In the
Pacific Northwest and
Alaska, salmon are
keystone species. and are estimated to discard up to half the salmon they've harvested uneaten on the forest floor,
Beavers Beavers also function as ecosystem engineers; in the process of tree-cutting and
damming, beavers alter the local ecosystems extensively. Beaver ponds can provide critical habitat for
juvenile salmon. An example of this was seen in the years following 1818 in the
Columbia River Basin. In 1818, the British government made an agreement with the U.S. government to allow U.S. citizens access to the Columbia catchment (see
Treaty of 1818). At the time, the
Hudson's Bay Company sent word to
trappers to extirpate all furbearers from the area in an effort to make the area less attractive to U.S. fur traders. In response to the elimination of beavers from large parts of the river system,
salmon runs plummeted, even in the absence of many of the factors usually associated with the demise of salmon runs. Salmon recruitment can be affected by beavers' dams because dams can: • Slow the rate at which nutrients are flushed from the water system; nutrients provided by adult salmon dying throughout the fall and winter remain available in the spring to newly hatched juveniles • Provide deeper
salmon pools where young salmon can avoid avian predators • Increase productivity through
algal photosynthesis and by enhancing the conversion efficiency of the
cellulose-powered
detritus cycle • Create slow-water environments where juvenile salmon put the food they ingest into growth rather than into fighting currents • Increase structural complexity with many physical niches where salmon can avoid predators Beaver dams are able to nurture salmon juveniles in estuarine
tidal marshes where the salinity is less than 10 ppm. Beavers build small dams of generally less than high in channels in the
myrtle zone. These dams can be overtopped at high tide and hold water at low tide. This provides refuges for juvenile salmon so they do not have to swim into large channels where they are subject to predation by larger fish.
Lampreys It has been discovered that rivers which have seen a decline or disappearance of anadromous
lampreys, loss of the lampreys also affects the salmon in a negative way. Like salmon, anadromous lampreys stop feeding and die after spawning, and their decomposing bodies release nutrients into the stream. Also, along with species like
rainbow trout and
Sacramento sucker, lampreys clean the gravel in the rivers during spawning. Their larvae, called ammocoetes, are
filter feeders which contribute to the health of the waters. They are also a food source for the young salmon, and being fattier and oilier, it is assumed predators prefer them over salmon offspring, taking off some of the predation pressure on smolts. Adult lampreys are also the preferred prey of seals and sea lions, which can eat 30 lampreys to every salmon, allowing more adult salmon to enter the rivers to spawn without being eaten by the marine mammals.
Parasites According to Canadian biologist Dorothy Kieser, the
myxozoan parasite
Henneguya salminicola is commonly found in the flesh of salmonids. It has been recorded in the field samples of salmon returning to the
Haida Gwaii Islands. The fish responds by walling off the parasitic infection into a number of cysts that contain milky fluid. This fluid is an accumulation of a large number of parasites. '', a
myxozoan parasite commonly found in the flesh of salmonids on the West Coast of Canada, in coho salmon
Henneguya and other parasites in the
myxosporean group have complex life cycles, where the salmon is one of two hosts. The fish releases the spores after spawning. In the
Henneguya case, the spores enter a second host, most likely an invertebrate, in the spawning stream. When juvenile salmon migrate to the Pacific Ocean, the second host releases a stage infective to salmon. The parasite is then carried in the salmon until the next spawning cycle. The myxosporean parasite that causes
whirling disease in trout has a similar life cycle. However, as opposed to whirling disease, the
Henneguya infestation does not appear to cause disease in the host salmon—even heavily infected fish tend to return to spawn successfully. According to Dr. Kieser, a lot of work on
Henneguya salminicola was done by scientists at the Pacific Biological Station in Nanaimo in the mid-1980s, in particular, an overview report which states, "the fish that have the longest fresh water residence time as juveniles have the most noticeable infections. Hence in order of
prevalence, coho are most infected followed by sockeye, chinook, chum and pink. As well, the report says, at the time the studies were conducted, stocks from the middle and upper reaches of large river systems in British Columbia such as
Fraser,
Skeena,
Nass and from mainland coastal streams in the southern half of B.C., "are more likely to have a low prevalence of infection." The report also states, "It should be stressed that
Henneguya, economically deleterious though it is, is harmless from the view of
public health. It is strictly a
fish parasite that cannot live in or affect
warm blooded animals, including man". According to Klaus Schallie, Molluscan Shellfish Program Specialist with the
Canadian Food Inspection Agency, "
Henneguya salminicola is found in southern B.C. also and in all species of salmon. I have previously examined smoked chum salmon sides that were riddled with cysts and some sockeye runs in
Barkley Sound (southern B.C., west coast of
Vancouver Island) are noted for their high incidence of infestation."
Sea lice, particularly
Lepeophtheirus salmonis and various
Caligus species, including
C. clemensi and
C. rogercresseyi, can cause deadly infestations of both farm-grown and wild salmon. Sea lice are
ectoparasites which feed on mucus, blood, and skin, and migrate and latch onto the skin of wild salmon during free-swimming, planktonic nauplii and copepodid larval stages, which can persist for several days. Large numbers of highly populated, open-net salmon farms can create exceptionally large concentrations of sea lice; when exposed in river estuaries containing large numbers of open-net farms, many young wild salmon are infected, and do not survive as a result. Adult salmon may survive otherwise critical numbers of sea lice, but small, thin-skinned juvenile salmon migrating to sea are highly vulnerable. On the
Pacific coast of Canada, the louse-induced mortality of pink salmon in some regions is commonly over 80%.
Effect of pile driving The risk of injury caused by
underwater pile driving has been studied by Dr. Halvorsen and her co-workers. The study concluded that the fish are at risk of injury if the cumulative
sound exposure level exceeds 210
dB relative to 1 μPa2 s. ==Wild fisheries==