Local Biological Indicators
Biological conditions experienced by juvenile salmon entering the northern California Current.
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 and naturally contain 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) from May through September—the strongest upwelling period in the NCC—to index the biological response to climatological indices during the summer upwelling months (Figure CB-02).
Although copepod richness changes seasonally, basin-wide processes, indexed by climatological indices such as the PDO and ENSO, can also influence coastal waters richness. During negative (cool) periods of the PDO, species richness is low. During warm events, when the PDO is positive (warm), or during El Niño events, species richness is high (Hooff and Peterson, 2006).
Copepod species richness has been slowly increasing over time. Figure CB-04 shows the winter and summer species richness since 1969 and from 1996 to the present. There is a general trend of increasing species richness. In 2009, Peterson 2009 reported species richness had already increased over the previous 40 years, at a rate of 4.4 species per year. While this increase in biodiversity may be due to warming waters associated with climate change, it is too soon to tell.
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).
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).
Significantly, two cold-water species found in the northern copepod index are lipid-rich Calanus marshallae and Pseudocalanus mimus. Therefore, by indexing northern copepod biomass, we may also track the amount of lipid (wax–esters and fatty acids) transferred up the food chain (Miller et al. 2017). These fatty compounds appear essential to the growth and survival of many pelagic fishes.
Conversely, the years dominated by warm water, or southern copepod species, have smaller species with low lipid reserves. Small pelagic fish may have lower fat content due to feeding on these "fat–free" warm–water copepod species. These 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 - September 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) or view below.
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 a winter ichthyoplankton biomass indicator 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 species included in the winter ichthyoplankton biomass and provides data on each species' life-history traits, size, and timing.
Figure WI-01 shows the total winter ichthyoplankton biomass composed of food items for juvenile salmonids and the proportion of the total composed of nearshore versus offshore taxa.
The ichthyoplankton biomass of nearshore species is typically higher during colder ocean conditions with a higher biomass of taxa such as larval Pacific sandlance, smelts, and sculpins. Alternatively, the nearshore ichthyoplankton community can shift to a higher composition of larval rockfish, anchovies, and sardines during warmer ocean conditions.
Figure WI-03 shows the Principal Coordinate (PCO) analysis of the winter ichthyoplankton prey important for juvenile salmon. Years dominated by taxa associated with warmer years fall along the positive PCO1 axis.
Juvenile Salmon Catch
The number of juvenile salmon caught during our surveys can serve as an index, or surrogate measure, 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 most generally occurred 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 - 2016 (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 2008-2013).