Salmon population levels are of concern in the Atlantic and in some parts of the Pacific. Salmon are typically anadromous - they rear and grow in freshwater, migrate to the ocean to reach sexual maturity, and then return to freshwater to spawn. Determining how environmental stressors and climate change will affect these fisheries is challenging due to their lives split between fresh and saltwater. Environmental variables like warming temperatures and habitat loss are detrimental to salmon abundance and survival. Other human influenced effects on salmon like overfishing and gillnets, sea lice from farm raised salmon, and competition from hatchery released salmon have negative effects as well.
Environmental pressures
The population of wild salmon declined markedly in recent decades, especially North Atlantic populations which spawn in the waters of western Europe and eastern Canada, and wild salmon in the Snake and Columbia River system in the Pacific Northwest. The decline is attributed to the following factors:
Marine variables
Sea lice The transfer of parasites from open-net cage salmon farming, especially sea lice, has reduced numbers of wild salmon. The European Commission (2002) concluded, "The reduction of wild salmonid abundance is also linked to other factors but there is more and more scientific evidence establishing a direct link between the number of lice-infested wild fish and the presence of cages in the same estuary.",Scientific Evidence of Sea Lice from Fishfarms Seriously Harming Wild Stocks. saveourskeenasalmon.org however it is impossible to verify this statement from the reference provided. It is reported that wild salmon on the west coast of Canada are being driven to extinction by sea lice from nearby salmon farms. Dr Krkosek and his research group found, with their electronic modeling predictions, two of the best runs ever recorded for wild salmon instead of the expected collapsed salmon fishery. This raises real questions over the validity of the models and the dire predictions. For Atlantic salmon smolts, it takes as few as eight sea lice to kill the fish. On the Pacific Coast where the smolts are much smaller, only one or two can be critical, often resulting in death. In the Atlantic, sea lice have been a proven factor in both Norwegian and Scottish salmon declines. In the Western Atlantic, there has been little research at sea, but sea lice numbers in the period after 2000 do not appear to be a significant factor in the critical decline of endangered inner Bay of Fundy salmon. The situation may have been different in the 1980s and 1990s, but we are unlikely ever to know the factual history in that regard.
Overfishing and gillnets In general, overfishing has reduced populations, especially commercial netting in the Faroes and Greenland. Several seafood sustainability guides have recommendations on which salmon fisheries are sustainable and which have negative impacts on salmon populations
There are many methods of harvest for the commercial salmon fishing industry, such as trolling, seining, and gillnetting. Gillnets are an extremely size-selective method of harvest, where a long net is placed in the path of the salmon's migration to their natal stream in hopes of entangling the salmon for commercial harvest. Fish too small to be caught pass through the net, fish too large cannot be entangled, only catching fish that fall somewhere in between. By selectively harvesting certain sizes of fish, governed by the mesh-size of the gillnet, some age-class and length-class fish are selectively removed from the population, progressively leaving phenotypically smaller spawners. Fecundity generally decreases with length. So, smaller fish produce fewer eggs than larger fish. There is also concern regarding the genetic information passed down from the fish. If the majority of spawning fish in a particular salmon run has gotten smaller due to the size-selective fishing methods, the run could eventually evolve to become smaller.
Gillnets are designed to harvest a specific sized fish. For example, Washington Department of Fish and Wildlife's 2010 Commercial Regulations had a minimum mesh size restriction for Chinook, and a minimum – maximum for sockeye.Washington Department of Fish and Wildlife. (2010) Puget Sound Commercial Salmon Regulations. Possible problems arising from this selective harvest are smaller reproducing adult fish, as well as the unexpected mortality of the fish which sustain injuries from the gillnet but are not retained in the fishery. Most salmon populations include several age classes, allowing for fish of different ages, and sizes, to reproduce with each other. A recent 2009 study looked at 59 years of catch and escapement data of Bristol Bay sockeye to determine age and size at maturity trends attributable to the selectivity of commercial gillnet harvests. The study found that the larger females (>550 mm) of all age classes were most susceptible to harvest. The study suggests that smaller, younger fish were more likely to successfully traverse the gillnet fishery and reproduce than the larger fish. The study also found that the average length of sockeye harvested from 1946 to 2005 was 8 mm larger than the sockeye who escaped the gillnet fishery to spawn, reducing the fecundity of the average female by 5%, or 104 eggs.
If a salmon enters a gillnet, but manages to escape, it often sustains injuries. These injuries can lead to a lower degree of reproductive success. A study aimed at quantifying mortality of Bristol Bay sockeye salmon due to gillnet-related injuries found that 11 – 29% of sockeye sustained fishery-related injuries attributable to gillnets, 51% of those fish were expected to not reproduce.
