Impact and implications of large-scale environmental anomalies on the spatial distribution of spawning of the Namibian pilchard and anchovy populations

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.

Section: What Changes Have Occurred In the Distribution Age Structurmentioning

confidence: 97%

Synthesis: climate effects on biodiversity, abundance and distribution of marine organisms in the Benguela

Jarre

Hutchings

Kirkman

et al. 2015

“…Here the coastline follows a slightly concave orientation and the shelf is fairly wide (Boyd et al. , 1987), which creates a relatively sheltered area conducive to near‐surface thermal stratification (le Clus, 1990). During the early 1980s, the preferred spawning locations of sardine were around 19°30′S, where the shelf is narrower and conditions are much more variable due to the close proximity to the Angola–Benguela frontal zone and the strong upwelling cell off Cape Frio (19°S) (Kirchner et al.…”

Section: Discussionmentioning

confidence: 99%

The selection of spawning location of sardine (Sardinops sagax) in the northern Benguela after changes in stock structure and environmental conditions

Kreiner¹,

Yemane²,

Stenevik

et al. 2011

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.

“…For the purposes of this study, November to May has been defined as the main reproductive season. Pilchard and anchovy in the northern Benguela are serial spawners ( Le Clus, 1990), and although they maintain some level of reproductive activity throughout the year, most of the spawning activity and larval development occurs during these seven months between 17° and 24°S ( O'Toole, 1977; Badenhorst and Boyd, 1980; Hewitson, 1987; Le Clus, 1990; Le Clus, 1991). It should be noted, however, that prior to the collapse of the pilchard stock in the mid 1970s, elevated levels of pilchard spawning activity also occurred around September and October off Walvis Bay ( Crawford et al ., 1987 ).…”

Section: Methodsmentioning

confidence: 99%

Environmental conditions, satellite imagery, and clupeoid recruitment in the northern Benguela upwelling system

Cole

1999

The relationship between oceanographic conditions and clupeoid (pilchard, Sardinops sagax, and anchovy, Engraulis capensis) recruitment in the northern Benguela upwelling system was investigated from 1981 to 1987 using a time‐series of mean weekly SST images. Two approaches were taken. The first involved correlating recruitment success with the number of weekly coastal `SST events' above various cut‐off temperatures during the main reproductive season. The second involved constructing a multiple regression model of recruitment success with two independent environmental variables: namely, the number of coastal `SST events' greater than 19°C, and an onshore retention index for the early life‐history stages. The retention index was derived from a spatial time‐series analysis of the SST images using principal components analysis. In general, pilchard recruitment showed a positive relationship with the `number of SST events' whilst anchovy recruitment had a negative relationship; 1987 was an outlier year, during which there were exceptionally high levels of both pilchard and anchovy recruitment. The multiple regression R2 values were high and significant for both species (pilchard R2 = 0.88, anchovy R2 = 0.96). The regression model also accounted for the 1987 outlier according to levels of onshore retention which, despite low inshore SSTs, were particularly high during the 1986/87 reproductive season. Although these results need to be validated with data from a longer time period, they show how satellite data might be used for predicting clupeoid recruitment success in the northern Benguela.