Methods

We have sampled juvenile salmon In May, June, and September since 1998 at offshore stations ranging from Newport, Oregon, to Father and Son, Washington (Figure JSS-01). Pelagic fish are collected from the upper 20 m of the water column using a 264 rope trawl (NET Systems, Inc.; 30 × 20 × 100 m).

Figure JSS-01.  Transects sampled for coho and yearling and subyearling Chinook salmon, 1998 –present.

For each trawl sample, all fish and invertebrates are identified and enumerated, and the lengths of 50 randomly selected individuals are measured.  For juvenile salmon, up to 60 individuals of each species and size class (i.e., subyearling or yearling Chinook, based on size) are measured and individually frozen.  Remaining samples are frozen in bulk for further examination in the lab. 

Oceanographic data collected at each station include sea surface temperature and salinity, depth profiles of salinity and temperature (collected with a Seabird SBE–19 and SBE-25 CTD), and water transparency (measured with a Secchi disk and/or a transmissometer).

During each year since 1998, 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 & 2008), under very high Columbia River flows of June 1999, 2008, and 2010 – 2013 during extremely low flows of June 2001, and during anomalously warm conditions in the coastal ocean due to lack of upwelling in June 2005. During this period, the Pacific Decadal Oscillation moved from a warm phase (pre-1999) to a cool phase (1999–2002), then to warm phase again (2003-2007) and then back to a cool phase (2008-2013). Thus, nature has handed us a grand experiment that allows us to observe how salmon and other ecosystem components respond to short-term climate variability and on what temporal scales these responses occur.

Results

Salmon Distribution— Average juvenile salmon abundance over all May cruises has been highest in the vicinity of the Columbia River for both yearling (spring) Chinook and coho salmon, whereas subyearling (fall) Chinook salmon are rarely caught in May Figure JSS-02).

In June, average abundance has been highest in the vicinity of the Columbia River and off the Washington coast (Figure JSS-02). Distributions of coho salmon have been more widespread, whereas both yearling and subyearling Chinook salmon were far less common off Oregon than Washington.

Figure JSS-02. Average catch of salmonids per km towed at stations in May 1999, 2006 – 2012, June 1998 – present, and September 1998 – 2012 for each life history type.

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. Due to funding constraints, September sampling ended in 2012.

Catches in all months were very patchy in that 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.

Annual variation in salmon abundance–The lowest June catches of Chinook and coho salmon were associated with an El Niño event in 1998, anomalously low upwelling period during May-June 2005, and the warm “Blob” in 2017 (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). June 2019 catches of subyearling Chinook salmon were on the high side, whereas catches of yearling Chinook and coho salmon were about average, and catches of mixed-age juvenile were below average. Subyearling Chinook salmon catch were the 5th highest of the 22 years, yearling Chinook salmon were ranked 9th, yearling coho salmon were the 12th highest catches, and mixed-age juvenile Chinook salmon were the 26th highest of the 22 June survey catches.

Figure JSS-03.  Annual variation in catches of juvenile coho and Chinook salmon during June trawl surveys, 1998-present.