Juvenile Salmon Sampling Program
Methods
We have been sampling juvenile salmon off the coasts of Washington and Oregon at the stations shown in the figure below, in June and September, since 1998. Pelagic fish are collected within the upper 20 m of the water column with a NET Systems Model 264 rope trawl (30 m wide X 20 m high X 100 m long), at stations ranging from Newport, Oregon north to La Push, Washington (Figure 25).
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| Figure 26. |
Transects sampled for coho and yearling and subyearling Chinook salmon, 1998–2006. |
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For each trawl sample, all fish and invertebrates are identified and enumerated. Lengths of 50 randomly selected individuals are measured. For juvenile salmon, up to 200 individuals of each species and size class (i.e., subyearling and yearling Chinook based on size) are measured and individually frozen at –20°C.
Oceanographic data collected at each station include continuous underway sampling of sea surface temperature and salinity, depth profiles of salinity and temperature with a CTD (Seabird SBE–19 plus) and water transparency (Secchi depth and transmissometer). A water sample is collected from a depth of 3 m for analysis of chlorophyll–a (filtered through glass fiber filters). The filtrate is frozen for later analysis of nutrient concentrations (nitrate, silicate, phosphate). Zooplankton is collected by vertical plankton tow (0.5 m diameter, 200 µm mesh) and an oblique bongo tow (60 cm diameter, 333–µm mesh bongo) from 20 m to the surface.
We (Robert Emmett) also carry out cruises every 10 days during which pelagic fish are sampled at night along transects off the Columbia River and Willapa Bay. This work provides data to index the abundance of fish predators (such as hake) and forage fish (which can serve as an alternate prey of the fish predators).
From 1998 to 2005, we have collected samples over a wide range of ocean conditions. These data have provided many insights into the role of ocean conditions in controlling survival and growth of coho and Chinook salmon. For example, we sampled during a very strong El Niño (June 1998) and a strong La
Niña (cold water) (1999), under very high Columbia River flows (June 1999) and extremely low flows (June 2001), and during anomalously warm conditions in the coastal ocean due to lack of upwelling (June 2005). Also during this 8–year period, the Pacific Decadal Oscillation moved from warm phase (pre–1999), to cool phase (1999–2002), then to warm phase again (2003–2005). Thus, nature has handed us a grand experiment that allows us to determine in what ways and how quickly salmon and other ecosystem components respond to short–term climate variability.
Results
Salmon Distribution––Average juvenile salmon abundance over all June cruises has been highest in the vicinity of the Columbia River and off the Washington coast (Figure 26). Distributions of coho salmon have been more widespread, whereas both yearling (spring) and subyearling (fall) Chinook salmon were far less common off Oregon than Washington.
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| Figure 27. |
Average catch of salmonids per km towed at stations in June and September, for each life history type, averaged over 1998–2004. A (+) represents locations where trawls were attempted but no fish were caught. The very largest diameter circle represents an average catch of >50 fish/tow/km–1. |
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In September, salmon catches were lower overall, and their distributions shifted to the north with the exception of fall Chinook, which was found mainly inshore throughout the study area. Large catches were consistently made at several stations along the La Push (48°N), Queets River (47.5°N) and Grays Harbor (47°N) transects, as well as at two stations associated with the plume: one 5 miles off Willapa Bay (46.6°N), and the other 7 miles off the mouth of the Columbia River (46.2°N).
Catches in both June and September were also very patchy in that we generally caught half of the fish in less than 5 trawls per cruise and did not catch any fish in 40% of the trawls. Patches most generally occurred for both yearling Chinook and coho off the Washington coast in June (Figure 27) and very near shore for yearling and subyearling Chinook in September.
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Figure 28. |
Annual variation in catches of juvenile coho and Chinook salmon during June trawl surveys, 1998–2005. |
Year–to–year variations in salmon abundance––The lowest June catches of Chinook and coho salmon were associated with an El Niño event in 1998 and an anomalously low upwelling period during May and June 2005 (Figure 28). Highest catches were during the cold phase of the Pacific Decadal Oscillation (1999 2003). Catches of yearling Chinook began to decline in September 2003 in concert with persistent warming, which began that summer and continued through 2005. The number of yearling Chinook caught in September was lower than in June, whereas subyearling Chinook was more abundant in September than June.
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