Fat content indicates individual condition and fuels annual reproductive cycles in many fish species. These cycles, coupled with ecosystem changes, can result in large intra- and inter-annual fluctuations in fat content. However, quantifying this variability is challenging when scientific sampling does not cover the full reproductive cycle. Scottish and Dutch fish processors routinely measure muscle fat content of North Sea autumn-spawning herring (Clupea harengus) throughout fishing seasons. We validated these high temporal resolution data by estimating a strong intra-annual signal in fat content, which matched herring reproductive status. Fat content increased from 4.5% 95% CI [0.03, 0.06] to 16.1% 95% CI [0.15, 0.17] during May and June before plateauing and decreasing to 9.1% 95% CI [0.08, 0.10] in September, which coincided with the onset of spawning. We also examined inter-annual variability in the seasonal pattern and the timing and magnitude of peak fat content from 2006 to 2020. Inter-annually, fat content differed from the mean pattern during the feeding or spawning periods in years 2013, 2015–2017, 2019, and 2020. There was no difference in the timing and magnitude of peak fat content between years. Our study validates the scientific use of routinely collected fat content data from pelagic fish processors.