Linked effects of dam-released floods and water temperature on spawning ofthe Clanwilliam yellowfish Barbus capensis

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.