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<i>Calanus finmarchicus</i> is a keystone zooplankton species that is commercially harvested and is critical in sustaining many important fisheries in the North Atlantic. However, due to their patchy population distributions, they are notoriously difficult to map using traditional ship-based techniques. This study involves the use of a combined approach of standardized ocean colour imagery and radiative transfer modelling to identify reflectance anomalies potentially caused by surface swarms of <i>C. finmarchicus</i> in the northern Norwegian Sea. Here, we have standardized satellite eRGB imagery that depicts a distinct ‘red’ patch, which coincides with in situ measurements of high surface concentrations of <i>C. finmarchicus</i>. Anomaly mapping using a novel colour matching technique shows a high degree of anomaly within this patch compared to the surrounding waters, indicating the presence of something other than the standard bio-optical model constituents influencing the optics of the water column. Optical closure between modelled and satellite-derived reflectance signals is achieved (and the anomaly is significantly reduced) through the addition of <i>C. finmarchicus</i> absorption into the model. Estimations of the surface concentrations of <i>C. finmarchicus</i> suggest between 80,000 and 150,000 individuals m⁻³ within the extent of the identified red patch. Furthermore, analysis of the impact of <i>C. finmarchicus</i> absorption on the OC3M algorithm performance points to the potential for the algorithm to over-estimate chlorophyll concentrations if <i>C. finmarchicus</i> populations are present in the surface waters.