Electronic poster - Watermeyer Shannon Griffits | "Ecopath 25 years": conference and workshops

Electronic poster - Watermeyer Shannon Griffits

Title
	Changes in the trophic structure of the southern Benguela before and after the onset of industrial fishing
Authors
	Watermeyer KE^1,2, Shannon LJ¹, Griffits CL¹ Marine Research Institute, MA-RE Institute & Zoology Department University of Cape Town Private Bag X3 Rondebosch, 7701 South Africa kwatermeyer[AT]gmail.com
Watermeyer KE, Shannon LJ, Griffits CL
Marine Research Institute, MA-RE Institute & Zoology Department University of Cape Town Private Bag X3 Rondebosch, 7701 South Africa
Abstract
Despite a human presence in the Benguela region for at least one million years, exploitation of marine resources by European seafarers only began in earnest in the 1400s. Ecopath with Ecosim was used to construct and compare mass-balanced foodweb models of the southern Benguela ecosystem, representing aboriginal (10 000 BP-1651), pre-industrial (1652-1909), industrial (1910-1974) and post-industrial (1975-present) eras of human influence. Biomass at higher trophic levels (TLs) decreased over the periods examined, whereas that of sardine and anchovy increased in the early 2000s. Fishing became an important predatory impact, taking over consumption of small pelagics and horse mackerel from declined natural predators such as hake as removals increased over time. Harvesting of apex predators such as seals and seabirds during the pre-industrial era meant that the mean TL of the catch declined markedly between the pre-industrial (1900) and industrial (1960) models. Total biomass, consumption, respiration, production and throughput decreased from the pristine model to 1960, increasing again in the 2000s, probably influenced by the abnormally high small pelagic biomass in the early 2000s. Three additional alternate scenarios with differing assumptions were examined for each era. Although biomasses and consumption of various groups in these scenarios differed from base models, TL of the community and piscivore groups, and the diversity indices, were not altered much, suggesting that outputs from such retrospective models in the form of derived, relative indicators may be more robust than comparisons of absolute flows, which provide supplementary inferences. Although fishing has certainly impacted ecosystem structure since its commencement, effects are in addition to natural (specifically environmental) forcing. Fishing stress at the ecosystem level and the collapse of small pelagic stocks may lead to a shift toward a bottom-up trophic control mechanism, increasing the impact of environmental events including climate change. It is thus possible that pristine systems were not as severely affected by environmental anomalies as are modern systems.
Reference
Watermeyer KE, Shannon LJ, Griffiths CL (2008) Changes in the trophic structure of the southern Benguela before and after the onset of industrial fishing. African Journal of Marine Science 30(2): 351ñ382