J. King et al., Linked effects of dam-released floods and water temperature on spawning ofthe Clanwilliam yellowfish Barbus capensis, HYDROBIOL, 384, 1998, pp. 245-265
In South Africa, modified flow regimes designed to maintain ecosystem funct
ioning at some pre-determined level are recommended for all rivers involved
in major water-resource developments. One component of such flow regimes o
f special interest in the winter-rainfall region of the country is the smal
l pulses of higher flow that occur in the dry season (November-April), here
called freshes. Research was carried out in the Olifants River, in the win
ter-rainfall region of South Africa, on the link between freshes and spawni
ng of an endemic, vulnerable cyprinid, the Clanwilliam yellowfish Barbus ca
pensis. Experimental releases from Clanwilliam Dam on the Olifants River in
a previous year showed that a greater than usual spawning success, assesse
d by larval recruitment levels, was correlated with hypolimnetic freshes re
leased during the species' breeding season (October-January). In the study
reported on here, a hypolimnetic fresh of the same size, duration and timin
g as those linked with successful spawning failed to induce spawning. Diffe
rences in the thermal regime of water releases appeared to be responsible.
Two warm (19-21 degrees C), epilimnetic freshes of 15 hours duration were t
hen released, and correlated with fish moving onto the spawn beds and exhib
iting pre-spawning behaviour. However, the fish moved downstream away from
the spawn beds when cold (16-18 degrees C) hypolimnetic baseflows were rele
ased for 15 hours between the two epilimnetic freshes and continually after
the second one. The presence of free embryos and larval B. capensis indica
ted, from their developmental stages, that spawning had probably occurred b
efore the experimental releases began, when warm (21-23 degrees C) epilimne
tic water was spilling over the dam, and had halted once spillage stopped a
nd hypolimnetic releases began. Indeed, the presence of dead and deformed y
oung suggested that the cold water may have had a detrimental affect on tho
se spawned during the earlier warmer spell.
A summary table details the conditions, including the hydraulic characteris
tics of the spawn beds, believed to induce spawning and support early life
stages of B. capensis. It is concluded that freshes released from Clanwilli
am Dam at the appropriate time (October-January) should be able to increase
spawning success of B. capensis, but only if the water temperature at the
spawn site is at or above 19 degrees C and stable or rising. However, succe
ssful spawning will not necessarily lead to high recruitment if water tempe
ratures are not then maintained at appropriate levels for some time for dev
elopment of the embryos and larvae. This implies that additional epilimneti
c releases will be required following critical trigger flows.