Local Biological Indicators
Biological conditions experienced by juvenile salmon entering the northern California Current.
Copepod Biodiversity
Copepods drift with the ocean currents, making them good indicators of the origins of water transported into the northern California Current (NCC). Copepod biodiversity (or species richness) measures the number of copepod species in a plankton sample. We can use species richness to index the types of water masses present in Oregon and Washington's coastal zone.
Off the Oregon and Washington coasts, species richness is low in the summer when sub-Arctic waters dominate, containing zooplankton assemblages of low diversity (Figure CB-01; Hooff and Peterson, 2006). Alternatively, coastal waters are fed by the poleward flowing Davidson Current during the winter. The current brings a highly diverse assemblage of subtropical copepods to the NCC.
We use the monthly copepod richness anomaly (seasonal cycle removed) to index the biological response to climatological indices (Figure CB-02). Although copepod richness changes seasonally, basin-wide processes, indexed by climatological indices such as the PDO and ENSO, can also influence the copepod species richness in coastal waters. During negative (cool) periods of the PDO, species richness is low, and during warm events, when the PDO is positive (warm) or during El Niño events, species richness is high (Hooff and Peterson, 2006).
Measuring copepod species richness over time can help us track whether climate change is impacting biodiversity. In 2009, we reported species richness had increased over the previous 40 years, at a rate of 4.4 species per year. While this increase in biodiversity over this time period may be due to warming waters associated with climate change, there are clear strong events that may be driving this pattern. Figure CB-04 shows the winter and summer species richness since 1969 and from 1996-present. There is a general trend of increasing species richness since the beginning of the time series until 2018. However, species richness since 2020 has decreased back to levels similar to those in the mid 1990s.
Northern and Southern Copepod Biomass Anomalies
Seasonal and interannual changes in copepod diversity are further illustrated by anomalies in the biomass of northern species (dominant in the coastal Gulf of Alaska and Bering Sea) and southern species (dominant in offshore Oregon waters and central and southern California coastal waters; Peterson and Miller, 1977).
The northern (cold–water or boreal) group includes the copepod species: Pseudocalanus mimus, Acartia longiremis, and Calanus marshallae. The southern (warm–water or subtropical) group includes the copepod species: Acartia tonsa, Calanus pacificus, Calocalanus spp., Clausocalanus spp., Corycaeus anglicus, Ctenocalanus vanus, Mesocalanus tenuicornis, and Paracalanus spp.
The northern group usually dominates the Washington and Oregon coastal zooplankton community in the summer, while the southern group typically dominates during the winter (Peterson and Miller, 1977; Peterson and Keister, 2003). However, El Niño events and phase changes of the PDO (Keister et al., 2011, Fisher et al., 2015) can alter these patterns.
Figure NSC-01 shows a time series of the PDO and the monthly biomass anomalies of northern and southern copepod species. We can see that when the PDO is in a negative phase during the summer, cold-water copepod species dominate the zooplankton assemblage off Oregon (indicated by positive anomalies of cold-water copepods).
Figure NSC-01. The Pacific Decadal Oscillation (upper) and monthly biomass anomalies of the northern (middle) and southern (lower) copepod taxa from 1969 to present. Biomass values are log base-10 in units of mg carbon m–3. Credit: NOAA Fisheries
In contrast, when the PDO is in a positive phase, and subtropical water dominates the coastal California Current, we see positive anomalies of warm-water copepods and negative anomalies of cold-water copepods (Hooff and Peterson, 2006; Chhak et al., 2009; Bi et al., 2011; Di Lorenzo et al., 2013).
Two cold-water species, Calanus marshallae and Pseudocalanus mimus, in the northern copepod index are lipid-rich. Therefore, by indexing northern copepod biomass, we are also tracking the amount of lipids (wax–esters and fatty acids) that are essential for the growth and survival of many pelagic fishes (Miller et al., 2017).
Conversely, years that are dominated by warm water, are also dominated by southern copepod species that are smaller in size and have little lipid reserves. During periods of high southern copepod biomass, small pelagic fish may have lower fat content from feeding on these relatively "fat–free", warm–water copepod species. These small pelagic prey species then have a lower fat content to pass to higher trophic levels.
Copepod Community Composition
We use the copepod community composition to track the seasonal and interannual changes in the copepod species. We base this indicator on an ordination technique called multidimensional scaling (MDS), which visually represents how similar the copepod community is among plankton samples. Figure CCI-01 shows the X- and Y-axis scores from the MDS averaged from May–Sept for each year.
Years dominated by cold-water copepods cluster on the negative X-axis. Years dominated by warm-water copepod species cluster on the positive X-axis. This relationship seems to be related to the PDO and alongshore advection, as shown in Figure CCI-02. A negative–phase PDO results in more boreal water coming into the NCC, and, therefore, more cold-water copepods (negative axis score). Alternatively, a positive–phase PDO results in more subtropical water coming in either from the south (as during the large El Niño events of 1983 and 1998) or offshore (as during the El Niño–like event of 2005), resulting in more warm-water copepods (positive axis score).
