I 



Biology and Ecological Niches in the Gulf of the Farallones 



Salmon 



Peter Adams 



Two species of salmon chinook salmon 

 (Oncorhynchus tshawytscha) and coho 

 salmon (O. kisutch) are commonly found 

 in the Gulf of the Farallones. Chinook 

 salmon fishing is the activity that brings the 

 most people out on the waters of the gulf. 

 In 1995, the chinook fishery in the gulf was 

 valued at more than $24 million. 



Chinook (or king) salmon are key pred- 

 ators in the gulf, and their distribution and 

 occurrence are related to their seasonal 

 diet cycle. Most chinook salmon found in 

 the gulf are 3-year-old fish returning from 

 the open ocean that are preparing to enter 

 the Sacramento River system, where they 

 will spawn and then die. After the eggs 

 hatch the following spring, the juvenile 

 salmon will grow for 7 months in freshwa- 

 ter. They then migrate to the ocean, where 

 they will live for 2 years before returning 

 to the gulf. Coho (or silver) salmon along 

 the California coast are listed as a threat- 

 ened species under the Endangered Spe- 

 cies Act, and their capture has been pro- 

 hibited since 1993. Native-run (versus 

 hatchery) chinook salmon are also a candi- 

 date for a threatened-species listing. 



Chinook salmon returning from the 

 open ocean move into the Gulf of the Far- 

 allones in February and March, when they 

 are found off the Golden Gate from Boli- 



nas Point in the north to Point San Pedro 

 in the south. While in this area they 

 feed almost equally on Pacific herring and 

 anchovies. The herring have just migrated 

 back to their feeding grounds outside 

 of the Golden Gate from San Francisco 

 Bay, where they spawned from November 

 through February, and anchovies are gath- 

 ering in nearshore waters before moving 

 into the bay beginning in April. 



In April, chinook salmon are found 

 from north of the Golden Gate to Point 

 Reyes and offshore to the Farallon Islands. 

 There they feed on invertebrates, largely 

 the euphausiid shrimp (krill) Thysanoessa 

 spinifera. Krill are taken as prey from 

 surface and subsurface swarms that occur 

 over a wide area of the gulf during April 

 and May. The pink to orange color of 

 salmon flesh during this period is due to 

 a carotenoid pigment in the exoskeleton of 

 the krill (for more information, see chapter 

 on Krill). This flesh color has become so 

 popular that now there are fisheries for 

 krill, which are freeze-dried and fed to 

 pen-reared salmon as a finishing product 

 to produce this color. 



For a brief 2- or 3-week period in 

 April, the Chinook's diet is dominated by 

 the megalopa larvae of the Dungeness crab 

 (Cancer magister) (see chapter on Free- 



Floating Larvae of Crabs, Sea Urchins, 

 and Rockfishes). These larvae are the last 

 pelagic (free-floating or swimming) stage 

 before the crabs sink to the bottom and 

 take on their adult shape. More than 7,000 

 Dungeness megalopa have been found in a 

 single chinook-salmon stomach. 



In May and June, chinook start feeding 

 on krill and juvenile rockfish offshore near 

 the Farallon Islands. These rockfish are 

 late pelagic-stage fish that as adults will 

 migrate to bottom habitats. In years when 

 juvenile rockfish are abundant, they are 

 the preferred prey and dominate the chi- 

 nook diet during these months, whereas 

 in low-abundance years, chinook salmon 

 feed mainly on krill. 



Sometime between mid-June and mid- 

 July, the chinook salmon abruptly move 

 from near the Farallon Islands to directly 

 in front of the Golden Gate, the so-called 

 "middle grounds." Here, chinook salmon 

 feed exclusively on anchovies, which had 

 moved into San Francisco Bay in May 

 and June to begin spawning in the warmer 

 water. After June, when the water in the 

 gulf warms up because of the absence of 

 cold upwelled water, anchovies move out 

 of the bay and into the gulf where they 

 continue spawning into October. Chinook 

 Salmon remain in front of the Golden 



Gate until October, but in lower and lower 

 concentrations as they move up the Sacra- 

 mento River system to spawn. The follow- 

 ing February, the next year's 3-year-old 

 chinook salmon begin to enter into the 

 gulf, and the cycle begins again. 



During strong El Nino years, the 

 normal sequences of chinook salmon prey 

 do not develop because the large increase 

 in ocean temperature disrupts the prey's 

 normal behavior. As a result, the aggrega- 

 tions of salmon that feed on these prey 

 do not form, and chinook salmon of a 

 given length weigh much less than normal. 

 California's commercial salmon catch also 

 drops severely, and the recreational catch 

 is far below average. 



42 Biology and Ecological Niches in the Gulf of the Farallones 



