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Stable isotope analysis of baleen plates is a widespread technique for studying baleen whales. Typically, subsamples along the growth axis of the baleen plate are extracted and analysed to examine time-related variation in their stable isotope signals. However, baleen plate tissue is composed of two different tissues: a pair of cortex layers flanking an internal medulla. These two histological components exhibit differential development, and their consolidation as a tissue is therefore likely non-synchronic. This could influence stable isotope results because the stable isotope signal may differ in each subsample according to the proportion of the two histological components extracted from the tissue. In this study, stable isotope analysis was combined with optical microscopy examination of fin whale (Balaenoptera physalus) baleen plates to understand the ontogeny of the two histological components. In both of them, the δ15N values followed a sinewave pattern along the growth axis of the baleen plate. However, the δ15N values of the cortex appeared to be advanced compared to those of the medulla. Additionally, the amplitude of the δ15N values in the oscillations was higher in the cortex than in the medulla. The histological examination revealed that these differences are caused by earlier and faster synthesis of the cortex layer compared to that of the medulla. Because the stable isotope ratios of the two layers differ, we propose that in this type of studies only the outer-most part (closest to the surface) of the cortex should be subsampled and analysed. Additionally, to include the most recently formed tissue, this subsampling should start well below the zwischensubstanz, or baleen "gum". [ABSTRACT FROM AUTHOR]

Werth et al. (Werth et al. Polar Biol 43:707–723, 2020) propose that in all mysticete whales the growth rate of baleen varies with the position they occupy in the mouth. If this were true, comparison between individuals and studies of results of chemical tracers in baleen would be compromised. Here we compile results from previous research conducted in balaenopterids using stable isotope ratios as tracers as well as images of baleen racks showing growth ridges and conclude that, at least in the Balaenopteridae family, the growth rate of baleen is constant along the whole baleen rack irrespectively of the position in the mouth, thus ensuring reliability of comparisons. [ABSTRACT FROM AUTHOR]

We investigated the sex- and size-related differences in the diet of whale sharks from the Arabian Sea (north-western Indian Ocean) using carbon and nitrogen stable-isotope analyses in white muscle. The samples were collected during the commercial fishing season between April and May of 2001 in Veraval (Gujarat, India). The overall isotope signature was similar to that of the pelagic-neritic zooplanktivore Ilisha melastoma, which suggests that both species are feeding on similar prey. In whale sharks, a positive relationship was found between δN and δC. This, together with a significant enrichment of both heavy stable isotopes with total length indicates that the contribution to the diet of small fish and/or larger zooplankton of higher trophic level increases with the movement from offshore areas to coastal areas as they grow. Gender differences in the isotopic ratios were not statistically significant, but small sample size cannot rule out completely the existence of some degree of spatial or dietary segregation between sexes. [ABSTRACT FROM AUTHOR]