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. 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. While Pacific herring are commonly eaten by juvenile salmonids, they are not present in the ocean in Jan-March (they are larvae in the estuaries), and are thus not part of the biomass index. This biomass index was previously related to adult returns of spring and fall Chinook to Bonneville Dam and coho survival (OPIH), but that is no longer the case (Figure WI-02).

 
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
   
 
Ammodytes
personatus
ClupeidaeCottidaeEngraulis
mordax
OsmeridaeSebastes
Common name
   Pacific sand
lance
Pacific
herring
SculpinNorthern
anchovy
Smelt Rockfish
Spawning season
   
 
Nov–MarFeb–AprJan–FebFeb–Jun Year–round¹
Jan–May
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 
Source
    Emmett
et al. 1991
Hart 1973 Emmett
et al. 1991
Emmett
et al. 1991
Hart 1973;
CDFG 2009
Love
et al. 2002
;
Matarese
et al. 1989
 
 
¹ winter peak

For the fourth year in a row, the biomass of fish larvae in winter was high but generally made up of offshore taxa indicative of warm ocean conditions such as rockfishes (Sebastes spp.) and northern anchovy (Engraulis mordax) (Auth et al. 2018). The 2018 winter biomass of fish larvae that salmon prey upon was the 4th highest in the 21-y time series and the biomass in 2015-2018 were all within the top 7 highest biomass years (WI-01).

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.

Recent work (Daly et al. 2017) has shown that the type of fish that are available for salmon to consume is also an important predictor of food conditions and success of salmon. 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 2018. 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.

In contrast to the winter ichthyoplankton biomass in 2018, the overall community composition of winter ichthyoplankton in 2018 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.