persistence of forage fish ‘hot spots’ and its association with foraging steller sea lions in...

Persistence of forage fish ‘hot spots’ Persistence of forage fish ‘hot spots’ and its association with foraging and its association with foraging

Steller sea lions in southeast AlaskaSteller sea lions in southeast Alaska

Scott M. GendeScott M. Gende

National Park Service, Glacier Bay Field Station, 3100 National Park Service, Glacier Bay Field Station, 3100 National Park, Juneau, Alaska, USA; National Park, Juneau, Alaska, USA;

[email protected][email protected]

Michael F. SiglerMichael F. Sigler

National Marine Fisheries Service, Alaska Fisheries National Marine Fisheries Service, Alaska Fisheries Science Center, Auke Bay Laboratory, Juneau, Alaska, Science Center, Auke Bay Laboratory, Juneau, Alaska,

USA; [email protected]; [email protected]

1. Identify aggregations of pelagic fish prey in space and time.

2.Examine whether these prey ‘hot spots’ persist within and across seasons.

3.Examine which characteristics of prey aggregations are associated with predator aggregations.

4.Model foraging effort (efficiency) as it varies with these characteristics.

Objectives:Objectives:

Upper Lynn canal, Upper Lynn canal, southeast Alaskasoutheast Alaska

~40 linear km~40 linear km

Methods:Methods:

1. Hydroacoustic surveys for 1. Hydroacoustic surveys for pelagic prey conducted June 2001-pelagic prey conducted June 2001-May 2004May 2004

2. Periodic midwater trawls to 2. Periodic midwater trawls to sample prey energy and confirm sample prey energy and confirm echo soundecho sound3. Concurrent observations of top 3. Concurrent observations of top predators including Steller sea lions predators including Steller sea lions and humpback whalesand humpback whales

4. Transformed data from 4. Transformed data from estimates of biomass to energy estimates of biomass to energy densities integrated across the densities integrated across the water columnwater column5. Blocked data into tenths of a 5. Blocked data into tenths of a latitudinal minute such that each latitudinal minute such that each ‘block’ constituted approximately ‘block’ constituted approximately 1.83 km)1.83 km)

Methods:Methods:



4.4. Blocked data into tenths of a Blocked data into tenths of a latitudinal minute such that each latitudinal minute such that each ‘block’ constituted approximately ‘block’ constituted approximately 1.83 km1.83 km5. Transformed data from 5. Transformed data from estimates of biomass to energy estimates of biomass to energy densities integrated across the densities integrated across the water columnwater column

Methods:Methods:



5. Transformed data from 5. Transformed data from estimates of biomass to energy estimates of biomass to energy densities integrated across the densities integrated across the water columnwater column

kJ x 10kJ x 1066/km/km22

4. Blocked data into tenths of a 4. Blocked data into tenths of a latitudinal minute such that each latitudinal minute such that each ‘block’ constituted approximately ‘block’ constituted approximately 1.83 km)1.83 km)

Results:Results:

Strong seasonal variation in prey energy Strong seasonal variation in prey energy density; consistent across three yearsdensity; consistent across three years

0

5,000

10,000

15,000

20,000

25,000

30,000

JUN

AUGO

CTDEC

FEBAPR

JUN

AUGO

CTDEC

FEBAPR

JUN

AUGO

CTDEC

FEBAPR

Avera

ge e

nerg

y d

en

sit

y in

stu

dy

are

a

(Million

s k

J/km

2)

2001 2004

0

5,000

10,000

15,000

20,000

25,000

30,000

JUN

AUGO

CTDEC

FEBAPR

JUN

AUGO

CTDEC

FEBAPR

JUN

AUGO

CTDEC

FEBAPR

Cold winter months (Nov-Feb) are Cold winter months (Nov-Feb) are hothot

Avera

ge e

nerg

y d

en

sit

y in

stu

dy

are

a

(Million

s k

J/km

2)

2001 2004

SeasonSeasonal haul-al haul-outout

> 20000

10000-20000

5000-10000

1000-5000

1-1000

Prey energy density and SSL locations: Prey energy density and SSL locations: Nov 03Nov 03

>70%>70%

50-70%50-70%

30-50%30-50%10-30%10-30%

<<10%10%

Prey energyPrey energy % of SSL% of SSL


> 20000

10000-20000

5000-10000

1000-5000

1-1000

Prey energy density and SSL locations: Prey energy density and SSL locations: Dec 03Dec 03

>70%>70%

50-70%50-70%

30-50%30-50%10-30%10-30%

<<10%10%



> 20000

10000-20000

5000-10000

1000-5000

1-1000

Prey energy density and SSL locations: Prey energy density and SSL locations: Jan 04Jan 04

>70%>70%

50-70%50-70%

30-50%30-50%10-30%10-30%

<<10%10%



> 20000

10000-20000

5000-10000

1000-5000

1-1000

Prey energy density and SSL locations: Prey energy density and SSL locations: Feb 04Feb 04

