Dissolved Gas Supersaturation (DGS) and Gas Bubble Trauma (GBT) in fish is a physical cause - biological effect relationship which has received the attention of environmental scientists for the past several decades. In British Columbia, DGS has been identified as a potential threat to fish populations in many water courses throughout the province. In this report, DGS is examined in terms of its causes, environmental levels, and potential impacts on fresh water and marine environments. Where sufficient information exists, DGS water quality guidelines are developed for the protection of fresh water and marine life. These guidelines are derived primarily from information describing the adverse physiological effects of DGS on fish and invertebrates. Additional factors, such as environmental variables and organism behavioural patterns which can intensify or mitigate these effects, are considered in the guideline derivation. At this time, no other water uses (i.e., drinking water, agricultural, recreational, or industrial) could be identified which would require guidelines for DGS.

The scientific literature upon which this report is based was identified through computer searches of several North American scientific database providers. These included:

· ASFA (AQUATIC SCIENCES AND FISHERIES ABSTRACTS): 1978 to January, 1993
· AQUAREF (ENVIRONMENT CANADA): 1970 to March, 1993
· BA (BIOSIS PREVIEWS): 1969 to March, 1993
· CA (AMERICAN CHEMICAL SOCIETY): 1977 to March, 1993
· CAB (CAB ABSTRACTS): 1972 to February, 1993
· ELIAS (ENVIRONMENT CANADA LIBRARY NETWORK): 1976 to September, 1992
· ENVIRO (ENVIROLINE DATABASE): 1971 to January, 1993
· NTIS (NATIONAL TECHNICAL INFORMATION SERVICE): 1964 to March, 1993
· WAVES (ENVIRONMENT CANADA): to March, 1993

In addition, many papers were identified in bibliographies from the primary literature. For example the review of DGS and GBT by Weitkamp and Katz (1980) provided 138 references which were not identified by the computer database searches. This was also true of several other papers (Colt et al. 1986, White et al. 1991). In all, over 380 papers were examined for their suitability for the guideline derivation process.

1.1 Dissolved Gas Supersaturation

Dissolved gas supersaturation is a condition which exists in many natural and man-made water bodies throughout the world. It occurs when the partial pressures of atmospheric gases in solution exceed their respective partial pressures in the atmosphere. Figure 1 shows the relationship between gas solubility and temperature for the two major atmospheric gases, oxygen and nitrogen. When dissolved gas concentrations of oxygen and nitrogen are above their respective saturation lines in the figure, they are in a supersaturated state. Conversely, when concentrations of these gases are below the saturation lines, they are under-saturated. Also shown in Figure 1 is the vapour pressure of water as a function of temperature. Water vapour plays an important role in the reporting of dissolved gas levels and in the biological effects of DGS. However, it is generally treated as always being in a saturated state at the prevailing water temperature.

Figure 1: Solubility of Oxygen and Nitrogen in Water

Individual atmospheric dissolved gases (oxygen, nitrogen, and trace gases such as argon and carbon dioxide) can often be supersaturated without adverse effects on aquatic and marine organisms. However, when the sum of the partial pressures of all dissolved gases exceeds atmospheric pressure, there is the potential for gas bubbles to develop in water and in the aquatic and marine organisms which inhabit the water. This causes a condition known as gas bubble trauma. GBT and its physiological consequences to fish and other organisms will be described more fully in Section 1.2.

Throughout the literature, a variety of methods have been used for the reporting of dissolved gas tensions. The sum of the partial pressures of all dissolved gases is referred to as the Total Gas Pressure (TGP), while the difference between TGP and atmospheric pressure is defined as Delta P. Both TGP and Delta P are usually reported in mm Hg (millimetres of mercury) or sometimes in sea level or local atmospheres. Many authors report TGP as a percent of sea level or local atmospheric pressure (TGP%). For reasons which will be presented in Section 4.1, Colt (1984) recommends that delta P, rather than TGP or TGP%, be used as the preferred method of reporting dissolved gas tensions.

1.2 Gas Bubble Trauma

Dissolved gas super-saturation can produce a variety of physiological signs which are harmful or fatal to fish and other aquatic and marine organisms (Renfro 1963, Stroud and Nebeker 1976, Weitkamp and Katz 1980, Cornacchia and Colt 1984, Johnson and Katavic 1984, Gray et al. 1985, Fidler 1988, White et al. 1991). As a class, these signs are referred to as gas bubble trauma (Fidler 1984) or gas bubble disease (Bouck 1980). The major signs of GBT which can cause death or high levels of stress in fish are:

· Bubble formation in the cardiovascular system, causing blockage of blood flow and death (Jensen 1980, Weitkamp and Katz 1980, Fidler 1988).
· Overinflation and possible rupture of the swim bladder in young (or small) fish, leading to death or problems of overbuoyancy (Shirahata 1966, Jensen 1980, Fidler 1988, Shrimpton et al. 1990a and b).
· Extracorporeal bubble formation in gill lamella of large fish or in the buccal cavity of small fish, leading to blockage of respiratory water flow and death by asphyxiation (Fidler 1988, Jensen 1988).
· Sub-dermal emphysema on body surfaces, including the lining of the mouth. Blistering of the skin of the mouth may also contribute to the blockage of respiratory water flow and death by asphyxiation (Fidler 1988, White et al. 1991).

