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 Oncorhynchus nerka, spawn in close physical proximity in tributaries to Takla Lake, British Columbia but are reproductively isolated and genetically distinct. Using genetic markers, we were able to investigate, for the first time, 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 throughout Takla 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 were predominant (71–75%) in the west arm whereas kokanee were predominant (82%) in the north arm, a difference attributed to the distribution of spawners in the brood years studied. Within arms, the morphs were intermixed with no detectable difference in relative abundance by depth or among trawl catches. Both morphs were highly selective in their diet, especially in the north arm where fish densities and grazing pressure were lower. As age 0 juveniles, sockeye salmon were significantly larger than kokanee (53 vs. 39 mm on average) but their food habits were virtually identical. Thus we found no evidence of behaviour that would reduce niche overlap between these incipient species.