The Ecology: Incubation
1. SEDIMENT FREE WATER
Natural, healthy streams normally run clear water over a clean gravel bottom.
Impacts of sedimentation on the Kokanee salmon:
- sediments cover eggs and clog Kokanee gills to reduce the flow of oxygen;
- water temperature increases due to the sediments absorbing heat.
Oxygen is needed in the growth and development of the embryo inside the egg.
The embryo extracts oxygen as water passes over the egg’s membrane.
Impacts of low oxygen levels on developing Kokanee:
- growth may be slowed;
- embryo and alevin may die.
Remember — fish are poikilotherms (cold-blooded).
Eggs and alevin are dependent on appropriate water temperatures for growth and development.
In the alevin, developmental rate and metabolic needs increase in warmer water, but growth rate is reduced resulting in a smaller than normal fry.
Fry emerge after a period of increasing temperatures and this emergence time is adapted to optimize food availability upon entering the lake; with warmer temperatures there is more zooplankton in the water.
Eggs settle in crevices between the rocks.
Proper crevice size provides protection from:
- harmful light;
- losses downstream;
- predators (other fish and birds).
Smaller gravel is needed because it is more porous and the porous gravel allows water to flow through it to cover the eggs that are hidden deep down in the gravel.
5. TREE REMOVAL (LOGGING, HOUSING, AGRICULTURE)
- loss of bank stability results in erosion and this leads to siltation of the gravel in which the salmon spawn;
- no shade cover along the river increases water temperature.
6. WATER QUANTITY (DRAWDOWN)
Changes in stream flow will fluctuate with irrigation and domestic uses causing problems with temperature, siltation and oxygen levels.
Severe damage occurs along the shoreline where drying kills eggs and fry of shore spawners.
7. FLOOD CONTROL
Mission Creek was originally located on a flood plain with meandering streams and annual flooding occurring all the way down to Gordon Road.
This provided clean gravel bars and pools for Kokanee. Now straight dikes along much of the prime spawning habitat are causing channelization and gravel distribution downstream.
As a result, the original 8 km of spawning habitat has been reduced to 3 km.
8. HARMFUL FORESHORE DEVELOPMENT
Foreshore damage usually occurs when owners unintentionally destroy foreshore habitat (Littoral Zone).
Natural vegetation along shorelines provides refuge for animals that live there.
Aquatic vegetation (milfoil) removal reduces habitat needed for nursery fishes to grow.
9. INSTREAM GRAVEL IMPROVEMENT
Between 0.5 and 1 inch diameter gravel is preferred for spawning by Kokanee salmon (Peachland Creek is a good example of this).
10. PROVISION OF SPAWNING CHANNELS
The survival of Kokanee eggs are limited by stream flow and by silting of gravel that restricts the flow of water. If these factors are controlled and optimum stream conditions prevail then a greater production of young salmon will result.
11. SHORE SPAWNING HABITAT PROTECTION
By identifying shore spawning habitat, we can:
- set it aside for wildlife;
- influence developers to make the area compatible to the already existing environment.
12. MISSION CREEK SPAWNING CHANNEL
The spawning channel was established in 1988 and rebuilt in 1995 by the BC Ministry of Environment along with various community groups in order to compensate for the reduction of Kokanee salmon in Okanagan Lake.
The Mission Creek spawning channel is located 7 km upstream from the outfall of Mission Creek into Okanagan Lake.
The channel is approximately 1 km long and 3.6 meters wide. It provides 3200 square meters of sized spawning gravel arranged on 28 reaches separated by weirs approximately 30 cm high.
The most important times in the life of the Kokanee in terms of survival are spent during incubation in the stream and during growth in the lake.
The growth period for the Kokanee salmon fry takes place in the lake. It is at this stage that they must find the food they need to survive and once again there are certain requirements that must be met in order for them to do so. Twenty years ago Mission Creek provided a spawning and rearing environment for 80 – 90% of the tributary spawning Kokanee of Okanagan Lake.
The Ecology: Growth
1. KOKANEE GROWTH
Kokanee growth may be followed from egg to adult maturity and spawning. Both shore-spawning and stream-spawning Kokanee salmon spawn in the fall of the year.
Eggs incubate in the gravel over the winter, hatch into alevin ( Fig 1 ) in the new year, and emerge from the gravel as fry from March to June. The precise period of incubation from egg deposition to emergence is dependent on water temperature ( Fig 2 ) .
Fry begin feeding upon entry into the lake in spring. Up to this time, a length of approximately 23 mm, they have lived on yolk provided in their eggs by their mother. Further growth is very dependent on food availability. Typically they reach over 20 cm during their third feeding summer and spawn that fall. Those smaller than 20 cm postpone spawning to the next fall at age four. Some choose to grow very large (55 cm) before spawning. All die after spawning. Fig 2 shows the typical growth rate of Okanagan Kokanee salmon and typical size at spawning.
