| The influence of habitat fragmentation on population dynamics within watersheds is not well understood, particularly for species known to disperse great distances. Effective conservation management, however, relies in part on an understanding of the role connectivity plays in maintaining viable populations over time and space. To evaluate the effects of landscapes and spatial structure on population persistence, demographic and genetic surveys were conducted on native amphidromous Gobioid fishes of the Hawaiian Archipelago. First, the accuracy and biases of a novel population estimation technique, snorkel surveys, was compared with individual and batch mark-recapture techniques. Density estimates from visual surveys and mark-recapture were strongly correlated (P< 0.0001), although visual survey estimates were generally higher than mark-recapture estimates and undersampled large fish. Estimates from batch mark-recapture were not significantly different from those from visual surveys at the same sites, though they did exhibit wider variance. These results indicate that visual surveys provide an efficient means to generate useful estimates of the abundance of Awaous stamineus, and potentially other stream fish as well. Next, we compared population densities at adult habitats with land use/land cover data from multiple spatial assessment units (SAU), as well as stream community and local habitat data. General linear models and Akaike Information Criterion (AIC) were used to select best-fit models for each species (R2 values ranged from 0.25 to 0.40). The two Sicydine species (Lentipes concolor and Sicyopterus stimpsoni), which migrate the furthest upstream, were strongly associated with downstream land uses. However, the densities of L. concolor and S. stimpsoni were most strongly negatively associated with the presence of non-native Poeciliid fishes in the watershed (P < 0.0001, selection frequency = 98% and 100% respectively). These findings indicate that conservation of migratory species would benefit from the prioritization of non-native species removals and habitat restoration along migratory pathways. Finally, microsatellite analyses indicated little spatial structure over the range studied, and was consistent with high gene flow within and between watersheds. Demographic estimates from mark-recapture suggested differences in apparent recruitment between watersheds that may be related to the presence of invasive Poeciliid fish, or other watershed-specific environmental conditions. Further studies are needed to specifically test the role of in-stream barriers and altered flow on recruitment dynamics and gene flow in amphidromous species. Results from this study imply that restoration efforts at the watershed scale could promote recovery of native populations. |
