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Size-based criteria [length frequency distributions (LFDs), size ranges, size class structure and biomass size spectra] were applied to investigate changes between mesopelagic historical (HA, 1966-1979) and present fish assemblages (PA, 2014-2015) on the basis of a total of 35 566 length measurements. Under-sampling, collection bias, time-averaging, and environmental change were considered as sources of uncertainty. In PA comparisons, size-based criteria allowed for a distinction between seasonal (spring vs. summer) and environmental (oxygen minimum zone vs. tropics) factors. In HA-PA comparisons, significant differences in LFDs were indicated in 20 out the 28 species-region combinations, however, without association to changes in size ranges. In 8 species, younger size classes increased in dominance, whereas in 10 cases older size classes increased. In two species, a shift in modal length was observed. At community level, smaller specimens increased in relative abundance in the subtropical and tropical regions in PA samples. Slopes of normalized biomass size spectra steepened in 2015 for the tropical (-0.59 to -1.03) and subtropical region (-1.03 to -1.28) and are in line with published modelling results for unfished assemblages. The slope for the temperate region was -0.50 in 1966 - 1979. It is concluded that observed differences in length structure are owing to environmental changes.