Dead fish swimming: a review of research on the early migration and high premature mortality in adult Fraser River sockeye salmon Oncorhynchus nerka

Abstract

Adult sockeye salmon Oncorhynchus nerka destined for the Fraser River, British Columbia are some of the most economically important populations but changes in the timing of their homeward migration have led to management challenges and conservation concerns. After a directed migration from the open ocean to the coast, this group historically would mill just off shore for 3-6 weeks prior to migrating up the Fraser River. This milling behaviour changed abruptly in 1995 and thereafter, decreasing to only a few days in some years (termed early migration), with dramatic consequences that have necessitated risk-averse management strategies. Early migrating fish consistently suffer extremely high mortality (exceeding 90% in some years) during freshwater migration and on spawning grounds prior to spawning. This synthesis examines multidisciplinary, collaborative research aimed at understanding what triggers early migration, why it results in high mortality, and how fisheries managers can utilize these scientific results. Tissue analyses from thousands of O. nerka captured along their migration trajectory from ocean to spawning grounds, including hundreds that were tracked with biotelemetry, have revealed that early migrants are more reproductively advanced and ill-prepared for osmoregulatory transition upon their entry into fresh water. Gene array profiles indicate that many early migrants are also immunocompromised and stressed, carrying a genomic profile consistent with a viral infection. The causes of these physiological changes are still under investigation. Early migration brings O. nerka into the river when it is 3-6° C warmer than historical norms, which for some late-run populations approaches or exceeds their critical maxima leading to the collapse of metabolic and cardiac scope, and mortality. As peak spawning dates have not changed, the surviving early migrants tend to mill in warm lakes near to spawning areas. These results in the accumulation of many more thermal units and longer exposures to freshwater diseases and parasites compared to fish that delay freshwater entry by milling in the cool ocean environment. Experiments have confirmed that thermally driven processes are a primary cause of mortality for early-entry migrants. The Fraser River late-run O. nerka early migration phenomenon illustrates the complex links that exist between salmonid physiology, behaviour and environment and the pivotal role that water temperature can have on population-specific migration survival.

Physiology, Behavior, and Conservation

“…Studies using biotelemetry to study survival, behavior (e.g., entry timing, migration speed), and physiology (e.g., tissue samples of telemetered animals, parallel laboratory cardiorespiratory studies) reveal that there are stock-specific thermal thresholds and clear relationships between impaired migration behavior (including failed migration and subsequent death) and physiological status Hinch et al 2012). The cardiorespiratory capacity of Pacific salmon is constrained such that fish are unable to migrate beyond a given stock-specific thermal threshold Eliason et al 2011).…”

Aerobic scope increases throughout an ecologically relevant temperature range in coho salmon

“…Within this paradigm of local adaptation, it is perhaps not surprising that the ever-increasing peak summer temperatures of the Fraser River, British Columbia, Canada, superimposed on normal annual variability, have caused high en route mortality in sockeye salmon in some years (Patterson et al, 2007;Farrell et al, 2008;Martins et al, 2011;Hinch et al, 2012). Consequently, understanding the underlying cause(s) of migration failure at high temperatures will be fundamental to efforts aimed at conserving the diversity of wild Pacific salmon.…”

“…Consequently, understanding the underlying cause(s) of migration failure at high temperatures will be fundamental to efforts aimed at conserving the diversity of wild Pacific salmon. Indeed, a progressive decline in AS above historic modal river temperatures has been proposed as one of several potential mechanisms for population-specific en route mortality at high temperatures Hinch et al, 2012). With climatic change occurring at an alarming rate and regarded as a key pressure acting upon Pacific salmon conservation and management (Patterson et al, 2007;Crozier et al, 2008;Bryant, 2009;Martins et al, 2011;Reed et al, 2011), AS has emerged as a potentially useful functional measurement with which to make predictions about the thermal niche of aquatic ectotherms and hence potential fitness outcomes under future warming scenarios.…”

Productivity and life history of sockeye salmon in relation to competition with pink and sockeye salmon in the North Pacific Ocean

“…Initial scientific analyses indicated the long-term decline in abundance was likely associated with a decline in productivity at sea rather than low parental spawner abundances or other factors in fresh water. Factors in fresh water are known to adversely impact Fraser sockeye salmon (e.g., Hinch et al 2012), but the observed long-term decline in productivity could not be explained by freshwater processes alone Connors et al 2012). Recently, a number of investigations have explored early marine factors associated with the unexpectedly small sockeye salmon return in 2009 (Rensel et al 2010;Miller et al 2011;Beamish et al 2012; Thomson et al 2012;McKinnell et al 2014).…”