Matt Sloat Department of Fisheries and Wildlife Oregon State University

1. Density-dependent reinforcing mechanisms of anadromy in partially migratory salmonid populations Matt Sloat Department of Fisheries and Wildlife Oregon State University

2. Partial migration- populations in which only a portion of individuals adopts a migratory life history.

3. Salmonid species exhibiting partial migration: steelhead/rainbow trout (Oncorhynchus mykiss) coastal cutthroat trout (O. clarki) masu salmon (O. masuo) brown trout (Salmon trutta) brook charr (Salvelinus fontinalis)

4. Partial migration- populations in which only a portion of individuals adopts a migratory life history. What are proximate mechanisms causing individuals to adopt alternative life histories? What is the importance of the critical period?

5. Stream salmonids actively defend territories used for feeding on drifting invertebrates. Within a given environment growth depends on access to food which is driven by social dominance. Hartman 1965

6. Competition can result in displacement of large numbers of individuals Elliott 1994

7. Fish with higher relative metabolic rates tend to become dominant (Metcalfe 1995). Similar results for: rainbow trout (McCarty 2001) masu salmon (Yamamoto et al. 1998) brown trout (Titus and Mosegaard 1991)

8. How does natural selection during the critical period influence life history expression?

9. Migrants Residents Somatic growth Waste Metabolic costs Fish with higher metabolic costs tend to become migratory. Brook charr (Morineville and Rasmussen 2003) Similar results for brown trout (Forseth et al. 1995)

10. Hypothesis • Standard metabolic rate is a good predictor of competitive dominance in stream salmonids. • If intraspecific competition favors individuals with higher SMR, then increases in competitive intensity should increase strength of selection for fish with higher SMR. • If having higher metabolic costs increases the probability of becoming anadromous, increasing competition may increase the propensity for anadromy.

11. Experimental test of density effects on selection for relative metabolic rate in O. mykiss

13. Experimental channels inflow flow direction 0.3 m 2.5 m bricks emigration trap

14. emergence check

16. Preliminary results • Patterns of emigration • Final density • Otolith size of territorial fish under • different treatments

17. p1 p3 p2

18. Final density Final channel densities Stocking density

19. CC B A Probability density Otolith size

20. Discussion: • If there is selection for SMR with increasing intensity of intraspecific competition what are the implications for life history expression?

21. Shift in mean MR to higher values. Less variance in individual physiologies. More variance in individual physiologies Metabolic rate Metabolic rate Recruits (parr) Stock (eggs or alevins) Weak selection on MR. Wider range of realized growth because of wider range of physiologies and decreased energy expenditure defending territories. Selective emigration or mortality of low MR individuals. Greater proportion of individuals prone to migration. Modified from Titus (1990)

22. Summary • Standard metabolic rate is a good predictor of competitive dominance in stream salmonids. • If intraspecific competition favors individuals with higher SMR , then increases in competitive intensity should increase strength of selection for fish with higher SMR. • Since migrants have higher relative metabolic costs, increasing competition may increase the propensity for anadromy.

23. Future work (an incomplete list): • Refinement of analyses. • Respirometry of offspring from resident • and anadromous parents. • 3) Measurement of other traits correlated • with life history expression.

24. Acknowledgments: Funding provided by USFS PNWRS and USGS FRESC, Corvallis. ORAFS Carl Bond Memorial Scholarship Thanks to Gordie Reeves, Jason Dunham, Brian Neilson, India Sloat, Don Stevens, Don Ratliff, Rich Madden, Jack Palmer, Michael Reeves.