Food Habits and Small Scale Habitat Utilization of
Atka mackerel (Pleurogrammus monopterygius) in
the Aleutian Islands, Alaska
University of Washington JISAO
Alaska 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.
Steller sea lion trawl exclusion zones
10 nm
Trawl exclusion zones were fully
established in the early 1990’s
around sea lion rookeries and
haulouts at 10 and 20 nautical
miles.
20 nm
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 1second intervals.
Stomach Fullness
What 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.
Water column structure
47% hauls stratified
53% hauls well mixed
degrees (C)
degrees (C)
Depth (meters)
2
2.5
3
3.5
4
4.5
5
5.5
6
6.5
2
7
0
0
20
20
40
40
60
60
80
80
100
100
120
120
140
140
160
Stratified
160
2.5
3
3.5
4
4.5
5
5.5
6
6.5
7
Well mixed
If temp difference between the surface (5 meters) temp and bottom temp < 1.5°C = well mixed
If 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
Stomach Fullness (% body weight)
Inside vs. outside trawl exclusion zone
0.03
H0 : no difference in feeding
intensity inside vs. outside
trawl exclusion zone:
0.02
Not rejected. (t-test, P=0.18, not
significant)
0.01
N = 20
0.00
0.04
Inside
N=8
Outside
Water column stratification
H0 : no difference in feeding
intensity in well mixed
water vs. stratified:
0.03
0.02
0.01
N = 11
N=8
Mixed
Stratified
0
Not rejected. (water column
structure was marginally nonsignificant 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)
Results - Seguam Pass 2002
Diet Composition by total weight
June
August
100%
100%
50%
50%
N=274
0%
•Diet comp changes
temporally
October
100%
N=146
amphipods
atka eggs
copepods
euphausids
fish
other
50%
0%
3
4
5
6
Age Classes
7+
N=180
0%
•Age 3 do not
consume fish
•Euphausiids
dominate diet in
August by weight
* Other: cephalopods, misc. inverts
Results – diet composition Amchitka Island 2003
July
Diet Composition by total weight
100%
N=411
amphipods
arrow worms
atka eggs
copepods
euphausiid
fish
other
50%
0%
•Diet comp differs
temporally
October
100%
•Diet comp differs from
Seguam study area
50%
N=201
0%
3
4
5
6
Age classes
7+
•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