1/14/2013: Fall Midwater Trawl

Results Point to the Obvious: Less Water Means Less Fish

Dr. Jon Rosenfield is a conservation biologist who represents The Bay Institute in various planning efforts to protect and restore the San Francisco Estuary and its watershed, including its numerous endangered species.

January 9, 2013
During the last months of 2012, we nervously awaited two close calls with disaster: the Mayan (Not)pocalypse and the federal government’s “Fiscal Cliff.” Ultimately, neither of these dire predictions came to pass and, hopefully the terms themselves are headed to the same historical scrap heap as past non-events like “Y2K”.

But the end of the year also revealed increasing evidence that our Bay-Delta ecosystem is currently experiencing an unprecedented environmental collapse, one I hereby dub “The Physical Cliff.” Unfortunately, we’re already over the edge of this cliff, and the prediction that we’re going to hit the ground below the cliff’s edge is based in science much more sound than that used by Mayan doomsday cosmologists or modern-day political prognosticators. Fortunately, we can still reverse course of this trajectory and end our freefall from the Physical Cliff, but only if we respond quickly.

The Data
Government agencies have been performing aquatic community sampling programs for years in the Bay-Delta estuary. One of these programs, the California Department of Fish and Wildlife’s Fall Midwater Trawl, has caught fish and zooplankton (things that fish eat, like shrimp) at dozens of stations in the estuary’s upstream reaches between September and December annually since 1967. This program makes available one of the best long-term records of aquatic species abundance to be found anywhere in the world.

The Fall Midwater Trawl results from 2012 reinforced frightening patterns that indicate that we have already exceeded the Bay-Delta’s limits – we are falling off the Bay-Delta’s Physical Cliff. Last year, abundance of once common native species such as longfin smelt and Delta smelt – both of which are at-risk of disappearing entirely – declined substantially from levels detected just one year earlier and close to all-time record lows experienced in the past few years. Other fish and fish prey species have declined dramatically as well; even non-native fish species like threadfin shad and American shad were at record or near-record low levels providing further indication of the seriousness of this widespread problem. The Fall Midwater Trawl’s results are confirmed by results from other long-term sampling programs.
Figure 1: Trends in abundance of four fish species as recorded by Midwater Trawl from 1967-2012. On each graph, each of the horizontal lines represents abundances 10x that of the line below it. The fact that both native (smelt) and non-native (shad) have also declined precipitously in recent years is added evidence of the Bay-Delta ecosystem's collapse.

Horses of the Apocalypse
There are many drivers that have contributed to the disappearance of what were once truly staggering numbers of Central Valley fish and wildlife. More than 95% of riparian, wetland, and floodplain habitats were destroyed by the early part of the 20th Century (see TBI’s report Sierra to the Sea), and restoring them is both feasible and a key component of working our way back up the Physical Cliff. But success in recovering native species and functional habitats will also depend on reining in another driver of ecosystem decline in the Bay-Delta and its watershed: the large scale diversion and impoundment of fresh water resources.

Increasing amounts of water have been diverted from Central Valley rivers and streams since at least the mid-1800’s. After World War II, diversions escalated rapidly when the gigantic federal Central Valley Project came online in the late 1940s. Two decades later the State Water Project began exporting directly from the Sacramento-San Joaquin Delta. Despite growing environmental concerns, those diversions reached historic peak levels in the past decade, perpetuating a pattern of nearly permanent and extremely severe drought for the past three decades.

Not surprisingly to those of us who study aquatic ecology, many fish and wildlife species in the Central Valley respond positively to the volume and timing of freshwater flow. (See TBI's report
Gone with the Flow for detailed discussion about the strong, durable and ubiquitous relationships between flow and species' abundance.) So, the widespread declines in species abundance reflected in 2012’s Fall Midwater Trawl results were entirely foreseeable.

Freshwater flows into and through the Delta were extremely poor since the early 2000’s (10 of 12 years were “critically dry” or “super-critically dry." In 2011, wet conditions returned to the Central Valley – wet enough that the Delta experienced what would historically have been a "below normal" year, i.e., not good but not cataclysmic. Most of the fish species in the Delta responded positively and in a few years from now, we expect better returns from the ocean of salmonids that migrated to sea during 2011. But last year was drier than 2011 and, because of the extremely high rates of water diversion throughout the Central Valley, Delta outflows looked unpromising: most of our native fish populations declined to near their lows once again, as a result.

{C}Figure 2: Winter-spring freshwater flows out of the Delta as they would have been without diversions (unimpaired) and as they were (actual) over the past eight decades. Colors indicate hydrological categories for each year from wet (W, wettest 20% of years, blue bars) to super-critical years (SC, driest 2.5% of years, black bars). In the last 45 years, human water diversions have created a nearly permanent and severe drought. Super-critical dry years occurred only once during that period but, for fish in the estuary, 18 years were that dry. Nature provided 10 wet years during this period, but fish in the estuary only experience 4 years with wet conditions.

{C}The simple take home message from last year is the same one we've been hearing repeatedly over the past 45+ years: when there is more water, there are more fish; less water, less fish. Humans are the real engineers of this Physical Cliff; in all but the wettest years we divert so much water from the Bay-Delta and its watershed that the estuary suffers a severe drought. Beneficial conditions for our native fish and wildlife occur only during the wettest years, when precipitation exceeds our ability to divert water.

We can stop the ecosystem’s freefall and actually re-scale the Physical Cliff by simply leaving enough water in our rivers and Delta to ensure adequate habitat conditions occur more frequently. Pending decisions by the State of California and the federal government about the amount of flow required for Bay-Delta ecosystem protection could make the difference, if decision makers do the right thing – but the bottom of the cliff is rushing towards us very quickly.