Brook Trout Spawning in Salt Water

Researchers in New England have filmed evidence of brook trout (Salvelinus fontinalis) spawning in a brackish salt water estuary. While the behavior has been observed by naturalists for over 100 years, this was the first time that the dynamic has been recorded to video. The findings have potential implications for scientists in helping to restore populations of sea-run brook trout, an endangered form of the species locally known as "salter brookies".

In November of 2016, supermoons and king tides in Massachusetts created unusually high levels of sea water pushing into the mouths of area rivers and streams, creating higher levels of salinity than are normally experienced in brook trout spawning beds that are adjacent to estuaries. While the adult form of sea-run brook trout exhibit an anadromous life cycle–spawning in fresh water streams and maturing in the open ocean–it has been generally assumed that eggs and milt from the fish require fresh water with no saline content for the critical first stages of fertilization and hatching, and that this is what drives the migratory spawning behavior in the first place.

Researcher Steve Hurley made the video recordings while conducting regular maintenance checks on electronic monitoring stations near the mouth of Red Brook on The Trustees of Reservations Theodore Lyman Reserve near Plymouth, MA. Hurley reported to the Sea Run Brook Trout Coalition website what he observed.

"Trout had been spawning in the area for a couple of weeks and the water levels I observed were at least 2 feet higher than the normal water level. I encountered a group of cadets from the Massachusetts Maritime Academy environmental monitoring class doing some water quality measurements and decided to show them the brook trout spawning areas to avoid. I was surprised to see the fish still hanging out near the redds. The class measured salinity levels near my Antenna 2 and measured 25 ppt at the bottom (full strength sea water is about 35 ppt) and 0.2 ppt at the surface. After downloading the PIT reader, I decided to try to film some underwater GoPro footage. This requires a long stick and careful placement to avoid spooking the trout."

What Hurley recorded and subsequently uploaded to YouTube was footage of fish engaging in spawning behavior in areas that are clearly being inundated by salt water. In addition to the salinity measurements noted above, this was evidenced by swirling currents with higher levels of salinity mixing with fresh water from the river, creating a subtle differential in the index of refraction between the two that was visible to the camera.

It is not clear if the eggs from the brackish water redds will become fertile, and if so, will result in fish that ultimately mature to become sea runs or the more familiar landlocked form of brook trout that are common in fresh water inland streams and lakes.

Restoration of viable populations of salter brookies would potentially create a regional sport fishery for a species that could rival the interest that currently exists for other anadromous salmonids such as steelhead in the Pacific Northwest and the sea-run brown trout of Argentina.

---