An assessment of biases associated with caging, tethering, and trawl sampling of summer flounder (Paralichthys dentatus)

Abstract

We provide an example of the type of bias assessment that should, but often is not, used in ecological studies using techniques such as caging, tethering, and trawl sampling. Growth rates of summer flounder (Paralichthys dentatus) from cage enclosures were compared to those from mark-recapture trials, and prey types (identified through stomach content analysis) compared between caged fish and wild fish collected from nursery sites over a 2-yr period. Site-specific growth rates estimated from the caging method were similar (± 15%) to those estimated from the mark-recapture trials. Prey types were nearly identical between caged and wild fish, although selectivity may have varied quantitatively. Caging summer flounder will generally be an appropriate tool with which to measure growth rates in the wild, but comparisons with an independent measurement method are necessary for validation. In tethering trials, predation was significantly greater on tethered than on untethered fish, indicating that tethering is not an appropriate tool with which to measure absolute rates of predation on juvenile summer flounder. The lack of effects of substrate (sand versus mud) and fish origin (hatchery-reared versus wild) on predation of tethered versus untethered fish indicates that tethering trials will not indicate treatment-specific differences when none exist. Tethering may be an acceptable method for comparing relative rates of predation on different substrates and between hatchery-reared and wild juvenile summer flounder in the field, although true differences in treatment levels could be masked by tethering. Beam trawl efficiency estimates for juvenile summer flounder were similar between beach and marsh habitats, but differed significantly between marsh sites, indicating that site-specific trawl efficiency estimates may be critical to accurately assess juvenile flounder the appropriateness of comparisons of size-frequency information between the sites and habitats used in this study. Caging, tethering, and beam trawl sampling are appropriate tools for measuring ecological parameters of juvenile summer flounder, but only if possible biases of each method are identified and compensated for when interpreting data collected using these methods.