Genotypic variation does not explain differences in growth of mummichogs Fundulus heteroclitus from simple and complex tidal marsh landscapes

Decimal coded wire tags were used to measure individual growth rates of mummichogs (Pisces: Fundulidae) within tidal marshes on Sapelo Island, Georgia, USA. Mummichogs (n = 17 508) were marked and released at 3 sites with different levels of complexity in their tidal channel drainage networks. The proportion of recaptures (average 19%), which varied from 6.4 to 43.3% among sites and release dates, was inversely related to relative complexity of the tidal drainage networks. Gender-specific differences were detected in size metrics and growth rates. Mean (+/- SD) growth rates (mm d(-1) total length, TL) of individuals at large for 28 to 56 d prior to recapture ranged from 0.062 +/- 0.044 to 0.274 +/- 0.060 for males and 0.071 +/- 0.036 to 0.279 +/- 0.062 for females, with the least and most rapid growth rates associated with complex and simple tidal drainage networks, respectively. A 'common garden' experiment was conducted to test the hypothesis that there were genotypic differences in growth potential seemingly associated with landscape structure. Two size classes (40-50 and 55-65 mm TL) of individually marked mummichogs from drainage networks characterized as simple (high growth) and complex (low growth) were raised in the laboratory for 42 d on each of 2 daily rations (10 and 30% wet body mass) of minced fresh-frozen grass shrimp. There was no significant difference in mean (+/- SD) daily growth rates of mummichogs from the 2 sites (simple: 0.283 +/- 0.124 and complex: 0.299 +/- 0.131) when reared under the same conditions. Although there was no evidence of a genotypic difference in growth potential between these local populations of mummichogs, a possible association between landscape structure and gene expression reflected in the spatial variation of fish growth within tidal marsh ecosystems remains to be explored.