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Recruitment, Energetics, & Coastal Assessment: Research Feature

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

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Results and Discussion (continued)

 
Catch from the two-boat tow net. Photo NOAA Fisheries.  

A second component of this research involved sampling along five transects placed perpendicular to the pro-delta.  These transects cross the Yukon River plume front and provide an opportunity to investigate juvenile Chinook salmon in an estuarine environment.  On the pro-delta, the majority of juvenile Chinook salmon were captured at the three northernmost stations with only a small proportion of the catch occurring at the station directly off the southern tributary mouth (SM).  This was a surprising finding given the high CPUE of sampling in the southern tributary.  As the Yukon river flows outward toward the ocean, the earth’s rotation causes it to be deflected northward, a phenomenon known as the Coriolis effect. This induces a general north-northeasterly flow of water around the delta.  The higher CPUE for juvenile Chinook in the northern stations suggests that the fish may be following these prevailing currents north into Norton Sound possibly to access more abundant food resources or to escape predation. 

Species composition between pro-delta transects and between transect stations varies seasonally, possibly in relation to the strength and location of the river plume.  Nearly all the salmon were captured in the upper 3 meters of the water column, and abundance was higher in areas of active mixing between the river and marine waters.  Post sampling processing of CTD and catch data suggests that juvenile Chinook salmon may be utilizing these frontal boundary areas either as a transitional environment or for foraging.  At the present time, we do not have the equipment to detect these fronts in the field during the sampling.  Occasionally, we can detect a color change between the river and marine waters, and we have conducted opportunistic sampling in these areas.  However, prevailing weather conditions on the pro-delta do not make this a reliable or consistent method of front detection. We have investigated the use of an airborne drone equipped with thermal and visual cameras that could detect the location of these frontal boundaries and assist with targeted sampling of these areas.  However, at the present time we have not obtained funding to pursue this method of data collection.

Catch of juvenile Chinook salmon on the pro-delta was much lower than catch in the river, with the majority of fish captured in June.  Juvenile Chinook salmon are captured during surveys in Norton Sound and the southern Bering Sea in late August and early September, suggesting that the salmon may have a relatively short period of residency in the Yukon River plume before heading fully into marine waters.  This would be similar to findings of juvenile Chinook salmon use of plume environments in other large river systems. 

Although the focus of this research was on juvenile Chinook salmon, we have also collected a large amount of data on other juvenile and adult fish in both the tributary and pro-delta habitats.  Juvenile coregonids (whitefish and ciscos), burbot, and sheefish were the most abundant species in our tributary sampling in 2014 and 2015.  Arctic lamprey, both the larval ammocoete and smolt stages, were also regular components of our tributary catch, and the smolt stage was also captured on the pro-delta.  In 2016 we will be providing genetic samples to the University of Alaska to further research on this unusual species. 

On the pro-delta, the location of the seaward edge of the Yukon River plume changes throughout the summer.  In June when river discharge peaks, the warmer river water extends as a wedge across the entire platform forming a distinct front with the offshore marine waters.  Later in the summer, this structure breaks down and, depending on the level of discharge, the river water spreads out in a layer that can extend to the outermost sampling stations.  Temporal and spatial changes in stratification and temperature affect community composition on the pro-delta between sampling stations on a transect and between transect locations.  The most abundant species in the pro-delta samples are juvenile and larval Pacific herring, juvenile and larval rainbow smelt, juvenile saffron cod, and nine-spine sticklebacks. 

Conclusion

We have provided an overview of ongoing research into juvenile Chinook salmon habitat use, out migration, and condition on the Yukon Delta.  Preliminary data analysis shows interannual variation in juvenile Chinook salmon abundance and out migration timing.  CPUE in the 2 years of this study was lower than in 1986 when Chinook salmon abundance was high, but migration patterns between the study years are similar.  Our research shows spatial and temporal differences in outmigration timing and strength between the three main Lower Yukon tributaries, which may reflect outmigration strategies of fish originating in different parts of the Yukon River system.  At the outset of our research, one theory held that the peak of the juvenile Chinook salmon outmigration occurred prior to ice-out on the lower river.  Our research suggests that peak migration timing occurs sometime after ice-out and may be related to temperature, prey availability, or other factors.  

By Katharine Miller

 

<Results and Discussion

 

 


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