Intertidal fish communities may make poor indicators of environmental quality: Lessons from a study of mangrove habitat modification

Utilization of fish as indicators of environmental quality in estuarine systems has received much recent attention, although some results have raised questions about using these taxa for such a role. We attempted to identify a fish response to a horticultural practice which removes portions of mangrove forest canopy to create unobstructed vistas. Fish were sampled monthly from six pairs of sites within the mangrove-forested intertidal zone of Rookery Bay, FL over two years. Following a first year of baseline sampling, the mangroves in one site from each pair of sites were pruned in accordance to Florida State law (Mangrove Trimming and Preservation Act of 1996), resulting in a mean 46% and 50% reduction in canopy coverage and litter fall, respectively. However neither the abundance nor species composition of fish in trimmed versus control plots were significantly different (Two-Way Repeated Measures ANOVA's for paired plot differences in fish density and biomass; nonmetric multidimensional scaling, and ANOSIM) suggesting that fish did not respond to mangrove canopy damage. Our results show that due to rather high spatial and temporal variability in fish abundance, it would be difficult to detect any but extreme changes of these taxa in the mangrove intertidal zone, and this may be a common feature of the fish communities of mangrove intertidal zones and other coastal systems. We suggest that improved use of fish as indicators of intertidal habitat quality would benefit from improved information on environmental factors which determine fish distribution and abundance in these areas. Moreover, since a majority of the fish in the intertidal zone is habitat generalists, we propose that individual-level measures of fish response to habitat quality may be more effective than fish abundance.

► Removal of a large portion of mangrove forest canopy did not result in statistically detectable changes in intertidal fish.
► High spatial and temporal variability of intertidal fish abundance makes it an insensitive indicator of habitat condition.
► The authors suggest improvements to future monitoring plans that may increase the efficacy of fish as indicators.