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Habitat Requirements for Lake Rearing Sockeye Salmon Populations
General Life History
The distribution of lake rearing sockeye salmon populations is based on the requirement
of a nursery lake for the freshwater rearing of juveniles. The largest populations in British
Columbia are located in the Fraser and Skeena river drainages where tributary systems are often
connected to large lakes. Other large populations exist in the Nass River system in northern BC,
the Somass River system on the west coast of Vancouver Island and in both Rivers and Smith
inlets in central BC. Smaller stocks spawn in systems with nursery lakes throughout the
province. The majority of mature adult sockeye return to their river of origin to spawn during
the summer or autumn as four year old fish that have spent two years in the ocean. The
exceptions to this pattern include small fish (primarily males referred to as jacks) that return to
spawn after one year of ocean residence and those fish that remain in the ocean for a third year.
Migration distances from the ocean to freshwater spawning grounds range from a few kilometers
to over 1,000 km. The initiation of the spawning migration for each unique stock is timed to
capitalize on ideal conditions in the freshwater habitat that will ensure appropriate egg
development. In most cases, stock specific migration timing is remarkably consistent between
years.
When mature sockeye salmon reach the spawning grounds, the female selects a nest site
and begins digging a pit referred to as a redd, where eggs will be deposited. Most spawning
areas are located in rivers or streams that are tributary to lakes; however, some stocks spawn in
lake outlet streams while others spawn along the lake shore. The digging process removes sand,
silt and fine gravel from the nest site creating a favourable environment for incubation of the
eggs. Once the nest is complete, the female deposits the eggs which are fertilized by one or more
males and then moves to the area immediately upstream of the nest and begins digging another
pit. The material removed by this digging action covers the fertilized eggs to protect them from
predation and from being washed away by the scouring action of the river or stream. This
process may be repeated several times resulting in multiple nests containing eggs from one
female.
The length of time required for the eggs to incubate depends to a large extent on water
temperature. In general, the lower the water temperature, the longer the incubation period
required. Upon hatching, the juvenile sockeye (called alevins) move downward in the gravel
varying distances depending on gravel size. At this point, the young fish have an attached yolk
sac that provides the required nutrition. Towards the end of incubation, alevins move up through
the gravel to emerge as fry. This occurs after nightfall between April and early June and
generally coincides with the complete absorption of the yolk sac. In streams that are tributary to
lakes, fry swim downstream toward the nursery lake often moving passively with the current. In
lake outlet spawning areas, newly emerged fry move toward the stream bank where flow is
reduced and swim upstream against the current toward the nursery lake. Timing of lake entry by
fry is determined by many factors that ensure juveniles encounter the best possible conditions for
feeding, predator avoidance and growth.
When juvenile sockeye enter the lake they spend the first few months close to the
shoreline where they feed on larval and adult insects. Once they have moved to deeper offshore
waters, juveniles feed on a variety of zooplankton species that are plentiful in the upper water
column. In this environment, they exhibit a complex pattern of daily vertical migration that
varies between lakes and changes seasonally. In general, they occupy surface waters at night
moving down to deeper waters during the day. This pattern of movement serves to maximize
feeding opportunities while minimizing exposure to predators. It also ensures that they receive
the greatest benefit from food ingested by controlling metabolism and maximizing growth.
In the vast majority of cases, juvenile sockeye from lake rearing populations in BC leave
the nursery lake after one year of residence. In some less productive lake environments, two
years of lake rearing occur before migration to the ocean. Juvenile sockeye typically leave the
nursery lake as water temperatures increase in the spring. A process called smoltification begins
to occur in which both physical and behavioral changes take place in preparation for life at sea.
These include becoming more silver in colour, slimmer and more streamlined in shape and
undergoing the physiological changes required for tolerance to seawater. During the
downstream migration, the smolts travel in schools and migrate actively (faster than the current
in slow moving water). Most movement takes place at night in order to reduce the risk of
exposure to predators.
