No abstract

The effects of climate events on the feeding ecology and trophic dynamics of Pacific salmon (Oncorhynchus spp.) in offshore waters of the central Gulf of Alaska were investigated during early summers (1994)(1995)(1996)(1997)(1998)(1999)(2000), based on analyses of stomach contents, and carbon and nitrogen stable isotopes (d 13 C and d 15 N). Gonatid squids (mainly Berryteuthis anonychus) were the dominant prey of all salmon species except for chum salmon (O. keta). During the 1997 El Niñ o event and the 1999 La Niñ a event, squids decreased sharply in the diets of all Pacific salmon except coho salmon (O. kisutch) in the Subarctic Current, and chum salmon diets changed from gelatinous zooplankton (1995-97) to a more diverse array of zooplankton species. A d 13 C and d 15 N analysis indicated that all salmon species occupied the same branch of the food web in 1999-2000. We hypothesize that high-seas salmon adapt to climate-induced changes in their prey resources by switching their diets either within or between trophic levels. To understand the effects of climate change on Pacific salmon in the Gulf of Alaska, biological oceanographic research on B. anonychus and other important prey resources is needed.

The examination of population‐specific adaptations of introduced salmonids to the wide range of environments found in Patagonia (southern South America) can help unveil some of the genetic and environmental contributions to life history variation. The rainbow trout Oncorhynchus mykiss introduced into Argentina originated from a few parental stocks. Although some of these stocks were anadromous, all of the dozens of established populations described until now have been freshwater resident. In this paper we provide the first documentation of the presence of an anadromous run of rainbow trout in the Santa Cruz River, the second largest river of Argentinean Patagonia. Microsatellite analysis revealed that anadromous and resident fish from the Santa Cruz River are genetically indistinguishable, probably representing alternative life histories within the same population. Both wild types are very different from the fish of Danish origin that were reared in a local hatchery, suggesting that they are descended more directly from California stocks or that they have been affected by strong drift or selection. Marine growth and freshwater residence are comparable to those of California steelhead. River entrance peaks in early fall. Population age structure and scale pattern analysis indicate that fish enter the river at age 3 but that most do not spawn until their next river entry as 4‐year‐olds. An unusual aspect of Santa Cruz anadromous fish is that they are long‐lived and highly iteroparous. For instance, 20% of the fish analyzed had experienced as many as five spawning events.

Eight temperature‐recording data storage tags were recovered from three salmonids in Alaska (pink and coho salmon and steelhead trout) and five chum salmon in Japan after 21–117 days, containing the first long‐term records of ambient temperature from Pacific salmonids migrating at sea. Temperature data imply diel patterns of descents to deeper, cooler water and ascents to the surface. Fish were found at higher average temperatures at night, with narrower temperature ranges and fewer descents than during the day. Fish tagged in the Gulf of Alaska were at higher temperatures on average (10–12°C) than chum salmon tagged in the Bering Sea (8–10°C). Chum salmon were also found at a wider range of temperatures (−1–22°C vs 5–15°C). This is probably related both to the different oceanographic regions through which the fish migrated, as well as species differences in thermal range and vertical movements. Proportions of time that individual fish spent at different temperatures seemed to vary among oceanographic regions. Steelhead trout may descend to moderate depths (50 m) and not be limited to the top few metres, as had been believed. Japanese chum salmon may seek deep, cold waters as they encounter warm surface temperatures on their homeward migrations. Temperature data from all fish showed an initial period (4–21 days) of day and night temperatures near those of sea surface temperatures, suggesting a period of recuperation from tagging trauma. A period of tagging recuperation suggests that vertical movement data from short‐term ultrasonic telemetry studies may not represent normal behaviour of fish. The considerable diurnal and shorter‐term variation in ambient temperatures suggests that offshore ocean distribution may be linked more to prey distribution and foraging than to sea surface temperatures.