Herring spawn seen from space

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Optical sensors on two of the Sentinel satellites spotted a herring spawning event near Vancouver Island in March 2018.

Date & Time
09 March 2018 18:22 UTC and 10 March 19:22 UTC
Sentinel-2 and 3
Level 1 and 2 reflectances

By Hayley Evers-King and Ben Loveday (PML), Maycira Costa and Andrea Hilborn (University of Victoria)

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.

Figure 1: The herring spawn is clearly apparent as a bright blue patch in this Sentinel 3A OLCI imagery from 9 March 19:22 UTC. Processed by PML/NCEO/NEODAAS.
Figure 2: A close up view of the area obtained from the Sentinel 2B MSI instrument on 10 March 18:22 UTC provides more details of the local extent of the spawning event. Processed by PML/NCEO/NEODAAS.

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.

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