Ecology of salmon in their final days of life

Andrew Dittman

Adult Salmon Ecology1. Physical disturbance by adult salmon

2. Salmon carcasses as a resource3. Effects on freshwater systems4. Effects on terrestrial systems5. Feedback loops and bears

1. Migration and maturation

2. Redd-digging and egg-laying

3. Death

The final days in the lives of Pacific salmon

Decomposition or consumption

Migration and maturation. . .

Migration and maturation. . .

Photo:Daniel SchindlerPhoto: Daniel Schindler

Redd-digging and egg-laying. . .

Photo:Daniel Schindler

Redd-digging and egg-laying. . .

Death. . .

3. Death. . .

Ouch!

Death. . .

Scavenging

Sue JohnsonCarcass decomposition

Salmon as excavators

The digging of redds by females disturbs the gravel substrate of streams, affecting sediment transport, algae, and insects.

No salmon

Many salmon

June SeptemberAugustJuly

Suspended particulate load

Cottonwood Cr.

Pick Cr.

Jon MooreFilter paper after processing equal volumes of water from the creeks throughout the season

0

0.1

0.2

0.3

0.4

0.5

June July August June July AugustSept Sept

alga

l bio

mas

s (μ

g ch

loro

phyl

l-acm

-2)

Salm

on m

-2

0

0.1

0.2

0.3

0.4

Algal dynamics and salmon densities

2001 2002

Pick Creek Jon Moore

0

500

1000

1500

2000

0

200

400

600

Aqua

tic in

sect

s·m

-2Salmon disturbance reduces aquatic insect density

Stream with salmon

Stream with no salmon

June July August Sept(Mean + 1 SE)

Salmon as fertilizer

Salmon migrate to sea at a small size and return at a much larger size (though most die at sea). Their return migration and death is a rare way to fight the otherwise inexorable downstream flow of nutrients to the sea.

Magnitude of salmon carcass net import of biomass to freshwater

pink chum sockeye coho chinook

Smolt wt (g) 0.22 0.4 10 18 10

Kg/1000 smolts

0.22 0.4 10 18 10

Marine survival

0.028 0.014 0.131 0.104 0.031

Adult wt (kg) 1.63 3.73 2.69 3.02 7.22

Kg/1000 smolts

45.64 52.22 352.43 314.08 223.82

Net import 207 x 131 x 35 x 17 x 22 x

Pacific Northwest Geology 101

Nitrogen and Phosphorus limit production

Sources:P — from rocks and soil (via weathering)N — from the atmosphere (via N-fixation)

Why are salmon-derived nutrients important?

1. Nitrogen and phosphorus are limiting nutrients to freshwater and terrestrial ecosystems.

2. Salmon bodies are rich in N and P

3. Salmon can be very dense, and inevitably die

Correlation between phosphorus and fish standing crop in B.C. lakes

0 1 2 3Total Phosphorus (μg/L)

log

Stan

ding

Cro

p (k

g/ha

)

2.8

2.2

1.6

1.0

0.4

0

Stockner. 1987. Can. Sp. Publ. Fish. Aquat. Sci. 96

Import of biomass, nitrogen and phosphorous to Bear Creek, SEAK, 1999

Chum Pink

Biomass of returning adults (kg)

5463 4663 5800 4760

Total kg 10126 10560Kg per meter 14.5 15.1

Nitrogen PhosphorousTotal kg imported 621 83

kg per meter 0.89 0.12

0102030405060

June July Aug Sept0100200300400500600

TN (µ

g L

-1)

TP (µ

g L

-1)

0102030405060

0100200300400500600

TN (µ

g L

-1)

TP (µ

g L

-1)Creek with salmon

Creek without salmon

Water chemistry

Data: Jon Moore

Where do the nutrients go, and how do they get there?

Aquatic vs. terrestrial pathways

AQUATIC

Predatory fishes

Invertebrates

Microorganisms

Spawning fish eggs and carcasses

Primary and secondary producer density downstream and upstream from carcasses

Den

sity

(num

ber p

er c

m2 )

or

A

sh-fr

ee d

ry m

ass

(mg/

cm2 ) 400

300

200

100

0

Wipfli et al. 1998. CJFAS 55

Macroinvertebrates Biofilm

carcasses

upstream (control)

Average density of biofilm and benthic invertebrates at six salmon carcass loading levels

0 1.45 2.9 4.35 5.8 7.25

180

135

90

45

0

Den

sity

or A

sh-fr

ee d

ry m

ass

per c

m2

Carcass wet mass (kg)

biofilm

invertebrates

Resident fishes (e.g., trout, charr, sculpins) rely heavily on eggs for their annual food reserves

Dolly Varden

Even small salmon, charr and trout readily eat salmon eggs.

