Improving Southeast Alaska

Geoduck Harvest Strategies using

Age Composition and Growth

Katie Palof*, M.Donnellan, K.McNeel, K.Hebert, and C.Siddon

Alaska Department of Fish & Game

LW Symposium 5-15-15

Data limitations for Alaska geoducks

• Lack of Alaska specific biological reference points

▫ Natural mortality

▫ Growth

▫ Age composition of beds

• Population structure

• Interactions between fishery and biology

Objectives

• Characterize age and growth in SE Alaska

• Indirectly estimate natural mortality

• Evaluate harvest strategies

http://www.adfg.alaska.gov/index.cfm?adfg=geoduck.rangemap

Geoducks (Panopea generosa)

• Large burrowing clam

• Settles within first year

• Lower intertidal to subtidal to depths of 110m (6-18 m most common)

• Long lived (114 + years in Alaska)

• Broadcast spawn

• Dive fishery in SE Alaska since 1990s▫ Rotational beds

▫ GHL 2% annually

▫ Last 5 years ~ 4 to 5 million ex-vessel value per year

http://archives.evergreen.edu/webpages/curricular/2010-2011/marinelife1011/web/panopea_generosa.html

Photo: Katie Palof

Age and growth

• Final refuge (1 m) age 4 to 5 (BC)

• Fully recruited at 6 yrs (BC)• Fast growth in first 10 years• Growth rate and maximum size

varies among environment and geography

• Natural mortality▫ BC, M = 0.01 to 0.04

Campbell et al. 2004

Methods

• Sample collection

▫ 2012 and 2014 – 9 locations total

▫ Age, length, weight , etc.

Methods

• Sample collection

▫ 2012 and 2014 – 9 locations total

▫ Age, length, weight , etc.

• Von-Bertalanffy growth relationships

• Catch-curve analysis - total mortality, Z

▫ Traditional

▫ Compositional analysis (Schnute & Haigh 2007)

Length- Age relationship

• Low variability

• Overall growth similar

• Small sample size?

• Young ages under-represented

Age composition

Table 1. Age distribution for 2012 and 2014 Southeast Alaska samples

AGE

Geographic area Mean SE Min Max n

12~1-Cone Island 54.01 0.86 13 80 223

12~2-East San Fernando 55.03 1.15 10 89 200

12~3-Nakat Inlet 53.37 1.35 8 100 211

12~4-Vallenar Bay 49.06 1.22 7 83 187

14~1-Vegas/Hotspur Islands 46.53 0.97 9 102 213

14~2-Tlevak Strait 46.15 1.60 6 112 193

14~3-Warren and Kosciusko Islands 40.98 1.05 13 77 212

14~4-Taigud/Kolosh Islands 28.00 1.67 6 81 100

14~5- Biorka/Legma Islands 35.06 1.51 2 69 120

All (pooled) 47.01 0.45 2 112 1659

Age frequency

• Variability in age frequency

• Small sample size vs. no of ages

• Sitka samples

1_Cone Is.

Age

Fre

quency

0 40 80

040

80

2_East San Fernando

Age

Fre

quency

0 40 80

040

80

3_Nakat Inlet

Age

Fre

quency

0 40 80

040

80

4_Vallenar Bay

Age

Fre

quency

0 40 80

040

80

5_Vegas/Hotspur Is.

Age

Fre

quency

0 40 80

040

80

6_Tlevak Strait

Age

Fre

quency

0 40 80

040

80

7_Warren/Kosciusko Is.

Age

Fre

quency

0 40 80

040

80

8_Taigud/Kolosh Is.

Age

Fre

quency

0 40 80

040

80

9_Biorka/Legma Is.

Age

Fre

quency

0 40 80

040

80

All geoduck ages combined

Age in 2012

Fre

qu

en

cy

0 20 40 60 80 100 120

02

04

06

08

0

Age frequency

• Variability in age frequency

• Small sample size vs. no of ages

• Sitka samples

1_Cone Is.

Age

Fre

quency

0 40 80

040

80

2_East San Fernando

Age

Fre

quency

0 40 80

040

80

3_Nakat Inlet

Age

Fre

quency

0 40 80

040

80

4_Vallenar Bay

Age

Fre

quency

0 40 80

040

80

5_Vegas/Hotspur Is.

Age

Fre

quency

0 40 80

040

80

6_Tlevak Strait

Age

Fre

quency

0 40 80

040

80

7_Warren/Kosciusko Is.

Age

Fre

quency

0 40 80

040

80

8_Taigud/Kolosh Is.

Age

Fre

quency

0 40 80

040

80

9_Biorka/Legma Is.

Age

Fre

quency

0 40 80

040

80

All geoduck ages combined

Age in 2012

Fre

qu

en

cy

0 20 40 60 80 100 120

02

04

06

08

0

Catch curve – traditional

• Recruitment not constant

• Suggest high age at full selectivity

• Does not match biology

• Total mortality (Z)

▫ All years : 0.037 to 0.045

▫ 2012 : 0.096 to 0.147

▫ 2014 : 0.030 to 0.043

Catch curve – Schnute and Haigh 2007

• Compositional analysis

• Fits model based on proportions at age distributions

• Incorporates recruitment anomalies

▫ Estimates spread

• Designed for one sample of age data

Alternative catch curve results• Alternative catch curve

approach fits data well

• Z estimated to be:

▫ 2012: 0.0235 to 0.0354

▫ 2014: 0.0262 to 0.0273

▫ 2012/2014 :

0.0302 to 0.0383

• Overall total annual mortality is ~ 3 to 3.8%

▫ Fishing mortality 2 to 2.5%

▫ Natural mortality 1.3%??

Discussion

• Growth and mortality objectives met

• Harvest strategy

▫ Current goal is that F = M annually

Regional harvest 2 to 2.5% annually

M estimate is 0.5 to 1.3

Current harvest may be high

▫ How to deal with infrequent recruitment pulses?

All geoduck ages combined

Age in 2012

Fre

qu

en

cy

0 20 40 60 80 100 120

01

02

03

04

05

0

Future work

• Understanding recruitment pulses

▫ Environmental influence?

▫ Pattern with geography or other spatial variables

▫ Climate change/ ocean acidification

• Determine the most appropriate harvest strategy

Thanks! Questions?

• Southeast Alaska Fisheries Dive Association (SARDFA) for their support in providing geoduck samples.

• Crew of the Fish & Game R/V Kestrel

• Scientific diving crews in both 2012 and 2014 who did all the hard physical work

• Team at the Fish & Game Age lab