Mechanical Management of Burrowing Shrimpon WA Coast

26th Conference for Shellfish Growers, Washington Sea Grant, Alderbrook Resort and Spa March 11-12, 2019

Cinde Donoghue Aquatic Assessment and Monitoring TeamWA DNR

Photo by Phillip Colla

Burrowing shrimp (ghost shrimp) Neotrypaea californiensis• a native species• an important food source in

Willapa Bay for Green sturgeon, Dungeness crab

• Creates extensive burrows in sandy mud flats.

• Disrupts sediment to the extent it negatively affects shellfish ground culture.

• Burrowing shrimp in Willapa Bay have been controlled through an Integrated Pest Management approach over the past 20 years.

• DNR Aquatics began investigating feasibility of mechanical management in 2018.

To investigate feasibility of mechanical alternatives for burrowing shrimp, DNR’s Aquatics Assessment and Monitoring Team conducted two experiments:

1) Proof of Concept study to examine 3 different mechanical methods: flooding, wet harrowing and dry harrowing.

2) Supplemental to further investigate dry harrowing method which appeared most promising.

Proof of Concept (POC) Study

1) Flooding –hydraulic pumping at low tide

2) Dry Harrowing- Marshmaster towing “roller chopper” across mudflat at low tide

3) Wet Harrowing -pulling weighted agricultural harrow across mudflat at high tide

Tested 3 different mechanical methods:

Experimental site - Grassy Island, Willapa Bay• 3 flood plots• 3 wet harrow plots• 3 dry harrow plots (2 passes

with roller chopper) • 9 control plotsAll plots 0.5 acres in size.

Before and after treatment monitoring of: • shrimp density, • shrimp biomass,• sediment compaction• sediment grain size• burrow count

1 m

Weight Movement

5 lb. Dropper Weight

Base Plate

Sediment Compaction

Before and after shrimp density (#shrimp/core) for each treatment type

Dry Harrowing Flooding Wet Harrowing

Effort involved for each treatment type

Focus on Dry Harrow method • 4 passes with roller-chopper• Investigate recolonization

from non-treated areas• Increased number of pumped

cores sampled• Added manual sample cores

Supplemental Study

Mean shrimp density collected per pumped core at Control and Treated plots at t0 (pre treatment), t1 (3 weeks post), and t2 (6 weeks post).

Shrimp Density Pumped Cores

Shrimp Density manual cores

Mean shrimp density collected per manual core at Control and Treated plots at t0 (pre treatment), t1 (3 weeks post), and t2 (6 weeks post).

Size Class Ranges Large Medium Small Extra Small

Carapace Length (mm) > 17.42 17.42 - 12.49 12.49 - 8.28 8.28 >

Total Length (mm) > 69.95 69.96 - 49.25 49.25 - 30.94 30.95 >

Mass (g) > 6.85 6.85 - 2.41 2.41 - 0.62 0.62 >

Mean size class (mm)

Large Medium Small Extra Small

Source WDNRBosley &

Dumbauld(2011)

WDNRBosley &

Dumbauld(2011)

WDNRBosley &

Dumbauld(2011)

WDNRBosley &

Dumbauld(2011)

CarapaceLength ±SD (mm)

20.33±

2.36

13.26±

1.97

14.58±

1.69

10.75 ±

0.31

10.32±

1.41

8.55±

1.49

6.16±

1.60

6.28±

1.78

Shrimp size class thresholds and means

0

2

4

6

8

10

12

14

2.25 3.5 4.75 6 7.25 8.5 9.75 11 12.25 13.5 14.75 16 17.25 18.5 19.75 21 22.25 23.5 24.75 26

0

2

4

6

8

10

12

14

2.25 3.5 4.75 6 7.25 8.5 9.75 11 12.25 13.5 14.75 16 17.25 18.5 19.75 21 22.25 23.5 24.75 26

t1

0

2

4

6

8

10

12

14

2.25 3.5 4.75 6 7.25 8.5 9.75 11 12.25 13.5 14.75 16 17.25 18.5 19.75 21 22.25 23.5 24.75 26

t2

t0

Dry harrowing showed a reduction in treated vs. control sites in all size classes, for each sample time.

x-small small medium large

carapace length (mm)

Num

ber o

f shr

imp

Mean shrimp densities from pumped (left) and manual cores (right) along a distance gradient from treatment edge, at times t0, t1 and t2.

