Author
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Radomski, Andrew |
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Zimba, Paul |
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Submitted to: Journal of the World Aquaculture Society
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 1/2/2009 Publication Date: N/A Citation: N/A Interpretive Summary: Double-crested cormorants are a frequent and major avian predator on channel catfish and other aquaculture species throughout the southeastern USA. Cormorant movements and occurrence within the aquaculture production region are understood, however no studies have focused on how cormorants select foraging sites within these aquaculture systems. The objective of this study was to determine if cormorant abundance is associated with specific (ultraviolet to visible) light reflectance spectra within commercial catfish ponds. Three commercial farms were used in this study; each farm had four ponds classified as 'preferred' and another four ponds classified as ‘avoided' based on observing cormorant numbers for three months prior to reflectance measures ( >100 hrs observation for each pond). Reflectance and pond turbidity were measured weekly for seven weeks during the period of maximal over-wintering cormorant numbers. Water samples were collected at the surface (2.5 inch depth) and to depths of light penetration at two locations (upwind and downwind sides) within each pond to determine spectral reflectance and turbidity. Average pond reflectance values were not significantly different between ponds preferred and avoided by cormorants; however, upwind reflectance differed significantly from downwind reflectance between cormorant use groups. The accuracy of separating upwind ponds preferred or avoided by cormorants was over 62% and lower wavelength colors corresponded to known UV-A sensing compounds in cormorants whereas higher wavelengths shared characteristics associated with algal abundance. Cormorants utilize ponds that have lower algal abundance as compared to avoided ponds. Technical Abstract: Double-crested cormorants (Phalacrocorax auritus) are a frequent and major avian predator on channel catfish (Ictalurus punctatus) and other aquaculture species throughout the southeastern USA. Although cormorant movements and occurrence within the aquaculture production region are understood, no studies have focused on the mechanisms utilized by these avian predators to select foraging sites within these high density aquaculture systems. The objective of this study was to determine if cormorant abundance is associated with specific (ultraviolet to visible) light reflectance spectra within commercial catfish ponds. Three commercial farms were used in this study; each farm had four ponds classified as 'preferred' and another four ponds classified as ‘avoided' based on observing cormorant numbers for three months prior to reflectance measures ( >100 hrs observation for each pond). Reflectance and pond turbidity were measured weekly for seven weeks during the period of maximal over-wintering cormorant numbers. Water samples were collected at the surface (1 cm depth) and secchi depth (10-41 cm) at two locations (upwind and downwind sides) within each pond to determine spectral reflectance and turbidity. Average pond reflectance values were not significantly different between ponds preferred and avoided by cormorants; however, upwind reflectance differed significantly from downwind reflectance between cormorant use groups. Using cross-validation, the accuracy of separating upwind ponds preferred or avoided by cormorants was over 62%. Repeated measures ANOVA identified the reflectance at 370, 380, 460, 500, 510, and 620 nm as significantly different between ponds preferred or avoided by cormorants. The lower wavelengths (380-410 nm) correspond to known UV-A sensing compounds in cormorants whereas the other wavelengths share characteristics associated with algal biomass. Cormorants utilize ponds that have lower algal biomass as compared to avoided ponds. |
