Habitat use, movement, and vulnerability of sedentary fishes in a dynamic world

In this thesis I quantify habitat use, movement, and vulnerability for seahorses and other sedentary fishes to understand how they may respond to habitat related threats. Surviving habitat change depends on either acclimatizing or escaping. Sedentary animals could be especially vulnerable if they require specific habitats or their ability to escape is reduced. However, dynamic coastal environments could promote greater flexibility in fish living there.

Populations of the sedentary seahorse Hippocampus guttulatus living in a dynamic estuary seem flexible in their habitat use, but individuals may be less so. I explored H.guttulatus habitat associations using underwater surveys and displacement experiments in the Ria Formosa lagoon. Seahorse populations declined substantially (73-94%) compared to previous surveys, but declines were unassociated with measured habitat changes. At low densities, H.guttulatus lived in a range of habitats but in warmer, deeper locations.

When displaced, H.guttulatus moved to a variety of environments but individual seahorses moved towards locations with familiar depths and current speeds. Individual variability could help protect populations in degrading habitat but individuals may still need to move to survive. Hippocampus guttulatus can move further than their small home ranges would suggest, which might help them escape habitat loss and degradation.

In aquarium experiments I showed that small acoustic tags could be used on captive H.guttulatus with minimal effects on movement and behaviour. I displaced tagged H.guttulatus and found they moved ten times further than typical home range movements. While H.guttulatus shows some capacity for acclimatizing and escaping, their sedentary nature could confer vulnerability to habitat loss and degradation.

Models simulating fish movement across artificial seascapes predicted sedentary fish should be more sensitive to habitat loss and fragmentation than more mobile fish. Comparative analyses of movement and vulnerability among marine benthic fish species supported these results. Among fishes threatened by habitat degradation, those that rarely move beyond home ranges were more threatened than those that migrate or disperse.

My thesis identified general relationships between movement and vulnerability in marine fishes, which could help prioritize conservation, while raising further questions about the additional effects of habitat specialization and environmental variability.

Caldwell, I.R. (2012). Habitat use, movement, and vulnerability of sedentary fishes in a dynamic world. Doctoral dissertation, University of British Columbia, Canada.