Large–scale Ocean and Atmospheric Indicators

Local and Regional Physical Indicators

Local Biological Indicators

Indicators Under Development

Introduction to Pacific Northwest Oceanography

Ocean Sampling Methods

Northern and Southern Copepod Anomalies

To explore the relationship between water type, copepod species richness, and the PDO, we developed two indices based on the affinities of copepods for different water types.  The dominant copepod species occurring off Oregon at NH 05 were classed into two groups: those with cold–water and those with warm–water affinities.  The cold–water (boreal or northern) group included the copepods Pseudocalanus mimus, Acartia longiremis, and Calanus marshallae.    The warm–water group included the subtropical or southern species Mesocalanus tenuicornis, Paracalanus parvus, Ctenocalanus vanus, Clausocalanus pergens, C. arcuicornis and C. parapergens, Calocalanus styliremis, and Corycaeus anglicus. 

Figure 16.	The Pacific Decadal Oscillation (upper), northern copepod biomass anomalies (middle) and southern copepod anomalies (lower), from 1969 through 2005.  Biomass values are log base–10 in units of mg carbon m–3.

The cold–water group usually dominates the Washington/Oregon coastal zooplankton community in summer, whereas the warm–water group usually dominates during winter (Peterson and Miller 1977; Peterson and Keister 2003).  This pattern is altered during summers with El Niño events and/or when the PDO is in a positive (warm) phase.  At such times the cold–water group has negative biomass anomalies and the warm group positive anomalies.  Figures 16 and 16a show time series of the PDO, along with biomass anomalies of northern and southern copepod species averaged over the months of May through September.  Changes in biomass among years can range over more than one order of magnitude.  When the PDO is negative, the biomass of northern copepods is high (positive) and biomass of southern copepods is low (negative), and vice versa.

Figure 16a.	The Pacific Decadal Oscillation (upper) and northern copepod biomass anomalies (lower) from 1969 through 2008.  Biomass values are log base–10 in units of mg carbon m–3.  Values are slightly different from those previously published because they were derived from monthly rather than quarterly averages.  Monthly average data available upon request from Bill.Peterson@noaa.gov.

Figures 17 and 17a show the same data, but as scatter grams, with copepod anomalies plotted against the PDO.  In both cases, the northern and southern copepod species anomalies are correlated with the PDO.  We theorize that the correspondence between the PDO and northern and southern copepod anomalies is due to physical coupling between the sign of the PDO, coastal winds, water temperatures, and the types of source water (and the zooplankton which they contain) that enter into the northern California Current and the coastal waters off Oregon. 

Figure 17.	Regression of northern (upper) and southern copepod anomalies (lower) vs. the PDO.  Units of biomass are mg carbon m³.

Figure 17a.

Regression of northern copepod biomass anomalies with the PDO illustrate the close relationship between the PDO and northern copepods.  Units of biomass are mg carbon m³.  Strongly negative PDO values lead to high biomass of cold–water copepods and vice versa.  The regression line shown was calculated after excluding the outlying data points from 1998 (an El Niño year) and 2005 (an anomalously warm ocean year).

When winds are strong from the north (leading to cool water conditions and a PDO with a negative sign), cold–water copepod species dominate the ecosystem.  During summers characterized by weak northerly or easterly winds, (such as during 1996, 1997, 2004 and 2005, the PDO is positive, warm water conditions dominate, and offshore animals move onshore into the coastal zone. 

Perhaps the most significant aspect of the northern copepod index is that two of the cold–water species, Calanus marshallae and Pseudocalanus mimus are lipid–rich species.  Therefore, an index of northern copepod biomass may also index the amount of wax–esters and fatty acids being fixed in the food chain.  These fatty compounds appear to be essential for many pelagic fishes if they are to grow and survive through the winter successfully.  Beamish and Mahnken (2001) provide an example of this for coho salmon.

Conversely, the years dominated by warm water, or southern copepod species can be significant because these species are smaller and have low lipid reserves.  This could result in lower fat content in the bodies of small pelagic fish that feed on these species as opposed to cold–water species.  Therefore, salmon feeding on pelagic fish, which have in turn fed on warm–water copepod species, may experience a relatively lower probability surviving the winter. 

The "northern copepod index" appears to be a good predictor of the survival of hatchery coho salmon.  Figure 18 shows the correlation between OPIH coho and northern copepod biomass anomalies.  The correlation is based on the year of ocean entry, that is, OPIH values for year + 1 are regressed on copepod biomass in year 1. 

Figure 18.	Regression of OPIH coho survival on the northern copepod biomass anomalies.  Values for copepod anomalies in 2006-2007 are indicated by arrows.

Figure 19.	Regression of Snake River Chinook smolt to adult return rates (from Scheuerell and Williams 2005) against the northern copepod biomass index.  The regression excludes the datum from the year 1998. Predicted SARs are shown by the red circles.

Using the relationship shown in Figure 18, and the value of the northern copepod anomaly for summer 2005, we can predict that the percent survival of coho salmon that return in fall 2006 will be very low, on the order of less than 1% of the 2005 juvenile migrant population.  This value is comparable to the lowest values ever recorded since the OPIH times series began in 1969.  We attribute such low survival to the late spring transition in 2005, anomalously warm ocean conditions, and low plankton biomass that persisted from May through mid–July 2005, a critical period for juvenile coho salmon.

A similar analysis was performed for Snake River spring Chinook (Figure 19) using the smolt–to–adult return rates published by Scheuerell and Williams (2005).  The regression was not as strong as that for coho, being significant only at the P = 0.02 level, and accounted for only 28% of the variability.  Snake River spring Chinook that went to sea in 2004 could return at a rate of 2%, similar to returns in 2001 and 2002.  However, given the extremely low value of the northern copepod index in 2005, we may expect one of the lowest returns of Snake River spring Chinook in 2007.