Since 1995, high levels of sockeye salmon mortality have been observed in the Fraser River. Salmon have been found dead upriver, before they reach their spawning grounds, and at their spawning grounds before they have the chance to spawn. These phenomena were associated with the unusually early migration of the salmon. Late-run sockeye arrive at the mouth of the Fraser River at the same time as they have historically, but they are not “holding” in the near-shore marine environment as they used to do. In fact, in 2000 and 2002, most of the late-run stocks entered the river at the same time as the summer-run stocks. Two studies (funded by the Pacific Salmon Commission and the Department of Fisheries and Ocean) conducted in 2002 showed that 96% of late-run sockeye that reached their spawning grounds weeks earlier (in mid August) than the normal time had died. The late-run sockeye that migrated at the normal time (in mid September) had only a 10% mortality rate.
The high mortality rate associated with the early entry of the late-run sockeye salmon is an important management issue. The reason why these fish are entering the Fraser River early and experiencing such high levels of premature mortality are still unclear. Possible causes that have been postulated include problems with energy status, osmoregulatory dysfunction, endocrine disturbance, environmental variation, parasites and disease, contaminants and the threat of marine predation. The potential mechanisms associated with some of these factors are briefly outlined below.
Energy status - Salmon do not feed after entering the river and must complete their upstream migration/spawning with the energy reserves acquired in the ocean. Late-run sockeye do not generally feed during their “holding” period in the Strait of Georgia (prior to migrating upriver) so they have limited energy/time to complete their migration and to spawn. It has been speculated that unusually low energy reserves could have triggered late-run sockeye to initiate freshwater migrations earlier than usual. This early river entry can result in late-run sockeye encountering high river flows similar to flows encountered during early summer migrations in the Fraser River. Fast water flows elevate active metabolism in fish resulting in large energy drains. When these late-run salmon start to enter the river earlier than normal it causes them to experience flow conditions to which can be arduous. Once the late-run salmon (with already limited energy reserves) initiate early migration and encounter more difficult river conditions than they are used to, they could run out of energy before reaching their spawning grounds or reproducing. The excessive energy depletion during migration or spawning may contribute to premature death in the late-run sockeye. Interestingly, our research group has found evidence contrary to this prediction. Specifically, we have revealed that the fish which migrate early actually have higher energy reserves at time of ocean entry than those that hold in the Ocean.
Osmoregulatory status - It is critical for salmon to adjust their osmoregulatory systems before moving from the sea to freshwater. They must remain in osmotic and ionic balance at all times. An uncoupling of the endocrine signals that prepares adults to migrate upstream may cause early freshwater entry. Interestingly, our research has revealed that the fish which migrate early have more variable gill enzyme levels than those that migrate at normal times. We have postulated that these early migrants are encountering patches of freshwater in the ocean that lead to remature downregulation of gill enzyme activity forcing fish to enter prematurely.
Endocrine system - The reproductive hormone, gonadotropin-releasing hormone (GnRH), is critical in gonad maturation, spawning behaviours during salmon migration and migratory timing. Disruption to the release of this hormone in the normal cycle, from the exposure to waterborne toxicants or other stressors, could impact the timing of freshwater entry and premature aging in salmon. It is also possible that reproductive hormone titres may be differentially expressed in normal vs early migrants. In fact, our research has revealed that early migrants tend to have elevated levels of hormones indicating advanced reproductive development and potentially premature senescence.
In River Condition - Changes in the Fraser River discharge/temperature can impact migration timing in Fraser sockeye as well as mortality. In 1997 and 1999, high velocity regions in the Fraser Canyon became obstructions to sockeye passage. Deaths in the lower river were partially due to excessive exhaustion. High discharge coinciding with high fish densities can create migration bottlenecks in regions of high flow. It is also possible that the oxygen concentration in the local region can be depleting due to high densities. This could also affect swimming performance and migration rates. Suspended sediment concentrations increase in the Fraser River during periods of high discharge causing physical lesions, disorientation and death (in high concentrations). Water temperature has a commanding effect on the biology of fish and high (either acute or chronic) temperatures have been implicated to mortality. Our research has revealed that water temperature may indeed be one of the most important factors in determining the migration success of individual fish.
Parasites and Disease - Parasites and disease may affect the migratory behaviour and premature mortality in migrating salmon. A myxozoan parasite of the kidney may influence cues of the Fraser River sockeye to migrate upriver in an attempt to escape the parasitic infection in the coastal zones. The transmission of parasites in salmon occurs when they migrate through the estuary and into the lower river. The parasites have been identified to cause lesions within kidney glomeruli, which contribute to the en route mortality. The traditional holding behaviour displayed by late-run sockeye may be a tactic to avoid premature infection from this parasite. Warm water temperatures along the spawning routes for many late-run stocks may lead to rapid development of infection from the parasite. Delaying migration until water temperatures cool in the fall may limit the severity of the disease. Similarly, a change in the “zone of transmission” or in parasite development rates may contribute to early migration. Parasitic infection is typically severe by the time salmon reach their spawning grounds.
The mechanisms responsible for these patterns of early migration/high mortality discussed in here are complex and likely represent the interaction of numerous stressors. This problem requires further research through strategic experimentation and continual monitoring of the physiological condition/performance of a set of representative stocks in different regions. That effort could help to identify management steps that could ultimately save millions of dollars in lost harvest opportunities while ensuring the perpetuation of the many diverse sockeye salmon populations. In 2006, our group will be involved in a comprehensive research project funded by the Pacific Salmon Commission that will bring together researchers from academia (UBC, Carleton), government (DFO, PSC), and industry (Kintama, LGL). The goal will be to understand the factors responsible for early migration and to further understand the consequences of early migration. Carleton grad students Kyle Hanson and Mike Donaldson will be working on this project.
To see more on this study, click on pdf.