question: Why do salmon fish die after spawning ?
comment: The safest possible place to lay their eggs is upstream, far from any big sea or river predators. That's how they give their young the best chance of survival. But it takes an enormous amount of stamina to get upstream, and there's no food up there. So once they've laid their eggs, without the energy to make the return trip and feed, there's no way to survive. It's a necessary sacrifice for the next generation.
passages: Pre-spawn mortality has been observed in other species of fish, including Chinook and sockeye salmon and steelhead. The cause of pre-spawn mortality in these fish in the Fraser River (sockeye salmon) and Klamath River (Chinook salmon and steelhead trout) differ than that observed in the coho salmon. The cause of pre-spawn mortality for the sockeye salmon in the Fraser River, BC, and Bristol Bay, Alaska, can be attributed to factors such as fish returning to spawn too early, warmer water temperatures, low dissolved oxygen and disease or parasites. In the Klamath River, mortality before spawning was due to disease, thermal stress, a low water flow and increasing numbers of salmon migrating within the same time period.
   Some species of fish exhibit mass simultaneous mortality as part of their natural life cycle.
  
  Fish kill due to spawning fatalities can occur when fish are exhausted from spawning activities such as courtship, nest building, and the release of eggs or milt (sperm). Fish are generally weaker after spawning and are less resilient than usual to smaller changes in the environment. Examples include the Atlantic salmon and the Sockeye salmon where many of the females routinely die immediately after spawning.
  
  Section::::Causes.:Water temperature.
   There are a number of possibilities as to the cause of pre-spawn mortality. It is possible that this mortality is due to acute cardiorespiratory toxicity to the fish. The symptoms (discussed above) and rapid rate of mortality are consistent with this type of toxicity. The exact chemical combination or mixture responsible for this early mortality is not known. However, a few possible explanations are discussed below.
  
  Section::::Effects of runoff.:Narcotic effects.
   Upon displaying specific symptoms (below), mortality of the coho salmon can be expected within a few hours. Mortality usually occurs before these fish have had a chance to spawn. In fact, in observing the female coho salmon carcasses, most of the fish have retained greater than 90 percent of their eggs. Pre-spawn mortality occurs in both male and female salmon, however, it is difficult to determine if male salmon have spawned, thus, pre-spawn mortality is usually signified by egg retention of female salmon. Pre-spawn mortality has also been observed in both wild and hatchery fish alike.
  
  Section::::Symptoms.
   Pre-spawn mortality is a phenomenon where adult coho salmon, "Oncorhynchus kisutch", die before spawning when returning to freshwater streams to spawn. This occurrence has been observed in much of the Puget Sound region of the Pacific Northwest. During fall migration, salmonids (trout and salmon) pass through urban watersheds which are contaminated with stormwater runoff. As the coho salmon pass through these waters, many will show symptoms of lethargy, loss of equilibrium and disorientation, and die within a few hours of showing these symptoms. These symptoms and behaviors are prevalent after rain events. Mortality often occurs before salmon have the opportunity to spawn, which is determined by cutting open female carcasses and observing for unfertilized eggs. Rates of pre-spawn mortality could impact the local salmon populations. Based on model projections, if rates continue, populations of coho salmon could become extinct within the next few decades.
   This symptom consists of the salmon repeatedly opening and closing its mouth. This action in some species of fish (e.g. eels) is used to push water over the gills to meet respiratory demands. For coho salmon, this could be an indication that the fish is not getting enough dissolved oxygen, signifying stress to the respiratory system or that the muscular system has been affected and this gaping is a forced action.
  
  Section::::Symptoms.:Fin splaying.
   Additional combinations of effects may result from contaminants and pathogens or contaminants and physiological effects. As anadromous fish migrate from the ocean to freshwater streams, many changes take place in the osmoregulatory and ionregulatory systems due to the changes in composition between freshwater and saltwater. These changes in combination with chemical pollutants may cause the observed symptoms and death of these coho salmon. However, these specific symptoms and resulting pre-spawn mortality are not observed in other anadromous fish, thus, the coho salmon may be a more sensitive species than other salmonids.
  
  Section::::Effects of runoff.:Metals.
   Narcosis is a phenomenon that occurs when a combination of organic toxicants, each at low concentrations, have a toxic effect on an organism. Narcosis has a nonspecific mode of action, where it does not target one specific site of action, but instead affects the membranes and protoplasm. These individual organic toxicants affect the fish at sublethal levels, however, the combination of contaminants can lead to mortality of the organism. However, if the stressor is removed narcotic effects can be reversed. Fish influenced by narcosis may display lethargic symptoms. Narcosis can also result in respiratory-cardiovascular responses to fish.
  
  Section::::Effects of runoff.:Mixtures.
   Section::::History.
  
