Migration timing affects salmon survival

If timing is everything, what happens when human disturbance delays animal migration?
In animal migration, timing is critical. Factors affecting survival and breeding success such as the availability of food, quantity of breeding sites, and predation vary with time. So what happens when human disturbance alters migration timing? A new study addresses that question for the migration of two species of Pacific salmon - the Chinook salmon Oncorhynchus tshawytscha and steelhead Oncorhynchus mykiss.

Chinook salmon. Image credit Zurekus used via creative commons license.According to the study published in the Journal of Applied Ecology, researchers from the National Marine Fisheries Service found that the timing of juvenile migration to the sea affects subsequent survival to adulthood. Pacific salmon migrate together from fresh water where they rear as juveniles to the northern Pacific Ocean where they obtain more than 90% of their biomass before returning to spawn in fresh water as mature adults. In many cases, the construction of hydropower dams has shifted the migration timing of salmon.
The researchers Mark Scheuerell, Richard Zabel and Benjamin Sandford used data from passive integrated transponder tags to estimate differences in juvenile to-adult survival in years between 1998 and 2007 and to determine migration timing. They found that the estimated survival of Chinook was generally highest for juveniles migrating in early to mid-May and then decreased across the remainder of the migration season. Survival for steelhead was much higher in early to mid-May for all years studied except 1999 when estimated survival peaked during late May.  The authors offer a possible explanation for the results:

"Salmon populations may have adapted their timing of juvenile migration to exploit optimal foraging conditions and avoid predators in the estuary and nearshore ocean…. Our results show that earlier migrants currently survive at much higher proportions than later migrants, suggesting that overall juvenile-to-adult survival may have been higher previous to development of the hydropower system."

Study recomendations for management...

"Based on the results described here, one focus of the management of Snake and Columbia River salmon populations would be to increase their downstream migration rate so that they arrive in the estuary earlier. This can be achieved by increasing river flow from storage reservoirs or increasing the amount of water spilled over the dams, which allows individuals to pass the dams more quickly."

Source:	Journal of Applied Ecology
Title:	Relating juvenile migration timing and survival to adulthood in two species of threatened Pacific salmon (Oncorhynchus spp.)
Authors:	a) Mark Scheuerell, a) Richard Zabel and b) Benjamin Sandford
	a) National Marine Fisheries Service, Seattle, WA; b) National Marine Fisheries Service, Pasco, WA

Comments

[Bobolink]

Thanks for bringing this article to my attention. It's very interesting. Hopefully, someone will do something about it.