Northwest Fisheries Science Center

Winter Ichthyoplankton

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.

Bar chart showing estimates of winter ichthyoplankton from 1998 to present with the total that are prey items for salmon shown in blue sections of each bar. Figure WI-01.  Estimates of total winter ichthyoplankton biomass from 1998 to present.  Proportions composed of fish larvae considered prey items for juvenile salmon are represented by blue bars.

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
Scientific name
Common name
   Pacific sand
Smelt Rockfish
Spawning season
Nov–MarFeb–AprJan–FebFeb–Jun Year–round¹
Time to hatching (d)
21 14 9–14 2–4 10–40 N/A
Size at hatching (mm)
5 7.5 4–5 2–3 3–6 3–6
Time to juvenile stage (d)
90–120 d60 d60 d70 d90 d120–150 d
Juvenile size (mm)
30 25–40 15–20 25 20 25–30
Mean size when eaten by salmonids (mm)
42 34 22 60 39 34 
et al. 1991
Hart 1973 Emmett
et al. 1991
et al. 1991
Hart 1973;
CDFG 2009
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.

Figure WI-02. Spring and fall Chinook salmon adult counts at Bonneville (lagged by 2 years), and coho salmon survival (OPIH, lag 1 year) vs. the log of the winter ichthyoplankton salmon prey biomass. Number symbols indicate the year of juvenile salmon outmigration.

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).

Figure WI-03. Figure WI-03. Principal Coordinate Analysis (PCO) of annual composition of winter ichthyoplankton typically eaten by salmon averaged over January-March with warm years on the right side of PCO axis 1.

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).

Figure WI-04. Figure WI-04. Relationship of spring and fall Chinook salmon adult counts at Bonneville (lagged by 2 years), and coho salmon survival (OPIH, lag 1 year) vs. the PC1 ichthyoplankton species composition value from Figure WI-03. Number symbols indicate the year of juvenile salmon outmigration.

Of particular note during the January-March of 2017 were:

  1. The percent of the total winter ichthyoplankton that were common salmon prey in 2017 was high at 80%.
  2. Taxa were present for the third winter in a row that are considered southern California winter spawned larvae: Pacific hake (Merluccius productus) and Pacific sardine (Sardinops sagax). In 2016 and 2017, juvenile salmon ate young of the year sardines, which are a new prey item for them in the 20 year time series. Additionally, another new prey to juvenile salmon in 2017 were small pyrosomes.
  3. Sardine larvae were present in winter 2017, but not in high amounts, and were located at inshore (NH 01 and 10) stations; some were > 10 mm long.