Ecological interactions between juveniles of reproductively isolated anadromous and non-anadromous morphs of sockeye salmon, Oncorhynchus nerka, sharing the same nursery lake

Sockeye salmon and kokanee, the anadromous and non-anadromous morphs of Onc orhynchus nerka, spawn in close physical proximity in tributaries to Takla Lake, British Columbia but are reproductively isolated and genetically dist inct. Using genetic markers, we were able to investigate, for the first tim e, ecological interactions between the morphs as juveniles sharing the same nursery lake. Trawl and hydroacoustic surveys conducted in August of 1988 and 1991 revealed that juvenile O. nerka were distributed fairly evenly thr oughout Talka Lake with average densities ranging from 351-558 fish ha(-1) in the north arm to 585-769 fish ha(-1) in the west arm. Sockeye salmon wer e predominant (71-75 %) in the west arm whereas kokanee were predominant (8 2%) in the north arm, a difference attributed to the distribution of spawne rs in the brood years studied. Within arms, the morphs were intermixed with no detectable difference in relative abundance by depth or among trawl cat ches. Both morphs were highly selective in their diet, especially in the no rth arm where fish densities and grazing pressure were lower. As age 0 juve niles, sockeye salmon were significantly larger than kokanee (53 vs. 39 mm on average) but their food habits were virtually identical. Thus we found n o evidence of behaviour that would reduce niche overlap between these incip ient species.