The 1980s – a decade of change in the Benguela ecosystem

Shannon, L. V.; Crawford, R. J. M.; Pollock, D. E.; Hutchings, L.; Boyd, A. J.; Taunton-Clark, J.; Badenhorst, A.; Melville‐Smith, Roy; Augustyn, C. J.; Cochrane, Kevern L.; Hampton, Ian; Nelson, G.; Japp, D. W.; Tarr, R. J. Q.

doi:10.2989/02577619209504707

Cited by 48 publications

(34 citation statements)

References 29 publications

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“…if the amount of density-dependence was affected by extrinsic forces with population crashes occurring when poor conditions occurred at high densities , there has been a sustained decrease across all three Namibian populations (at a rate of 10% per annum at the smallest colony) despite low densities. All five west coast Cape gannet colonies are dependent on the highly productive Benguela upwelling ecosystem (Shannon et al 1992). However, the contrasting fortunes of the three Namibian and two South African colonies accord with strong regional differences in the health of fish stocks in this oceanographic system.…”

Section: Discussionmentioning

confidence: 99%

Using behavioural and state variables to identify proximate causes of population change in a seabird

et al. 2005

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Changes in animal population size are driven by the interactions between intrinsic processes and extrinsic forces, and identifying the proximate mechanisms behind population change remains a fundamental question in ecology. Here we report on how measuring behavioural and state proxies of food availability among populations experiencing different growth rates can be used to rapidly identify proximate drivers of population trends. In recent decades, the Cape gannet Morus capensis has shown a major distributional shift with historically large colonies in Namibia decreasing rapidly, whilst numbers at South African colonies have increased, suggesting contrasting environmental conditions in the two regions. We compared per capita growth rates of five of the six extant colonies with foraging range (using miniaturised Global Positioning System loggers), foraging work rate, food delivery rates and body condition of breeding adults. We found significant associations between the rate of population change, individual behaviour, energetic gain and body condition that indicate that recent population changes are associated with extrinsic effects. This study shows that behavioural and state data can be used to identify important drivers of population change, and their cost-effectiveness ensures that they are an appealing option for measuring the health of animal populations in numerous situations.

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Section: Discussionmentioning

confidence: 99%

Using behavioural and state variables to identify proximate causes of population change in a seabird

et al. 2005

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“…Mills 1995). The increase in jellyfish abundance in the northern Benguela appears to have coincided with a period of declining pelagic fish catches there (Shannon et al 1992, FAO 1995, and it has been suggested that these phenomena are directly linked. Although the diets of C. hysoscella and A. aequorea are not well described, related species are known to prey upon fish eggs and larvae (e.g.…”

Section: Introductionmentioning

confidence: 99%

Acoustic observations of jellyfish in the Namibian Benguela

Brierley¹,

Axelsen²,

Buecher³

et al. 2001

Mar. Ecol. Prog. Ser.

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Multi-frequency acoustic data (18, 38 and 120 kHz) were collected in conjunction with pelagic trawl sampling for gelatinous macrozooplankton during a cruise to the Namibian Benguela in September 1999. Sampling focused specifically on the scyphozoan Chrysaora hysoscella and the hydrozoan Aequorea aequorea, both of which occur in large numbers, are probably of major ecological importance, and physically hamper pelagic fishing and diamond extraction activities. C. hysoscella was detected predominantly at an inshore station and A. aequorea was found in greatest abundance further offshore in deeper water. Echo-sounder observations were linked directly to net catches, and relationships between catch density (number of individuals m -3) and nautical area scattering coefficients (s A ) at each frequency were determined for both species in order to estimate target strength (TS) using the comparison method. TS for C. hysoscella (mean umbrella diameter 26.8 cm) was -51.5 dB at 18 kHz, -46.6 dB at 38 kHz and -50.1 dB at 120 kHz; for A. aequorea (mean central umbrella diameter 7.4 cm) TS was -68.1 dB at 18 kHz, -66.3 dB at 38 kHz and -68.5 dB at 120 kHz. These TS values compared favourably with previously published estimates for related species. Jellyfish were caught at high numerical densities (maxima 3 C. hysoscella per 100 m 3 , 168 A. aequorea per 100 m 3 ). These high densities, combined with the not unsubstantial TS at frequencies used for fisheries surveys, imply that jellyfish could potentially bias acoustic estimates of fish abundance. We suggest a simple multifrequency approach that could be used to discriminate between echoes from jellyfish and some commercially important pelagic fish in the northern Benguela ecosystem.

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“…The comparison of the two systems provides a framework for developing a more comprehensive view of the factors that influence the sensitivity of biological production to wind forcing and for a better understanding of the underlying dynamics of EBUS ecosystems in general . This can also be useful for predicting the response of EBUS to potential wind changes induced by climate change (Bakun, 1990;Shannon et al, 1992;Mendelssohn, 2002;McGregor et al, 2007).…”

Section: Introductionmentioning

confidence: 99%

What controls biological production in coastal upwelling systems? Insights from a comparative modeling study

Lachkar

Gruber

2011

Biogeosciences

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Abstract. The magnitude of net primary production (NPP) in Eastern Boundary Upwelling Systems (EBUS) is traditionally viewed as directly reflecting the wind-driven upwelling intensity. Yet, different EBUS show different sensitivities of NPP to upwelling-favorable winds (Carr and Kearns, 2003). Here, using a comparative modeling study of the California Current System (California CS) and Canary Current System (Canary CS), we show how physical and environmental factors, such as light, temperature and cross-shore circulation modulate the response of NPP to upwelling strength. To this end, we made a series of eddyresolving simulations of the two upwelling systems using the Regional Oceanic Modeling System (ROMS), coupled to a nitrogen-based Nutrient-Phytoplankton-ZooplanktonDetritus (NPZD) ecosystem model. Using identical ecological/biogeochemical parameters, our coupled model simulates a level of NPP in the California CS that is 50 % smaller than that in the Canary CS, in agreement with observationally based estimates. We find this much lower NPP in the California CS despite phytoplankton in this system having nearly 20 % higher nutrient concentrations available to fuel their growth. This conundrum can be explained by: (1) phytoplankton having a faster nutrient-replete growth in the Canary CS relative to the California CS; a consequence of more favorable light and temperature conditions in the Canary CS, and (2) the longer nearshore water residence times in the Canary CS, which permit a larger buildup of biomass in the upwelling zone, thereby enhancing NPP. The longer residence times in the Canary CS appear to be a result of the wider continental shelves and the lower mesoscale activity characterizing this upwelling system. This results in a weaker offshore export of nutrients and organic matter, thereby increasing local nutrient recycling and reducing the spatial decouCorrespondence to: Z. Lachkar (zouhair.lachkar@env.ethz.ch) pling between new and export production in the Canary CS. Our results suggest that climate change-induced perturbations such as upwelling favorable wind intensification might lead to contrasting biological responses in the California CS and the Canary CS, with major implications for the biogeochemical cycles and fisheries in these two ecosystems.

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The 1980s – a decade of change in the Benguela ecosystem

Cited by 48 publications

References 29 publications

Using behavioural and state variables to identify proximate causes of population change in a seabird

Using behavioural and state variables to identify proximate causes of population change in a seabird

Acoustic observations of jellyfish in the Namibian Benguela

What controls biological production in coastal upwelling systems? Insights from a comparative modeling study

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