Factors that Influence Fish Size in British Columbia Lakes

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Factors influencing fish size in BC still waters.


By Peter Caverhill


For those who love to paddle still water ponds and lakes in search of fly rod adventure, British Columbia is the nirvana. The choices range from tiny azure jewels at high altitude, to sea level expanses of dark fresh water, that are only steps off salty, clam-infested beaches.

This province has such a variety of still water (lake) opportunities that it truly causes the mind to reel. Some offer tiny, colorful trout in endless, easy profusion while others grudgingly give up salmon-sized trophies every other day or season.

Most fly fishers are intrigued, and perhaps even besotted, by the lure and search for really large trout. It isn't always clear why some still waters have large trout and others have only small ones. The influencing factors are complex, but we humans have a penchant for converting the complex into simple black and white.

What follows is probably just such a simplification which hopefully, will provide you with a few more tools to unravel the secrets of still water fishing.

Where the lake is located, the surrounding geology, and the climate are most important determiners of productivity and the ability to grow large trout.

Other more local factors, such as shape of the lake bottom, amount of available spawning, presence of other fish, lake elevation, water quality and man's activities also contribute to the potential for large trout. Consider these factors the next time you are planning a safari to new still waters.

Location in the Province

Still waters located in different parts of British Columbia may not be the same in their basic capability to produce large trout.

In 1956, a scientific paper was published by Drs. T. Northcote and P. Larkin which looked into the productivity of BC lakes. They were able to divide the province into 10 regions, according to the different ability of each region's waters to support and grow aquatic life (productivity).These regions reflect the differences in geology and climate that occur in the province.

Northcote and Larkin were looking for several specific lake parameters that could be measured, and that would provide a ready indicator of productivity. They found one factor, the total dissolved solids (TDS) content of the water, to be a very useful indicator of the general level of productivity of lakes province wide.

TDS is the amount of minerals and nutrients (phosphorus, calcium and others) that is dissolved in lake water. It is measured in parts per million (ppm). Very low TDS levels are considerably less that 100 ppm, and moderate to high levels are much greater than 100 ppm.

We can get a rough guideline on a lake's potential to grow fish flesh by understanding the productivity regions of the province which are defined primarily by TDS and a few other factors .

Table 1 explains the different productivity regions of BC, including their location, TDS, and other characteristics.

Table 2 provides TDS readings for some well known lakes in various parts of the province.

Factors like the TDS level help to explain why it takes almost a miracle for lakes near Vancouver to grow trout larger than a foot, and why a fingerling rainbow stocked into a Merritt pothole will be 4 pounds in only two years!



Still water productivity will be influenced by surrounding geological conditions:- notably, the chemical composition of the rocks and soils, and how easily this material will be available to lake waters.

Phosphorus, nitrogen and calcium are particularly important nutrients for all aquatic life. Where areas are rich in these nutrients, and where they are easily available through erosion, lakes have higher productivity. These conditions occur in the central interior of the province.

On the coast, conditions do not favor productivity because so much of the area lacks the appropriate nutrients, or these nutrients are tied up in very difficult to erode granite formations. Exceptions to this, are the lakes that reside in the southern part of Vancouver Island and the Gulf Islands. Here, the rainfall is lower, and nutrients are more available in surrounding soils.


Precipitation, and temperature are climatic factors that also have a major influence.

In areas where precipitation is very high (particularly the coastal mainland and parts of Vancouver Island), lakes will not be able to retain eroded nutrients, as they will quickly be flushed away. A high lake flushing rate, combined with surrounding substrates that are very difficult to erode, guarantees low productivity. These waters often have well-defined and numerous tributary streams.

Low precipitation, and easily eroded surroundings endowed with nutrients (like the Interior), is a formula for abundant aquatic life and a potentially rapid rate of growth for fish. Small lakes with this lucky set of conditions are extremely rich, because the available nutrients have lots of time to go into solution, and they stick around for a long time. These waters often have no well-defined tributary streams, except possibly for a short period in the early spring, during snow melt and runoff.

The average annual summer and winter temperature can also influence lake productivity. Lakes that have long, very cold winters and short cool summers (like those in the far north of the province) have shorter periods when water temperatures are optimal. This reduced “growing season” means that fish will take much longer to achieve an interesting size.

The Lake Bottom

BC has an incredible variety of lake types, ranging from very large bodies of water with steep sides and great depths, to tiny, shallow, saucer-like potholes.

The shape of the lake bottom is another factor to consider when searching for waters that might hold larger fish.

Lakes that are shallow (have very low average depth) are generally able to produce more life than lakes that are steep and deep (have very high average depth). Productive lakes have plenty of shallows where sunlight is able to penetrate. This stimulates plant life and aquatic fish foods, such as plankton, scuds and insects.

Granite. Rain. TDS = 4 ppm. The formula for tiny trout

Available Spawning

Lots of good spawning for trout is seldom desirable when the objective is large fish. Abundant spawning and early rearing means lots of young fish will enter the lake and compete for a fixed food supply. The more fish competing, the smaller in size they will be.

Unproductive coastal lakes, with greater rainfall, usually have more streams and spawning areas than very productive interior waters, where rainfall is low and spawning streams are in short supply. Many interior trout waters have no natural spawning, and fish presence is entirely dependent on annual stocking.

Other waters may have limited natural spawning, and it may not be available every year :- these can be trophy waters!