Sockeye smolts exhibit a variety of behaviors upon reaching saltwater. While some may
reside for a short period in the river estuary or in nearshore waters adjacent to the river mouth
most disperse into coastal waters more quickly. Movement to offshore areas begins with a
general migration to the northwest along the coast. During this time, sockeye feed on a variety
of organisms preferring zooplankton, insects and fish larvae. By July, they tend to from a band
approximately 40 km offshore that extends from southwest coast of Vancouver Island to the Gulf
of Alaska. Movement continues in a northwesterly direction at a rate of approximately 18
kilometers per day.
Once in the Gulf of Alaska, sockeye begin to move further offshore to open ocean waters.
This typically occurs in late autumn or early winter. Patterns of movement in the ocean are
complex and are influenced by many factors including temperature, currents and food
distribution. During this period, sockeye are widely distributed throughout the North Pacific
where they feed on large zooplankton, small fish and squid. For most sockeye salmon, sexual
maturation begins during their second year of ocean residence when they begin an accelerated
spring-early summer migration back to the river of origin.
Habitat Requirements by Life History Stage
The health of all Pacific salmon is closely linked to the availability of productive
freshwater, coastal and marine environments. Lake rearing sockeye populations require quality
spawning, incubation, lake rearing and ocean habitats to thrive and remain productive. The loss
of quality habitat in any of these vital areas will have a negative impact on stocks. Healthy
habitat is challenged by human competition for accessible land and fresh water, for ocean spaces
and for the interconnecting estuarine and coastal areas. In order to maintain productive lake
rearing sockeye populations throughout the Pacific coastal region, these important habitats must
be protected.
Spawning
Adult sockeye salmon require unimpeded access to their home spawning grounds in
order to successfully reproduce. Features such as dams, debris jams, waterfalls, or rock/mud
slides that block upstream migration can limit access to spawning areas and impact production.
Also, if conditions such as high water temperature or extreme high or low flows are encountered
when spawners arrive at their river or stream of origin, fish often mill about in the vicinity of the
river mouth, waiting for conditions to improve. Such delays in river entry can have a detrimental
affect on survival and on spawning success as fish are exposed to predation from marine
mammals and, since feeding has stopped in preparation for spawning, vital energy reserves are
used up. This is of particular concern for those sockeye populations with the longest spawning
migrations. In years when river conditions present a difficult migration environment due to
elevated water temperature or extreme flows, a high percentage of some stocks will run out of
energy and die before they reach the spawning grounds. For some large Fraser River sockeye
populations, rates of such pre-spawning mortality have been very high in some years. While it
may not possible to prevent these conditions in all situations, every effort must be made to
ensure that river temperatures and flows are not negatively impacted by human activity. In
those years when adverse migration conditions are anticipated, it may be necessary for fishery
managers to reduce the catch of stocks undertaking long spawning migrations to ensure that an
appropriate number of fish reach the spawning grounds.
Sockeye salmon exhibit the greatest diversity in selection of spawning habitat among the
Pacific salmon. They may spawn in inlet or outlet tributaries of a nursery lake, in suitable
habitat between lakes, along lakeshores, or on submerged beaches. In all of these habitats,
sockeye salmon require extensive, high quality spawning gravels for optimum production.
Areas selected by female sockeye for redd construction tend to be located where the substrate is
small enough to be moved by the fish and large enough to allow good intragravel water flow to
the incubating eggs and developing alevins. This ensures that the environment in the nest is
supplied with a constant flow of water to deliver dissolved oxygen and to remove waste. Those
sockeye that spawn on lake shores require access to undisturbed shorelines with quality gravel
and clean, upwelling groundwater to ensure high survivals. Field observations indicate that
while many spawning areas are presently under-utilized, other areas appear to be filled to
capacity in some years. This observation suggests that a lack of prime spawning areas can limit
sockeye production in some systems as spawners may be forced to build redds in less suitable
locations or on top of previously constructed redds resulting in reduced overall production.
