LONG-TERM EFFECTS OF SIZE-SELECTIVE FISHERIES & HATCHERY MATING PRACTICES ON AGE & SEX COMPOSITION OF CHINOOK SALMON RETURNING TO HATCHERIES David Hankin & Jackie Fitzgibbons

Fisheries Biology

Humboldt State University, Arcata, CA

FUNDING: THANKS TO -THE YUROK TRIBE & THE TRINITY RIVER RESTORATION PROGRAM

LIFE HISTORY OF CHINOOK SALMON

Anadromous and Semelparous Age at Maturity:

Males: Ages 2 (“jacks”) through Age 6

Females: Ages 3 through Age 6 Female Age Composition is used to Classify

Stock-Specific Maturation Schedule (Early-, Mid-, and Late-Maturing).

Female Age Composition in Several Oregon Coastal Streams

Early- Mid- Late-

In Wild Populations, Percentages of Males at Age are Not Tightly Correlated with

Proportions of Females at Age

FEMALE AGES STREAMS MALE AGES

3 4 5 6 2 3 4 5 6

50 44 5 1 Applegate 33 39 26 2 0

0 20 73 7 Nehalem 4 23 31 42 0

2 33 43 21 Trask 0 12 62 20 6

4 30 58 9 Salmon 35 22 28 13 2

Size at Age Increases Dramatically Through Age 4, Then More Slowly

Late-Maturing Stocks Produce the Largest Fish (Kenai R., Alaska)

Ocean Fisheries Can Dramatically Reduce the Probability of Spawning at an Old Age

Hatchery Production is HUGE:

• California’s Central Valley (Sacramento River):

32 million fish

• OR/WA Columbia River Hatcheries: 150-180 million fish

Recreational and Commercial Fisheries are Intensive

AGE AT MATURITY IS AN INHERITED TRAIT THAT IS STRONGLY INFLUENCED BY PARENTAL AGES

ELK RIVER HATCHERY AGE AT MATURITY EXPERIMENTS:

Hankin et al. 1993. Evidence for inheritance of age at maturity in chinook salmon. CJFAS.

Elk River Hatchery Experiments

Age-Specific Mating Experiments:

1974 BY: 3 x 3 VS 5 x 5

1979 BY: 2 x 4+ VS 4+ x 4+

1980 BY: 2 x 4+ VS 4+ x 4+

2 3 4 5 6

AGE

0

400

800

1200

Est

imate

d R

iver

Retu

rns

at

Age

1974 BY: 3x3 vs 5x5

Female Returns: 3x3Female Returns: 5x5

2 3 4 5 6

AGE

0

1000

2000

3000

Est

imate

d R

iver

Retu

rns

at

Age

1974 BY: 3x3 vs 5x5

Male Returns: 3x3Male Returns: 5x5

2 3 4 5 6

AGE

0

50

100

150

Obse

rved R

iver

Retu

rns

at

Age

1979 BY: 2x4+ vs 4+x4+

Female Returns: 2x4+Female Returns: 4+x4+

Spawning Behavior Research Suggests that Larger Males Enjoy Greater

Reproductive Success (Baxter HSU)

MANAGEMENT ISSUES AND MODELING QUESTIONS

Does size-selective ocean fishing, through shifting age composition of spawners to younger ages, select for earlier age at maturity (Ricker 1980, 1981)?

Does random mating of hatchery fish, especially random inclusion of jacks as male parents, cause unintentional (genetic) selection for earlier age at maturity (Hankin 1986-present!)?

KEY MODEL FEATURES

Matrixes of Conditional Maturation Probabilities – Device to Account for Heritability of Age at Maturity

Age- and Sex-Structured Model of a Hatchery Chinook Salmon Population That Incoporates:

1. Hatchery Mating Practices, & 2. Size-Selective Ocean Fisheries

Example Matrixes of Conditional Maturation Probabilities: AT AGE 2, FOR MALES

MALE PARENT AGE

FEMALE PARENT AGE

2 3 4 5 6

3 0.5810 0.2997 0.1786 0.0574 0.0287

4 0.5428 0.2800 0.1688 0.0536 0.0268

5 0.5280 0.2600 0.1549 0.0498 0.0249

6 0.4652 0.2400 0.1430 0.0460 0.0223

Example Matrixes of Conditional Maturation Probabilities: AT AGE 3, FOR FEMALES

MALE PARENT AGE

FEMALE PARENT AGE

2 3 4 5 6

3 0.4026 0.3103 0.2182 0.1280 0.0604

4 0.2740 0.2112 0.1484 0.0856 0.0429

5 0.1456 0.1122 0.0789 0.0455 0.0228

6 0.0726 0.0567 0.0394 0.0228 0.0140

MODEL ASSUMPTIONS

REASONABLE ASSUMPTIONS

Hatchery matings are all 1:1 No females mature at age 2 All eggs are equally likely to survive to age 2 50:50 sex ratio in ocean at age 2

