Impacts on Fish Habitat in Cultus Lake
and the Role Man made Debris may Play Within it
By Dale Carlisle
The idea that man made structures can be of value as fish habitat is not a new concept to most of us. Its origins lay in the recognition of the variety of life that colonizes and resides in, on and around maritime wreckage. As a result of these observations, the construction of numerous salt water artificial reefs worldwide, have been successfully undertaken. This is done either to develop otherwise barren parts of the ocean floor, or to enhance already existing, but declining, natural biotic systems. Man made debris also provides habitat to freshwater fish, though there is less emphasis given to this form of habitat intercession for a variety of reasons. While traditionally viewed as refuse and a detriment to the environment, some discarded “garbage” by virtue of its shape, size and composition, also plays an important role in the establishment or maintenance of fresh water fish populations regardless of the initial reasons for its introduction.
While the aquatic terrain of Cultus Lake may appear to be stable and unchanging to the casual observer from the surface, there is in reality, a continuous altering of both the bottom composition and in the succession of flora and fauna species. One of the results of this continued change is that pressure is sometimes placed upon less adaptable or marginalized species. The habitat, inadvertently created by deposited manmade debris, may provide some relief from this pressure for those threatened populations, or it might exacerbate the pressure against them, by aiding encroaching species.
In this three part article I will introduce the natural mechanisms that influence habitat in Cultus Lake, the aquatic environments I have observed, and some ways that I have seen manmade debris acting as surrogate habitat for various fish populations. It is in no way meant to be either exhaustive or academic. My intention is to illuminate one aspect of the lakes overall environment that may be overlooked, and to stimulate discussion regarding current fish habitat, future habitat, and potential strategies regarding refuse removal and natural or artificial habitat intercession.
Natural factors that shape the aquatic environment
Geology, Sediment flow and Alluvial fans
Cultus Lake came into existence, geologically, at some point after the last of a series of Ice Ages that ebbed and flowed across many parts of North America. As ice sheets moved up and down the Columbia Valley, they ground the bedrock before them, creating thick layers of gravel called glacial till that covered much of the valley floor. As the last of the ice flows receded, a large piece of glacier broke off and settled deep into this gravel bed, pushing out the sides to create what is called a “kettle” that then filled with water to form a primordial lake.
Even after the initial contour of the lake was established by this massive downward pressure, the general flow of water through it continued to impact the overall bathymetric profile. From its southernmost egress at Frosst Creek, to its northernmost drainage into Sweltzer Creek, water flow and sedimentation created deeper contours in the south and a shallowing of the bottom towards the northern part of the lake. Smaller creeks along the eastern and western shoreline have also influenced the lakes overall shape in a similar, though lesser, fashion.
Creeks also play a role in lake shaping via the formation of alluvial fans. Creek runoff entering the lake brings with it deposits of rock fragments, washed down from gullies in the mountains above. Over the centuries these deposits have formed crescent shaped rubble slopes underwater, reaching down and out into the lake. These alluvial fans aid fish habitat creation in two distinct ways:
First, the fragments of rock form numerous den sites for benthic dwellers such as the Coast Range Sculpin, as well as necessary gravel spawning beds for egg spreaders like the Cultus Sockeye Salmon.
Second, the gradual slopes along the borders of alluvial fans allow a broader littoral zone to form, more so than that which might be encountered with steep bedrock based shorelines. The loose substrates of alluvial fans also allow aquatic plants to root and colonize easier, creating protective shelter for many species of fish.
Lake maturation and the influence of organic matter
Like many other lakes in the Fraser Valley, Cultus is impacted by the collection of organic material within it. This material fuels the slow but relentless transition through the various stages of lake development from pioneer, to lake, through pond and finally towards marshland. Initially these effects would have been slow to manifest, because the sources of organic input would have been minimal, but, as the source of organic materials begin to multiply and intensify, the effects become more pronounced. With the onset of eutrophication, which will be discussed later, this maturation process begins to measurably impact fish habitat and species survivability. Organic materials that contribute to fish habitat may take the form of tree falls, debris from gully washes, above water shoreline plants, littoral zone plants, phytoplankton and algae.
Tree falls: Over the centuries, trees that border Cultus Lake have fallen into the lake through windfall, soil erosion or decay. These trees, some quite large, form a framework for habitat that can last for hundreds and hundreds of years. Hollows, roots and branches provide hiding spaces, while dens along the substrate provide nesting sites for others. Even larger species of fish find shelter beside, beneath and among submerged trees.
Debris from gully washes: The debris from tree falls higher up in the mountains also finds its way into the lake via the creek gullies that surround it. These inflows create underwater fields of various sized wood debris that stretch down into the depths. Within this debris both benthic and limnetic species often find shelter but the debris can also negatively impact habitat. Wood and bark/pulp often settle on and smother alluvial fan den sites, as both alluvial rock and wood debris come from the same creek source.
Shoreline plants: While shoreline plants, for the most part, do not directly provide habitat for Cultus Lake fish species, they do indirectly impact the lakes underwater makeup. Each fall and throughout the winter, cast off organic material from shoreline plants either falls or is blown into the lake where it is breaks down and decays to become part of the lakes overall bio load. As well as increasing dissolved nutrient levels in the water, the detritus also covers and fills in gravel bed spawning sites, benthic nesting sites, dens and other habitat structures. When the level of annual detritus addition to the lake was low, established habitats could remain viable for many decades, or even centuries, at a time but as the level of detritus deposits increase, as with latter lake staging and eutrophication, these sources of habitat can be covered over in a relatively short period of time.
Large expanses of cultivated grasses along the northern, eastern and southern shores also inadvertently promote populations of geese to occupy the lake for longer periods of time than they might otherwise have done in a wilder setting where food sources were less available. These birds defecate either in, or near the water, adding to the bio load of the lake and accelerating the cycle of eventual eutrophication.
Littoral zone plants: Littoral zone plants, which make up the majority of vascular species found in Cultus Lake, play an important role in habitat development in two ways.
First, they create ample nursery sites for developing juvenile fish of many species. Here, small clusters of fry can grow in relative safety from predation until they are large enough to participate successfully in open schooling behavior.
Second, littoral zone plants eventually die off and decay and this plant matter then adds to the level of detritus added to the lake. As the lake fills in with sediment and more shoreline falls within the favorable depth limits for littoral plant growth, the rate of detritus addition will increase. Individual plant species can also affect deposit levels as some plants have a slower, more constant rate of growth and decay while others engage in rapid annual growth/die off cycles, which serves to further exacerbate detritus accumulation.
Phytoplankton and algae: As dissolved nutrient levels within the lake increase, as they are in Cultus, the conditions become favorable for free floating plants such as phytoplankton and alga to form. These plants emerge with rapid growth cycles in the spring, as sunlight increases, and then die off when the available nutrients have been consumed. As eutrophication of the lake develops, the dissolved nutrient levels are no longer exhausted and these blooms and die offs occur in succession throughout most of the summer months adding tremendously to the amount of detritus accumulating in the lake.