Hunter, 1959); and Sashin Creek, Southeastern 

 Alaska (Merrell, 1962). For the brood years 

 studied, total mortality in these streams ranged 

 from 76 to 99.9 percent. Although these esti- 

 mates fail to differentiate among mortalities oc- 

 curring during (1) adult migration, (2) egg and 

 larval development, and (3) fry migration, other 

 evidence indicates that the largest portion of total 

 fresh-water mortality occui's between the time 

 eggs are deposited and fry emerge. 



Typical results are seen in mortality studies at 

 Hook Nose Creek (Hunter, 1959). Although 

 Hunter found that total deaths varied consider- 

 ably from year to year, losses before spawning 

 appeared to be consistently small. The number 

 of fry consumed by predators was fairly constant 

 from year to year and was usually a small frac- 

 tion of the potential egg deposition. Most deaths 

 occurred between spawning and fry emergence. 

 Hunter's data showed that over a 10-year period, 

 69-94 percent of the eggs potentially available 

 for deposition were lost before emergence of fry. 



Increased utilization of streams and watersheds 

 by logging, mining, and other multiple-use activi- 

 ties has caused concern about the welfare of 

 salmon. A thorough understanding of the factors 

 causing mortality in spawning beds will be re- 

 quired to evaluate the effects of multiple-use 

 activities on pink and chum salmon. 



In 1956 the Bureau of Commercial Fisheries 

 gave Saltonstall-Kennedy Act funds to the Fish- 

 eries Research Institute (FRI), University of 

 Washington, to study the effects of logging on 

 pink salmon streams in Alaska. These studies 

 concentrated on identifying the time and magni- 

 tude of mortality and determining the factors 

 responsible. This paper reviews past work on 

 factors causing mortality in spawning beds and 

 reports findings of field studies in three South- 

 eastern Alaska spawning streams. Field obser- 

 vations on mortality and associated environmental 

 factors thought to cause mortality are described. 



REVIEW OF ENVIRONMENTAL REQUIRE- 

 MENTS OF EGGS AND LARVAE 



Given an environment free of mechanical dis- 

 turbances, the growth, development, and survival 

 of salmon eggs and larvae depend largely upon 

 physical and chemical characteristics of the sur- 

 rounding water. Properties of water that affect 

 eggs and larvae include temperature, dissolved 



oxygen content, velocity, mineral and waste meta- 

 bolite content, and osmotic pressure. 



The spawning bed environment is greatly influ- 

 enced by weather and characteristics of the 

 streambed, stream, and watershed. The quality 

 of intragravel water 3 is influenced in part by the 

 hydrological regimen. Environmental changes 

 within spawning beds can accompany changes 

 in tide level, precipitation, and air temperature. 

 Periods of spawning and development very likely 

 coincide with the seasonal conditions that offer 

 maximum opportunity for survival of the young 

 salmon. 



SOURCES OF INTRAGRAVEL WATER 



To survive, eggs and larvae must receive an 

 ample supply of oxygenated water suitable in 

 temperature and free of toxic substances. The 

 source of intragravel water may govern to a large 

 extent its physical properties and its suitability 

 for eggs and larvae. 



Ground water and surface stream water are the 

 two primary sources of intragravel water. In 

 spawning beds of pink and chum salmon, surface 

 stream water is the primary source of intragravel 

 water (Sheridan, 1962a), while in spring-fed 

 spawning beds commonly used by other salmonid 

 species; e.g., sockeye salmon, ground water may 

 be an important source of intragravel water. 



Vaux (1961, 1962) showed that interchange be- 

 tween stream and intragravel water occurred when 

 certain hydraulic requirements of the stream and 

 streambed were satisfied. He formulated models 

 which showed the direction of interchange depends 

 on the curvature of the gravel surface profile. 

 Where the profile was concave, water upwelled; 

 where it was convex, a downdraft occurred. In 

 the absence of curvature, there was no inter- 

 change, provided permeability and gravel lied 

 depth did not vary. Vaux verified these relations 

 with field and laboratory experiments. Figure 1 

 illustrates the direction of interchange with change 

 in curvature of the stream bottom. 



WATER TEMPERATURE 



Water temperature controls the rate of growth 

 and the developmental and metabolic processes of 

 the salmon embryo. It also affects other water 

 quality characteristics, such as dissolved oxygen 

 concentration. 



» The term "intragravel water" refers to water occupying interstitial spaces 

 within the streambed. 



496 



U.S. FISH AND WILDLIFE SEKVICE 



