Persistence of prey hot spots in southeast Alaska Scott M. Gende National Park Service, Glacier Bay...

Persistence of prey ‘hot Persistence of prey ‘hot spots’ in southeast Alaskaspots’ in southeast Alaska

Scott M. GendeScott M. Gende

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

Scott_Gende@nps.govScott_Gende@nps.gov

Michael SiglerMichael Sigler

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

Juneau, Alaska, USA; Mike.Sigler@noaa.govJuneau, Alaska, USA; Mike.Sigler@noaa.gov

1. Are there high aggregations of pelagic fish prey in space and time?

2.Do these ‘hot spots’ persist through time?

3.What is the response of predators to these aggregations?

Questions:Questions:

~40 km study ~40 km study areaarea

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 km)1.83 km)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

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:

On average prey energy density is not On average prey energy density is not equal across monthsequal across months

10,000

15,000

20,000

25,000

30,000

FEBAPR

2001 2004

10,000

15,000

20,000

25,000

30,000

FEBAPR

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

2001 2004

SeasonSeasonal haul-al haul-outout

> 20000

10000-20000

5000-10000

1000-5000

Distribution of pelagic prey energy November Distribution of pelagic prey energy November 20032003

1-1000

> 20000

10000-20000

5000-10000

1000-5000

Distribution of pelagic prey energy December Distribution of pelagic prey energy December 20032003

1-1000

> 20000

10000-20000

5000-10000

1000-5000

Distribution of pelagic prey energy January Distribution of pelagic prey energy January 20042004

1-1000

> 20000

10000-20000

5000-10000

1000-5000

Distribution of pelagic prey energy February Distribution of pelagic prey energy February 20042004

1-1000

> 20000

10000-20000

5000-10000

1000-5000

Distribution of pelagic prey energy March 2004Distribution of pelagic prey energy March 2004

1-1000

> 20000

10000-20000

5000-10000

1000-5000

Distribution of pelagic prey energy April 2004Distribution of pelagic prey energy April 2004

1-1000

> 20000

10000-20000

5000-10000

1000-5000

Distribution of pelagic prey energy May 2004Distribution of pelagic prey energy May 2004

1-1000

Distribution of pelagic prey energy November Distribution of pelagic prey energy November 20032003

80-100%

60-80%

40-60%

20-40%

Proportion of observed Steller sea lions Proportion of observed Steller sea lions November 2003 November 2003

80-100%

60-80%

40-60%

20-40%

Proportion of observed Steller sea lions Proportion of observed Steller sea lions December 2003 December 2003

80-100%

60-80%

40-60%

20-40%

Proportion of observed Steller sea lions January Proportion of observed Steller sea lions January 2004 2004

80-100%

60-80%

40-60%

20-40%

Proportion of observed Steller sea lions Proportion of observed Steller sea lions February 2004 February 2004

R2 = 0.4084

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

R2 = 0.38

Avg. energy density of each block

ea lio

Strong relationship between the average Strong relationship between the average energy density of each block energy density of each block (winter)(winter) and the and the

distribution of Steller sea lionsdistribution of Steller sea lions

60-70%

Hot spot persistence: the probability of Hot spot persistence: the probability of encountering a hot spot across all winter monthsencountering a hot spot across all winter months

50-60%

60-70%

Hot spots do not persist during the non-winter Hot spots do not persist during the non-winter monthsmonths

50-60% 20-30%

Proportion of winter surveys when sea lions seen Proportion of winter surveys when sea lions seen foraging foraging

>40% 30-40% 20-30%

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

% of months when spot is hot

R2 = 0.02

No relationship between hot spot No relationship between hot spot location and foraging sea lions during location and foraging sea lions during

the non-winter monthsthe non-winter months

Non-Non-winterwinter%

ea lio

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

% of months when block is hot

ea lio

R2 = 0.02

R2 = 0.41

Sea lions consistently utilized the prey Sea lions consistently utilized the prey hot spots during the winter (Nov-Feb)hot spots during the winter (Nov-Feb)

Non-Non-winterwinter

WinterWinter

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?• Some hot spot areas persisted through time; Some hot spot areas persisted through time;

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 lions response is However, it appears that sea lions response is greatest in areas with highest prey persistence greatest in areas with highest prey persistence rather than highest prey densityrather than highest prey density

So what?So what?

Abundance of prey j

in the environmentEncounter rate

with prey j

Attack rates on

prey j

Capture rates on

prey j

Consumption rates on prey j

λj /Nj

aj / λj

cj /aj

Kj /cj

Relative Encounter

Attack

probability

Capture

success

Consumptionprobability

Foraging Efficiency

(Intake/Effort)

TT11 TT22 TT33 TT44

High density, low persistence of prey High density, low persistence of prey patchespatches

TT11 TT22 TT33 TT44

xxxx = mid efficiencyII

TT11 TT22 TT33 TT44

xxxx = mid efficiency

Low density, low persistence of prey Low density, low persistence of prey patchespatches

TT11 TT22 TT33 TT44

xxxx = mid efficiencyII

TT11 TT22 TT33 TT44

= low efficiency

TT11 TT22 TT33 TT44

= low efficiency

Low density, high persistence of prey Low density, high persistence of prey patchespatches

TT11 TT22 TT33 TT44

= low efficiency

TT11 TT22 TT33 TT44

= low efficiency

= high efficiency

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.

Persistence of prey hot spots in southeast Alaska Scott M. Gende National Park Service, Glacier Bay...

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