Other signs of GBT include exopthalmia and ocular lesions (Blahm et al. 1975, Bouck 1980, Speare 1990), bubbles in the intestinal tract (Cornacchia and Colt 1984), loss of swimming ability (Schiewe 1974), altered blood chemistry (Newcomb 1976), and reduced growth (Jensen 1988, Krise et al. 1990), all of which may compromise the survival of fish exposed to DGS over extended periods.

Each sign of GBT involves the growth of gas bubbles internal and/or external to the animal. However, for each sign there is a threshold level of delta P which must be exceeded before bubble formation or swim bladder overinflation can begin (Fidler 1988, Shrimpton et al. 1990a). Still, the activation of GBT signs is not an easily demonstrated cause and effect relationship. This is because bubbles which develop internal to the animal may form in many body compartments, disrupting neurological, cardiovascular, respiratory, osmoregulatory, and other physiological functions (Stroud and Nebeker 1976, Weitkamp and Katz 1980, Fidler 1988, Shrimpton et al. 1990a and b). Thus, depending on the level of DGS, there may be multiple signs present in affected animals. GBT may also increase the susceptibility of aquatic and marine organisms to other stresses such as bacterial, viral, and fungal infections (Meekin and Turner 1974, Nebeker et al. 1976b, Weitkamp and Katz 1980). All signs of GBT weaken fish, especially juvenile life stages, thereby increasing their susceptibility to predation (White et al. 1991). Consequently, mortality can result from a variety of both direct and indirect effects caused by DGS.

Figures 2 through 5 show some of the signs of GBT in rainbow trout exposed to high levels of DGS. Figure 2 shows skin blistering which has occurred in the mouth of an adult rainbow trout while Figure 3 shows sub-dermal emphysema on external surfaces of the head. Figure 4 shows a severe case of exopthalmia in a juvenile trout. Figure 5 is a microphotograph of gill lamella from a fish which has died from GBT. Bubbles in the afferent arteries are clearly visible.

Recent research (Fidler 1984 and 1988, Alderdice and Jensen 1985a and b, Colt 1986) suggests that GBT in fish can be divided into acute and chronic responses depending on the levels of DGS.

Acute GBT: Acute GBT usually involves delta P levels in excess of 76 mm Hg (Sea Level TGP% about 110%). However, the susceptibility of fish to these levels of DGS is highly dependent on age class or size. For example, Nebeker et al. (1978) found delta P levels up to 200 mm Hg (Sea Level TGP% about 126%) had no effect on eggs or newly hatched fry of steelhead trout (Oncorhynchus mykiss). At a delta P of 130 mm Hg, this resistance appeared to continue until the fish were about 16 days old, at which time bubbles began to form in the mouth, gill cavity, and yolk sac (Nebeker et al. 1978). The accumulation of bubbles in larval fish compromises swimming and feeding ability and the fish is eventually trapped at the water surface as a result of excess buoyancy. Juvenile and adult fish are more susceptible to GBT, with lethal signs appearing at delta P levels of 76 to 106 mm Hg (Weitkamp and Katz 1980, Gray et al. 1982, Fidler 1988). In these fish, sub-dermal emphysema of the mouth lining accompanied by blockage of gill water flow by extra-corporeal inter-lamella bubbles causes death in several days (Fidler 1988). At slightly higher levels of DGS, bubble growth in the cardiovascular system can lead to death in just a few hours (Fidler 1988, also Section 6.1.2).

Chronic GBT: Chronic GBT usually involves delta P levels between 20 and 76 mm Hg. Chronic signs include over-inflation of the swim bladder and bubble formation in the gut and buccal cavity (Weitkamp and Katz 1980, Colt 1986, Fidler 1988). Mortality levels are generally low and require extended periods of exposure before they are detected (Peterson 1971, Shrimpton 1985, Wright and McLean 1985, Colt 1986, Shrimpton et al. 1990a and b). For example, Wright and McLean (1985) found that exposure of juvenile chinook salmon (Oncorhynchus tshawytscha) to delta P levels ranging from 0 to 46 mm Hg for 122 days resulted in a mortality of 4.1% compared to 1.6% in control fish. Peterson (1971) found delta P levels of 15 to 40 mm Hg caused formation of a small bubble in the buccal cavity of incubating Atlantic salmon (Salmo salar) and improper development of the operculum. Although the gas bubble disappeared, heavy mortality occurred six to eight weeks later at first feeding. In larval striped bass (Morone

Figure 2: Subdermal Emphysema in the Mouth of a Rainbow Trout.