Incubation
Studies indicate that at a constant temperature, sockeye salmon have the longest
incubation period of the five pacific salmon species. During incubation, eggs and alevins
require a stable environment with an uninterrupted supply of clean, oxygen rich water. The
percentage of eggs and alevins that survive the incubation phase depends to a large extent on
stream and stream bed conditions. Flooding during the winter can result in a large amount of
gravel movement which reduces survival by causing eggs or alevins to be exposed and swept
downstream. Also, silt loads associated with flooding may hinder water circulation through the
redd reducing available oxygen to harmful or lethal levels. Studies have shown that a higher
proportion of fine sediment in the spawning gravel reduces survival and results in smaller
emergent fry. Activities that contribute to flooding or siltation of incubation areas can magnify
the impact of extreme winter conditions and reduce survival to emergence.
Juvenile Rearing
After emerging from the gravel, sockeye fry migrate directly to the nursery lake. During
this migration the fry are highly vulnerable to predators such as birds and large fish. Loss due to
predation during this stage varies but has been estimated to range from 15 to 90%. The
availability of appropriate cover can limit exposure to predators and reduce mortality on fry
during migration to the nursery lake. Upon reaching the lake, sockeye juveniles spend the first
few weeks in the near shore area where they feed primarily on insect larvae. Shoreline habitats
must remain favourable to the production of insect populations in order to sustain juvenile
sockeye during this life stage. With the growing settlement of lakeshore areas for recreational
purposes, it is important that shoreline areas remain in a natural state in order to maintain a
productive habitat for juvenile sockeye.
The longest period of freshwater residence for lake rearing sockeye salmon is spent in
the offshore waters of the nursery lake and juveniles require quality, productive offshore
habitat to thrive. During this period, sockeye juveniles employ various strategies to maximize
feeding while reducing exposure to predation which include vertical migrations and schooling
behaviors. In some lakes, species that are predators on juvenile sockeye have been enhanced or
introduced for recreational purposes. Such practices should be avoided in sockeye nursery lakes
and every effort made to maintain quality offshore lake habitats for juvenile sockeye.
There are many sources and types of pollutants that can affect the nursery lake habitat
including waste water, pesticides, toxic chemicals, petroleum products and organic compounds.
These substances can reduce the quality of the habitat for juvenile sockeye and for the aquatic
organisms on which they feed. All efforts must be made to minimize the introduction of
pollutants into sockeye nursery lakes.
Sockeye salmon smolts transiting from nursery lakes to the ocean require open migration
corridors that are free from obstacles that could delay or prevent passage. Since predation can
result in high mortality rates during this phase, it is important that migration paths have cover
available to smolts. Appropriate cover includes healthy, abundant streamside vegetation as well
as woody debris and large substrate material.
Ocean Phase
The transition zone from freshwater to saltwater provided by the river estuary is an
important habitat for all transiting juvenile salmon. Sockeye smolts require a natural,
productive estuary that can provide the necessary food resources while offering refuge from
fish and bird predators. The time spent in the estuary helps transiting sockeye smolts adjust to a
saltwater environment that is dramatically different from the freshwater habitat that has been
their home during the first part of their life. As they move into the ocean, another critical life
history phase begins. Research suggests that up to 90% of the natural ocean mortality occurs
during the first four months that sockeye are at sea. Since survival is generally size-selective,
favourable near-shore ocean productivity is important as it can result in faster growth and a
shorter time to reach the size required for increased survival. As a result, coastal ocean
productivity is a key habitat component that impacts the survival of sockeye salmon.
Sockeye travel far during the marine phase of their life reaching the northern, nutrientrich waters of Alaska and the Arctic. The distribution of sockeye in offshore waters is dependent
on ocean environmental conditions and on food availability. While detailed migration patterns
and other aspects of their marine ecology remain poorly understood, ocean residence is
recognized as a very important component of the life cycle of all Pacific salmon. During their
time at sea, sockeye migrate varying distances while increasing in size and acquiring the energy
reserves required for reproduction. While distribution patterns vary between years and stocks,
all sockeye utilize coastal and off shore habitats during a period of rapid growth that is critical to
reproductive success.
References
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