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In-use exhaust stream CH4 emissions from two dual fuel marine engines were characterized and strategies for CH4 reduction were identified and evaluated. For this, a low-cost, portable, wavelength modulation spectroscopy (WMS) system was developed. The performance of the developed WMS sensor was assessed using gas standards and demonstrated on a heavy-duty, diesel pilot ignited, direct injection natural gas research engine through comparison to a flame ionization detector. The WMS sensor was subsequently used to measure the exhaust-stream CH4 concentration from two diesel pilot-ignited, port-injected natural gas engines on a coastal vessel while under normal operation. Using cylinder deactivation to reduce the excess air ratio, lambda, and vessel operation changes to minimize operation at lower loads, the total CH4 emission were reduced by up to 33%. The measured, load specific CH4 emissions were subsequently used to identify an improved vessel operation strategy, with an estimated 56-60% reduction in CH4 emissions. These results demonstrate the importance of considering the real-world engine operation profile for accurate estimates of the global warming potential, as well as the utility of a WMS sensor for characterizing and mitigating in-use CH4 emissions.