Presence of Other Fish

The presence of other fish may be beneficial, or detrimental, in helping to determine the possible presence of bragging-size trout.

Lakes that are accessible to migratory fish (trout or salmon) may provide some very interesting fishing for larger trout. These trout may be present to take advantage of abundances of salmon juveniles or eggs. The trout may only be around for a short period of time, and successfully finding them will require detailed local knowledge.

In other circumstances, resident trout in a lake may be able to take advantage of a local population of forage species like shiners, chub or kokanee.

Becoming fish eaters may allow the trout in some generally unproductive waters to achieve trophy status. Some species of trout are more adapted to being fish eaters (ie cutthroat trout and certain stocks of rainbow trout).

In many lake situations, other fish (especially non-salmonids like redside shiners and pike minnows aka. squawfish) can mean serious competition for the trout population. These other fish may out-compete trout for food, or they may be trout predators. The result can be a lake containing a few small trout, with the possibility of some old monsters that have been able to survive the competition/predation by eating their enemies.

Lake Elevation

Just as the climate can influence a lake's “growing season”, so can elevation. The higher the elevation of a lake, the shorter is the period of optimal temperature for organisms to grow. Therefore, as elevation increases, the expectation of finding really big fish should also diminish. However, rules do not exist without exceptions. High elevation waters that have high basic productivity (as indicated by their TDS) with limited spawning and low angling harvest, can provide a supply of portrait-sized fish.

Other Water Quality Aspects

Water contains a variety of materials and gases, either dissolved or in suspension. Water is also subject to various conditions (ie. be hot or cold). Water quality is the sum of these contents and conditions. These may or may not be beneficial, depending on our perspective - in this case it's things that influence the potential for large trout being present.

We have already looked at some basic water quality aspects such as the presence of dissolved minerals (nutrients). Other water quality aspects such as dissolved oxygen and extremes of temperature are important.

For some lakes, very low levels of dissolved oxygen or extremely high water temperatures may mean that trout cannot survive. Many of the ultra-shallow but highly rich interior lakes are in this category.

Winter conditions may produce extremely low, or no, oxygen levels and fish may die (winter kill). Occasionally, summer temperatures may create lethal conditions (summer kill). It is only man's intervention, in the form of winter aeration or the stocking of trout species that are more tolerant to high temperatures or low oxygen, that allow fish to be there.

Bog ponds/lakes, whether they be in the unproductive coast or the rich interior, have water that is very acidic and low in productivity. Every region of the province will have some lakes in this category, so the big fish- scouting angler may decide to give these a pass.

Lakes that are glacially influenced may be cloudy (turbid) with glacial “flour” that is suspended in the water, and may be very cold. The suspended particles will affect sunlight penetration of the water, and thus the amount of photosynthesis and plant growth that can occur. The average temperature during the growing season will be lower, reducing productivity.

All shallows, little rain, TDS 300 ppm. The proof is in the net!

Man's Activities

As BCs population increases, and the demand for more still water angling grows, there are a number of factors related to human activity that may influence the presence of large trout.

Lake shore and upland development can affect fish populations by changing the quality of lake waters. Mostly, this is negative. It increases turbidity (suspended sediment) levels and adds pollutants to the water that may be toxic to fish and fish food.

In some rare cases, organic pollution (septic tank runoff) may add nutrients and increase the lake's productivity and ability to grow larger trout.

Population growth (with more anglers) creates a greater demand for stocking lakes, and increased stocking potentially results in more waters with bigger fish. Some of these waters will have ideal conditions for large trout.Many interior waters can only have trout populations if they are stocked annually.

Sometimes hatchery stocking adds too many fish, resulting in a decrease in average fish size.

Fish culture and small lake management is becoming more sophisticated, especially in the science of selecting special brood stocks to suit certain lake conditions. These “designer” trout can do well and grow large, where other traditional rainbow brood stocks will not.

With growth in the angler population invariably comes the illegal transfer and introduction of unwanted (and often highly detrimental) species like redside shiners into high quality trout fisheries. Shiners, and their sort, eventually out-compete rainbow trout for a lakes's available food.

Ignorant or uncaring anglers illegally using live bait (fish or insects) can transport undesirables and leave them to explode in a new environment.

Harvest of trout by anglers has an influence on the availability of the larger specimens. In very rich lakes, where trout grow rapidly and the number of large fish is substantial, harvest may not have a serious impact on all but the very largest members of the population.

In less rich waters, large fish are rare and can easily be cropped -off as angler use and take increases. In the past, some of these unproductive waters had a reputation for monster fish. Weaver Lake and a few other waters near Vancouver and elsewhere on the coast, produced rainbow trout of 10 pounds or more. These undoubtedly were very old fish, and only existed because very few anglers were able to harvest them. One of these lakes had absolutely whopper rainbow, but in addition to very low angler harvest, the fish had another secret :-they dined richly on salamanders!

Locating large trout in BC still waters can be an interesting and rewarding science. This adventure is certainly more effective when you consider those factors that make fish large.

Choose a potentially productive area (surrounding geology and climate), and a lake with the right qualifications in terms of high nutrient level, moderate elevation, more shallows than deep water, and remote from the hordes. In doing this, you greatly increase your chance to capture that fish of a lifetime!

© 2000 Interactive Broadcasting Corporation

Peter's Arcticles...



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Factors that Influence Fish Size in British Columbia Lakes