Fraser River sockeye salmon: data synthesis and cumulative impacts

assumption that exposure occurs where there is spatial and temporal overlap of potential factorswith the post-smolts. As sockeye salmon pass through the Strait of Georgia, Queen CharlotteSound, and along the coast they are exposed to the physical and biological conditions of theseason.Strong evidence of exposure would require a much more precise knowledge of the spatial andtemporal patterns of both the sockeye salmon and each specific potential stressor, recognizingthat migration timing varies among stocks. The mere presence of a potential stressor does notnecessarily mean that exposure has occurred. For example, finding a particular contaminant atone sampling location in the Strait of Georgia does not mean that sockeye salmon were exposedto it. If an infection is detected in fish, that may implicate exposure to that pathogen, but overlapwith the pathogen alone may not. Furthermore, “exposure” in a general sense to potential sourcesmay not necessarily correspond with exposure to actual detrimental conditions. Johannes et al.(2011) show that farm area and total farm inputs (i.e. chemical fertilizers and insecticides) havebeen increasing around the Strait of Georgia, yet improved management practices have reducedrunoff from farms waste, which is the element that is most likely to directly affect sockeyesalmon.Exposure to many predators can only be assumed based on the likely overlap in space and timebecause knowledge of the distribution and diet of many predators is lacking. Christensen andTrites (2011) have searched the scientific literature for diet information for potential predatorsbut the data are often relatively sparse, old or only available for particular species. Physicalevidence that particular predators have consumed sockeye salmon post-smolts providesconvincing evidence of exposure, but a lack of such data does not support any conclusions aboutexposure - most sockeye that are eaten are simply never seen again and thus exposure cannot beconfirmed. The importance of potential predators is often based on knowledge about thecomposition of a predator’s diets and whether or not sockeye salmon is a preferred prey species.However, it is possible that a relatively abundant predator could have a substantial impact onsockeye, even if sockeye comprised only a small, possibly undetectable fraction of the predator’sdiet (Christensen and Trites, 2011). For example, if 0.1% of the diet of spiny dogfish weresockeye smolts, spiny dogfish would consume 14.5 million smolts within the Strait of Georgiaand yet over a thousand dogfishes might need to be sampled before finding one containingsockeye salmon smolts (Christensen and Trites, 2011, p. 77). To provide some frame ofreference, the average number of smolts leaving Chilko Lake from 1997-2005, prior toaccounting for any mortality while outmigrating to the ocean, was approximately 22.5 million.The key point here is that a predator could have a substantial impact on a prey species even ifthat prey is a negligible proportion of the predator’s diet. For example, the existing evidenceshows that juvenile salmon represents a very small proportion of the diet of Steller sea lions. The59