Biologists See Slight Rise in 2016 Juvenile Delta Smelt Numbers, Though Still Below 2012 Estimates
An adult Delta smelt. Service biologists and researchers are, for the first time, able to estimate the number of juvenile smelt throughout the estuary using California Department of Fish and Wildlife surveys along with new mathematical modeling. Credit: Steve Martarano/USFWS
By Steve Martarano
November 18, 2016
Using the State of California’s June Delta smelt surveys, the U.S. Fish and Wildlife Service estimates there were nearly 114,000 juvenile smelt in the estuary this past summer. This is up from an estimate of 63,000 juveniles in 2015, but still considerably down from 2012 estimates of 5.2 million.
However, surveys did not detect smelt this September and October, and November numbers have not been confirmed.
For the first time, Service scientists are able to estimate the number – or abundance – of juvenile smelt throughout the estuary using the State’s surveys and new mathematical modeling. Not only are the scientists estimating current numbers, but they are also able to go back in time and estimate past population sizes. For example, they now estimate that there were 16.4 million juvenile Delta smelt in the estuary in 1996.
A California Deptartment of Fish and Wildlife bologist conducts Delta smelt monitoring operations near Rio Vista, California, in May, 2016. Credit: CDFW
These surveys and subsequent abundance estimates focus on when Delta smelt are transitioning from their larval stage into juvenile fish. This happens when they are about 20 mm, or just under one inch long.
The State’s June 2016 survey found a total of 17 juveniles, compared to 15 in 2015. Service scientists use these numbers and the new mathematical modeling to estimate overall abundance. The State’s catches in both 2015 and 2016, however, were two orders of magnitude less than previous high years. For example, they caught 1,015 juveniles in June 1996.
Juvenile Delta smelt sampling locations. Click for larger image.
Credit: Jon Myatt/USFWS
“While the number of fish found compared to last year isn’t that much different, the good news is they were more spread out and caught in more locations,” said Service statistician Ken Newman, who noted that Delta smelt were found in nine different locations in 2016 compared to five in 2015.
Newman also noted that the number of juvenile fish produced by each adult, a measure of reproductive success, was considerably higher in 2016 than in 2015 as the estimated number of adults in 2016 was roughly one-tenth the number of adults in 2015.
“This apparent increase in reproductive success, compared to 2015, suggests that 2016 was environmentally a better year for Delta smelt,” Newman said, adding that increased outflow in April and May could have benefitted the fish as anticipated.
While it’s possible there may have been more juvenile Delta smelt in June 2016 compared to last year, the low fall surveys to date could mean many didn’t survive. The State conducts the surveys from September to December each year at 122 stations.
Steve Martarano is the public affairs officer for the San Francisco Bay-Delta Fish and Wildlife Office, located in Sacramento, Calif.
