[env-trinity] What caused the Sacramento River fall Chinook stock collapse?

[Sari Sommarstrom]

Helpful to read the official summary...

NMFS Report on the Sacramento River Fall Chinook Salmon Decline - 2009
Report to the Pacific Fishery Management Council (PFMC)
http://swr.nmfs.noaa.gov/media/SalmonDeclineReport.pdf
Executive summary
         In April 2008, in response to the sudden collapse of Sacramento 
River fall Chinook salmon (SRFC) and the poor status of many west coast 
coho salmon populations, the Pacific Fishery Management Council (PFMC) 
adopted the most restrictive salmon fisheries in the history of the west 
coast of the U.S. The regulations included a complete closure of commercial 
and recreational Chinook salmon fisheries south of Cape Falcon, Oregon. 
Spawning escapement of SRFC in 2007 is estimated to have been 88,000, well 
below the PFMC's escapement conservation goal of 122,000-180,000 for the 
first time since the early 1990s. The situation was even more dire in 2008, 
when 66,000 spawners are estimated to have returned to natural areas and 
hatcheries. For the SRFC stock, which is an aggregate of hatchery and 
natural production, many factors have been suggested as potential causes of 
the poor escapements, including freshwater withdrawals (including pumping 
of water from the Sacramento-San Joaquin delta), unusual hatchery events, 
pollution, elimination of net-pen acclimatization facilities coincident 
with one of the two failed brood years, and large-scale bridge construction 
during the smolt outmigration (CDFG, 2008). In this report we review 
possible causes for the decline in SRFC for which reliable data were available.
         Our investigation was guided by a conceptual model of the life 
history of fall Chinook salmon in the wild and in the hatchery. Our 
approach was to identify where and when in the life cycle abundance became 
anomalously low, and where and when poor environmental conditions occurred 
due to natural or human-induced causes. The likely cause of the SRFC 
collapse lies at the intersection of an unusually large drop in abundance 
and poor environmental conditions. Using this framework, all of the 
evidence that we could find points to ocean conditions as being the 
proximate cause of the poor performance of the 2004 and 2005 broods of 
SRFC.We recognize, however, that the rapid and likely temporary 
deterioration in ocean conditions is acting on top of a long-term, steady 
degradation of the freshwater and estuarine environment.
         The evidence pointed to ocean conditions as the proximate cause 
because conditions in freshwater were not unusual, and a measure of 
abundance at the entrance to the estuary showed that, up until that point, 
these broods were at or near normal levels of abundance. At some time and 
place between this point and recruitment to the fishery at age two, 
unusually large fractions of these broods perished. A broad body of 
evidence suggests that anomalous conditions in the coastal ocean in 2005 
and 2006 resulted in unusually poor survival of the 2004 and 2005 broods of 
SRFC. Both broods entered the ocean during periods of weak upwelling, warm 
sea surface temperatures, and low densities of prey items. Individuals from 
the 2004 brood sampled in the Gulf of the Farallones were in poor physical 
condition, indicating that feeding conditions were poor in the spring of 
2005 (unfortunately, comparable data do not exist for the 2005 brood). 
Pelagic seabirds in this region with diets similar to juvenile Chinook 
salmon also experienced very poor reproduction in these  years. In 
addition, the cessation of net-pen acclimatization in the estuary in 2006 
may have contributed to the especially poor estuarine and marine survival 
of the 2005 brood.
         Fishery management also played a role in the low escapement of 
2007. The PFMC (2007) forecast an escapement of 265,000 SRFC adults in 2007 
based on the escapement of 14,500 Central Valley Chinook salmon jacks in 
2006. The realized escapement of SRFC adults was 87,900. The large 
discrepancy between the forecast and realized abundance was due to a bias 
in the forecast model that has since been corrected. Had the pre-season 
ocean abundance forecast been more accurate and fishing opportunity further 
constrained by management regulation, the SRFC escapement goal could have 
been met in 2007. Thus, fishery management, while not the cause of the 2004 
brood weak year-class strength, contributed to the failure to achieve the 
SRFC escapement goal in 2007. The long-standing and ongoing degradation of 
freshwater and estuarine habitats and the subsequent heavy reliance on 
hatchery production were also likely contributors to the collapse of the 
stock. Degradation and simplification of freshwater and estuary habitats 
over a century and a half of development have changed the Central Valley 
Chinook salmon complex from a highly diverse collection of numerous wild 
populations to one dominated by fall Chinook salmon from four large 
hatcheries. Naturally-spawning populations of fall Chinook salmon are now 
genetically homogeneous in the Central Valley, and their population 
dynamics have been synchronous over the past few decades. In contrast, some 
remnant populations of late-fall, winter and spring Chinook salmon have not 
been as strongly affected by recent changes in ocean conditions, 
illustrating that life-history diversity can buffer environmental 
variation. The situation is analogous to managing a financial portfolio: a 
well-diversified portfolio will be buffeted less by fluctuating market 
conditions than one concentrated on just a few stocks; the SRFC seems to be 
quite concentrated indeed.
         Climate variability plays an important role in the inter-annual 
variation in abundance of Pacific salmon, including SRFC. We have observed 
a trend of increasing variability over the past several decades in climate 
indices related to salmon survival. This is a coast-wide pattern, but may 
be particularly important in California, where salmon are near the southern 
end of their range. These more extreme climate fluctuations put additional 
strain on salmon populations that are at low abundance and have little 
life-history or habitat diversity. If the trend of increasing climate 
variability continues, then we can expect to see more extreme variation in 
the abundance of SRFC and salmon stocks coast wide.
         In conclusion, the development of the Sacramento-San Joaquin 
watershed has greatly simplified and truncated the once-diverse habitats 
that historically supported a highly diverse assemblage of populations. The 
life history diversity of this historical assemblage would have buffered 
the overall abundance of Chinook salmon in the Central Valley under varying 
climate conditions. We are now left with a fishery that is supported 
largely by four hatcheries that produce mostly fall Chinook salmon. Because 
the survival of fall Chinook salmon hatchery release groups is highly 
correlated among nearby hatcheries, and highly variable among years, we can 
expect to see more booms and busts in this fishery in the future in 
response to variation in the ocean environment. Simply increasing the 
production of fall Chinook salmon from hatcheries as they are currently 
operated may aggravate this situation by further concentrating production 
in time and space. Rather, the key to reducing variation in production is 
increasing the diversity of SRFC. There are few direct actions available to 
the PFMC to improve this situation, but there are actions the PFMC can 
support that would lead to increased diversity of SRFC and increased 
stability. Mid-term solutions include continued advocacy for more 
fish-friendly water management and the examination of hatchery practices to 
improve the survival of hatchery releases while reducing adverse 
interactions with natural fish. In the longer-term, increased habitat 
quantity, quality, and diversity, and modified hatchery practices could 
allow life history diversity to increase in SRFC. Increased diversity in 
SRFC life histories should lead to increased
stability and resilience in a dynamic, changing environment. Using an 
ecosystem based management and ecological risk assessment framework to 
engage the many agencies and stakeholder groups with interests in the 
ecosystems supporting SRFC would aid implementation of these solutions.

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