Growth and shrinkage is sex-dependent
in Antarctic krill
Geraint Tarling1, Simeon Hill1, Helen Peat1, Sophie Fielding1, Christian Reiss2, Angus Atkinson3
1. British Antarctic Survey, Natural Environment Research Council, Madingley Rd, Cambridge, CB3 0ET, UK
2. Southwest Fisheries Science Center, 8604 La Jolla Shores Drive, La Jolla, CA 92037, USA
3. Plymouth Marine Laboratory, Prospect Place, The Hoe, Plymouth PL1 3DH, UK
A number of taxa can shrink
Degrowth is a common trait in taxa with indeterminate growth
Coelenterates (Anthopleura elegantissima)
Urochordates (Clavelina moluccensis)
Echinoderms (Diadema antillarum)
Molluscs (Chiton pelliserpentis)
Degrowth is constrained in vertebrates and arthropods
….with a few exceptions
Marine Iguana - larger individuals decreased in body length during ENSO years
Amblyrhynchus cristatus
Wikelski and Thom 2000
Crustaceans – must moult in order to grow
Terminal moult
e.g. copepods
Seasonal or infrequent moulting
e.g. crabs and lobsters
Decrease in body length at moult is not widely observed
Euphausiids – moult continuously
Body length
Euphausia superba
Terminal moult
Time (in weeks)
?
Laboratory evidence that starved krill shrink in body-length
Ikeda and Dixon (1982)
45% decrease in body mass, 7 mm decrease in body length over 211 days
Do krill shrink during winter in their natural environment?
Abyssal feeding by krill - could provide winter food banks
3000m
Clarke and Tyler 2011
Schmidt et al. 2011
Accurate age-structures are required
by fisheries yield models to regulate harvesting
Ecosystem-based fishery management model for the Southern Ocean
Constable et al. (2000)
Krillbase – spatially comprehensive database of
krill body-length frequency information since 1920s
N70 net
Atkinson et al. (2008)
Circumpolar abundance of Antarctic krill
RMT8 trawl
Krillbase sampling effort by month
2500
Net records
2000
1500
1000
500
0
1
2
3
4
5
6 7
Month
8
9
Poor winter coverage
10 11 12
Net records
CCAMLR krill fishery observer programme
Sampling effort by month
10000
9000
8000
7000
6000
5000
4000
3000
2000
1000
0
1
2
3
4
5
6
7
Month
8
9
10
Good winter coverage
11
12
Spatial coverage of Krillbase and CCAMLR data
SW Atlantic region
Further datasets for
1) South Georgia
2) Western Antarctic
Peninsula
Krillbase
Proportion (KRILLBASE)
0.12
Summer
Juvenile (Krillbase)
Female (Krillbase)
Male (Krillbase)
All krill (CCAMLR)
0.10
0.06
0.05
0.08
0.04
0.06
0.03
0.04
0.02
0.02
0.01
0.00
0.00
0
10
20
30
40
50
60
Total body length (mm)
Therefore, individuals <35 mm body length were not considered
Proportion (CCAMLR summer)
Accommodation of sampling bias
Body-length frequency trajectories
differed seasonally between sexes
1400
Early summer
(Oct to Dec)
1200
1000
800
600
Female
Male
400
200
0
35
40
45
50
55
60
65
4000
Frequency
Late summer
(Jan to Apr)
3000
2000
1000
0
35
40
45
50
55
60
65
6000
Overwinter
(May to Sep)
5000
4000
3000
2000
1000
0
35
40
45
50
55
Total body length (mm)
60
65
Sex-ratio of larger individuals was highly skewed
towards males during overwinter
1.0
KRILLBASE summer
CCAMLR summer
CCAMLR winter
0.9
Proportion of males
0.8
Winter
0.7
0.6
0.5
0.4
Summer
0.3
0.2
0.1
0.0
35
40
45
50
55
60
65
Total body length (mm)
Is differential degrowth between sexes the only explanation?
Is immigration or emigration a factor?
1.0
0.9
Proportion of males
0.8
0.7
0.6
0.5
0.4
0.3
Scotia-Weddell winter
WAP winter
South Georgia winter
0.2
0.1
0.0
35
40
45
50
55
60
65
Total body length (mm)
No sex-biased migration from one region to the next
Is there a sex-bias in seasonal mortality?
1.0
Median 0.439
Median 0.463
Median 0.411
Early summer
Late summer
Overwinter
Proportion of males
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
Overall sex-ratio is the same between seasons –
even in favour of females during overwinter
Different trajectories of growth between males and females
60
Total body length (mm)
55
Female observations
Male observations
50
45
40
35
30
25
01 04 07 10 01 04 07 10 01 04 07 10 01 04 07 10 01 04 07 10 01
Mode 1
Mode 2
Mode 3
Month
Largest body-lengths switch between males and females depending on season
Different trajectories of growth between males and females
60
Male observations
Switched-growth Von-Bertallanffy
Total body length (mm)
55
50
45
40
35
30
25
01
01
02
Mode 1
02
03
03
Mode 2
04
04
05
Mode 3
Month
Males grow fast in the early season
05
06
Different trajectories of growth between males and females
60
Switched-growth Von-Bertallanffy
Female observations
Total body length (mm)
55
50
45
40
35
30
25
01
01
02
Mode 1
02
03
03
Mode 2
04
04
05
05
06
Mode 3
Month
Females reach larger body lengths by midsummer, but are shorter during overwinter
Different trajectories of growth between males and females
60
Switched-growth Von-Bertallanffy
Female observations
Sine-wave-growth Von-Bertallanffy
Total body length (mm)
55
50
45
40
35
30
25
01
01
02
Mode 1
02
03
03
Mode 2
04
04
05
05
06
Mode 3
Month
Female growth conforms to a sine-wave growth function
Different trajectories of growth between males and females
60
Switched-growth Von-Bertallanffy - Male
Sine-wave-growth Von-Bertallanffy - Female
Total body length (mm)
55
50
45
40
35
30
25
01
01
02
Mode 1
02
03
03
Mode 2
04
04
05
05
06
Mode 3
Month
Female and male growth are out of phase – but reach similar maxima
Krill ovaries are large and regress over winter
Ov - ovary
>40% of wet body mass during summer
The seasonal regression of the ovary in females may result in a shrinkage of body-length
Conclusions
1. Krill shrink in body length in the natural environment
2. Shrinkage occurs in females but not males during overwinter
3. Shrinkage is more likely to be a function of the life-cycle
than a response to starvation
4. Different growth trajectories of male and female krill
must be factored into fishery-production and -management
models
Acknowledgements: KRILLBASE contributors, particularly Valarie Loeb, Volker Siegel
and Evgeny Pakhomov. CCAMLR fishery observers and the CCAMLR data centre, for
allowing analysis of their data under the rules outlined in CCAMLR-XXII, paragraphs
12.1 to 12.6