Enhancing the contrast and visibility of daily growth increments in fish otoliths etched by proteinase K buffer

The otoliths of the amphidromous gobies Stiphodon elegans and Sicyopterus j aponicus, a marine eel Strophidon sathete and a freshwater fish Varicorhinu s barbatulus were digested by proteinase K buffer (PKb). which removed the proteins and retained the major calcified structure to reveal conspicuous d aily increments. Compared with etching by ethylenediaminetetraacetate (EDTA ). etching with PKb revealed more visible daily increments and enhanced the contrast. Moreover,. by using PKb, both the daily increments and annuli co uld be observed simultaneously. The better performance of PKb as an etching agent can be attributed to the digesting of predominantly only the organic matrix and leaving the calcified structure almost intact. PKb provides ano ther effective means to reveal otolith microstructure so as to examine and count daily growth increments. (C) 1999 The Fisheries Society of the Britis h Isles.