No evidence of lateral shrimp movement to recolonize the treated plots

Mean sediment penetration was significantly less (sediment more compact) in treated plots compared with untreated plots

0

0.05

0.1

0.15

0.2

0.25

t0 t1 t2

mea

n gr

ain

size

(mm

)

sample time

Sediment Grain Size: Before and After Dry HarrowingControl and Treatment Plots

control treatment

No significant difference in grain size between control and treated plots pre and post treatment

Sediment grain size analyzed using standard sieve shaker

• Shrimp density in dry harrowed plots dropped significantly from pre-treatment densities (by an average of 89%).

• After another four weeks (six weeks post-treatment, @ t2) this low shrimp density (0.73 ± SE .23) shrimp/core persisted.

• Dry-harrowing impact on shrimp densities was greater than natural shrimp population variability

Findings from experiments on Grassy Island, Willapa Bay, April through Sept 2018

The POC and Supplemental study results provide evidence that dry harrowing may have some potential to play a role in the management of burrowing shrimp yet specific technical and broader questions still remain:

Technical Questions• What is the sediment compaction threshold that allows for productive ground culture? • Is there a strong relationship between shrimp density and sediment compaction? • How long do the effects of mechanical treatments persist? • How much does timing with respect to shrimp life cycle influence treatment effect?

Broader issuesAre the methods: • viable on a larger, commercial scale? • compatible with shellfish ground culture? • economically realistic?

y = 0.0325x + 12.738R² = 0.0065

0

5

10

15

20

25

30

40 45 50 55 60 65 70 75 80 85

# sh

rimp/

m2

# burrows/m2

Stoney Pt, Willapa BayJuly 2015

y = 0.0624x + 11.263R² = 0.0237

0

5

10

15

20

25

0 10 20 30 40 50 60 70

# sh

rimp/

m2

# burrows/m2

Grassy Island, Willapa BayMarch, 2015

y = 0.1908x + 1.0646R² = 0.4521

0

2

4

6

8

10

12

0 5 10 15 20 25 30 35 40 45

# sh

rimp/

m2

# burrows/m2

Midde Sands, Willapa BayAugust 2015

y = -0.5117x + 47.169R² = 0.2654

0

10

20

30

40

50

60

0 10 20 30 40 50 60 70 80

# sh

rimp/

m2

# burrows/m2

Middle Sands, Willapa BayMay 2016

y = 0.0861x + 8.8105R² = 0.071

02468

101214161820

40 45 50 55 60 65 70 75 80 85

# sh

rimp/

m2

# burrows/m2

Ellen Sands, Willapa BayJuly 2015

y = 0.05x + 1.3769R² = 0.1206

0

1

2

3

4

5

6

7

8

0 10 20 30 40 50

# sh

rimp/

m2

# burrows/m2

Nemah North, Willapa BayJuly 2015

y = 0.1881x + 2.9434R² = 0.2109

02468

101214161820

0 10 20 30 40 50 60

#shr

imp/

m2

#burrows/m 2

North Long Island, Willapa BayJuly 2015

y = -0.0004x + 1.4043R² = 2E-06

0

1

2

3

4

5

6

7

8

0 5 10 15 20 25 30

# sh

rimp/

m2

# burrows/m 2

Oysterville, Willapa BayJuly 2015

y = 0.1802x + 1.9702R² = 0.3622

0

5

10

15

20

25

30

0 10 20 30 40 50 60 70 80 90 100

Grassy Island, Middle Sands, Mill ChannelRhodesia Beach, Oysterville, Stony Pt., Ellen Sands

June 2014

y = -0.0231x + 2.6041R² = 0.0016

0

5

10

15

20

25

0 5 10 15 20 25 30

# sh

rimp/

m2

# burrows/m2

Port of Peninsula, Willapa BayJuly 2015

y = -0.179x + 14.303R² = 0.0985

05

1015202530354045

0 10 20 30 40 50 60

# sh

rmp/

m2

#burrows/m2

Rhodesia Beach, Willapa BayJuly 2015