Application of the stratified-Petersen estimator to estimate numbers of masu salmon Oncorhynchus masou smolts

Multiple marking and recapture were conducted to estimate numbers of masu s almon Oncorhynchus masou smelts in a river in Hokkaido, northern Japan. A p artial fence and a rotary-screw trap, located approximately 5 km apart, wer e used to capture downstream migrating smelts from May to early July 1998 a nd 1999. Marking and recovery data were initially stratified at 7-day inter vals although data for some weeks were subsequently pooled to increase samp le sizes. Diagnostic chi-square tests indicated that pooled Petersen estima tes, which were formed by pooling all of the marks applied and all of the r ecoveries, were not consistent, and the stratified-Petersen, i.e., maximum likelihood Darroch, estimates were appropriate. The stratified-Petersen est imate was 30% higher than the pooled Petersen in 1998, and was 16% higher i n 1999. The migratory behavior of masu salmon smelts changed during each ru n and the capture efficiency of the trap varied as water flow fluctuated. U nder these conditions, heterogeneity in the capture probability of smelts a pparently occurred, which violated a critical assumption required for the p ooled Petersen estimator.