MODEL ASSUMPTIONS

“SIMPLIFYING” ASSUMPTIONS

No Freshwater Harvest Size at age k is independent of parental ages Ocean exploitation rates are constant and do

not vary between years

Model Variables, Model Parameters, & Model Form:

Model Variables

Model Parameters

p1 = probability of survival from egg to age 2

pk = (conditional) probability of surviving from age k to k+1

iF(i,j) = conditional probability of a female maturing at age i given

age i male and age j female parents

uk = ocean exploitation rate at age k

fj = average number of eggs per age j female

max = maximum number of eggs that can be hatched and reared

Model Form

MODEL SCENARIOS

UNEXPLOITED VS EXPLOITED HATCHERY MATING POLICY:

1. Completely Random Mating of All Adults, Including Jacks; OR

2. Completely Random Mating Except for Some Selection Against Jacks.

Modeling Results % Jacks in Returns (Unexploited)

1/Z ('Systematic Jacks') or Y ('Maximum Jacks')

Per

cent

age

Jack

s20

2530

3540

1.00 0.75 0.50 0.25 0.10

Other Age Groups Should Also Be Affected By Jack Mating Policy (Unexploited Equilibrium)

2 3 4 5 6

010

30

Male Age at Maturity

Per

cent

of

Ret

urns Random Matings

1/Z=0.25Y=0.1No Jacks Used

2 3 4 5 6

010

30

Female Age at Maturity

Per

cent

of

Ret

urns

Exploitation and Random Mating Together Shift Age Structure Toward Males & Younger Ages

2 3 4 5 6

020

4060

Male Age at Maturity

Per

cent

of

Ret

urns

Unexploitedu3=.2, u4=.35u3=.3, u4=.53

2 3 4 5 6

020

4060

Female Age at Maturity

Per

cent

of

Ret

urns

Selection Against Jacks Can in Part Compensate for Age Structure Shifts Due to Ocean Fishing

Proportion of Jacks Used as Male Parents (1/Z)

Per

cent

Jac

ks in

Pop

ulat

ion

2030

4050

60

1.00 0.75 0.50 0.25 0.10

Unexploited

Moderate Exploitation

Heavy Exploitation

2 3 4 5 6

020

40

60

Male Age at Maturity

Perc

ent

of

Retu

rns

Unexploitedu3=.2, u4=.35u3=.3, u4=.53

Completely Random Mating

2 3 4 5 6

020

40

60

Male Age at Maturity

Perc

ent

of

Retu

rns

No Jacks Used

Jack Mating Policy Effects on Age Composition are not Limited to Age 2 Males: Male Returns

Jack Mating Policy Effects on Age Composition are not Limited to Age 2 Males: Female Returns

3 4 5 6

010

2030

Female Age at Maturity

Per

cent

of

Ret

urns

Unexploitedu3=.2, u4=.35u3=.3, u4=.53

Completely Random Mating

3 4 5 6

010

2030

Female Age at Maturity

Per

cent

of

Ret

urns No Jacks Used

Conclusions from Modeling

Completely Random Mating Should Theoretically Select for Early Age at Maturity.

Jack Usage is Critical in An Early-maturing Stock Type.

“Different” Hatchery Mating Protocols Might Mitigate, in Part, for Fishery Selection?

NEED MORE MATING EXPERIMENTS!!

MORE DIFFICULT QUESTIONS TO ANSWER IN FUTURE:

How does size-selective ocean fishing affect long-term trends in age at maturity in wild population with non-random mating?

How much additional selective effect due to mating policy and inheritance of age at maturity vs. direct selective effects of “fishing down”?

MORE DIFFICULT QUESTIONS TO ANSWER IN FUTURE:

How is “size at age” inherited and to what degree does it influence maturation?