Great Outdoors columnist James Murray (File photo)

Great Outdoors columnist James Murray (File photo)

Column: Fish under stress from summer’s high temperatures

Great Outdoors by James Murray

By James Murray


As residents of the Southern Interior, we are not the only ones who have been suffering through a long hot summer.

A recent statement put out by the Freshwater Fisheries Society of BC (FFSBC), authored by Sue Pollard, says that fish too have been undergoing a lot of stress.

Over the summer we experienced some of the most severe temperatures on record.

On June 28, the town of Lytton broke the Canadian record three days in a row.

The rest of the Southern Interior also experienced record dry conditions due to little snow last winter and low amounts of rainfall in the spring.

All the while, fish habitat was drying up at a record rate.

The FFSBC report stated the reduction in water levels will inevitably mean a reduction of physical habitat. Reduced water levels also mean that water bodies heat up faster.

As water increases in temperature, it has a lower capacity to retain dissolved oxygen.

At the same time, elevated temperatures increase metabolic rates in organisms, which increases the demand for oxygen.

For animals like fish that breathe via gills, this can be disastrous.

Fish are ectotherms, said the FFSBC. This means they cannot regulate their body temperature, and body temperature reflects that of their watery environment.

As water temperatures heat up fish start to undergo greater levels of physiological stress.

According to other FFSBC studies, survival rates for fish in streams can be different compared to lakes.

Assuming water continues to flow in a stream, water movement generally keeps water mixed and oxygenated (although a loss of riffles reduces this capacity), but with a more uniform temperature except where cold water seepages from springs occur.

Fish will seek refuge in proximity to these seepages and in deeper, shaded pools to reduce energy expenditure. This can result in very “clumped” fish distributions. Water temperatures in some streams can increase quickly when temperatures stay high. However, they are also more responsive than lakes when air temperatures drop again to tolerable levels.

As the summer sun heats lake surfaces, waters stratify, where the warmest waters settle in the shallow surface layer while the coolest waters remain in the deepest layer.

This stratification intensifies as summer progresses, with less and less mixing between the two layers.

The surface layer maintains high oxygen levels due to wind action. The deep, cool layer where fish may seek refuge becomes depleted of oxygen over the summer due to use by fish and other organisms, as well as lack of mixing with the surface.

These two layers may force an increasingly narrow wedge between the two for cool-water fish like trout trying to balance temperature and oxygen needs. In extreme cases, this can result in a summer fish kill.

Read more: Salmon and other sea life affected by recent heat waves, experts say

Read more: Extreme heat waves are putting lakes and rivers in hot water this summer

Falling temperatures in the autumn restore mixing, and deeper layers are usually replenished with oxygen. The FFSBC suggests that recreational anglers can be proactive and take additional steps to help protect fish when summer temperatures start to climb – such as taking extra care to minimize air exposure, and avoid taking photos of fish to reduce handling.

Be selective on when and where you fish, fishing in early morning and evening periods when air temperatures are lower, and limit your time fishing isolated pools in streams where water levels have dropped drastically.

These pools may concentrate fish, making them more vulnerable to angling.

If we do our part to help lessen the pressures faced by fish populations this summer, maybe, just maybe, we will be able to enjoy better fishing next summer.

Thank you to all the men and women fighting the wildfires.

Outdoors and Recreation