December 10, 2007
The Mattole estuary is the gateway through which all Mattole salmonid populations pass to enter and leave the river. Historically, the estuary likely functioned as a nursery where juvenile steelhead and Chinook salmon thrived in deep pools, a well-developed slough system, and complex, shaded streambank habitats. These conditions once nurtured thousands of juvenile Chinook salmon and steelhead. They spent their first summers in the lagoon's protected and productive waters until autumn rains forced the mouth open.
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In the summer of 2007, a situation developed which, in our experience, there was no precedent. An unseasonal rain approaching 1 inch throughout the watershed fell on July18th. On July23rd MSG divers noted an entirely unexpected phenomenon. Tens of thousands of juvenile Chinook had entered the estuary. They had apparently left the cold water reaches where they had been rearing and headed downstream, perhaps mobilized by impulses that anticipated a river mouth opening. Subsequent monitoring of upstream rearing areas confirmed emigration from these areas. Since the regular seasonal mouth opening was not expected until late October, the MSG was extremely concerned about the possibility of a major loss.
The gravity of this situation in the estuary can only be understood in terms of our ongoing efforts to monitor salmonid fresh water life histories in the Mattole and, hopefully, to increase the likelihood of their survival. While this was an aberrant event, it points directly to a critical reoccurring problem for salmon survival related to summertime river conditions for which we have been struggling to find proper responses.
Water Quality
MSG estuary water quality monitoring is conducted to determine whether or not lagoon water quality (water temperature, dissolved oxygen, specific conductance, and pH) is suitable for continued over-summer rearing of juvenile salmonids. Dive monitoring has been conducted annually since 2001 to document oversummer salmonid survival and distribution in the estuary/lagoon.
Steelhead and Chinook in the Estuary
During the sumer of 2007 the peak number of both steelhead and Chinook young-of-the-year observed in the lagoon was on July 23rd, following the summer rain on July 18th. The rain likely flushed them out from oversummering habitat in the Mattole headwaters and cooler tributary locations. Following the peak of young-of-the-year (yoy) observed, observations of both species of yoy declined.
From May to October a total of approximately 62,000 steelhead young-of-the-year (<4"), 70,000 steelhead parr (4"-8"), and 7,000 1 year+ steelhead (>8") were observed by MSG divers. The greatest number of steelhead young-of-the-year observed was approximately 30,000 in late July.
Of the approximately 62,000 Chinook observed over the course of the season, 49,000 were under 4" and 12,000 were between 4-8". Prior to the mouth closure on July 3, numbers of Chinook observations were low. This suggests these fish were emigrating at a substandard size for ocean survival.
By the end of the monitoring season in October, there were very few salmonid observations. On the last dive only 6 steelhead yoy, 31 steelhead parr, 1 steelhead 1 year+, and 1 Chinook were observed.
Water Temperature
Water temperature is one of the most important parameters to assess the suitability of the lagoon for rearing salmonids. Water temperature influences juvenile salmonid growth, competition among species, and vulnerability to parasites, diseases and pollutants. Temperature data recorded in the lagoon in 2007 suggest thermal conditions were sub-optimal, especially following river mouth closure, which likely compromises the growth, health, and vulnerability to predators of salmonids rearing there.
Salmonids rearing in the Mattole lagoon in summer 2007 were exposed to temperatures above the positive growth threshold for most of the duration of the summer following lagoon formation.
Prior to river mouth closure, average water temperature in the lower lagoon remained below the upper limit for positive growth due to the input and mixing of the saline water. After river mouth closure, temperatures in the lower estuary were higher at times than the upper estuary as input and mixing of the saline water was not as much of a factor.
Later in the season, average water temperature indicated both the upper and lower estuary was not suitable for the growth of juvenile salmonids. However, since weekly average temperatures remained sublethal, we believe mortality was not a direct result of high water temperature but rather a combination of factors. Likely factors include a combination of acute and chronic temperature stress, increased metabolic activity and lack of feeding at higher temperatures, and decreased ability to withstand predation, other water quality factors, and other challenges.
Temperature data would suggest that salmonid rearing conditions in the lower lagoon were likely better than the upper portions of the lagoon. However, MSG snorkel surveys found very few fish in the lower lagoon and greater numbers of fish in the upper lagoon where more complex habitat exists.
This suggests that there are factors other than water temperature that are influencing fish distribution in the lagoon. Such factors could include competition, poor feeding opportunities, limited cover, and/or reduced flow.
Lack of physical habitat complexity and cover from predation likely contributes to limited fish utilization of the lower lagoon for oversummering habitat. The MSG has installed three structures in the lagoon to supply additional cover. Proposals for tree-planting in the riparian zone around the lagoon are also high priority to improve habitat.
In the MSG's opinion, the observations in 2007 with the unseasonal rainfall, which triggered a premature migration of juvenile Chinook into the estuary, was an uncommon occurrence. The existing inadequate conditions in the estuary made these salmonid populations extremely susceptible to predation. MSG's Chinook Survival Enhancement and Rescue Rearing programs provide Mattole salmonids a mitigating defense against these unseasonal occurrences and consequent population declines. Preliminary scale analysis suggests that most of the returning adults in the Mattole River had oversummered in freshwater. This year, we may have lost a significant proportion of the population that would contribute the most to adult returns because of this event.
In order to avert and protect Mattole salmonids from further population decline, the MSG encourages all stakeholders and local state and federal agencies to work with the MSG and with one another, to attain a long term solution.
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TOC for Forest & River News, Winter 2007