Table 1 summarizes juvenile chinook captured in intermittent tributaries during the 1996 season. Numbers collected in a given stream reflect effort expended in the stream rather than population.
The Lincoln-Peterson mark/recapture technique (Brower et al. 1990) was used to estimate the population of juvenile chinook within the sample sites of Blue Tent and Mud Creeks (Table 2). Since collections made above and below the sample area yielded no marked fish the population being sampled may be assumed to be confined to the sample area during the time of estimation. The large discrepancy in density between the two creeks relates at least in part to their difference in size. The sample reach of Blue Tent averaged 5.5 m wide, and was mostly shallow run or riffle. Pools were small and seldom exceeded 0.5 m in depth. In Mud Creek, by contrast, the sample reach averaged 10 m wide and consisted mostly of deep runs or pools. Average depth was about a meter with some areas in excess of 1.5 m.
| Table 2. Number per linear meter of stream | |
|---|---|
| Blue Tent Creek | 0.74 |
| Mud Creek | 5.9 |
Figure 1 (Mud Creek) and Figure 2 (Blue Tent Creek) show size distribution of captured chinook and classify them into races according to the Sacramento River Daily Length Table generated by the California Department of Water Resources Environmental Services Office (R. R. Johnson, et al., 1992).
Use of the Daily Length Table to categorize tributary-rearing chinook into races has limitations since chinook grow faster in tributaries than in the river (See section on growth rate and race classification). However it may provide a useful tool for comparing the two creeks. Early in the season (December through January) both creeks contain mostly winter run juveniles sufficiently distinct in size to be confidently categorized. Confidence is increased by the 93 coded-wire-tagged fish recovered and verified to be winter race (Table 1).
Beginning in January, but especially from February on, many similar-sized juveniles are present, some of which are large enough to be categorized as spring chinook, with smaller individuals falling into the range to be fall chinook. Considering the uncertain status of the spring chinook, it is important to try to understand what races are represented in these tributaries. In February, in Blue Tent Creek we begin to see a large group of small fish, all but one or two of the largest of which fall into the fall chinook size classification. As the season progresses and the distribution shifts to the right, gradually more and more fall into the spring chinook size classification. This is exactly what would be expected if they were all were fall chinook growing faster than the rate from which the daily length charts were calculated (Figure 2).
However, the size distribution data from Mud Creek are very different than those from Blue Tent Creek. In Mud Creek, throughout the season a high proportion of rearing juveniles fall into the size range categorized as spring chinook. Between mid December and March, data are unfortunately sparce due to sampling difficulties with high water (less a problem in the smaller Blue Tent Creek), but a group of fish clearly in the spring size range was present. As sampling became easier in March, this group became more obvious, particularly when compared with Blue Tent data. On March 2, the largest chinook observed in Blue Tent Creek was 62 mm fork length. On the same date in Mud Creek two thirds of the chinook exceeded 62 mm. (Figure 1). Since both creeks were running enough for salmon to migrate into them by mid December and our data show similar growth rates in each (0.94 mm/day in Blue Tent; 1.1 in Mud), it is clear that there are fundamental differences between the juvenile chinook populations in each. In addition to differences in size distributions on similar dates, there is also a difference between maximum size attained in the two streams before emigrating. In Blue Tent Creek, few juveniles over 85 mm fork length were observed, whereas in Mud Creek many reached 90 mm or more. Since spring chinook typically smolt at a larger size than fall chinook (Fisher, 1992), the absolute larger size attained in Mud Creek also suggests the presence of the spring race. (see also relevant data and discussion in the 1995 report)
A logical explanation for the difference in size distributions is that Mud Creek serves as a rearing habitat for spring chinook while Blue Tent does not. There is little, if any, spring chinook spawning habitat upriver from the mouth of Blue Tent Creek to provide a source of spring chinook juveniles. By contrast, Mud Creek joins the river downstream from Mill Creek and Deer Creek and joins Big Chico Creek directly. Since Mill, Deer and Big Chico Creeks serve as spawning habitat for spring chinook, some spring juveniles may be moving into Mud Creek for rearing. The limited data for down-migrating fry in Big Chico Creek suggest movement of fry in December and January, probably in conjunction with storm flows. Those fry need a place to rear, and Mud Creek appears to provide good rearing habitat. (An alternative, but more convoluted, explanation is that there exists an early-spawning group of fall chinook, that for some reason only spawns in the river downstream of Red Bluff so that fry have no access to Blue Tent Creek.)
