 Cramer
Fish Sciences
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Summary Postcard No. 6 |
April 30 -
May 20, 2007
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Overall for this pilot season
of Coded Wire Tagging Operations on the Stanislaus River, we
have tagged 934 juvenile Chinook with 87 mortalities (9.3%
mortality) and 869 tagged Chinook released (93% tag
retention) (Figure 1).
Figure 1. Number of Juvenile Chinook Tagged and Released
at Caswell (including mortality and average tag retention).
Flows increased on April 22nd with subsequent
sharp increases in fish number (see
Caswell Rotary Screw Trap Update #7). However, tagging
operations were suspended during this time while problems
with fish health were investigated. Consultation with
project partners included several conference calls; meeting
of the Stanislaus River Fish Group (SRFG) on May 2nd,
and review of a preliminary fish health report from Scott
Foott (see
pathology report here). Field reports after
increased flows reached Caswell indicated fish condition
remained relatively poor during the initial increase. By May 3rd, sustained high
flows, catch numbers in the 20-40 range, and markedly
improved fish health created conditions suitable to
reinitiate our tagging operations (Figure 2).
Figure 2. Technician tagging a juvenile Chinook smolt at Caswell.
[click on the photo for
enlarged view]
From May 3rd to May 9th, tagging
operations went very well with low mortality (3.64%) and
high tag retention (94%), however mortality levels increased
on May 10th and 11th, and gill
abnormalities and other skin lesions indicative of
columnaris infections were observed in some of the
mortalities (Figure 3). Tagging was ceased on May 11th
due to fish health and low catch numbers. By May 18th,
catch had increased along with apparent fish health (normal
gill color, energy level), and 28 fish were tagged.
However, overnight mortality was excessive (57%) and tagging
operations were once again ceased.
Figure 3. Abnormal gills (apparent white-grey, columnaris lesions)
from a Chinook mortality.
[click on the photo for
enlarged view]
Additional specimens
have been sent to Scott Foott for evaluation and preliminary
results again indicate columnaris infections. Field
technicians are currently recording gill color (1 - pale to
5 - deep maroon) of all Chinook in Caswell RST catch, an
important diagnostic of columnaris and other infections.
Our field observations of fish health have been
consistent with columnaris infections (e.g., abnormal gills,
low energy). Problems with columnaris are known to be common in freshwater systems, and
are usually triggered by temperature increases above 15°C
(59°F). Temperatures of 20-21°C (68-69.8°F) are known to be
optimum for rapid spread of infection. Columnaris infects
the gills, prevents proper osmoregulation and oxygen uptake,
facilitates additional bacterial and fungal infections on
necrotic gill tissue, and kills the fish within days (Scott
Foott, personal communication).
We will continue to monitor the prevalence of gill
abnormalities in RST catches at Caswell, discuss additional
investigations with USFWS, CDFG and BOR, and reinitiate
tagging only when fish condition improves. Further details
regarding reports from the fish health laboratory and field
results will be available in upcoming updates.
Please review the final
Detailed Study Plan for Coded Wire Tagging of Chinook Salmon
in the Stanislaus River
for more information on the
CWT study.
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