By Andre Martel) 28 IV Equipment and Chemicals 31 V Forms and Labels 32 VI Conversion Tables 38 mussel population in I-urope underwent a population linois Natural History Survey Biological Notes No. 138 decline 5 to 10 years after invasion of a new area (Walz 1973, 1975). Only by monitoring local populations can the occurrence of this decline be noticed. The timing of the population decline is particularly important for biologists conducting long-range population studies of zebra mussels and their effects on native species. 1.4 Definition of and Need for Standard Sampling Fundamentally, any protocol used by consensus can be a standard. An ideal standard protocol fits broad user needs for information and is easy to use. Standardization is most important for individuals who plan to compare their data with results from other studies or from other years within the same study. These individuals include scientists, biological consultants, fisheries managers, and individuals from industries and public utilities. Standard methods should never be used, however, at the cost of data quality. Priority should be given to the ability to replicate results within a study, and then to comparability among studies. Zebra mussel settlement, growth, and density appear to be affected by light, temperature, depth, currents, substrate composition, substrate texture, substrate type, pH, ionic concentrations, and local fauna. For example, because of substrate preferences, equal numbers of juveniles measured on different settlement plate materials may not reflect equal population densities. Use of a standard sampling protocol is thus essential for comparing data among sites and between studies. Reporting ancillary data such as temperature, depth, and substrate types is equally important. 1.5 Early Detection of Zebra Mussels Personnel at power plants, water treatment facilities,