Intermediate eelgrass patch complexity predicts age-0 fish density.
Thistle, Maira*,1, Schneider, David 1, Gregory, Robert1, 2, Wells, Nadine2, 1 Memorial University of Newfoundland, St. John's, NL, Canada2 Fisheries and Oceans Canada, St. John's, NL, Canada
ABSTRACT- Interactions between fish and habitat components are known to influence spatial and temporal distribution patterns at many scales. In Newfoundland coastal waters, a number of juvenile fish species, including Atlantic cod (Gadus morhua), are associated with eelgrass (Zostera marina). However the strength of the relationship is variable at different eelgrass sites. We investigated the relationships between a number of spatial characteristics of eelgrass and juvenile cod abundance. We used aerial photographs to determine perimeter and area at multiple scales, fractal dimensions of perimeter (DP) and area (DA), and two indices combining perimeter and area complexity at these scales, A=f(P) and P/A, at sites in two study areas, Newman Sound (∼25 km) and a broader coastal survey (∼600 km). Age 0 Atlantic cod density was estimated using a seine net. Patterns between cod density and eelgrass perimeter, area, DP, and DA were inconsistent at both scales. We found parabolic relationships between cod density and both A=f(P) and P/A, indicating highest fish densities at sites of intermediate patchiness and edge regularity. Within Newman Sound, mean cod densities (± 1 SE) were 0.0104 ± 0.0048 fish/m2 in a continuous meadow, 0.1609 ± 0.0694 fish/m2 in a discontinuous, irregular meadow, and 0.0369 ± 0.0104 fish/m2 in a highly fragmented, highly irregular site. A single value of A=f(P) may describe sites with very different complexities. This duality was greatly reduced in P/A. We discovered similar parabolic relationships between P/A and both age-0 rock cod (G. ogac) and white hake (Urophycis tenuis). The spatial arrangement of eelgrass is important to juvenile fish distribution, suggesting that this relationship may be due to sites of intermediate complexity which likely provide both optimal protective cover and opportunity to feed.
Key words: eelgrass, habitat complexity, multiscale analysis
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