  Pre-spawn mortality was first observed in the Puget Sound after efforts had gone into restoring urban watersheds in hopes of drawing salmon back to these areas. Restoration consisted of removing culverts, debris, and any other barriers. Restoration projects were successful in that salmon returned to these runs, however, in initial surveys of restoration success, the pre-spawn mortality symptoms were observed. It is now recognized that in restoring watersheds, water quality and contamination must also be considered for successful spawning events of coho salmon if the population is to persist.
   The term "prespawn mortality" is used to refer to fish that arrive successfully at the spawning grounds, and then die without spawning. Prespawn mortality is surprisingly variable, with one study observing rates between 3% and 90%. Factors that contribute to these mortalities include high temperatures, high river discharge rates, and parasites and diseases. However, "at present there are no reliable indicators to predict whether an individual arriving at a spawning area will in fact survive to spawn."
   Prior to spawning, depending on the species, salmon undergo changes. They may grow a hump, develop canine-like teeth, or develop a kype (a pronounced curvature of the jaws in male salmon). All change from the silvery blue of a fresh-run fish from the sea to a darker colour. Salmon can make amazing journeys, sometimes moving hundreds of miles upstream against strong currents and rapids to reproduce. Chinook and sockeye salmon from central Idaho, for example, travel over and climb nearly from the Pacific Ocean as they return to spawn. Condition tends to deteriorate the longer the fish remain in fresh water, and they then deteriorate further after they spawn, when they are known as kelts. In all species of Pacific salmon, the mature individuals die within a few days or weeks of spawning, a trait known as semelparity. Between 2 and 4% of Atlantic salmon kelts survive to spawn again, all females. However, even in those species of salmon that may survive to spawn more than once (iteroparity), postspawning mortality is quite high (perhaps as high as 40 to 50%).
   The run up the river can be exhausting, sometimes requiring the salmon to battle hundreds of miles upstream against strong currents and rapids. They cease feeding during the run. Chinook and sockeye salmon from central Idaho must travel and climb nearly before they are ready to spawn. Salmon deaths that occur on the upriver journey are referred to as "en route mortality".
   Salmon of pre-spawn mortality display unusual behaviors and symptoms, such as swimming in circles or exhibiting other erratic swimming patterns. Other symptoms include lethargy, disorientation, loss of equilibrium, gaping, and fin splaying. Death of the coho salmon occurs within hours after observing these symptoms.
  
  Section::::Symptoms.:Fish behavior and swimming patterns.
   Fin splaying occurs when the pectoral fin of the salmon is rigid and extended perpendicular to the body of the fish instead of laying flat against the fish. In other fish, e.g. kelp bass, this action is a startle response.
  
  Section::::Nonpoint source pollution.
  
  Section::::Nonpoint source pollution.:Urban and stormwater runoff.
  
  Many of the contaminants that are responsible for degrading urban watersheds come from nonpoint source pollution. This consists of a wide array of contaminants coming from a variety of sources and locations.
   Section::::Spawning.
   Section::::Spawning.
   Coho salmon that are affected by pre-spawn mortality show behavioral symptoms that are easily identified. The fish displayed behaviors like swimming near the surface of the water, swimming in circular patterns, or consistently running into the river bank. This may be due to a loss of equilibrium and orientation of the fish.
  
  Section::::Symptoms.:Gaping.
   Coho salmon ("Oncorhynchus kisutch") increase in weight was found to be more than 60% after four generations of selective breeding. In Chile, Neira et al. (2006) conducted experiments on early spawning dates in coho salmon. After selectively breeding the fish for four generations, spawning dates were 13–15 days earlier.
  
  Cyprinids
   The steelhead is especially susceptible to enteric redmouth disease. Considerable research has been conducted on redmouth disease, as its implications for steelhead farmers are significant. The disease does not affect humans.
  
  Section::::Species.:Coho salmon.
  
  The Coho salmon is the state animal of Chiba, Japan.
  
  Coho salmon mature after only one year in the sea, so two separate broodstocks (spawners) are needed, alternating each year. Broodfish are selected from the salmon in the seasites and "transferred to freshwater tanks for maturation and spawning".
   there was a record salmon returning to spawn in 2001 (from the juveniles in 2000) compared with a 97% collapse in 2002 (from the juveniles in 2001).
  
  The authors also note that initial studies had not investigated bacterial and viral causes for the event despite reports of bleeding at the base of the fins, a symptom often associated with infections, but not with sea lice exposure under laboratory conditions.
   Juvenile salmon: The Most Crucial Stage of the Life Cycle
   Highly elevated cortisol levels mediate the post-spawning death of semelparous "Oncorhynchus" Pacific salmon by causing tissue degeneration, suppressing the immune system, and impairing various homeostatic mechanisms. After swimming for such a long distance, salmon expend all of their energy on reproduction. One of the key factors in salmon rapid senescence is that these fish do not feed during reproduction so body weight is extremely reduced. In addition to physiological degradation, pacific salmon become more lethargic as mating goes on, which makes some individuals more susceptible to predation because they have less energy to avoid predators. This also increases mortality rates of adults post-mating.
   Section::::Reproduction.
  
  Although the species is widely regarded as sexually monomorphic, males have been suggested to grow more quickly, and in some naturally occurring strains, males are noted to possess dark blotches on the base of their dorsal fins. The species reaches sexual maturity around one year of age, and continues to reproduce for 9–10 years. Frequency and timing of spawning may be related to the occurrence of rain. "A. ocellatus" fish are biparental substrate spawners, though detailed information regarding their reproduction in the wild is scarce.
   AquAdvantage salmon have the potential to feed more efficiently than wild-type salmon. This leads to an accelerated growth rate during their first year after hatching. These fish have the capability to grow 11 times faster than wild-type salmon. This characteristic allows GM salmon to mature more rapidly and gives them the ability to reproduce in less than two years (about 700 days). However, studies suggest this accelerated maturity of GM salmon does not provide a reproduction advantage over wild-type.
  
  Section::::Concerns.:Smoltification.
   Salmon start the run in peak condition, the culmination of years of development in the ocean. They need high swimming and leaping abilities to battle the rapids and other obstacles the river may present, and they need a full sexual development to ensure a successful spawn at the end of the run. All their energy goes into the physical rigours of the journey and the dramatic morphological transformations they must still complete before they are ready for the spawning events ahead.
  