The inclusion of sub-detection limit LA-ICPMS data, in the analysis of otolith microchemistry, by use of a palindrome sequence analysis (PaSA)

O. Ben-Tzvi, A. Abelson, S. D. Gaines, M. S. Sheehy, G.L. Paradis,M. Kiflawi

Limnol. Oceanogr. Methods 5:97-105 (2007) | DOI: 10.4319/lom.2007.5.97

ABSTRACT: Fish otoliths are a reliable source of historical data regarding individual fish. Optical and chemical data obtained from otoliths are used to reveal the fish’s age, natal areas, dispersal trajectories, and more. However, whereas optical methods are well established and widely practiced, methods for obtaining and interpreting chemical data continue to evolve. Despite rapid advances in the past decade, analytical approaches to otolith microchemisty continue to develop as new opportunities and limitations emerge. Otolith chemistry often reflects environmental conditions such as the chemical composition, temperature, and salinity of the ambient water. However, it is not always clear how these interact to produce the chemical signature observed in the otolith. Such gaps and inconsistencies in our knowledge may result partly from valuable information being discarded when measured concentrations fall below customarily defined detection limits (DL). Below we review some of the inaccuracies that may arise when analyzing chemical data obtained from laser-ablation inductively coupled mass spectrometry (LA-ICPMS). We argue that measurements traditionally defined as sub-DL can contain valuable information that would otherwise be discarded. We base our argument on the analysis of sub-DL signals obtained from both sides of otoliths core. Specifically, we show that these signals often form sequences that are symmetrical about the core (palindrome), and that the probability of obtaining even the simplest palindromic sequence by chance falls well below the customary 5% significance level. We conclude by discussing how this microchemical data can be analyzed in a manner that is insensitive to errors in concentration readings.