Diet assessment of commercially important fishes in Lake Tanganyika using stable isotope techniques

Start date: 1 February, 2017 End date: 26 April, 2017 Project type: Master's Thesis (prior to 2018) Project code: A30223 Institutions: Aarhus University (AU), Denmark Grant recipient: Henrik Høiberg Jessen Total grant: 19,000 DKK



Lake Tanganyika is a large rift valley lake in eastern Africa. The pelagic fishery in the lake, which is largely made up of the three species Lates stappersii, Limnothrissa miodon and Stolothrissa tanganicae, is a hugely important source of food and income, supplying approximately 25-40% of the animal protein consumed in the riparian nations, and employing roughly one million people. Catch per unit effort is declining in the lake, and there are disagreements as to the primary underlying cause, with overfishing and climate changes being the most popular hypotheses. Because of this, models that account for both fisheries pressure and climate changes are being developed as part of the CLEAT project. This study aids in this endeavor, by establishing a pelagic food web for use in this modelling process, along with estimations of fish growth, zooplankton densities and limnological variables.

The limnological variables, temperature and oxygen saturation, showed a shallowing of the mixed depth, consistent with earlier reports on the rate of shallowing, and a general warming of the upper layer of the lake, also consistent with climate change induced warming reported earlier.

Counts of primary increments on otoliths from the three species indicated approximately linear growth at the life stages examined, but these linear models were found unlikely to explain growth for the entire growth range of any of these fish; Von Bertalanffy growth seemed more appropriate for this. Back calculated hatch dates based on these counts suggested continuous, rather than restricted, spawning periods.

A stomach analysis of L. stappersii showed a change in diet composition throughout the life cycle of the fish, shifting from copepods, to fish larvae supplemented by shrimps, to whole fish prey as the fish grows. Stable carbon and nitrogen isotope values from fish muscle were largely consistent with this, but more interestingly, showed no major differences among the fish species when size was accounted for, suggesting that fish diet is a function of size, rather than species, likely because more palatable prey is consumed as soon as the fishes size allows for this. Further, the stomach analysis indicated that shrimps were a relatively unimportant source of food for the L. stappersii, despite earlier studies claiming the fishes have the ability to sustain themselves entirely on shrimps, possibly indicating selective feeding on the likely more palatable fish larvae or whole fish prey when available. Fish below 80mm total length were likely to consume primarily copepods, supplemented by shrimps. Around 80mm a shift to primarily feeding on fish larvae, still supplemented by shrimps and/or copepods, was observed. In the case of L. stappersii, the fish grow large enough to consume whole fish when they reach a total length of 250mm, after which whole fish, namely S. tanganicae, compose the majority of the food consumed. These results should help to strengthen further modelling and management, by giving a recent estimation of carbon flow from the primary producers to the pelagic fishery in the lake, as well as new knowledge on the growth pattern of these species.