Comparing trophic flows in the southern Benguela to those in other upwelling ecosystems

Abstract

A balanced trophic flow model of the southern Benguela ecosystem is presented, averaging the period 1980-1989 and emphasizing upper trophic levels. The model is based largely on studies conducted within the framework of the Benguela Ecology Programme and updates the results of an expert workshop held in Cape Town in September 1989. Small pelagic fish other than anchovy Engraulis capensis and sardine Sardinops sagax, mainly round herring Etrumeus whiteheadi and mesopelagic fish, were important components of the food web in the southern Benguela. Severe balancing difficulties were encountered with respect to the semi-pelagic resources (hake Merluccius spp.) and demersal top predators (sharks), indicating the need for further research on the interaction of these groups with their ecosystem. The model is compared to other existing trophic flow models of ecosystems in major upwelling areas, i.e. the northern Humboldt Current (4-14°S), the California Current (28-42°N) and the southern Canary Current (l2-25°N), and to two independently constructed models of the northern Benguela ecosystem. These models are compared using network analysis routines of the ECOPATH software, focusing on the interactions between the five dominant fish species (anchovy, sardine, horse mackerel Trachurus trachurus capensis, chub mackerel Scomber japonicus and hake) that support important fisheries in all systems. The upwelling systems rank by size rather than species dominance. The ratio of catches and primary production differs between systems, partly because of differences in fishing regimes. Predation on the five dominant fish groups by other fish in the system was the most important cause of fish mortality in all models. Fishery catches are generally a larger cause of mortality for these groups than predation by mammals. The ecological cost of fishing appears to be comparatively low in the southern Benguela, because catches are low compared with the primary production, but also because the fishery is relatively low in the foodweb. However, in view of the very tight foodweb demonstrated in the model. it is likely that an increase in fishing pressure would cause severe trade-offs with respect to other components of the southern Benguela ecosystem.

Long-term changes in the mean trophic level of Central Chile fishery landings

“…Another factor that could have biased the observed trend in TL m is the relatively low trophic level of small pelagic fish in Central Chile (TL=2.02) compared to TLs of similar species in comparative ecosystems (Jarre-Teichmann et al, 1998). The low TL of small pelagic fish in Central Chile is explained by the fact that, in this system, unlike other upwelling systems, these species seem to feed almost exclusively on phytoplankton (Arrizaga, 1983;Arrizaga et al, 1993).…”

Jellyfication of Marine Ecosystems as a Likely Consequence of Overfishing Small Pelagic Fishes: Lessons from the Benguela

“…Food-web models using the Ecopath with Ecosim approach were also used in the region for describing within-system structural changes over time (Heymans et al 2004, Watermeyer et al 2008a or for comparisons with other ecosystems (Jarre-Teichmann et al 1998, Shannon et al 2008, 2009). These modeling studies (e.g., Shannon et al 2009) have suggested that the energy flows within ecosystems can be disrupted by changes in the abundance of small pelagic fishes.…”

Ecosystem approach to fisheries management in the northern Benguela: the Namibian experience

“…The modelling approach used was ECOPATH with ECOSIM (EwE; Pauly 1992, Walters et al 1997). Local scientists and international experts at the workshop agreed on how the structure of existing ECO-PATH models of the region (Jarre-Teichmann et al 1998, Shannon andJarre-Teichmann 1999) should be modified in order to tailor the model to the fishing scenarios to be tested. The major product of this phase of the project was an updated ECOPATH model that described trophic flows in the northern Benguela in the late 1990s, following the anomalous environmental conditions experienced during 1994 and early 1995 (Roux and Shannon 2000).…”