Update: Fixed typo below. The U.S. Fish and Wildlife Service estimates 13,000 smelt are in the Delta, not 1,300.
Belatedly, here are some observations from native fish guru Peter Moyle about the latest Delta smelt surveys and the lack of a rebound last year, despite more water flowing through the system.
The bottom line: The numbers are low enough that any year-to-year differences that we might notice in the surveys “are not meaningful,” Moyle says.
He offers a shred of hope, noting that 200 smelt were found in a separate survey in December, “but one good sampling doth not a rebound make.”
“1. First, remember there are three sampling programs that are used to monitor delta smelt, summer townet, fall midwater trawl, and fall & spring Kodiak trawl. The first two, designed originally to sample striped bass young, continue to catch smelt at record low levels. The Fall KT has had the same pattern until this month (December) when it caught over 200 smelt (227). The FKT program is basically aimed at smelt so this could be interpreted as a population response to the wet cool conditions, pure luck, or focused sampling on a few places the smelt aggregate. The UC Davis Fish Conservation and Culture Laboratory folks caught nearly 200 smelt near Decker Island (in Sacramento River) a few weeks ago, to augment their breeding program. This ‘hot spot’ is often where smelt aggregate. USFWS estimates the total adult population is 13,000 fish (https://www.fws.gov/sfbaydelta/species/delta_smelt.cfm) but the take of smelt in these two programs suggests that the agencies are betting more smelt exist out there that the estimate indicates. It is important to recognize that when the numbers of fish become as low as they are today, one or two fish can make a big difference in the index for any species; this means there is a strong stochastic component to the numbers. The trends for the species do suggest record low numbers but we have reached a point where year to year differences are not meaningful. The ‘rebounds’ you notice are unlikely to be real. Delta smelt, longfin smelt, and striped bass are sufficiently different in their spawning habits and areas that different factors may be affecting survival of larvae and juveniles when numbers are low.
“2. The fall midwater trawl and summer townet surveys were designed to assess the abundance of juvenile striped bass in the main channels of the Delta, but FMT is most often cited because it catches pre-spawning adult smelt and has a long record. It is fortuitous that delta smelt and longfin smelt used the same areas as juvenile stripers, so the FWT probably broadly samples them reasonably well, although the program does not sample key spawning for each species areas particularly well. It is a lousy sampling program for threadfin shad and splittail so their indices do not merit attention. But I tend to accept the results of the FMT as our best indicator of smelt trends, recognizing its imperfections, especially if other sampling programs show the same pattern.
“3. Assuming a rebound in smelt is even detectable, there is at a least a possibility of improved conditions leading to improved numbers. But the indications from all sources (except possibly December FKT) are that smelt numbers are extremely low, random factors (e.g. a school of predatory silversides being present in an important spawning area) could prevent recovery and lead to extinction.
“4. Another factor to consider is that delta smelt are extremely sensitive to warm waters (greater than 20 degrees C) and this is just as important as flow for smelt survival. 2016 was another year of early spring warming, a trend we’ve been seeing for nearly a decade. The cool temps we are seeing now are good for the remaining fish but if Delta waters warm up early again we will not likely see a rebound in the smelt population. or at least not one that is detectable. A ‘test’ of this idea occurred in 2011 which had the coldest and latest spring-summer transition we’ve seen and there was a detectable increase in the smelt population.
“5. Curiously, the FMT may no longer be especially good at assessing juvenile striped bass numbers. Our studies in Suisun Marsh, for example, do not show extreme decline in juvenile bass. Striped bass can live up to 40 years, partly at sea, and large females produce millions of eggs, so this species has the considerable potential to respond positively to improved conditions, especially if zooplankton are abundant in the spring window when larval/small juvenile bass are present.
“The eternal optimist in me wants to see the recent Kodiak trawl catch as a positive sign of smelt resilience but one good sampling bout doth not a rebound make.”