Iteroparity and Steelhead: what we know and don’t know

Iteroparity and Steelhead: what we know and don’t know John R. McMillan Oregon State University

Introduction to Reproduction • Reproductive strategies (big bang v. bet-hedge) • Annual strategy in plants and semelparous strategy in animals • Reproduce one time • Perennial strategy in plants and iteroparous strategy in animals • Reproduce more than one time • Salmonid species • Pacific salmon – semelparous (Altukhov et al. 2000) • All other species – iteroparous (Wilson 1997)

Selection and Traits • Life history theory (Stearns 1976; Charlesworth 1994; Crespi and Teo 2002) • Semelparity • Select for higher juvenile survival • Increased egg size • Select for no adult survival • Increased investment in egg weight, female nest guarding, secondary sexual characteristics, breed under higher densities • Iteroparity • Low or inconsistent juvenile survival • Smaller egg size • High adult survival • Reduced investment in eggs, secondary sexual characteristics, lower breeding density, and no female nest guarding

Selection and Traits • Life history theory (Stearns 1976; Charlesworth 1994; Crespi and Teo 2002) • Semelparity • Select for higher juvenile survival • Increased egg size • Select for no adult survival • Increased investment in egg weight, female nest guarding, secondary sexual characteristics, breed under higher densities • Iteroparity • Select for lower juvenile survival • Smaller egg size • Select for higher adult survival • Reduced investment in egg weight, secondary sexual characteristics, no female nest guarding, breed under lower densities

Steelhead Patterns • Steelhead • One-time reproduction generally most common • Highly variable rates of repeat spawning (0 – 79 %) • Iteroparous individuals • Typically female (Burgner et al. 1992; Wertheimer and Evans 2005) • Typically smaller sized (Teo and Crespi 2002; Hendry and Stearns 2004) • More common in ocean-maturing life history (Busby et al. 1996) • Latitude and distance from sea • Highest levels at latitudinal extremes(e.g., Russia, Savvaitova et al. 1999; South America, Riva-Rossi 2007) • Similar observations for Atlantic salmon (Jonsson and Jonsson 2004) • Perhaps because energy consumption increases with water temperature • Lowest levels appear to be in furthest inland populations(Meehan and Bjornn 1991; Narum et al. 2008)

Steelhead Patterns • Steelhead • One-time reproduction generally most common • Highly variable rates of repeat spawning (0 – 79 %) • Iteroparous individuals • Typically female (Burgner et al. 1992; Wertheimer and Evans 2005) • Typically smaller sized (Teo and Crespi 2002; Hendry and Stearns 2004) • More common in ocean-maturing life history (Busby et al. 1996) • Latitude and distance from sea • Highest levels at latitudinal extremes(e.g., Russia, Savvaitova et al. 1999; South America, Riva-Rossi 2007) • Similar observations for Atlantic salmon (Jonsson and Jonsson 2004) • Lowest levels appear to be in furthest inland populations(Meehan and Bjornn 1991; Busby et al. 1996; Narum et al. 2008)

Latitudinal Distribution of Repeat Spawn Rates for Steelhead Derived largely from Busby et al. 1996 Russia AK - Canada Washington - Oregon North South

Why is Iteroparity Important? • Recruitment • First-time spawners don’t always replace themselves (e.g., Waddell Creek, Keogh River; Hal Michael, personal communication) • Repeat spawners in Atlantic salmon can produce disproportionate share of recruitment (Chadwick 1987; Mills 1989)

Fitness • Increased lifetime fitness • Greater fecundity • Repeat spawning female steelhead produced twice as many offspring as one-time spawners (Seamons et al., in prep) • Spread risk over multiple generations (Fleming and Reynolds 2004; Hendry and Stearns 2004)

What Human Factors Influence Iteroparity? • Environment • Habitat conditions experienced by adults during return to ocean • Dams • Climate • Oversummering habitat in some cases • Sport and commercial fisheries • Level of adult exploitation and encounter rates during return to ocean • Selection against larger fish could select against iteroparous individuals • This has been suggested to have occurred in Atlantic salmon (see Jonsson and Jonsson 2004)

Fisheries • Sport • Rate of adult exploitation & encounter by sport anglers during return to ocean? • Energy expenditure • Commercial • Selection against larger fish could select against iteroparous individuals • This has been suggested to have occurred in Atlantic salmon (see Jonsson and Jonsson 2004)

Patterns, Processes, Uncertainties • Lots of hypotheses, little data • Does iteroparity matter? • What causes iteroparity? • Natural v. human influences • How does this factor into our management regimes? • Abundance v. diversity

Iteroparity and Steelhead: what we know and don’t know