Food Habits and Small Scale Habitat Utilization of Atka mackerel (Pleurogrammus monopterygius) in

the Aleutian Islands, Alaska

University of Washington JISAOAlaska Fisheries Science Center FIT – Fisheries Interaction Team

Kimberly Rand and Sandra Lowe

Atka mackerel background• Largest groundfish biomass in the Aleutian Islands.

• Important prey item in Steller sea lion diets, especially in the Aleutian Islands.

• Nest guarding, demersal, batch spawners, 50% maturity at 3.6 years of age (McDermott and Lowe 1997)

• Based on archival tag informations, Atka are found to vertically migrate in the water column (Nichol and Sommerton 2002)

• Primarily eat zooplankton (~90%)

• Atka do cannibalize eggs from the demersal nests.

10 nm

20 nm

Steller sea lion trawl exclusion zones

Trawl exclusion zones were fully established in the early 1990’s around sea lion rookeries and haulouts at 10 and 20 nautical miles.

What is EFH for feeding?• Explore the spatial and temporal variation in Atka mackerel feeding patterns in 2 local aggregations in the Aleutians.

• “Those waters and substrate necessary to fish for spawning, breeding, feeding, or growth to maturity“

– H0 : Average stomach fullness is the same inside vs. outside trawl exclusion zones (disturbed vs. undisturbed habitat)

– H0 : Average stomach fullness is the same in well mixed water vs. a stratified water column

– Examine diet composition for each age class by area and month

Study Areas

Methods• Mark-recapture study platform for data collection

• Hauls were made in areas of high Atka mackerel abundance

• Approximately 320 hauls totaling 2,638 specimens collected

• Haul was observation – from each haul selected ~10 fish (50/50 sex ratio)

• Stomach fullness = % body weight of fish

• Stomach fullness = proxy for feeding intensity (data log transformed)

• Depth and temperature were recorded with an MBT at 1-second intervals.

Stomach FullnessWhat factors affect spatial variance?

– Inside vs. outside trawl exclusion zones– Length of fish (larger fish can eat disproportionately

more food)– Time of day– Depth– Temperature– Water column structure was determined using

temperature profiles. Highly variable and dynamic throughout the Aleutian passes.

0

20

40

60

80

100

120

140

160

2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7

degrees (C)

Water column structure

0

20

40

60

80

100

120

140

160

2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7

degrees (C)

Well mixedStratified

Dep

th (

met

ers)

53% hauls well mixed47% hauls stratified

If temp difference between the surface (5 meters) temp and bottom temp < 1.5°C = well mixedIf temp difference between the surface (5 meters) temp and bottom temp > 1.5°C = stratified

Results – Stomach fullness Seguam pass 2002

•Hauls concentrated on 2 distinct local aggregations

•Feeding intensity differs between these 2 local aggregations

•Feeding significantly declines from June to October

June - Seguam pass 2002

0

0.01

0.02

0.03

0.04

0.00

0.01

0.02

0.03

N = 20 N = 8

Sto

mac

h F

ulln

ess

(% b

ody

wei

ght)

Inside Outside

Mixed Stratified

N = 8N = 11

Inside vs. outside trawl exclusion zone

Water column stratification

H0 : no difference in feeding intensity inside vs. outside trawl exclusion zone:

Not rejected. (t-test, P=0.18, not significant)

H0 : no difference in feeding intensity in well mixed water vs. stratified:

Not rejected. (water column structure was marginally non-significant in June ~ P=0.07)

Results – Stomach fullness Amchitka Island 2003

H0 : no difference in feeding intensity inside vs. outside trawl exclusion zone:

Rejected. (t-test, P=0.018, n=20)

H0 : no difference in feeding intensity in well mixed water vs. stratified:

Water column structure + trawl zone (ANOVA, n=18):Water column: P=0.06 Trawl zone: P=0.0004

(Too few data points to detect interaction)

Results – Stomach fullness Amchitka Island 2003

H0 : no difference in feeding intensity inside vs. outside trawl exclusion zone:

Rejected. (t-test, P=0.023, n=38)

H0 : no difference in feeding intensity in well mixed water vs. stratified:

Water column structure + trawl zone (ANOVA, n=37):Water column: P=0.85 Trawl zone: P=0.04

(No Interaction)

Water column structure All areas and years combined

• Regardless of area, year or month does water column structure explain the variance in average stomach fullness?

• Combined all data – 2002 (Seguam pass, Tanaga Island)– 2003 (Amchitka Island, Seguam pass)– 2004 (All 3 study areas)

• Given area, water column structure (mixed vs. stratified) significantly explained average stomach fullness variance (ANOVA, P=0.03, n=214, no interaction)

0%

50%

100%

Results - Seguam Pass 2002

August

0%

50%

100%

3 4 5 6 7+

amphipods

atka eggs

copepods

euphausids

fish

other

0%

50%

100%

Die

t Com

posi

tion

by

tota

l w

eigh

t June

October

N=274 N=180

N=146

Age Classes

•Diet comp changes temporally

•Age 3 do not consume fish

•Euphausiids dominate diet in August by weight

* Other: cephalopods, misc. inverts

0%

50%

100%

amphipodsarrow wormsatka eggscopepodseuphausiidfishother

0%

50%

100%

3 4 5 6 7+

July

October

N=201

N=411

Age classes

Results – diet composition Amchitka Island 2003 D

iet C

ompo

siti

on b

y to

tal

wei

ght

•Diet comp differs temporally

•Diet comp differs from Seguam study area

•Atka mackerel eggs large portion of October diet* Other: cephalopods, misc. inverts

Conclusions• Although there were no significant differences in average

stomach fullness means inside vs. oustide trawl zone at Seguam, there appeared to be an area important to feeding inside the trawl zone.

• Average stomach fullness differed relative to trawl exclusion zones at Amchitka during July and October - spawning season. This was indirect effect of spawning habitat located inside trawl exclusion zones as the majority of the diet was comprised of eggs.

Conclusions• Water stratification was marginally non-significant at both

Seguam and Amchitka in explaining average stomach fullness variance, possibly due to low sample size.

• When combining all study areas and years, average stomach fullness was significantly greater in a well mixed water column than a stratified water column.

• Water column stratification and oceanographic processes may be important for Atka mackerel feeding (Essential Fish Habitat)

• Diet composition differed by age class, inter-annually and spatially.

Thank you• EFH for funding

• Sandra Lowe• Dan Cooper

• Susan Piquelle• Susanne McDermott

• Peter Munro• Liz Conners

• Libby Logerwell• The entire FIT team

• Cascade Fishing / NPFF• The AFSC Age and Growth

• The AFSC Stomach Lab• Crew F/T Pacific Explorer

• Crew F/T Seafisher• Especially the Atka mackerel!

•All the data collectors!•John Hargrove•Yvonne Ortiz

•Bob Lauth•Dan Cooper

•Barney Baker•James Nimz

•Ingwar•Scott McKlillip•Alan Harvison

•Jerry Hoff•Justin Keesee

•Eric Dobbs•Elaine Herr•Ty Yasenak

•Sandi Neidetcher