Carbaryl, Carbofuran, and Methomyl - National Marine Fisheries ...

Natural Mortality Factors
Available data indicate high natural mortality rates for salmonids, especially in the open
ocean/marine environment. According to Bradford (1997), salmonid mortality rates
range from 90 to 99%, depending on the species, the size at ocean entry, and the length of
time spent in the ocean. Predation, inter- and intraspecific competition, food availability,
smolt quality and health, and physical ocean conditions likely influence the survival of
salmon in the marine environment (Bradford 1997; Brodeur, Fisher et al. 2004). In
freshwater rearing habitats, the natural mortality rate averages about 70% for all salmonid
species (Bradford 1997). Past studies in the Pacific Northwest suggest that the average
freshwater survival rate (from egg to smolt) is 2 to 3% throughout the region (Marshall
and Britton 1990; Bradford 1997). A number of suspected causes contributing to natural
mortality include parasites and/or disease, predation, water temperature, low water flow,
wildland fire, and oceanographic features and climatic variability.
Parasites and/or Disease
Most young fish are highly susceptible to disease during the first two months of life. The
cumulative mortality in young animals can reach 90 to 95%. Although fish disease
organisms occur naturally in the water, native fish have co-evolved with them. Fish can
carry these diseases at less than lethal levels (Kier Associates 1991; Walker and Foott
1993; Foott, Harmon et al. 2003). However, disease outbreaks may occur when water
quality is diminished and fish are stressed from crowding and diminished flows (Spence,
Lomnicky et al. 1996; Guillen 2003). Young coho salmon or other salmonid species may
become stressed and lose their resistance in higher temperatures (Spence, Lomnicky et al.
1996). Consequently, diseased fish become more susceptible to predation and are less
able to perform essential functions, such as feeding, swimming, and defending territories
(McCullough 1999). Examples of parasites and disease for salmonids include whirling
disease, infectious hematopoietic necrosis (IHN), sea-lice (Lepeophtheirus salmonis),
Henneguya salminicola, Ichthyopthirius multifiliis or Ich, and Columnaris
(Flavobacterium columnare).
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