Prevalence of Marine-Mammal Tooth and Claw Abrasions on Adult Anadromous Salmonids Returning to the Snake River

Abstract

During 1990, a relatively high percentage of adult anadromous salmonids returning to the Snake River had marine‐mammal tooth and claw abrasions. From 1990 through 1993, annual incidence of these injuries ranged between 14.0 and 19.2% for spring–summer chinook salmon Oncorhynchus tshawytscha and between 5.4 and 14.2% for steelhead O. mykiss. Open wounds were observed on about one‐third of the fish with abrasions. Although these results suggest that predation by pinnipeds may be an important mortality factor for Snake River spring–summer chinook salmon, further research is needed to accurately define the magnitude of the predation.

Regurgitation Rates of Intragastric Radio Transmitters by Adult Chinook Salmon and Steelhead during Upstream Migration in the Columbia and Snake Rivers

“…Transmitter retention was evaluated only for Chinook salmon and steelhead that reached Lower Granite Dam on their way to upriver spawning areas or hatcheries. A detection system near the top of the Lower Granite fish ladder automatically diverts about 90% of fish with CWTs to an adult collection facility (Durkin et al 1969;Harmon et al 1994). More than 91% of the radio-tagged fish that passed Lower Granite Dam in the 4 years of this study were first diverted into the collection facility.…”

Stress Indices in Migrating Juvenile Chinook Salmon and Steelhead of Wild and Hatchery Origin before and after Barge Transportation

“…Factors believed to contribute to the decline of Snake River chinook salmon and steelhead runs include mortality of emigrating juvenile salmonids in the hydroelectric system on the Snake and Columbia rivers (Raymond 1979(Raymond , 1988, predation on juveniles and adults in the estuary by growing populations of avian (C. B. Schreck and L. E. Davis, Oregon State University, personal communication; D. Roby, Oregon State University, personal communication) and mammalian (Harmon et al 1994) predators, and decreased marine survival resulting from cyclic changes in weather and oceanic conditions (Mantua et al 1997). To reduce exposure of migrating juvenile salmonids to the hydroelectric system, the U.S. Army Corps of Engineers collects between 11 and 21 million juveniles annually (Hetherman et al 1998) at four dams on the Snake and Columbia rivers (Figure 1), transports them around the remaining downstream dams in trucks or barges, and releases them 235 km upstream from the mouth of the Columbia River.…”

Pattern and prevalence of predator damage on adult Atlantic salmon, Salmo salar L., returning to a river system in north‐east Scotland

“…Ignoring such minor scale loss, overall levels of damage to salmon from the Conon system were within the range reported from surveys of salmonids caught in other river systems. For example, in both North American (Beach, Geiger, Jeffries, Treacy & Troutman 1985; Harmon et al . 1994) and Scottish (Rae & Shearer 1965; Parrish & Shearer 1977) rivers, levels of damage varied between < 5% and 25% depending upon year, season and the species of salmonid under study.…”

“…In the past, such damage has typically been attributed to seals. For example, Harmon et al . (1994) stated that the size of wounds and scratches on salmonids from the Snake River indicated that damage was inflicted by harbour seals.…”

“…In particular, there is uncertainty over the relative impact of the various fish, bird and mammalian predators which are known to take salmonid prey. In the absence of quantitative data on salmon mortality rates, levels of skin damage observed on returning fish have sometimes been used as an index of predation (Parrish & Shearer 1977; McDonald 1991; Harmon, Thomas, McIntyre & Paasch 1994). Furthermore, such damage to fish in coastal fisheries has invariably been attributed to seals.…”