Riparian Management and Fish Productivity

Peggy Wilzbach and Ken CumminsUSGS CA Cooperative Fish Research UnitHumboldt State University

Habitat isn’t enough. Fish need food!

• Many streams with pristine habitat support low production of salmonid fishes

• Some of the most productive streams have suboptimal habitat (but abundant food)

The greatest salmonid production is realized in hatchery channels!

Options for increasing salmonid production? • Direct addition of food organisms

(not realistic over long-term)• Nutrient or organic matter

enrichment• Increasing autotrophic production

Case study: Effects of riparian canopy opening and salmon carcass addition on the abundance and growth of resident salmonids

Wilzbach, M.A. et al. 2005. Can. J. Fish. Aquat. Sci. 62: 1-10.

buffer

Experimental Design

Tectah

Tarup

Pac

ific

Oce

an

Pac

ific

Oce

an SF Rowdy

Little Mill

Peacock

Savoy

0 5 10 15

kilometers

N

Lower Smith River Lower Klamath River

Savoy Creek: closed canopy

Tarup Ck– Open Section

Time of day (h)1 2 3 4 5 6 7 8 9 10 1112 13 1415 16 17 18 19 20 21 22 23 24

µmol

•s-1

•m-2

0

50

100

150

200

250

300

350

Closed Open

Incident Radiation: Savoy Ck, July 6, 2003

Stream temperature did not differ between cut and uncut reaches in these coastal streams

Total Salmonid Biomass

Treatment effects:riparian date

(but not carcasses)

June 2002

-4

-2

0

2

4

6

8October 2002

UncutCut

June 2003

Carcass treatment

None Added

Cha

nge

in b

iom

ass

afte

r man

ipul

atio

n(g

l m

- 2)

-4

-2

0

2

4

6

8October 2003

None Added

Specific Growth Rates

Significant treatment effects: riparian, date, riparian* carcass

None Added

Spec

ific

grow

th ra

te (%

l da

y-1 )

0.0

0.1

0.2

0.3

0.4

0.5

Uncut Riparian Cut Riparian

overwinter 2002

Carcass treatment

None Added

oversummer 2002 overwinter 2003

None Added

Conclusions:• Increased light was more effective than carcass addition

in enhancing salmonid productivity of study sites

• Carcass enhancement may fail to increase salmonid production in settings where light is limiting or other factors prevent its successful use

• Selective trimming of riparian alder should be evaluated as a management tool for enhancing salmonid production

Can the food-generating capacity of a stream be readily assessed? Across broad spatial scales, highly

productive streams are associated with:

• moderate temperatures, groundwater inputs

• relatively low vegetative canopy coverage

• hard waters, relatively high concentrations of inorganic nutrients

• Within regions, need to directly measure prey availability

• Macroinvertebrate drift more accurately reflects prey availability than does the benthos

• The ratio of behavioral to accidental drifters may provide a good index of prey availability during low flow conditions

Behavioral drifter: predictably available on a diel basis

Accidental drifter: without predictable pattern of drift entry; “windfall” diet items

-0.1 0.0 0.1 0.2 0.3 0.40

10

20

30

40

50

60Pe

rcen

t of d

rift m

ass

from

beh

avio

ral d

rifte

rs

(ASI

N S

QR

T)

Specific growth rate of salmonids (% · day-1)

R2 = 0.42

Summer

-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.60

10

20

30

40

50

60

70

Perc

ent o

f Ter

rest

rial I

nver

tebr

ates

in D

rift

(ASI

N S

QR

T)

Specific growth rate of salmonids (%· day-1)

R2 = 0.62

In our study, % of terrestrial inverts was negatively related to fish growth

to establish the amount, spatial pattern of light gain, and riparian composition needed to optimize local food supplies - without sacrificing beneficial functions of riparian vegetation or cumulating temperature loadings downstream.

Research needs: