrow directly into the muddy substrate when threatened and that, unlike another fish species common to this area, the horse mackerel Trachurus capensis, the gobies tolerate the presence of jellyfish. © Inter-Research 2011.
Kainge P.,National Marine Information and Research Center Nat |
Wieland K.,Technical University of Denmark |
Feekings J.,Technical University of Denmark
African Journal of Marine Science | Year: 2015
Diel patterns in survey trawl catches for the Cape hakes Merluccius capensis and M. paradoxus off Namibia were studied in order to examine the effect of diel bias on catchability, and its implication for survey abundance estimation and the consistency of the survey time-series. Catch rates (numbers per haul) by species and length from summer demersal biomass surveys conducted during the period 2002–2013 were used, together with a computation of the corresponding light-level data from which the solar zenith angles were obtained. Generalised additive models were fitted to assess the relationship between the catches and a number of explanatory variables. Significant covariates were zenith angle, depth and geographical position. The final models explained 78% and 59% of the variability in catch rates of M. capensis and M. paradoxus, respectively. For M. capensis, the response to zenith angle increased sharply for values above 100°, which represents the time between sunset and sunrise. For M. paradoxus there was a moderate increase in the response to zenith angle during the night. In cases where some fishing took place at night in shallow water, the survey results for M. capensis were more greatly affected than was the case for M. paradoxus, which is related to the different depth preference of the two species. Fishing in depths shallower than 400 m outside daylight hours should therefore be avoided in order to reduce bias and ensure consistency in abundance estimates from surveys. © 2015 NISC (Pty) Ltd.
Iitembu J.A.,National Marine Information and Research Center Nat |
Miller T.W.,Ehime University |
Ohmori K.,Ehime University |
Kanime A.,National Marine Information and Research Center Nat |
Wells S.,National Marine Information and Research Center Nat
Fisheries Oceanography | Year: 2012
Two species of hake, Merluccius capensis (MC) and Merluccius paradoxus (MP), together account for most of Namibia's fisheries catch and they are collectively the most important secondary consumers in the Benguela Current ecosystem. To better resolve their feeding behavior in the northern Benguela Current ecosystem, we examined the size-specific ontogenetic shift in MC and MP using stable isotopes of δ 13C and δ 15N as measures of relative source production and trophic level. We also compared hake isotope values to those of their major zooplankton (four species) and nekton (12 species/groups) prey. Results from δ 15N showed a significant positive relationship with size in both MC and MP; however, the slopes of the two species were significantly different, with MP displaying a steeper trophic shift. A significant increase in δ 13C with size was observed in MC but not MP. In all size classes (except ≤20-29cm) MC expressed significantly higher δ 13C values, generally matching their respective shelf-slope adult distributions. Relative to zooplankton and nekton prey, smaller hake of both species (20-39cm) were trophically indistinguishable at a trophic level (TL) of 3.3, indicating predominant zooplanktivory. The largest MC of 60-70cm had TLs of approximately 3.5-3.6, whereas MP of the same size were slightly higher, at 3.7-3.8 TL, indicating greater piscivory. This is the first spatially and ontogenetically extensive stable isotope analysis of MC and MP in the Benguela Current. Further extension of this analysis throughout the hake range would provide considerable insight into the trophic dynamics of this commercially and ecologically important group. © 2012 Blackwell Publishing Ltd.