Persistence of prey ‘hot spots’ in southeast Alaska

1. Persistence of prey ‘hot spots’ in southeast Alaska Scott M. Gende National Park Service, Glacier Bay Field Station, 3100 National Park, Juneau, Alaska, USA; Scott_Gende@nps.gov Michael Sigler National Marine Fisheries Service, Alaska Fisheries Science Center, Auke Bay Laboratory, Juneau, Alaska, USA; Mike.Sigler@noaa.gov

3. Questions: • Are there high aggregations of pelagic fish prey in space and time? • Do these ‘hot spots’ persist through time? • What is the response of predators to these aggregations?

4. ~40 km study area

5. Methods: 1. Hydroacoustic surveys for pelagic prey conducted June 2001-May 2004 2. Periodic midwater trawls to sample prey energy and confirm echo sound 3. Concurrent observations of top predators including Steller sea lions and humpback whales 4. Transformed data from estimates of biomass to energy densities integrated across the water column 5. Blocked data into tenths of a latitudinal minute such that each ‘block’ constituted approximately 1.83 km)

6. Methods: 1. Hydroacoustic surveys for pelagic prey conducted June 2001-May 2004 2. Periodic midwater trawls to sample prey energy and confirm echo sound 3. Concurrent observations of top predators including Steller sea lions and humpback whales 4. Transformed data from estimates of biomass to energy densities integrated across the water column 5. Blocked data into tenths of a latitudinal minute such that each ‘block’ constituted approximately 1.83 km)

7. Methods: 1. Hydroacoustic surveys for pelagic prey conducted June 2001-May 2004 2. Periodic midwater trawls to sample prey energy and confirm echo sound 3. Concurrent observations of top predators including Steller sea lions and humpback whales 4. Transformed data from estimates of biomass to energy densities integrated across the water column 5. Blocked data into tenths of a latitudinal minute such that each ‘block’ constituted approximately 1.83 km)

8. Methods: 1. Hydroacoustic surveys for pelagic prey conducted June 2001-May 2004 2. Periodic midwater trawls to sample prey energy and confirm echo sound 3. Concurrent observations of top predators including Steller sea lions and humpback whales 4.Blocked data into tenths of a latitudinal minute such that each ‘block’ constituted approximately 1.83 km) 5. Transformed data from estimates of biomass to energy densities integrated across the water column

9. Methods: 1. Hydroacoustic surveys for pelagic prey conducted June 2001-May 2004 2. Periodic midwater trawls to sample prey energy and confirm echo sound 3. Concurrent observations of top predators including Steller sea lions and humpback whales kJ x 106/km2 4. Blocked data into tenths of a latitudinal minute such that each ‘block’ constituted approximately 1.83 km) 5. Transformed data from estimates of biomass to energy densities integrated across the water column

10. Results:

11. On average prey energy density is not equal across months Millions kJ/kg2 2001 2004

12. Cold winter months (Nov-Feb) are hot Millions kJ/kg2 2001 2004

14. Distribution of pelagic prey energy November 2003 Seasonal haul-out > 20000 10000-20000 5000-10000 1000-5000 1-1000

15. Distribution of pelagic prey energy December 2003 Seasonal haul-out > 20000 10000-20000 5000-10000 1000-5000 1-1000

16. Distribution of pelagic prey energy January 2004 Seasonal haul-out > 20000 10000-20000 5000-10000 1000-5000 1-1000

17. Distribution of pelagic prey energy February 2004 Seasonal haul-out > 20000 10000-20000 5000-10000 1000-5000 1-1000

18. Distribution of pelagic prey energy March 2004 Seasonal haul-out > 20000 10000-20000 5000-10000 1000-5000 1-1000

19. Distribution of pelagic prey energy April 2004 Seasonal haul-out > 20000 10000-20000 5000-10000 1000-5000 1-1000

20. Distribution of pelagic prey energy May 2004 Seasonal haul-out > 20000 10000-20000 5000-10000 1000-5000 1-1000

21. Distribution of pelagic prey energy November 2003 Seasonal haul-out

22. Proportion of observed Steller sea lions November 2003 Seasonal haul-out 80-100% 60-80% 40-60% 20-40% 0-20%

23. Proportion of observed Steller sea lions December 2003 Seasonal haul-out 80-100% 60-80% 40-60% 20-40% 0-20%

24. Proportion of observed Steller sea lions January 2004 Seasonal haul-out 80-100% 60-80% 40-60% 20-40% 0-20%

25. Proportion of observed Steller sea lions February 2004 Seasonal haul-out 80-100% 60-80% 40-60% 20-40% 0-20%

26. Strong relationship between the average energy density of each block (winter)and the distribution of Steller sea lions R2 = 0.38 % of months sea lions found foraging within that block Avg. energy density of each block

27. Hot spot persistence: the probability of encountering a hot spot across all winter months Seasonal haul-out >70% 60-70% 50-60%

28. Hot spots do not persist during the non-winter months Seasonal haul-out >70% 60-70% 50-60% 20-30%

29. Proportion of winter surveys when sea lions seen foraging Seasonal haul-out >40% 30-40% 20-30%

30. No relationship between hot spot location and foraging sea lions during the non-winter months % of months sea lions found foraging within that block R2 = 0.02 Non-winter % of months when spot is hot

31. Sea lions consistently utilized the prey hot spots during the winter (Nov-Feb) R2 = 0.41 % of months sea lions found foraging at the spot Winter R2 = 0.02 Non-winter % of months when block is hot

32. Are prey aggregated in time and space? • Overwintering herring schools result in high prey aggregations Nov-Feb and occur in consistent locations. • Do these prey ‘hot spots’ persist? • Some hot spot areas persisted through time; the probability of encountering a high concentration of prey exceeded 70% for some areas • Do predators respond to this persistence? • Strong relationship (during the winter) between sea lion distribution and distribution of prey. However, it appears that sea lions response is greatest in areas with highest prey persistence rather than highest prey density

33. So what?

34. Abundance of prey j in the environment Nj Relative Encounter rate λj /Nj Encounter rate with prey j λj Attack probability aj / λj Foraging Efficiency Attack rates on prey j aj (Intake/Effort) Capture success cj /aj Capture rates on prey j cj Consumption probability Kj /cj Consumption rates on prey j Kj

35. High density, low persistence of prey patches T1 T2 T3 T4 x x x x

36. High density, low persistence of prey patches T1 T2 T3 T4 x x x x

37. High density, low persistence of prey patches T1 T2 T3 T4 I x x x x = mid efficiency E

38. High density, low persistence of prey patches T1 T2 T3 T4 I x x x x = mid efficiency E Low density, low persistence of prey patches x x x x

39. High density, low persistence of prey patches T1 T2 T3 T4 I x x x x = mid efficiency E Low density, low persistence of prey patches x x x x

40. High density, low persistence of prey patches T1 T2 T3 T4 I x x x x = mid efficiency E Low density, low persistence of prey patches I x x x x = low efficiency E

41. High density, low persistence of prey patches T1 T2 T3 T4 I x x x x = mid efficiency E Low density, low persistence of prey patches I x x x x = low efficiency E Low density, high persistence of prey patches x x x x

42. High density, low persistence of prey patches T1 T2 T3 T4 I x x x x = mid efficiency E Low density, low persistence of prey patches I x x x x = low efficiency E Low density, high persistence of prey patches x x x x

43. High density, low persistence of prey patches T1 T2 T3 T4 I x x x x = mid efficiency E Low density, low persistence of prey patches I x x x x = low efficiency E Low density, high persistence of prey patches I x x x x = high efficiency E

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