It is not understood why some postpone spawning until they are considerably larger than others but large males do have advantages in securing mates on the spawning ground, and large female do lay many more eggs than small females.
In contrast to other salmonid species, Kokanee salmon are plankton feeders throughout their lives. As such they would be placed at the third thophic level in a typical food chain presentation ( Fig 3 ). A major food species throughout the first summer are copepods, while daphnia are preferred when available later in the summer. However, when their major food source consists of carnivorous zooplankton then Kokanee would be at the fourth level. For example, Kokanee larger than 25 cm in length feed on the introduced carnivorous zooplankton Mysis shrimp (Mysis relicta)
( Fig 4 ).
During several points in their life cycle Kokanee experience growth difficulties related to feeding. Upon fry emergence from the gravel they only have about five days to establish exogenous feeding or they die of starvation. Stream spawners must migrate down the stream and into the lake to begin feeding. Shore-spawners may emerge and feed immediately.
The seasonal timing of first feeding appears synchronized to the spring period when lake production of Kokanee food is increasing. We think that this is the explanation for the later fall spawning of shore-spawners compared to stream-spawners. Streams are colder therefore incubation is longer so back calculation from optimal lake conditions for spring fry feeding would necessitate stream spawning earlier in the fall (September 1 to October 15) compared to shore spawning (October 15 to November 15).
Survival through the first winter as fry in the lake also appears difficult. Lack of food leading into their first winter either due to competition or a colder than normal fall, may leave them without sufficient body storage to see them through the winter. This is exacerbated if the winter period is extended due to a late spring so that the typical spring increase in lake productivity is delayed. Some scientists think that this may be the major factor in the decline of Kokanee numbers in Okanagan Lake. The Mysis shrimp is hypothesised to outcompete the young Kokanee for their optimal food heading into their first winter.
Survivors of the first exogenous feeding winter then have a high expectation of surviving to adulthood if they are not consummed by predators. Kokanee are food for numerous fish and waterfowl species. In the Okanagan Rainbow trout, Burbot, Mergansers, Common Loon, typically eat all sizes of Kokanee, and at spawning time sculpin, suckers, carp, and mallards are often observed to consume Kokanee eggs.
Plants and algae, when they grow, incorporate various nutrient elements (especially phosphorus) into their tissues. These incorporated nutrients are passed along the food chain to become part of animal tissues.
These nutrients exist naturally in waterways but when in excess can cause problems such as massive algae blooms and aquatic weed growth (milfoil).
Phosphorus normally enters the rivers and lakes from runoff waters and from decomposing vegetation and carcasses of spawned out salmon.
3. FOOD SUPPLY
Kokanee compete for zooplankton with species such as the freshwater shrimp (Mysis relicta), Rainbow trout, whitefishes, northern squawfish and suckers.
A low food supply reduces the Kokanee growth rate resulting in insufficient body reserves to survive their first winter.
4. LOW PREDATION ON KOKANEE
Large Rainbow trout feed exclusively on Kokanee therefore the Kokanee must avoid the trout to survive.
5. WATER DAMS
These dams cause a reduced flow of phosphorus thus reducing lake productivity. The phosphorus settles out of the water behind the dam.
6. TERTIARY SEWAGE TREATMENT
As far as we know, outfall from sewage treatment plants into Okanagan Lake does not impact the lake’s water quality other than reducing the phosphorus level.
Phosphorus loading to Okanagan Lake has been cut from 59,100 kg in 1970 to 3,500 kg in 1994 due to the improved sewage treatment plant.
7. INTRODUCTION OF EXOTIC SPECIES
Mysis relicta, the Oppossum shrimp, was introduced into Okanagan Lake in 1966 as a food source to enhance Kokanee production.
In 1973 Mysis were found in Rainbow trout and in plankton hauls. Since then they have become a common organism in the fauna of Okanagan Lake and are thought to outcompete young Kokanee for the zooplankton on which both populations feed.
8. ADDITION OF NUTRIENTS
Most of the decaying vegetation and other waste products carried down by the streams during spring runoff are in a form not immediately available for plant growth.
Such materials may settle out before they are decayed, and become locked in the bottom sediments of a lake thus can not be utilized.
The level of Okanagan Lake’s productivity is known as being oligotrophic (nutrient poor).
9. REMOVAL OF COMPETITION
If the lake’s productivity is increased there will be more food resources for all organisms so there will be less competition among the Kokanee and other species.
Perhaps the Mysis shrimp should be reduced in numbers. How this will be done is still unknown. But regardless of what is decided it is important to realize that the interaction between the Mysis, Kokanee and zooplankton are complex; thus quick fix solutions are not possible.
10. DETERMINATION OF AGE
Scales and otoliths (inner ear bones) are used to determine the age of a salmon.
The scales of the salmon grow, and as they do they form rings just like those of a tree trunk.
These rings form bands which can be counted from the widely-spaced rings occuring in the spring and summer months when food supply is abundant, and the more tightly-packed rings which indicate the winter months when growth is slow from limited food supply.