Tissue from dead salmon is also consumed by fishes.

Condition factor of juvenile coho in control and carcass-laden sites

Sept. Oct. Nov. Dec. Jan. Feb. Mar.

Con

ditio

n Fa

ctor

1.4

1.3

1.2

1.1

1.0Carcasses in Creek

carcasses added

control

Bilby et al. 1998. CJFAS 55: 1909-1918

How can we determine whether the effects are related to salmon tissue?

Stable Isotopes 101Stable isotopes are natural isotopes of

common elements (Carbon and Nitrogen).The ratio of the rare to common element

(e.g., 15N:14N) is the isotopic signature.Different sources of these elements have

distinct and consistent ratios, including differences between marine and freshwater systems. Thus the stable isotopes help track “marine derived nutrients”.

Proportion of salmon-derived nitrogen and carbon in the biota of salmon-bearing streams

Biota Average percent N Average percent CRiparian foliage 18 0Biofilm 21 25

Grazers 25 29Shredders 24 0Collector-gatherers 14 30Invertebrate predators 11 27Age-0 cutthroat 19 23Age-1 and 2 cutthroat 26 25Adult cutthroat 46 47Age-0 coho 31 40

Age-1 steelhead 32 61

Bilby et al. 1996. CJFAS 53:164-173

Enrichment of juvenile salmonids with nitrogen derived from carcasses reaches a plateau

R2 = 0.48

-0.2

0.0

0.2

0.4

0.6

0.8

0 250 500 750 1000

carcasses per km

inde

x of

N 1

5 en

richm

ent

Bilby et al. 2001

Adult salmon

InsectsJuveniles

Growth Survival

More adult salmon

2º

1ºPositive feedback loop?

Terrestrial dispersal of salmon-derived nutrients

1. Large predators and scavengers move carcasses into riparian areas.

2. Carcasses are consumed by small, mobile predators and scavengers.

3. Floods deposit carcasses into floodplain.

4. Subsurface water transports dissolved nutrients to trees in the riparian zone.

TERRESTRIAL

Riparian birds and other vertebrates

Invertebrates

Vegetation

Carcasses and digested fish

1. Direct movement of carcasses

Photo: Sue Johnson

2. Indirect movement of salmon nutrients

δ15Ν values in white spruce (Picea glauca): Lake Aleknagik, AK 1997

-6

-5

-4

-3

-2

-1

0

Spawning Sites Reference Sites

MeanFoliarδ15Ν < 25m

from stream

> 50mfrom

stream

James Helfield, Ph.D. dissertation

δ15N values in white spruce as a function of bear activity, Lake Aleknagik, AK 1997

-12

-8

-4

0

4

8

12

Bear Spawn. Ref.Spawning

Sites < 5 mReference

Sites < 5 mBear

Kitchens

Mea

n fo

liar δ

15Ν

Adult salmon

Vegetation

Habitat

Juveniles

Growth Survival

More adult salmon

3º

Positive feedback loop?

AQUATIC TERRESTRIAL

Predatory fishes Riparian birds and other vertebrates

Invertebrates

Microorganisms

Spawning fish eggs and carcasses

Invertebrates

Vegetation

Carcasses and digested fish

Predation by bears on salmon: ecology and behavior

Susan Johnson

Bears are highly omnivorous but salmon are very important as food

0

5

10

15

20

25

30

35

1 3 5 7 9 11 13 15 17 19 21 23 25 27Days alive in the stream

% o

f the

sal

mon

bear killssenescent

Bears tend to kill newly arrived (unspawned) salmon if they can

Bears selectively consume salmon to maximize energy intake, not biomass

Brown bears that eat more meat (mainly salmon) are larger than those eating

primarily vegetation300

200

100

0

Mea

n fe

mal

e m

ass

(kg)

0 25 50 75 100Dietary meat %Hilderbrand et al. 1999

Brown bears are most dense in areas where they eat lots of meat

0 25 50 75 100Dietary meat %

Den

sity

(bea

rs/1

000

km2

)

Hilderbrand et al. 1999

Brown bear populations with more dietary meat (salmon) have larger litters than those eating primarily vegetation

0 25 50 75 100

2.6

2.4

2.2

2.0

1.8

1.6

Dietary meat %

Mea

n lit

ter s

ize

Hilderbrand et al. 1999

Salmon are very important for bears, and bears have influenced the evolution of salmon life histories. In addition, the interaction between them has ramifications for nutrient cycling in aquatic and terrestrial systems.