>70%>70%

50-70%50-70%

30-50%30-50%10-30%10-30%

<<10%10%



> 20000

10000-20000

5000-10000

1000-5000

1-1000

Prey energy density and SSL locations: Prey energy density and SSL locations: Mar 04Mar 04

>70%>70%

50-70%50-70%

30-50%30-50%10-30%10-30%

<<10%10%



> 20000

10000-20000

5000-10000

1000-5000

1-1000

Prey energy density and SSL locations: Prey energy density and SSL locations: Apr 04Apr 04

>70%>70%

50-70%50-70%

30-50%30-50%10-30%10-30%

<<10%10%



> 20000

10000-20000

5000-10000

1000-5000

1-1000

Prey energy density and SSL locations: Prey energy density and SSL locations: May 04May 04

>70%>70%

50-70%50-70%

30-50%30-50%10-30%10-30%

<<10%10%



Proportion of surveys where above Proportion of surveys where above average prey densities were located: average prey densities were located:

winter months (Nov-Feb)winter months (Nov-Feb)

>70%

60-70% 50-60%


>70%

60-70% 50-60% 20-30%

Proportion of surveys where above average Proportion of surveys where above average prey densities were located: prey densities were located: non-winter non-winter

months (Mar-Oct)months (Mar-Oct)


Prey persistence relative to locations of Prey persistence relative to locations of foraging sea lions: foraging sea lions: winterwinter

>70%

60-70% 50-60%

Prey persistence

>40% 30-40% 20-30%

Foraging SSL

0%

10%

20%

30%

40%

50%

0% 10% 20% 30% 40% 50% 60% 70%

Persistence (Proportion of surveys patch was hot)

Pro

port

ion

of

mon

ths s

ea lio

ns

fou

nd

fora

gin

g w

ith

in p

atc

hWinter: R2 = 0.41

0%

10%

20%

30%

40%

50%

0% 10% 20% 30% 40% 50% 60% 70%

Non-winter: R2 = 0.01

0%

10%

20%

30%

40%

50%

0% 10% 20% 30% 40% 50% 60% 70%

Pro

port

ion

of

mon

ths s

ea lio

ns

fou

nd

fora

gin

g w

ith

in p

atc

hPersistence: R2 = 0.41

Average Density (Proportion of surveys patch was hot)

0%

10%

20%

30%

40%

50%

- 5,000 10,000 15,000 20,000 25,000

Density: R2 = 0.36

1.1. Are prey aggregated in time and Are prey aggregated in time and space?space?• Overwintering herring schools result in high Overwintering herring schools result in high

prey aggregations Nov-Feb and occur in prey aggregations Nov-Feb and occur in consistent locations.consistent locations.

2.2. Do these prey ‘hot spots’ persist?Do these prey ‘hot spots’ persist?• The probability of encountering a high The probability of encountering a high

concentration of prey exceeded 70% for some concentration of prey exceeded 70% for some areasareas

3.3.Do predators respond to this Do predators respond to this persistence?persistence?• Strong relationship (during the winter) between Strong relationship (during the winter) between

sea lion distribution and distribution of prey. sea lion distribution and distribution of prey. However, it appears that sea lion’s response is However, it appears that sea lion’s response is strongest in areas with highest prey strongest in areas with highest prey persistence, not necessarily highest densitypersistence, not necessarily highest density

So So what?what?

A foraging effort model:A foraging effort model:

How will foraging effort of sea How will foraging effort of sea lions vary with density or lions vary with density or persistence of prey hot spots?persistence of prey hot spots?

xxx

TT11 TT22 TT33 ....T....T1010

Prey distribution: High density, low Prey distribution: High density, low persistencepersistence

xxx

Low density, low Low density, low persistencepersistence

xxx

Low density, high Low density, high persistencepersistence

PersistencePersistence

LowLow HigHighh

0

6

12

18

24

30

10 9 8 7 6 5 4 3 2 1Fora

gin

g E

ffort

Fora

gin

g E

ffort

Prey density = Prey density = HighHigh

Random walkRandom walkBayesian foragerBayesian forager

0

6

12

18

24

30

10 9 8 7 6 5 4 3 2 1

Prey density = Prey density = MidMid


LowLow HigHighh


LowLow HigHighh

0

40

80

120

160

10 9 8 7 6 5 4 3 2 1

Prey density = Prey density = LowLow

Density may not be Density may not be the only the only characteristic of prey characteristic of prey aggregations that are aggregations that are important to important to predators; predators; persistence may be persistence may be just as important, just as important, particularly for those particularly for those that do not have the that do not have the ability to search large ability to search large areas efficiently.areas efficiently.

Special thanks to : Dave Csepp, JJ Volldenweider, and Jamie Womble. This project funded by the Auke Bay Laboratory, National Marine Fisheries Service

persistence of forage fish ‘hot spots’ and its association with foraging steller sea lions in...

Documents

sample prey energy

pelagic prey

energy densities

prey energy density

blocked data

prey hot spots

yearsaverage energy

hotaverage energy density