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We demonstrate that assimilation of water temperature measurements by sea turtles into an operational ocean nowcast/forecast system improves representation of mesoscale eddies and front variations in the Kuroshio-Oyashio Confluence region. For a period from August to December in 2009, vertical ranges of the measurements by six turtles covered from surface to maximum 150 m depth, and the number of the measurements in the region was comparable to that of usual sampling by research vehicles and profiling floats. Comparison of the turtle measurements and products of an operational ocean nowcast/forecast system JCOPE2M elucidated low-temperature bias in JCOPE2M due to assimilation of high-resolution satellite sea surface temperature data. Additional assimilation of the turtle data into a modified JCOPE2M system with correction of the low-temperature bias led to reasonable modification of horizontal temperature/salinity gradient associated with northward moving warm core rings separating from the Kuroshio Extension and southward moving the Oyashio water intrusion branches. Effects of the turtle data assimilation propagated to far east of the Kuroshio-Oyashio confluence region along the subarctic boundary and front through advection. Another experiment assimilating the in situ profile data excluding the traditional temperature/salinity profiles data but including only the turtle data showed that the turtle measurements captured the warm core rings better than the Oyashio intrusion branches, suggesting possible optimum design of future ocean observing systems composed of different kinds of animals and/or autonomous underwater vehicles. more...