http://hdl.handle.net/1834/1014 2013-05-18T01:47:08Z 2013-05-18T01:47:08Z Kreiner, Anja Yemane, Dawit Stenevik, Erling K. Moroff, Nadine E. http://hdl.handle.net/1834/4625 2013-02-28T13:40:01Z 2011-01-01T00:00:00Z

Title: The selection of spawning location of sardine (Sardinops sagax) in the northern Benguela after changes in stock structure and environmental conditions Authors: Kreiner, Anja; Yemane, Dawit; Stenevik, Erling K.; Moroff, Nadine E. Abstract: Most reports on the distribution of spawning areas of sardine (Sardinops sagax) in the northern Benguela originate from the 1970s and 1980s. The northern Benguela system was in a high upwelling regime during those decades. Since the early 1990s upwelling favourable winds have decreased and a trend of increasing sea surface temperature (SST) has been observed. Changes in the structure of sardine stock in the northern Benguela have been observed and it has been suggested that a reduced biomass and changes in stock structure has led to decreased spawning in the favourable southern locations, thus preventing a recovery of the sardine stock. The present paper on the contrary shows that there has been a shift in spawning location from the less favourable northern areas in the early 1980s to spawning areas further south in the 2000s. Thus, the failure of the northern Benguela sardine stock to recover since its collapse in the late 1960s cannot be explained by spawning in less favourable areas. The shift in preferred spawning location to more southern areas since the 1980s was to be expected with a general warming of the northern Benguela system. Alternative explanations for the failure of the sardine stock to recover such as a reduction in average length as well as length at 50% maturity, leading to a reduction in reproductive output, increased predation pressure, and increased low oxygen waters are proposed.

2011-01-01T00:00:00Z Kirchner, C. H. Bartholomae, C. H. Kreiner, A. http://hdl.handle.net/1834/4671 2013-03-16T17:42:31Z 2009-01-01T00:00:00Z

Title: Use of environmental parameters to explain the variability in spawnerrecruitment Authors: Kirchner, C. H.; Bartholomae, C. H.; Kreiner, A. Abstract: This study attempts to explain the variability in recruitment of sardine in the northern Benguela and to develop potential models by including environmental information to predict recruitment. Two different recruitment and spawner number datasets were available: a VPA-developed dataset, for the period 1952–1987, and data from a simple age-structured model for 1992–2007. In all, four environmental indices were used: the degree of the intrusion of the warm Angola Current into northern Namibia, termed the Angola–Benguela front index; the extent of the upwelling area off central Namibia; average sea surface temperature (SST) over the northern and central Namibian shelf; and wind stress anomalies at Lüderitz as an indicator of upwelling strength. Contrary to general belief, it was found that extremely high recruitment can happen at low spawner levels. This occurred in years in which a large upwelling area existed in association with the minimum southward intrusion of the Angola Current. These effects override the normal negative linear relationships with SST and the positive linear relationship with wind. However, when the area of upwelling is average or small, the effects of spawner biomass, SST and wind become important factors in the variability of recruitment. To estimate exceptional recruitment, the upwelling and front indices were included in the model. To measure medium and weak recruitment, spawner numbers and the SST and wind anomaly formed part of the model. These models can be used simultaneously to predict recruitment before annual acoustic surveys take place and thus aid management decisions.

2009-01-01T00:00:00Z Monteiro, P.M.S. Van Der Plas, Anja Pascall, A. http://hdl.handle.net/1834/2129 2012-05-19T07:58:20Z 2007-01-01T00:00:00Z

Title: Cross-shelf biogeochemical characteristics of sediments in the central Benguela and their relationship to overlying water column hypoxia Authors: Monteiro, P.M.S.; Van Der Plas, Anja; Pascall, A. 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.

2007-01-01T00:00:00Z Lett, Christophe Veitch, Jennifer Van der Lingen, Carl D. Hutchings, Larry http://hdl.handle.net/1834/4667 2013-03-16T17:35:58Z 2007-01-01T00:00:00Z

Title: Assessment of an environmental barrier to transport of ichthyoplankton from the southern to Authors: Lett, Christophe; Veitch, Jennifer; Van der Lingen, Carl D.; Hutchings, Larry Abstract: The Lüderitz upwelling cell and Orange River cone (LUCORC) area, a transboundary region between South Africa and Namibia, is considered to be an environmental barrier to transport of ichthyoplankton from the southern to the northern Benguela upwelling ecosystems. We use environmental data and modelling to assess the potential mechanisms responsible for this barrier: environmental data were extracted from the 1 × 1° World Ocean Atlas 2001 database and used to build maps of annual mean salinity, temperature, chlorophyll, dissolved oxygen and nutrient concentrations; outputs of a regional circulation model were used in an individual-based model to assess the transport of passive particles from the southern to the northern Benguela. The data show no clear environmental barrier at sea surface, but the model results suggest that particles released there would be largely transported offshore. The model also shows that particles released below the surface could be transported alongshore from the southern to the northern Benguela, but low subsurface temperatures would increase ichthyoplankton mortality and hence be a strong limiting factor to northward transport. We conclude that the combination of a surface hydrodynamic and a subsurface thermal barrier could limit the possibility for ichthyoplankton of epipelagic species to be transported from the southern to the northern Benguela, but that ichthyoplankton of mesopelagic species, having a wider tolerance to low temperatures, would be less affected.

2007-01-01T00:00:00Z