Thanks Bugs for getting me the report, It seems that zooplankton prey available in different marine areas during the migration of sockeye play a huge roll of where they go missing.
Here is some of the discussion from the PSF paper on
Quantifying Survival of Age Two Chilko Lake Sockeye Salmon
during the First 50 Days of Migration
"Several factors could differentially affect early marine survival in the CSOG and NEVI
regions. The NEVI area includes the northern-most 1/5th of the Strait of Georgia, and continues
north to encompass the Discovery Islands, Johnstone Strait, the Broughton Archipelago, and
Queen Charlotte Strait, and it is vastly more complex than the CSOG. The area north of
Johnstone Strait is world-renowned for its rich underwater biodiversity (Britnell 2010), and
offers whale watching and other eco-tourism opportunities (Destination BC 2017). The
Johnstone Strait, however, has little primary production (and thus zooplankton prey) because
wind and currents keep it well mixed to depths well below the photic zone (Thomson 1981;
McKinnell et al. 2014), and this leads to decreased juvenile salmon growth rates (Journey et al.
2018). Following the 2009 Fraser River sockeye crash, a trophic gauntlet hypothesis was put
forth by McKinnell et al. (2014) which describes the extreme ocean and climate events occurring
in this region and Queen Charlotte Sound that may have led to poor survival of juvenile sockeye
in 2007, two years prior to the adults’ return. Extreme environmental conditions could lead to
either decreased growth and size-based selection by predators as a result, or outright starvation if
continued for long enough. For instance, (Tucker et al. 2016) observed that Cassin’s auklets
preferentially preyed on smaller salmon in poor condition in southern Queen Charlotte Sound,
area directly 610 north of our study site. The SOG, on the other hand, is one of the most
productive inland seas. Nutrient input and spring phytoplankton bloom timing means that there is
an abundant prey resource pool for migrating juvenile salmon (Harrison and Mackas 2014), and
growth rates are higher (Journey et al. 2018). In the Discovery Islands area between the SOG and
Johnstone Strait, there appears to be an abundant food supply in some years (Price et al. 2013),
but not in others (Neville et al. 2016); McKinnell et al. (2014) discuss the potential production
mechanisms associated with this transition zone
Predation by marine mammals, particularly pinnipeds, has gained more attention as more
studies reveal the preferred diets that these animals consume. The harbour seal (Phoca vitulina
richardsi) population in British Columbia, and particularly the SOG, has rebounded to historic
levels since the species was protected in 1970 (DFO 2009) and there is evidence that they feed
on salmon species of conservation concern, including sockeye (Thomas et al. 2016). ()Even if
juvenile salmon comprise only a small proportion of the total diet, this results in large numbers
of fish (Thomas et al. 2016; Howard et al. 2013; Chasco et al. 2017). As the NEVI region
includes the northern-most area of the SOG, the lower survival we estimated for NEVI could be
partly attributed to fish becoming be more vulnerable to predation as they are concentrated in the
northern SOG and narrower waterways of the Discovery Islands where there are numerous seal
haul outs (DFO 2009; Yurk and Trites 2000)().
Finally, the NEVI area, unlike the SOG, has numerous open net-pen salmon farms, and has
been fraught with controversy regarding the possible effect on wild salmon (Young and
Matthews 2010). The potential for interaction between wild and farmed salmon was highlighted
in a 2012 federal inquiry into the decline of sockeye salmon (Cohen 2012). The inquiry called on
the Department of Fisheries and Oceans Canada (DFO) to enforce stricter regulations and
recommended prohibiting salmon farmsin the Discovery Islands region if DFO cannot
confidently say “the risk of serious harm [to wild salmon] is minimal” (Cohen 2012). For
example, farmed salmon may transmit sea lice to migrating wild salmon, possibly reducing
foraging success and growth rates of wild salmon (Godwin et al. 2015; Godwin et al. 2017).
There are also a host of viral and bacterial pathogens associated with farmed salmon which may
be potentially harmful to wild salmon (Johansen et al. 2011) although the transmission of disease
has been poorly documented. Travel times reported here indicate that juvenile salmon migrate
quickly through this area, but the risk of serious harm is largely unknown, emphasizing the
importance of evaluating the effect of salmon farm exposure times on wild salmon survival."
