If the coho population of a coastal stream is much lower than what it was in historic times, the best thing to do is to add young fish to the system in hopes of bringing it back to the original capacity. Right? Wrong. We concluded that release of hatchery coho presmolts into coastal streams has resulted in a replacement of late spawning wild fish by early spawning hatchery fish with poor survival. Our introduction of hatchery presmolts has hurt coastal coho populations rather than helped them.
The results show that the summer rearing densities of the juveniles were increased by 50% due to stocking of presmolts. However, we found a 40-50 percent decrease in wild coho juveniles in the stocked streams. We also found that, despite the 50% increase in rearing densities in the stocked streams, adult abundance usually did not increase.
The release of hatchery coho presmolts was intended to help restore wild coho populations. We found that juvenile densities in streams where hatchery presmolts returned to spawn were 46% lower than streams without presmolts. The early spawning hatchery fish apparently do not contribute substantially to juvenile production. The reduction in juvenile abundance corresponds to a similar reduction in late spawning wild adults.The contribution of presmolts to ocean fisheries is about 0.1%, i.e., to add 1000 adults to the ocean fisheries, we need to release 1 million presmolts.