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

John R. McMillanOregon 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)

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)

North South

Russia Washington - OregonAK - Canada

0.00

0.25

0.50

0.75

1.00

% R

espa

wne

rsLatitudinal Distribution of Repeat Spawn Rates for Steelhead

Derived largely from Busby et al. 1996

• 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)

Why is Iteroparity Important?

• 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