16 864 nautical miles and 73 days later…

On November 1, the RV SONNE returned from its 285th cruise to the Benguela upwelling system to the port of Emden, Germany. During this TRAFFIC project expedition, water and nutrient samples, phytoplankton, zooplankton, fish, and cephalopods were collected which have now arrived at the participating institute and are ready to be analyzed. While the TRAFFIC project is comparing the energy that travels through the food web (Trophic transfer efficiency) of each subsystem, the role of the researchers at the Thünen Institute of Sea Fisheries is to focus on the diversity and feeding ecology of mesopelagic fishes and cephalopods. Mesopelagic fish play an important role in the food web because they feed on zooplankton and small fishes and are preyed upon by predators such as seals, tuna, and hake. However, little is known about their position in the food web in the Benguela’s northern and southern subsystems.

The Rectangular midwater trawl was used to collect mesopelagic fishes and cephalopods (squids and octopus) and line and hook fishing took place each night to catch larger squids. Sampling took place at night, when mesopelagic fish and zooplankton are at the surface to feed and to avoid predators in daylight. PhD candidate Sabrina Duncan is comparing mesopelagic fish diversity and community composition between the northern and southern subsystems as well as the role of different mesopelagic species in the food web through biochemical analysis. A Thünen MSc student will be using stomach content analysis and biochemical analysis to compare the diet and position of squid in the food web, between each subsystem. Some of these analyses are already taking place and others are planned for the next months.

Because the first samples were collected during February and March of 2019, seasonal comparisons will also be made between samples, since seasonality affects the upwelling in the area and consequently environmental parameters such as oxygen concentration in the water. Some preliminary observations were made during the cruise, regarding the seasonality. One of these was the presence of high amounts of jellyfishes (Chrysaora fulgida) on the shelf of the northern Benguela Upwelling System. During the austral summer high numbers of jellyfishes were found in the northern subsystem, however the dominating species was Aequorea sp. and C. fulgida was not present. Jellyfishes can impact the trophic food web because they feed on the same prey as small pelagics, such as copepods and euphausids and are known as a ‘dead-end’ species in the food web because very few organisms feed on them (creating a dead-end in the food chain). After 73 days at sea, the crew was welcomed with the fall weather of Northern Germany.

The team enjoyed the station work, many nights of sampling, and the exciting catches that were observed in the nets and everyone is looking forward to analyse the samples and to come together again to present the results.

If you have questions, please contact Sabrina Duncan.

For more information about the TRAFFIC project, click here.