1. A First Step into the Pelagia: Linking Bottom Water Hypoxia to the Vertical Distribution of Fishes on the Northwestern Gulf of Mexico Shelf Elliott Hazen, Kevin Craig, Caroline Good, and Larry Crowder Duke University Marine Lab, Beaufort, NC

2. Introduction Tommy Munro (July 19-Aug 1, 2004) Two sites in GoM 41 station subset Day: 1200-1800 Night: 00-600

3. Hypotheses 1.There is less total biomass within the hypoxic zone compared to normoxic bottom waters. 2.Horizontal distribution of biomass will be greater at hypoxic edges declining with distance. 3.Vertical distribution of fish differs between areas with normoxic and hypoxic bottom waters

4. Methods CTDs measured: Temperature Salinity Dissolved oxygen Paired trawls Flat and Mongoose Downward looking 120 kHz SIMRAD EK60 echosounder Volume backscatter (Sv) in decibels (dB)

5. Data Analysis Sonar 5 Pro for acoustic data, -90 dB threshold, 1 meter bins Spatial Statistics –Thin plate spline interpolations for DO –Spatial regression on all stations (Mantel 1967) –Generalized Additive Models (GAMs) for non- linear and linear components Run separately for stations with hypoxic (< 2.0 mgO 2 ·L -1 ) and normoxic bottom waters

6. Hypoxic mean = -60.1 dB Normoxic mean = -58.2 dB Results Site A Site B Site A mean = -62.7 dB Site B mean = -66.5 dB

7. Acoustic Station Trawls Bumper Croaker Anchovies Other Fish 53% 38% 8% 4%

8. Mantel’s Test Depth (m) Temp. Salin. Time of Day Hyp.Thick O2O2 SpaceBackscatter 0.176 0.150 0.087 0.021 Bathym. Dist.Edge 0.040 0.035 0.042 0.012 0.11 0.050 0.038 0.12

9. Normoxic / Hypoxic Station 38 Station 55

10. G62 – Hypoxic, Night, DO= 1.0 mgO 2 ·L -1 11,618 croaker, 88 other fish Stations 17m 1m

11. G106 – Normoxic, Night, DO= 4.5 mgO 2 ·L -1 1415 bumper, 14 other fish Stations 7m 1m

12. Vertical Profiles 7

13. Relative Biomass Even distribution

14. GAMs Normoxic Hypoxic

15. GAMs Normoxic Hypoxic

16. GAMs Normoxic Hypoxic

17. GAMs Normoxic Hypoxic

18. Conclusions 1.Evidence of fish biomass within the boundaries of the hypoxic zone 2.The relationship with the horizontal edge was complex and not linearly significant 3.Vertical compaction above hypoxic bottom waters –Vertical avoidance –Altered diel migration –Potential food web interactions

19. Acknowledgements NOAA Center for Sponsored Coastal Ocean Research

20. G47 – Hypoxic, DO= 0.5 mgO 2 ·L -1 3 total fish: 1 croaker, 2 bumper Stations

21. Trawl At Acoustic Stations Bumper Anchovies Other Fish

22. Catches nAnchovyBumperCroakerOther_Fishtotal_fish day-norm163278379122304143445958 night-norm851834731453094624019 day-hyp720612609804344653 night-hyp113907016473536234799