Comparing Cousins - The Delta and Longfin Smelt
The delta smelt has been considered a bellwether for the health of the Delta for the past two decades. With new regulations from the U.S. Fish and Wildlife Service now in place to protect the tiny threatened species from extinction, protections for its evolutionary cousin, the longfin smelt, may not be far behind.
So how are longfin and delta smelt similar? A comparison of their life history strategies sheds some light.
- Both are small estuarine fishes of the northern smelt family Osmeridae.
- Both are open-water or pelagic species that spawn in the upper estuary including the Delta.
- Longfin smelt can survive in saltier water than delta smelt. Longfin smelt are fully anadromous – they spawn in freshwater and spend part of their life in saltwater. Due to this ability to live in saltwater, longfin inhabit the entire San Francisco Estuary and even the coastal Pacific Ocean. Also, additional longfin smelt populations inhabit a small number of other bay and coastal river systems north along the Pacific coast to Alaska.
- Delta smelt are semi-anadromous – they spawn in freshwater and spend part of their life in brackish water. Water that is saltier than about 19-20 parts per thousand parts of water kills delta smelt and most delta smelt hang out where the salinity is only about 1-3 parts per thousand. For reference, ocean water usually averages about 35 parts per thousand. So delta smelt are confined to the less salty portions of the upper San Francisco Estuary. The delta smelt is found nowhere else in the world.
- Both species seem to like to “hide in the dark,” but they have different ways of doing this. Delta smelt live mainly in the upper estuary and are strongly associated with turbid (muddy) water. (The upper estuary is Suisun Bay, Suisun Marsh and the Delta.) Longfin smelt live throughout the estuary and are sometimes associated with muddy water, but also inhabit deep water areas in San Francisco Bay where light intensity in the water is not strong.
- Longfin smelt live longer than delta smelt. Most longfin smelt spawn when they are two years old. Most die after spawning, but some live to three years old. Most delta smelt spawn when they are one-year-old and die after spawning too, although a few live to two years old.
- Longfin smelt are usually a little bigger and have a larger mouth than delta smelt, enabling them to eat larger prey.
- Longfin smelt spawn in colder water than delta smelt. The longfin spawn from about December-April, with a peak in January-February, while delta smelt spawn from about late February-June, with a peak in April-May.
- The abundance of longfin smelt at the end of their first year of life can be predicted by Delta outflow (the amount of river flow leaving the Delta and flowing into downstream bays) occurring during their spawning and early life. Spawning/early life outflow conditions for delta smelt do not predict their abundance at the end of the year.
- Sensitive egg and larvae periods for longfin smelt occur earlier in the year than for delta smelt, and some sources of mortality for longfin smelt differ from those of delta smelt (e.g., longfin smelt occur as bycatch in commercial shrimp fishery in San Francisco Bay).
- Longfin smelt used to be much, much more abundant in the San Francisco Estuary than delta smelt. Through time, their relative abundances have been declining and getting more similar. Matt Nobriga, a senior environmental scientist at the Department of Fish and Game, said the longfin decline is “the most substantial sustained native fish decline in the San Francisco Estuary in the past 30 or so years.”
- There is evidence that the overbite clam, Corbula amurensis (previously known as Potamocorbula amurensis) which invaded the estuary in the latter 1980s, has affected the abundance of longfin smelt, but there is not similar evidence that delta smelt abundance was affected by this clam.
Longfin and Delta smelt popultion infographic
courtesy M. Nobriga Department of Fish and Game