link to AFSC home page
Mobile users can use the Site Map to access the principal pages

link to AFSC home page link to NMFS home page link to NOAA home page

Recruitment, Energetics, & Coastal Assessment: Research Feature

Early Marine Ecology of Juvenile Chinook Salmon on the Yukon Delta (cont.) (pg 1, 2, 3, 4)

Research Reports
ABL Reports
HEPR Reports
NMML Reports
REFM Reports
Quarterly Index
Quarterly Home

Results and Discussion

Our second field season was completed in August 2015.  We will be returning to the Yukon River in 2016.  Laboratory analyses of diet and energetic status for fish captured in 2014 and 2015 have not been completed.  All other results should be considered preliminary at this time. 

To understand how changes in adult Chinook salmon runs have affected juvenile Chinook salmon abundance and distribution, we compared data from the first 2 years of the present study with the only prior study of lower Yukon River juvenile Chinook salmon which was undertaken in 1986 when salmon were abundant and returns were high.  In 1986, tributary sampling was restricted to stations in the south tributary.  Two of the sampling stations used in 2014 and 2015 were located as close as possible to stations sampled in 1986 to further a comparison between these studies.  Using data from these stations, we found that catch per unit effort (CPUE) and outmigration timing were highly variable between the 2 years of our study and when compared with 1986 (Fig. 2).  The CPUE was generally higher in 1986 over a longer period of time than in 2014 and 2015, which may reflect the higher abundance of salmon in the river, but we do not have enough data at present to speculate on the much lower CPUE in 2014 as compared with 2015.   In the southern tributary, outmigration patterns for 2015 and 1986 are similar, with two distinct peaks of outmigration occurring approximately 2 weeks apart.  This pattern is much less evident in 2014.

Figure 2. Catch per unit effort of juvenile Chinook salmon in the southern tributary by year and week (CPUE expressed as fish/10 minute tow).

A primary contribution of this research was to expand the spatial scale of sampling to include all three of the main lower tributaries, and additionally sample 15 stations on the pro-delta between the south tributary and Stuart Island east of the north tributary mouth.  Comparing CPUE from 2014 and 2015 across tributaries highlights spatial and temporal differences in juvenile Chinook salmon habitat use and outmigration strategies (Fig. 3).  For 2014, the dual peaks in outmigration that were absent from the south tributary (SM) data become more evident in the northern (NM) and middle (MM) tributary data.   For 2015, all tributaries show more than one peak in outmigration, with the timing of these peaks occurring at different times on the southern tributary than on the northern and middle tributaries.  These peaks may represent different cohorts of fish or may indicate that fish from different parts of the Yukon River system have distinct migration corridors in the individual tributaries.  We hope that genetic samples collected from all of the juvenile Chinook salmon that were captured can help us tease out whether spatial and temporal differences in migration patterns represent distinct groups of fish. 

Figure 3. Comparison of CPUE between the three main Lower Yukon tributaries in 2014 and 2015 (CPUE expressed as fish/cubic meter of water)

Figure 4. Comparison of average water temperatures between years  

Figure 5. Juvenile Chinook salmon CPUE by tributary in 2015.


Physical factors may also influence outmigration timing.  Average water temperatures were approximately 1.3°C higher in 2015 than in either 2014 or 1986, and the water temperature increased faster and stayed higher longer (Fig. 4).  For the southern tributary stations, the maximum water temperature of approximately 17 °C occurred near the second week of July in 1986 and 2015.  In 2015, water temperatures reached a high of 18.1°C on June 26 and remained above 18°C until the July 17.  In 1986 and 2015 the first outmigration peak in the southern tributary occurred when water temperatures were close to 14 °C, an event that happened two weeks earlier in 2015 than in 1986.   When the average length of the fish is compared across the3 years (Fig. 5), it is evident that fish in the first peak of outmigration in 2015 were substantially smaller than fish from the same period in 2014 and 1986.  This may suggest that temperature induced fish to migrate from the river earlier in 2015 and before they had the opportunity to feed and grow.  

Prolonged high temperatures in 2015 may also have affected the type and availability of prey.  Fish caught near the end of the sampling period in both 2014 and 1986 are significantly larger than fish from 2015 during the same timeframe.  Limited data from stomach content analyses for 2014 indicates that fish are primarily consuming terrestrial drift and the larval stages of aquatic insects.  A comparison of 2014 and 2015 stomach contents may highlight differences in prey type and stomach fullness.   Spatial and temporal differences in stomach contents and stomach fullness may also occur across tributaries.    To further investigate the role of prey types and availability on fish size and condition, our 2016 field data collection will include sampling for prey items, such as invertebrate drift in the tributaries.  We will be assessing not only what prey items are available for the fish to eat, but also the nutritional quality of the prey they consume.  





            | Home | Site Map | Contact Us | FOIA | Privacy | Disclaimer | | Accessibility | Print |           doc logo