A close look at the productivity of the Benguela Upwelling System

Background and Objective

Coastal upwelling systems produce a major portion of global fisheries resources, and the Benguela Upwelling System ranges among the largest. However, considerable differences currently exist between the fish stocks and the populations of their prey in the southern and the northern sections of the Benguela Upwelling System. Environmental conditions as well as the intensity of fishing activities have contributed to this divergent development.

In TRAFFIC, German scientists investigate, together with partners from southern Africa, the mechanisms behind the difference in transfer efficiency and productivity of the food webs in both regions. Researchers of the Thünen Institute of Sea Fisheries and the South African fisheries research institution DAFF, jointly lead a subproject focusing on mesopelagic fishes. These widely distributed fish species conduct extensive diel vertical migrations and thereby contribute significantly to the flux of matter between different depth zones of the ocean. Nevertheless, their role in the productivity of the food chains supplying commercially exploited fish species has received only limited attention, thus far.

Target Group

Fisheries managers, scientists, general public   

Approach

During two sea-going expeditions, the international team will investigate the distribution patterns and the vertical migration behavior of mesopelagic fishes. Hydro-acoustic measurements as well as net hauls will be taken with a research vessel. Closing nets allow for targeted sampling of fish from different depth zones, and various methods will be applied to obtain insight into the feeding behavior of the fishes. 

Data and Methods

Investigations at sea involve hydro-acoustic documentation of the distribution patterns of the fishes with direct net sampling. Methods of onboard analyses and investigations in the laboratory ashore include: 

• microscopic investigations of stomach contents
• genetic identification of prey items not otherwise identifiable
• investigation of the daily rhythms of migratory and feeding behavior
• measurement of stable isotopes as indicators of the position of the mesopelagic fishes in the food web
• size spectra analyses as components of the trophic transfer efficiency
• geostatistical interpretation of the fishes’ distribution patterns

Our Research Questions

• Is there a systematic difference between abundance and distribution patterns of mesopelagic fishes in the northern and southern Benguela Upwelling System?
• Which positions in the food chain do mesopelagic fishes occupy?
• Which are the major prey organisms of the mesopelagic fishes, and which species act as their own predators?
• In which way do mesopelagic fishes contribute to the trophic transfer efficiency, i.e. to the efficiency in the flux of energy and matter through the respective food chains?

Links and Downloads

• First Thünen Africa-day
• Meteor-expedition into the South Atlantic
• The integrated training and knowledge exchange programme SPACES II: Science Partnerships for the Adaptation to Complex Earth System Processes in the Southern Africa
• Thünen "News article": Southern African ecosystems in a changing global context