Local Biological Indicators
Biological conditions experienced by juvenile salmon entering the northern California Current.
Copepods are plankton that drift with the ocean currents. They are good indicators of the type of water transported into the Northern California Current. Copepod biodiversity (or species richness) is a simple measure of the number of copepod species in a plankton sample. We can use it to index the types of water masses present in Oregon and Washington's coastal zone.
For example, the presence of subtropical species off Oregon indicates subtropical water is flowing into the northern California Current from the south. Likewise, seeing coastal, subarctic species indicates transport of coastal, subarctic waters from the north.
Figure CB-01 shows average copepod species richness (i.e., the average number of species from all plankton samples) for each month from 1996 to present at station NH-5.
Species richness is low during summer because sub-Arctic waters dominate. These waters naturally contain zooplankton assemblages of low diversity (Hooff and Peterson, 2006). During winter, the opposite pattern prevails—coastal waters are fed by the poleward flowing Davidson Current, which brings a highly diverse assemblage of subtropical copepods to the northern California Current.
We use the monthly copepod diversity anomaly (seasonal cycle removed) from May to September to index the biological response to climatological indices during the summer upwelling months. This is the period of strongest upwelling in the northern California Current.
Although copepod biodiversity changes greatly on a seasonal cycle, basin-wide climatological indices, such as the PDO and ENSO, can also influence diversity in coastal waters. During negative (cool) periods of the PDO, species diversity is low, and when the PDO is positive (warm) or during El Niño events, species diversity is high (Hooff and Peterson, 2006).
Copepod species richness has been slowly increasing over time. Figure CB-04 shows that species richness has increased at a rate of 4.4 species over the past 40 years. Although this increase in biodiversity may be due to climate change, it is probably too soon to draw this conclusion (see Peterson 2009).
Northern and Southern Copepod Anomalies
These seasonal and interannual changes in copepod diversity are further illustrated by anomalies of the biomass of “northern” species (which are dominants in the coastal Gulf of Alaska and Bering Sea) and “southern” species (which are dominants in oceanic waters offshore of Oregon and in coastal waters off central and southern California) (Peterson and Miller, 1977).
The cold–water (boreal or northern) group included the copepods Pseudocalanus mimus, Acartia longiremis, and Calanus marshallae. The warm–water group includes subtropical or southern species Acartia tonsa, Calanus pacificus, Calocalanus spp., Clausocalanus spp., Corycaeus anglicus, Ctenocalanus vanus, Mesocalanus tenuicornis, and Paracalanus parvus.
The cold–water group usually dominates the Washington and Oregon coastal zooplankton community in summer. The warm–water group usually dominates during winter (Peterson and Miller 1977; Peterson and Keister 2003). 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.
When the PDO is in a negative phase, waters that feed the California Current and upwell onto the shelf are more sub-Arctic. Coldwater copepod species dominate the zooplankton off Oregon during summer and positive anomalies of cold-water copepods.
In contrast, when the PDO is in a positive phase, 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 of the cold-water species, Calanus marshallae and Pseudocalanus mimus, found in the northern copepod index are lipid–rich. 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 along to higher trophic levels.
Copepod Community Structure
We use the copepod community structure to track the seasonal and interannual changes in the copepod species. The indicator is based 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 while 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 (CCI-02). A negative–phase PDO results in more boreal water coming into the northern California Current from the north. 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 from offshore (as during the El Niño–like the event of 2005).
Biological Spring Transition
We define the biological spring transition as the date each year when the copepod community has moved from a winter (warm water) community to a summer (cold water) community.
We suggest that the timing of the summer copepod community's onset may be a more useful indicator of the physical transition from seasonal downwelling to seasonal upwelling. The summer copepod community's onset indicates the first appearance of the food chain most favorable for salmon, sardines, and sablefish. Large, lipid–rich copepods, euphausiids, and juvenile forage fish dominate this community. They help fuel a lipid-rich and productive food web.
The end of the upwelling season marks a winter community's return for zooplankton and the beginning of the fall transition.
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).
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.
The majority of marine fishes in the northern California Current (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. We developed a winter ichthyoplankton biomass indicator as a proxy for potential salmon food during this critical growth period (Daly et al. 2013).
Table WI-01 lists species included in the winter ichthyoplankton biomass and provides data on their life-history traits and the size and availability of each.
Figure WI-01 shows the total winter ichthyoplankton biomass composed of food items for juvenile salmonids and the proportion of the total that is composed of nearshore taxa.
Since 2014, the ichthyoplankton biomass of nearshore species has been below average except for 2018 (a warm average year). Offshore taxa dominated the ichthyoplankton in 2019 (Figure WI-01), similar to patterns in 2015-17 (Auth et al. 2018).
Figure WI-03 shows the Principal Coordinate (PCO) analysis of the winter ichthyoplankton prey that are 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 June and September trawl surveys can serve as an index, or surrogate measure, of ocean survival for spring Chinook and coho salmon. Figure JSC-01 shows catch per unit effort (CPUE) during our trawl surveys from 1998 to the present.
June catch rates were very low for both species during 2005. Rates rebounded gradually from 2006-2008 and 2013, only to decline again. They were the lowest for both species in June 2017. Due to funding constraints, there were no September surveys after 2012.