Summer drought can rapidly transform fish habitat, especially in British Columbia’s southern watersheds. Understanding which habitats are most vulnerable, and how anglers can reduce added stress, can help protect fish during hot, dry conditions.
Fish habitats in B.C. are diverse. They range from glacier-fed streams and alpine lakes, which remain cold throughout the year, to valley-bottom and grassland watersheds, where water temperatures vary widely from summer to winter. Vulnerability to heat and drought also varies depending on habitat. Lake-headed and snow-fed streams, as well as shallow lakes (especially in southern regions), are generally more vulnerable to summer heat and drought than large, deep lakes and glacier-fed streams.
What is drought?
Drought is a period of abnormally dry conditions that may result in water scarcity. In B.C., drought may be caused by a combination of factors, including insufficient snow accumulation (low snowpack), early or accelerated snowmelt, hot and dry weather, or delayed seasonal rainfall.
Drought impacts on fish
Drought conditions, especially in the summer when temperatures are high, create cumulative impacts to fish beyond water scarcity. The accompanying reduction in habitat and warmer water temperatures increase the metabolic stress on fish, which can be magnified when fish are angled to exhaustion.
Habitat loss
As streamflow declines, less physical habitat is available. Under severe drought conditions, fish may be confined to isolated pools, while key riffle habitats can dry up and no longer add oxygen through aeration. In lakes, lower water levels reduce water volume, and compress habitat for all species. Habitat loss can also force predators and prey to share smaller areas of thermal refuge.
Warmer water
With less water, waterbodies heat up faster during hot summer months. As water’s temperature rises, its capacity to hold dissolved oxygen declines. At the same time, higher temperatures increase organisms’ metabolic rates and their demand for oxygen. For fish and other animals that breathe through gills, this combination can be disastrous.
Streams impacted by drought may also surpass the maximum temperature thresholds for biological processes in fish. Fish are ectotherms: they cannot regulate their body temperature, which means that it reflects the temperature of their watery environment. When water heats up to a certain level, fish begin to experience physiological stress. For many trout species, that threshold is an average weekly maximum temperature of approximately 20°C. Some species of fish, like char, are even more sensitive to warm water than trout. The more prolonged and severe the temperature elevation, the more likely for the impacts to result in death.
As lakes warm during the summer, the water stratifies, with the warmest water on the surface and the coolest water in the deepest layer. Stratification intensifies as summer progresses, and there is less mixing between the two layers.
Wind action helps the surface layer maintain high oxygen levels over the summer months, but the deep, cool layer – where fish seek thermal refuge – becomes depleted of oxygen. The habitat suitable for trout between these layers becomes increasingly narrow as summer progresses. In extreme cases, this can result in a summer fish kill.
Falling temperatures in autumn restore mixing, and deeper layers are usually replenished with oxygen.
Provincial Fisheries Drought Response Plan
British Columbia has developed a fisheries management drought response plan to address the increasingly frequent drought conditions experienced in streams around much of the province. This plan lays out guidelines for the protection of fish that reflect severity of drought, water supply, and water temperatures. Suggested tools include more conservative angling regulations during high-stress times; water-use planning to prioritize fish needs where there are multiple users; and angling closures. The plan proposes thresholds for maximum water temperature which, if exceeded, would mean recreational fisheries should be closed.
The plan recommends that
- trout-specific streams and watersheds be closed to recreational angling where mean weekly maximum temperatures exceed 20°C and/or maximum daily temperatures exceed 23°C;
- char-specific streams and watersheds be closed to recreational angling where mean weekly maximum temperatures exceed 16°C and/or maximum daily temperatures exceed 20°C;
- streams and watersheds may in addition be closed to recreational angling where the flows are reduced below 5% of the long-term mean annual discharge (at such levels, there are significant concerns that key rearing habitats may be dewatered).
Due to persistent year-to-year high frequency of thermal events, the provincial response has been to proactively close fisheries. One example is the Cowichan River, where July-August stream temperatures get as high as 25°C due to surface heating of Cowichan Lake. Summer closures on Vancouver Island include most streams on the southern half of the island from July 15 to August 31.
Anglers should check the B.C. Freshwater Fishing Regulations prior to heading out on a fishing trip to see if in-season regulatory changes have been made. Hot and dry summers will likely result in stream closures.
How can recreational anglers help?
When temperatures are elevated and oxygen is less available, fish become physiologically stressed. Recreational anglers can help minimize the added stress of their angling activity as summer temperatures start to climb. Consider the following tips, especially if you practice catch and release.
- Take extra care in minimizing air exposure. Keep fish in water, or avoid taking photos to reduce handling time.
- Fish in early morning and later evening, when air temperatures are lower.
- Avoid fishing in streams where temperatures exceed 16°C (for char fisheries) and 23°C (when not targeting char).
- Avoid fishing in shallow lakes where temperatures could exceed 20°C at depth.
- Avoid or limit your time fishing isolated pools in streams where water levels have dropped drastically. These pools can act as refugia, providing the only habitat in some systems, and may concentrate fish, making them more vulnerable to angling.
- Minimize fight time to limit fish exertion.
- Consider delaying fishing at vulnerable waterbodies until later in the season.
While relevant for the well-being of all fish, these recommendations are particularly pertinent for wild fish to support conservation objectives.
Resources:
Provincial Fisheries Management: Drought Response Plan
B.C. Drought Information Portal – Regional drought and water scarcity information, including stream-flow data
Keep Fish Wet – information on best angling practices, as well as information on increased temperatures and impacts on fish
Author: Staff, Freshwater Fisheries Society of BC.