[6] Questions Over Corps' Going Acoustic
A new report from NOAA Fisheries has raised some questions over the use of acoustic tags to measure survival of juvenile fish.
Based on data collected in 2007 on juvenile spring chinook tagged and released at Lower Granite Dam, the report found that acoustic-tagged [AT] fish generally had higher survival than PIT-tagged fish in the Snake River, "but consistently had lower survival in the Columbia River."
Since the Corps of Engineers has decided to use the acoustic tags as a gold standard for measuring survival at federal hydro dams, the findings have raised some serious questions about some of the results, especially since the differences in survival may be related to flows, time of release, and a potential tag effect. The acoustic tags are much larger than PIT tags and require a suturing technique to be implanted in the small fish. But researchers can pinpoint dam passage routes with AT fish, while PIT-tagged fish must enter a dam's bypass system to be detected.
The report says researchers had found similar results from another group that compared acoustic and PIT-tags in 2006 (Hockersmith et al, 2007), and found slightly higher survivals for AT fish in the Snake and slightly lower in the Columbia.
The 2007 data also showed a trend of higher survival for AT fish at two lower Snake dams, but higher survival for PIT-tagged fish at Columbia River dams. However, due to low numbers of replicates--about 400 AT fish were released each week for 10 weeks during April and May--the precision needed for a statistically significant difference in survival between the AT and PIT-tag groups was not quite there. If researchers want greater test sensitivity, the report says more AT fish are needed in future studies.
What the 2007 weekly data did show was that AT fish exhibited lower survivals to McNary Dam for the first five releases, a trip that usually took about a week. In one release, the difference in survival between the AT and PIT-tag groups was about 30 percent.
According to the report, the Hockersmith study had used low samples of AT fish, with far fewer replicates than called for by the study design. But the 2007 data used more fish, which in turn, imparted more statistical power to its results.
The report says, overall, the AT/PIT relative survival to McNary was .89 for fish released through May 5 and 1.00 for fish released from May 9- May 15. By the time the fish got to John Day Dam, the earlier AT groups had a 19 percent lower survival rate than the earlier PIT-tagged groups, and the later AT groups a 9 percent lower survival rate than the PIT-tagged groups.
However, the Corps has already changed the focus of its survival studies to the lower river. In 2009, it began an AT survival study that tracked fish from John Day Dam through the estuary.
The report pointed out that other recent field studies to compare survival between the two types of tags have shown no differences, but they were limited to distances of 100 km or less. The NMFS report said the higher survival rates for PIT-tagged fish only showed up over distances of 200 km or more.
The NMFS report said more work needs to be done to examine potential relationships between fish size and tagging effects--preliminary results have suggested that relative survival between AT and PIT groups in 2007 "was likely lower for smaller fish."
"Furthermore," said the report, "acoustic-tagged fish in this study fared better in the lab than in the river, suggesting that laboratory studies overall may not adequately represent the experience of fish migrating in the Columbia River Basin."
The researchers have tried to sort out what environmental and biological factors might be responsible for the observed differences. They said initial covariate analyses suggest that relative survival to McNary was associated with tag burden, fork length, fish condition factor, water temperature, and river discharge, but it was difficult to sort out which factors played the most important roles. So difficult, they said, that it took hours of time, "even on powerful computers. Analyses of 2007 data were delayed in part because of this difficulty."
The report also noted that 2008 data became available before the 2007 report was finished, and the added year of information will likely be of great benefit to the analyses, and will be reported when complete.
Initial survival results from 2008 presented at last year's Corps' research review showed similar differences in the AT/PIT survivals. In fact, the first seven out of the 10 weekly releases showed higher survivals for PIT-tagged fish to McNary, some 20 percent or more.
Flow in the Snake in 2008 was fairly low until after the May 3 release, when it climbed to 100 kcfs by May 8, declined some until around May 15, when the freshet began; the May 13 weekly release was the first time all year that AT survivals beat PITs at McNary--about 20 percent better.
In 2007, the spring flow regime was quite different. It increased steadily from late April to May 1, when it reached about 100 kcfs, then dipped the following week and peaked on May 13. The biggest differences in survival--about 20 percent less for the AT fish--showed up in the May 1 and May 3 release groups.
"Although we were unable to find a direct connection between environmental conditions and survival, said the report, "the survival/flow patterns we observed indicate that a connection is likely. Thus we recommend further analyses of these relationships."
The NMFS study also found that analyses of detection probabilities showed that AT fish did not likely behave in the same manner as PIT-tagged fish, but that neither was more susceptible to avian predation.
John Ferguson, fish ecology division head at the NMFS Science Center in Seattle, told NW Fishletter that his agency is no longer collecting data in this area of research, but after the 2008 data is completely analyzed, a clearer picture of tag effects may result.
The report found survival differences between AT and PIT-tagged subyearlings more pronounced. Subyearlings are typically smaller than yearling fish when they migrate, and their size may limit the effectiveness of the AT tags. Mean relative survival (AT/PIT) to McNary Dam was .42 before June 25, and only .33 after that date, when flows decline and water temperatures are on the rise. -B. R.
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