Competition at sea Competition between juvenile salmon entering the ocean, other wild salmon, and hatchery-released salmon in the ocean is a major factor that determines survival rates. Hatchery production has increased since 1970, and there is high spatial and trophic, dietary overlap between wild and hatchery sockeye, pink and chum salmon in the North Pacific. With more competition for prey and space in the ocean, the period of maturation in saltwater before salmon reach sexual spawning age grows. Longer times spent in the ocean causes a decline in salmon survival and abundance. Also, as the human population grows, the demand for seafood increases. With more commercial fisheries, it is difficult to maintain sustainable fishing levels.
Freshwater variables
Warming temperatures An increase in fresh water temperature can delay spawning and accelerate the transition to smolting. Warmer temperatures of streams during spawning and incubation have negative effects on salmon productivity due to pre-spawn mortality, reduced egg survival, and temporal changes during salmon embryo development. However, the effects of higher stream temperatures during juvenile rearing vary across populations and habitats. For some watersheds as temperatures increase, an increase in turbidity and reduction in invertebrate food availability is found to cause a reduction in growth rates of juvenile salmon. For other watersheds, productivity increases with more nutrients and food availability for juvenile salmon. The salmon growth rate and maturation rate increase causing an age at maturation increase. Salmon are then able to migrate to the ocean earlier.
Ulcerative dermal necrosis
Ulcerative dermal necrosis (UDN) infections of the 1970s and 1980s severely affected adult salmon in freshwater rivers.
Habitat The loss of suitable freshwater habitat, especially degradation of stream pools and reduction of suitable material for the excavation of redds, has caused a reduction in spawning. Historically, stream pools were largely created by beavers . With their extirpation, the nurturing function of these ponds was lost. Reduced retention of the nutrients brought by the returning adult salmon in stream pools has lowered population numbers. Without stream pools, dead adult salmon tend to be washed straight back down the streams and rivers, so the nutrients are not available for the hatchlings.
The construction of dams, weirs, barriers and other "food prevention" measures bring severe adverse impacts to river habitat and on the accessibility of those habitats to salmon, particularly in the Pacific Northwest, where large numbers of dams have been built in many river systems, including over 400 in the Columbia River Basin. Other environmental factors, such as light intensity, water flow, or change in temperature, dramatically affects salmon during their migration season. Modern farming methods and various sources of pollution have resulted in loss of invertebrate diversity and population density in rivers, thus reducing food availability. Reduction in freshwater base flow in rivers and disruption of seasonal flows, because of diversions and extractions, hydroelectric power generation, irrigation schemes, barge transportation, and slackwater reservoirs, inhibit normal migratory processes and increase predation for salmon.Pacific States Marine Fisheries Commission (1997) "When Salmon Are Dammed". Agricultural practices, such as the removal of riparian plants, destabilization of stream banks by livestock and irrigation processes, result in a loss of suitable low-gradient stream habitats.Orr, Raymond I. (June 18, 2003) "Northwest Salmon Make Legal Headway". Indian Country Today
Impact of climate change Climate change is affecting different types of salmon in ways from short term affects such as a decrease in population to long term affects such as evolution change. The change in temperature has left salmon most vulnerable during their egg stage, and the becoming adult stage of its life cycle (9) Climate change has caused salmon to spend less time in their natural freshwater habitat (10) (1). This type of stressor can affect the age class of salmon because it causes them to migrate at an early stage, which would cause them to delay maturation making them spend an additional year feeding in the ocean. Also, it can affect the availability for food which creates a chain reaction to their freshwater growth (1)(8).
The immediate affects consist of thermal requirements which is the critical thermal maximum for survival, and population crashes. When the water temperature increases the thermal requirements for the three species (Atlantic salmon, brown trout, and Arctic charr) are a necessity because if any other problems occur it can be predicted/anticipated by those of the biodiversity in the freshwater ecosystems (2). This can cause a population crash since the increased temperature will affect the eggs and may also bring in more diseases and illnesses towards the species (6). Although, the species may be able to adapt to the rising temperatures (7). These species of fish help support and provide for important fisheries in Western Europe, so, according to a scholarly journal their research finding say that salmon populations changes from environmental issues has a massive affect many other populations in distant habitats (2)(3). The long term affects from climate change produce more selection pressures in different parts of salmon's life such as Juvenile growth, development rates, thermal tolerance, and disease resistance. Which shows that because of increased anthropogenic activity, or human-caused, formed these additional stressors on Salmons stages of life (4)(5).
North America