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.

Total winter ichthyoplankton biomass was highest in 2001, 2008, 2010, 2015, and 2016 and lowest in 1998, an El Ni´┐Żo 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
 
Scientific name
   
 
Ammodytes
hexapterus
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

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

Figure WI-02. Spring and fall Chinook salmon adult counts at Bonneville (lagged by 2 years), and coho salmon smolt-to-adult survival (SAR, lag 1 year) vs. the log of the winter ichthyoplankton salmon prey biomass. Numbers indicate the warm (red) and cold (blue) years. Years in black were outliers and were excluded from the regression.

*outliers were excluded using Cook's Distance

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).

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

Figure WI-04. Spring and fall Chinook salmon adult counts at Bonneville (lagged by 2 years), and coho salmon smolt-to-adult survival (SAR, lag 1 year) vs. the PC1 icthyoplankton species composition value from Figure WI-03. Numbers indicate the warm (red) and cold (blue) years. Years in black were outliers and were excluded from the regression. Figure WI-04. Spring and fall Chinook salmon adult counts at Bonneville (lagged by 2 years), and coho salmon smolt-to-adult survival (SAR, lag 1 year) vs. the PC1 icthyoplankton species composition value from Figure WI-03. Numbers indicate the warm (red) and cold (blue) years. Years in black were outliers and were excluded from the regression.

Of particular note during Jan-March of 2016 were:

  1. Largest average size of larval rockfish recorded for the entire time series.
  2. Northern anchovy larvae were present across the sampling area in February and March.
  3. Sardine larvae present in March at the most inshore stations (NH-01) to NH-15. Only in 2015 has this occurred over the time series. While sardine larvae are not common salmon prey, we are noting their presence.