Distribution and risk-sensitive foraging of juvenile gadids in relation to fractal complexity of eelgrass (Zostera marina) habitat

Thistle, Maria E. (2006) Distribution and risk-sensitive foraging of juvenile gadids in relation to fractal complexity of eelgrass (Zostera marina) habitat. Masters thesis, Memorial University of Newfoundland.

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Abstract

The influence of habitat configuration on faunal distribution and success is largely unknown, despite a large body of literature concerning implications of habitat fragmentation. In Newfoundland coastal waters, a number of juvenile fish species, including Atlantic cod (Gadus morhua), are associated with eelgrass (Zostera marina), a habitat that naturally occurs in a variety of configurations and that is susceptible to anthropogenic fragmentation. Studies have shown that the strength of this association is variable at different eelgrass sites and across scales. Given this inconsistent association and the complex spatial configurations of eelgrass, I investigated the relationships between a number of spatial characteristics of eelgrass and density of three juvenile (age-0) fish species, Atlantic cod, Greenland cod (G. ogac), and white hake (Urophycis tenuis). I used data from aerial photographs to determine 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. Age-0 fish densities were estimated at each eelgrass site using a seine net. I found parabolic relationships between βP/A and density for all three species, indicating highest fish densities at sites of intermediate patchiness and edge regularity. Furthermore, I determined that βP/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. I assessed this hypothesis by measuring foraging success in age-0 Greenland cod at sites of varying eelgrass fragmentation, as measured by βP/A I found that foraging success depends on cod density. I then combined this relationship and the parabolic relationship between density and fragmentation to develop a model that predicts the observed relationship between foraging success and eelgrass site configuration. My thesis demonstrates that eelgrass habitat configuration influences the density and success of resident fish fauna. Furthermore, this research demonstrates that predictive models are possible in ecology.

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