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First published online October 5, 2006
Journal of Experimental Biology 209, 4174-4184 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02489
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Escape responses in juvenile Atlantic cod Gadus morhua L.: the effects of turbidity and predator speed

Justin J. Meager1,*, Paolo Domenici2,3, Alex Shingles3 and Anne Christine Utne-Palm1

1 Department of Biology, University of Bergen, PO Box 7800, Bergen N-5020, Norway
2 CNR-IAMC, Loc. Sa Mardini, 09072 Torregrande, Oristano, Italy
3 International Marine Centre, Loc. Sa Mardini, 09072 Torregrande, Oristano, Italy


Figure 1
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  		Fig. 1. Predicted effects of predator velocity and turbidity on escape responses of juvenile cod. Dotted lines indicate the influence of turbidity on visual fields [calculated from a visual model (Aksnes and Utne, 1997Go), using parameters from (Fiksen et al., 1998Go); the visual field in clear water exceeds 530 cm and is not shown]. Theory suggests that reactive distances are shorter to a slow predator attack (black triangle: 150 cm s–1) than to a fast predator attack (white triangle: 296 cm s–1), assuming a constant apparent looming threshold (Dill, 1974aGo) of 0.5 rad s–1 (Domenici, 2002Go). Reactive distance (distance to the predator's widest point) to the slow predator is within the visual field for all turbidity levels, but reactive distance to the fast predator is outside the visual range in the highest turbidity.

 

Figure 2
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  		Fig. 2. Experiment set-up. (A) Glass aquarium (300 cmx70 cmx50 cm). (B) Removable fish compartment (40 cmx54 cmx40 cm, bottomless). (C) Removable glass and Perspex barrier. (D) Predator model (conical shape). (E) Predator model track (200 cm long). (F) Overhead video camera (250 Hz, Redlake, Motion Scope PCI) with infrared filter (Optolite 50% IR). (G) Infrared lamp (Derwent 70W, 830 nm). (H) Reflective white board. (I) Tank where saltwater and kaolin were mixed by air-bubbling and circulation. (J) Pump (58 l m–1) introducing turbid water mix into aquarium through jets (K).

 

Figure 3
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  		Fig. 3. Responsiveness (%) of juvenile cod with increasing turbidity to the fast (black) and slow predator attack speeds (grey). N, total number of fish used in each treatment.

 

Figure 4
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  		Fig. 4. Frequency distribution of juvenile cod turning rates during escapes (black bars) and spontaneous swimming (white bars).

 

Figure 5
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  		Fig. 5. Effect of turbidity on kinematic escape categories of juvenile cod to predator model (slow responses, white; intermediate response, grey; fast responses: black). Responses to fast and slow predator speeds were pooled because there was no significant interaction between turbidity and predator speed (P=0.93).

 

Figure 6
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  		Fig. 6. Circular frequency distribution of escape trajectories of juvenile cod to predator model (black arrow) (trajectory category size: 5°).

 

Figure 7
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  		Fig. 7. Effect of turbidity on reactive distance (triangles) and apparent predator size (or retinal size) (circles) of juvenile cod to two predator attack speeds: (A) 296 cm s–1 or 4.9 BL s–1 and (B) 150 cm s–1 or 2.5 BL s–1. Values are means ± 1 s.e.m. Dotted lines are estimates of TRD (true reactive distance), assuming 50 ms latency (bottom) and 100 ms latency (top).

 

Figure 8
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  		Fig. 8. Putative escape success (PES, %) of juvenile cod to the fast (black columns) and slow predator attacks (grey columns), with increasing turbidity. (A) Overall PES (B) PES of responders only. N, sample size.

 

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© The Company of Biologists Ltd 2006