Biological Spring and Fall Transitions
We define the date of the biological spring transition each year as the day when the copepod community has changed from a winter (southern, warm-water) community to a summer (northern, cold-water) community.
We suggest that the timing of the summer copepod community may be a more useful indicator of the spring transition from downwelling to upwelling. The onset of the summer copepod community indicates the first appearance of the food most favorable for salmon, sardines, and sablefish. Large, lipid-rich copepods, euphausiids, and juvenile forage fish dominate this community and help fuel a productive, lipid-rich food web.
Alternatively, the arrival of the winter copepod community marks the fall transition and the end of the upwelling season.
Download the raw data file (.CSV).
Winter Ichthyoplankton Biomass
Juvenile coho and Chinook salmon and steelhead feed primarily on late-larval and early-juvenile fishes when entering coastal waters in early summer (Brodeur et al., 2007; Daly et al., 2009; Daly et al., 2014). Unfortunately, the late-larval and early-juvenile life stages of most marine fishes are difficult to sample effectively (Brodeur et al., 2011), which led us to explore alternative indices of potential fish prey.
Most marine fishes in the NCC spawn in late-winter and early-spring (Brodeur et al., 2008). Winter-spawned fish larvae that grow and survive through spring provide a food base for juvenile coho and Chinook salmon and steelhead during their first marine summer, and are easier to sample than their older counterparts. By tracking winter-spawned fish larvae, we developed winter ichthyoplankton biomass indicators as a proxy for potential food available to juvenile salmonids during their early marine, critical growth period (Daly et al., 2013).
Table WI-01 lists the top 11 species included in the winter ichthyoplankton indices, and provides data on each species' life-history traits such as larval habitat, size at hatching and transformation to juvenile life-stage, peak larval abundance, and mean size when eaten by juvenile salmon.
Figure WI-01 shows the Principal Coordinate Ordination (PCO) analysis of the winter ichthyoplankton prey important for juvenile salmon. Years dominated by taxa associated with colder ocean conditions fall along the negative PCO1 axis-1, while warmer ocean conditions align along the positive side of axis-1.
Figure WI-02 shows two indices of the winter ichthyoplankton: the Index of Coastal Prey Biomass (ICPB) anomaly, and the winter ichthyoplankton composition PCO axis-1 anomaly shown in figure WI-01. The anomaly of the ICPB has been typically positive during colder ocean conditions with a higher biomass of coastal fish taxa that salmon consume, and negative during warmer ocean conditions. Using the inverse of the PCO axis-1 scores, the ichthyoplankton community composition anomaly is typically positive during colder ocean conditions, with a higher composition of coastal taxa such as larval Pacific sand lance, smelts, and sculpins; and a more negative anomaly during warmer ocean conditions, when the community shifts to a composition of more offshore taxa such as larval rockfish, northern anchovies, and Pacific sardines. During most years, the indices are in congruence with each other, but this has not occurred for the last four years, including 2023.
Basin-scale ocean temperatures are correlated with the ichthyoplankton community composition axis-1 PCO scores (SSTarc; Johnstone and Mantua, 2014), as shown in Figure WI-03. Offshore larval fish taxa such as rockfish, northern anchovy, Pacific sardine, and rex sole make up a larger part of the community composition during warmer ocean conditions, and coastal taxa such as larval Pacific sand lance, sculpins, smelt, and lingcod are a larger part of the community composition during colder ocean conditions.
Juvenile Salmon Catch
The number of juvenile salmon caught during our surveys can serve as an index of ocean survival for yearling Chinook and coho salmon.
We have observed the highest average juvenile Chinook and coho salmon abundance during May cruises in the Columbia River vicinity. In May, we rarely catch subyearling (fall) Chinook salmon (Figure JSS-02).
In June, we see the highest average abundance in the vicinity of the Columbia River and off the Washington coast (Figure JSS-02).
Coho salmon distribution has been more widespread, whereas both yearling and subyearling Chinook salmon were far less common off Oregon than Washington.
Catches in all months were very patchy. We generally caught half of the fish in ~5% of the trawls per cruise, and did not catch any fish in 40% of the trawls (Peterson et al., 2010). Patches generally occurred most often for both yearling Chinook and coho salmon off the Columbia River and the Washington coast in May and June (Figure JSS-02), and very near shore for yearling and subyearling Chinook salmon in September.
The lowest June catches of Chinook and coho salmon were associated with:
- An El Niño event in 1998.
- An anomalously low upwelling period during May–June 2005.
- The outmigration of juveniles in 2017 following the marine heatwave in 2015–16 (Figure JSS-03).
Conversely, the highest June catches occurred during years with a negative signal (cold phase) of the Pacific Decadal Oscillation (1999–2003 and 2008–13).