Marine diets of juvenile coho and Chinook salmon are primarily made up of age zero winter—spawning juvenile fish such as rockfish, Pacific sand lance, cottids, Northern anchovies and smelts (Brodeur et al. 2007; Daly et al. 2009; Table 7). Measures of ichthyoplankton biomass prior to the ocean entry of juvenile salmon is currently a good indicator of adult salmon returns (Daly et al. 2013). Annual biomass estimates of key salmon prey in winter and early spring provide an indicator of survival in the months before juvenile salmon enter the sea because these estimates reflect the feeding conditions they will potentially encounter. Figure WI-01 shows the proportions of total winter ichthyoplankton biomass composed of food items for juvenile salmon.
Winter ichthyoplankton data shown here were from samples taken 1 January to 31 March on the Newport Hydrographic Line. All fish larvae were identified and lengths were measured on a subset of each species per sampling station. Length–to–biomass conversions were made using published values, and total biomass in mg carbon per 1000 m3 at each station was calculated for all sampled larval fish and a subset of fish biomass that included only fish prey typically eaten by juvenile salmon. Table WI-01 lists common prey eaten by juvenile salmon in their first marine summer and provides data on the size and availability of each.
|Table WI-01.||Common prey eaten by juvenile salmon during their first marine summer. Shown are the peak spawning season, hatch time and size, estimated days to reach the juvenile stage and average size of prey when eaten by juvenile salmon.|
|Common prey of juvenile salmonids|
|Pacific sand |
|Time to hatching (d)|
|Size at hatching (mm)|
|Time to juvenile stage (d)|
| ||90–120 d||60 d||60 d||70 d||90 d||120–150 d|
|Juvenile size (mm)|
|Mean size when eaten by salmonids (mm)|
et al. 1991
et al. 1991
et al. 1991
et al. 2002;
et al. 1989
|¹ winter peak|
Food biomass for out-migrating juvenile salmon in 2017 is high based on the winter ichthyoplankton biomass index, primarily due to a high biomass of rockfish larvae. The 2017 winter biomass of fish larvae that salmon prey upon was the highest in the 20-year time series. For the third year in a row, due to the anomalously warm ocean conditions this winter which typically predicts lower salmon survival of early ocean migrants, we are again uncertain about the accuracy of our current prediction based on the biomass of ichthyoplankton.
We have added a second predictor based on the prey composition of winter ichthyoplankton which predicts lower returns of salmon in 2017. This second indicator also has a relationship with salmon survival. This index suggests that in addition to the quantity of the prey (biomass), the type of fish prey (composition) is also important. Below is the Principal Coordinate community analysis of the winter ichthyoplankton prey that are important for juvenile salmon (Figure WI-03). Warmer years are positive on axis 1 (PCO1), including 2017. This new index relates well to returns of spring and fall Chinook and coho salmon (Figure WI-04).
While the ichthyoplankton biomass from the winter of 2017 (Figure WI-02) suggests that conditions for outmigrating juvenile salmon were good, the ichthyoplankton composition is ranked one of the lowest of the time series and suggests that conditions for outmigrating juvenile salmon were poor (Figure WI-04).
Of particular note during the January-March of 2017 were: