Predictive analysis of scale-dependent habitat association: Juvenile cod (Gadus spp.) in eastern Newfoundland

We used results of laboratory experiments to make predictions about a well-established phenomenon, scale-dependent association of organisms with habitat. Based on shelter seeking behaviour in laboratory experiments, we predicted that age 0 cod would be locally decoupled from vegetated habitat at spatial scales on the order of the limits of underwater visibility, becoming associated at larger scales. On transects at 10 sites in Newman Sound, Bonavista Bay, Newfoundland age 0 juvenile cod were usually found either in or adjacent to the band of tall eelgrass running parallel to the coast. At the scale of 5 m or less along transects perpendicular and parallel to the coast cod were decoupled from habitat, as expected. At the scale of 5–20 m cod were either decoupled or only weakly associated with habitat, contrary to expectation based on underwater visibility and shelter seeking behaviour in the lab. At the scale of 20–100 m along transects perpendicular to the coast cod were positively associated with tall eelgrass, while being negatively associated with short eelgrass. This reflects the negative association of tall with short eelgrass due to parallel band structure at this scale. At scales of 20–100 m along transects parallel to the coast cod were positively associated with short eelgrass. Decoupling of cod density from the amount of habitat at scales of 5–20 m was due to the formation of small shoals near the boundaries between tall and short eelgrasses. The observed change in association of numbers with habitat with change in scale, combined with the theoretical expectation that association with habitat decreases as the risk/reward ratio increases, allowed us to draw the conclusion that the ratio of risk to reward decreases as spatial scale increases from 10 to 100 m. The predictive approach we develop here can be extended to other scale-dependent phenomena, such as scale-dependent association of predator with prey.