Abstract:
We tested the hypothesis that increased hypoxia in Lake Victoria provides a
refugium for the cyprinid dagaa (Rastrineobola argentea) against hypoxia-sensitive Nile
perch (Lates niloticus).
2. Hypoxia in the main habitat of dagaa was rare during 1979–80, but lasted 3–5
months in 1987 and 1988, during which time adult dagaa spent the day just above the
oxycline instead of near the bottom.
3. Dissolved oxygen concentrations in the layer just above the oxycline were not critical
for Nile perch, but a mass kill of this species following a sudden upwelling of
hypoxic water suggested that oxycline-dwelling is risky.
4. Our data suggest no difference between dagaa and Nile perch in the level at which
dissolved oxygen starts to limit their vertical distribution in the water column, but
dagaa seems to be even more sensitive to extreme hypoxia (B1–2 mg O2 L 1) than
Nile perch.
5. We argue that oxycline-dwelling dagaa are not seeking a predation refugium but
that they are limited by low oxygen levels in reaching their feeding areas near the
bottom.