david saiget zigzag ranger district · 2005-10-21 · zigzag ranger district. bull run reserve...

Coastal Cutthroat trout Shoal Spawning in a high montane lake of

the Cascade Range of Oregon

David Saiget

Mt. Hood National Forest

Zigzag Ranger District

Bull Run Reserve established in 1895 which closed public access in the Bull Run Watershed which serves as the drinking water supply for Portland metro area

4.2 mile circumference - 486 ac

natural lake level fluctuation 12’/year

As the headwater source for Portland-metro area drinking water, lake drawdown can occur in times of high water need which may have an affect on the fish population

Objectives:-determine extent of shoal spawning

-collect spawner, redd, and fry counts

-observe shoal spawning behavior

low avg high (n=5)

050

100150200250300350400

# of Eggs in excavated tributary redds

Avg length of females 14-18 inches

03-May07-May

11-May14-May

18-May21-May

24-May28-May

01-Jun05-Jun

09-Jun13-Jun

01020304050607080

Num

bers

N. Point & boat rampEast Isle

Snag Patch SW corner 4A shoal

Shoal spawner counts, 2003

05

101520253035

No.

Fry

Experimental gravel Natural Road/pit run

Fry production by substrate type

2.5 3.7 4.2 4.3 4.9 5.1 5.5 6 6.2 6.3 6.4 6.8 8.5 9

Depth, feet

05

10152025

3035

Fry production by Water depth

07-M

ay12

-May

17-M

ay20

-May

24-M

ay29

-May

03-Ju

n08

-Jun

13-Ju

n18

-Jun

23-Ju

n28

-Jun

03-Ju

l08

-Jul

13-Ju

l18

-Jul

23-Ju

l28

-Jul

5

10

15

20

LakeTrib 4A

shoal ht of spawntrib ht of spawn

Shoal emergence Trib emergence

Lake vs. tributary water temperature

1st redds observed 1st fry observed

Conclusions

Warmer temperatures and earlier emergence

may represent an advantage for shoal fry over tributary fry in the early growth stages

Limited spawning habitat in the tributaries may drive spawning in the shoals

Changes in lake surface elevation which prevent tributary accessmay force spawners to utilize the shoals

Shoal and tributary spawning may represent two divergent life histories

Absence of up- or down -welling, low DO’s, or poor substrates may be limiting fry production in shoal areas

Low productivity, oligotrophic nature of the lake may have in effect changed shoal spawning opportunities into feedingopportunities for many of these fish

Despite the high predation of eggs and less than optimal substrates, the presence of emergent fry in the shoal areas suggests that spawning in the shoals is a successful life history adaptation

Funded by:

Mt. Hood National Forest

Zigzag Ranger District

City of Portland Water Bureau

1992 1994 1995 199510001300160019002200250028003100340037004000

Population Estimates from hydroacoustics