Title

Inferring Directional Migration of Fish in a Seasonally Varying Wetland

Event Name/Location

97th Ecological Society of America Meeting, Portland, Oregon, August 5-9, 2012

Document Type

Conference Proceeding

Publication Date

8-9-2012

Abstract

Background/Question/Methods

Movement by animals can have major influence on metapopulations and metacommunities. Florida’s Everglades ecosystem is an important wetland with annual variation in hydrology. Small fish rapidly move in and out of flooding or drying portions of the wetland more rapidly than predicted by reaction-diffusion. Modeling suggests that the rates at which wetlands are colonized can be explained by fish following a dynamic ideal free distribution, wherein fish climb fitness gradients (driven by food availability drought escape). We developed a sampling method capable of measuring fish activity, orientation of movement and displacement over the landscape. We investigated whether local-level directional movement can be scaled up to show regional-level migrations, whether behavioral states are distributed in a way that enhance migration to favorable environments and whether behaviors vary between hydrologic seasons or among regions with different hydrological regimes.

Results/Conclusions

We tested those hypotheses for the most abundant small fish (the eastern mosquitofish, Gambusia holbrooki) in two largely separated regions of the Everglades that have different hydrological properties. We confirmed our ability to demonstrate site-level directional movement by observing similar direction and magnitude of movement with multiple, simultaneous traps within sites. In short-hydroperiod sites, during periods of water recession mosquitofish appear to move in large pulses; these pulses were not observed in the long hydroperiod sites. Both movement speed and directional dispersion was strongly related to water-recession rate in short hydroperiod sites (directional movement was oriented in tighter clusters when water levels were falling), but not obviously in the direction of known refuges. In long-hydroperiod sites, directional dispersion was also non-random, but the speed of movement was much lower. These data show that geographic displacement (total movement of populations over the landscape) is greatest in short-hydroperiod wetlands as water-level falls. Intense pulses of movement were not observed during the period of rising water; this may be a result of differing fitness gradients driving migration or our inability to sample the earliest migrations because of limited ability to sample in the shallowest water levels. Overall, the fish appear to switch behavioral states between non-migratory activity during periods of stable water and a state of rapid directional movement during periods of change. We demonstrate that these small fish in a large, semi-continuous landscape move non-randomly and that their dispersal is consistent with models predicting movement in a manner that climbs fitness gradients as fish move away from drying regions to avoid stranding.

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