Optical sensors on two of the Sentinel satellites spotted a herring spawning event near Vancouver Island in March 2018.
Every year thousands of Pacific herring congregate for spawning events in areas around British Columbia, Canada. Males and females simultaneously release huge quantities of milt and eggs. From shore these spawning events are visible as extensive clouds of milky blue water.
In March of this year, scientists from the University of Victoria were able to view an example of this phenomena off the coast of Vancouver Island in images from optical sensors on two of the Copernicus Sentinel satellites — Sentinel-2 and Sentinel-3A.
The OLCI sensor aboard Sentinel-3A is optimised for remote sensing of coastal ocean colour (see Figure 1). It has good spectral resolution and sensitivity, a revisit time of two–three days, and a medium spatial resolution of 300 m. The MSI sensor aboard Sentinel-2B (Figure 2) brings benefits of higher spatial resolution, although its spectral characteristics are optimised for land, and its revisit time is longer (five days).
Using these two Copernicus satellites together provides a very useful balance between spatial, temporal and spectral resolution when looking at signals like this herring spawn in the coastal ocean domain.
The eggs (roe) can be harvested and eaten, both by local people, and through commercial companies who serve the roe as a delicacy in many Asian countries.
On Vancouver Island area the herring roe is an important harvest for a number of different groups of indigenous people. However, in 2018, egg harvesting was linked to outbreaks of cholera in the region between French Creek and Qualicum Bay, including in the images above. The herring spawning is thus an important marine resource that needs to be sustainably managed, and can present social, economic, and environmental challenges.
Heiltsuk Nation's traditional herring harvest makes sustainable sense (The Globe and Mail)
Rare outbreak of cholera linked to herring eggs still ‘evolving’ (Vancouver Courier)