Figure 3: Sub-dermal Emphysema on the Head of a Rainbow Trout.

Figure 4: Severe Exopthalmia in a Juvenile Rainbow Trout.

Figure 5: Intra-corporeal Bubbles in the Lamella of a Rainbow Trout.

saxatilis), gas super-saturation caused over-inflation of the swim bladder and bubbles in the gut (Cornacchia and Colt 1984). Clinical signs of GBT occurred at delta P levels as low as 22 mm Hg and mortality was increased at delta P levels of 42 mm Hg. Excess buoyancy resulting from over-inflation of the swim bladder and the accumulation of gas in the gut are also common clinical signs of GBT in larval marine fish exposed to low levels of DGS (Dannevig and Dannevig 1950, Henly 1952, Kraul 1983).

DGS can affect all aquatic and marine organisms, including fish, invertebrates, and plants. However, most research in the field has been focused on fresh water fish with the major emphasis on trout and Pacific salmon species (Weitkamp and Katz 1980, Colt et al. 1986).

1.3 Water Quality Guidelines

Throughout North America water quality criteria and guidelines have been developed for a variety of chemical compounds and water physical parameters. For example, the province of British Columbia has developed water quality criteria for particulate matter, cyanide, nitrogen, lead, microbiological indicators, chlorine, ammonia, PCBs, nutrients, algae, molybdenum, copper, aluminum, mercury, fluoride, and pH (BC Ministry of Environment, Lands and Parks 1992). However, the province has not previously developed water quality guidelines for DGS. While Canadian Water Quality Guidelines have been published by Environment Canada for a wide range of chemical compounds and water physical parameters (CCREM 1987), water quality guidelines for DGS have not been developed.

In the United States, the US Environmental Protection Agency has published DGS water quality guidelines which recommend a maximum TGP of 110% of local atmospheric pressure (US EPA 1986). This guideline is also adopted by most of the states. No guidelines for DGS by other national or international agencies could be found in the literature.

Because of existing high levels of DGS in some British Columbia rivers, lakes, and marine environments (MacDonald and Hyatt 1973, May 1973, Clark 1977, Maxwell 1985, Penney 1987) and the potential for GBT in fish, it is important that guidelines be established to protect these environments. Amplifying this need is the growing interest in the development of small hydroelectric facilities throughout British Columbia and other parts of Canada. These facilities often possess high potential for producing DGS in smaller streams and rivers (Fidler 1992). Consequently, guideline criteria are needed to protect aquatic environments from these installations. Finally, the discharge of nutrients into fresh water and marine environments by industry, municipalities, and agriculture can dramatically increase primary production in these environments. This can lead to high levels of DGS through photosynthesis (Woodbury 1941, Renfro 1963, White et al. 1991). When combined with dissolved gas solubility changes which are driven by solar heating, high levels of DGS can occur. Again, guidelines are needed to protect fresh water and marine environments from this form of DGS.

Since the work upon which the US EPA guideline was developed, there has been considerable research into the problem of DGS and GBT in fish (Fidler 1984 and 1988, Alderdice and Jensen 1985a and b, Jensen et al. 1986, Schnute and Jensen 1986, Smith 1987, Krise and Herman 1989, Krise et al. 1990, Shrimpton et al. 1990a and b, Krise and Smith 1991, White et al. 1991). More recent information indicates that in many situations the US EPA guideline (TGP = 110%) may not afford adequate protection for some fish populations and especially for juvenile life stages of Pacific salmon, rainbow trout, and other species (Cornacchia and Colt 1984, Fidler 1984 and 1988, Wright and McLean 1985, Shrimpton et al. 1990a and b, White et al. 1991). However, there is now a broader understanding of the physical and biological phenomena associated with DGS and GBT along with a greater database of detailed information upon which more protective water quality guidelines can be developed.

In the development of water quality guidelines for DGS, it should be recognized that the effects of DGS on fresh water and marine organisms is quite different from the action of most toxic chemicals. For example, water depth plays an important role in protecting fish from the effects of DGS (Section 6.1.3). In addition, many rivers and lakes have naturally occurring levels of DGS which are potentially lethal to fish (Section 5.1). Yet, wild fish appear to have developed strategies for surviving in these environments (Section 8.1.1.3). Fish hatcheries present a unique situation where shallow water depths, extended exposure periods, and crowded conditions may compound the effects of DGS on fish (Section 6.1.4). The passage of fish through turbo machinery in dams presents another unique environment where the effects of DGS are amplified by the low pressure fields in this machinery (Section 3.1). Finally, the signs of GBT in fish is strongly dependent on the size of the animal (Section 6.1.1.2).

As a result of the many mitigating and compounding effects of environmental and biological variables, the procedures for developing water quality guidelines for DGS are more complex than those traditionally used in the development of guidelines for toxic chemicals. Some of the background information which is relevant to the development of water quality guidelines for DGS is reviewed in the following sections.