If spring chinook are in fact rearing in Mud Creek, they are not readily separable by size from fall chinook. Unlike the winter chinook, which form a completely separate cluster on size distribution graphs, fall and spring chinook overlap totally. Differential development and growth rates in different habitats enable later-spawned individuals in some circumstances to catch up with or even pass earlier-spawned individuals. A definitive answer to the question of spring chinook rearing in intermittent tributaries will be dependent upon a technique for positive identification of spring chinook; either cwt marking of known spring chinook fry or differentiation by DNA. The difficulty of marking spring chinook fry which are perhaps being moved downstream by storm flow suggests that DNA studies are sorely needed. Until such definitive studies are conducted, the uncertain status of spring chinook demands that we assume some use intermittent tributaries for rearing.
Collections of benthic invertebrates in Mud, Blue Tent and Dibble Creek contained an abundance of midges, mayflies, and stoneflies. (Table 3). Stomach contents of juvenile chinook from tributaries in 1995 and 1996 were compared with those reported for juveniles collected from the Sacramento River (Schaffter et al. 1982). An average of 41 food items per stomach in the tributary samples compares favorably with an average of 22 food items per stomach reported for Sacramento River juveniles. Juveniles in tributaries fed consistently on Diptera and Epemeroptera with opportunistic use of Plecoptera, terrestrial insects, and cyprinoid fry. Condition factors in the tributaries (Figure 3 and 4). were high and generally increased the longer the juveniles had been in the tributaries, as shown by data for marked fish (Figures 5, 6 7). Consistent high flows and frequent hatchery releases into the Sacramento River made comparison of condition between river and tributary-rearing juveniles difficult this year. The few juveniles we were able to seine from the Sacramento River were in poorer condition than juveniles caught in the tributaries, but they may have been recent hatchery releases rather than juveniles rearing in the river.
Juvenile chinook grew rapidly in the tributaries. Growth rates were estimated from changes in size distribution and from changes in average fork length of freeze-branded fish in Mud Creek (Figure 5. Both techniques yielded identical rates: 1.14 mm/day. Growth rates in Blue Tent Creek were estimated from changes in size distribution of groups of juvenile chinook marked by either freeze-branding or caudal-clipping. (Figures 6 and 7 ). Both groups yielded similar rates, approximately 1.0 mm/day. Growth rates obtained this year from groups of marked fish in the tributaries were similar to rates obtained in previous years from modal shifts in size distribution, conclusively demonstrating faster growth of tributary-rearing juveniles as compared to river-rearing juveniles.
In Mud Creek fish in the size range classified according to the daily length charts as fall-run were branded. When recaptured a week later, one third of the fish had grown large enough to be classified as spring-run. Three weeks later, all the recaptures would be classified as spring-run (Figure 8). The growth rate in Mud Creek (1.14 mm/day) was more than twice the rate obtained from the daily length charts for Sacramento River chinook (0.48 mm/day). Virtually identical results were seen with the caudal-clipped juveniles in Blue Tent Creek (Figure 9)
It is apparent that large numbers of several races of Sacramento River juvenile chinook entered non-natal tributaries in the 1995-1996 season. Conditions in the tributaries were favorable with respect to temperature, food, and water clarity. Marked fish could be followed for a period of 3 to 5 weeks, during which time they grew rapidly. The marked fish did not remain in the tributaries after reaching a size of 80 to 85 mm. Presumably they smolted, and emigrated. Except for winter chinook which smolt at a larger size, we observed very few chinook larger than 100 mm in intermittent tributaries. Many fish larger than 75 mm showed evidence of smolting. In Spring, 1996, there was ample water flow in the tributaries for successful egress of the smolts. By mid-April, temperature in the river was close to optimum, and water clarity had improved. By contrast, maximum daily temperature in Mud Creek was approaching lethality and average temperature was well above optimum (Figure 10) . Although there were still many small juvenile chinook in the river, they no longer entered Mud Creek. By the end of April, essentially all juvenile chinook in Mud Creek had smolted and emigrated. Blue Tent Creek, having less flow, warmed faster. Essentially all juvenile chinook were gone from Blue Tent Creek by mid-April, even though a few were still entering Mud Creek. To: Discussion