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.
Total winter ichthyoplankton biomass was highest in 2001, 2008, 2010, 2015, and 2016 and lowest in 1998, an El Ni�o year Figure WI-01. . Winter ichthyoplankton data shown here were from samples taken 1 January to 31 March. 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|
Last year we added a second predictor based on the prey composition of winter ichthyoplankton. 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 salmon Figure WI-03. Warmer years are positive on axis 1 (PCO1), including 2016.
Food biomass for out-migrating juvenile salmon in 2016 is predicted to be above the long-term average based on the winter ichthyoplankton biomass index, due primarily due to a high biomass of rockfish and northern anchovy larvae. The 2016 winter (January to March) biomass of fish larvae that salmon prey upon was the fifth highest in 19 years. Due to the anomalously warm ocean conditions this winter, which typically predict lower salmon survival of early ocean migrants, we are again uncertain about the accuracy of our current prediction based on the biomass of ichthyoplankton
This year 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 salmon (WI-03). Warmer years are positive on axis 1 (PC1), including 2015. 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 2016 (Figure WI-02) suggests returns of spring Chinook salmon in 2018 will be just below 200,000, the ichthyoplankton composition suggests it will be lower, around 70,500, which is the lowest of the time series (Figure WI-04).
Of particular note during Jan-March of 2016 were: