Larval condition and vulnerability to predation: reply to comment by Suthers

Abstract

Discussion 1538 Scientists recognize that observations and experiments at a variety of spatial and temporal scales and in a variety of different systems are required to obtain a complete understanding of a particular phenomenon. However, scientists also recognize that an individual study can usually only focus on one particular system at a certain time and place and that many different studies are required to achieve a general conclusion. The objective of our study on larval capelin (Mallotus villosus) (Elliott and Leggett 1998) was to provide an experimental test of whether larvae in poor condition had lower relative survival rates than larvae in good condition when exposed to a predator. We employed an experimental design that had been successfully used by a variety of other researchers (Litvak and Leggett 1992; Pepin et al. 1992; Pepin and Shears 1995; Elliott and Leggett 1996) to test selective mortality hypotheses under laboratory conditions in small aquaria. In our abstract, we explicitly stated that the results of our study “suggest that RNA/DNA ratios are not useful indicators of vulnerability to predation for larval capelin.” Clearly, we did not intend to have this conclusion be considered a broad statement that applied to all other systems. As in all scientific disciplines, we presented our data with the intention that other researchers would conduct further studies using a variety of experimental systems and protocols to provide further tests that would either support or refute our results. After a variety of studies on this topic had been conducted, we would hope that a metanalysis would provide a more complete understanding of the effects of larval condition on vulnerability to predation. Ultimately, the results of this research could lead to the incorporation of lar val condition indices into assessments of the future survival probabilities of larvae into fisheries models. Unfortunately, in the comment by Suthers, our conclusions have been overstated and interpreted as a definitive re jection of the utility of RNA/DNA ratios and larval dry weight in the study of larval condition. As stated above, our conclusions pertained to larval capelin and only in the context of the experimental conditions employed. Furthermore, in the second to last paragraph of Elliott and Leggett (1998), we explicitly stated that our conclusions are based on our results, some of which are in contrast with previous studies of larval capelin in small enclosures: “This difference in out come may result from the use of different experimental conditions and predator species or from the possibility that different measures of larval size (length versus dry weight) may not be equivalent in their influence on the vulnerability of larvae to predation.” In other words, variation in the spe cies used or experimental protocol could have caused a different result (as has been reported in many other studies of selective mortality). A general conclusion on the influence