Fractal measures of habitat structure: maximum densities of juvenile cod occur at intermediate eelgrass complexity

Habitat patchiness is known to alter the relation of a population to both its predators and its prey. We developed a biologically interpretable measure of habitat complexity to test whether juvenile fish density depends on degree of patchiness. In Newfoundland coastal waters, juvenile fish species, including Atlantic codGadus morhua, are associated with eelgrassZostera marina. However, the association of density with eelgrass cover is dependent upon site and the scale of analysis. We tested for non-monotonic relationships (with an intermediate optimum) between 5 spatial characteristics of eelgrass and density of 3 juvenile (Age-0) fish species: Atlantic cod, Greenland codG. ogac, and white hakeUrophycis tenuis. We used aerial photography to determine eelgrass perimeter and area measurements at multiple scales, fractal dimensions of perimeter (DP ) and area (DA ), and a measure that combines perimeter and area complexity at these scales (βP/A ). Fish densities were estimated at each site using a seine net. We found parabolic relationships betweenβP/A and density for all 3 species, indicating highest fish densities at sites of intermediate patchiness and edge regularity. Furthermore, we determined thatβP/A provided a less ambiguous estimate of spatial configuration than other measures. This intermediate maximum may reflect a trade-off, whereby eelgrass sites of intermediate spatial complexity provide juvenile fish with both optimal protective cover and opportunity to feed. This pattern may apply to any species requiring open areas in which to forage as well as shelter to offset predation risk.