Habitat Requirements for Coastal Coho Salmon Populations

Habitat Requirements for Southern Chum Salmon Populations

General Life History

Chum salmon have the most extensive distribution of all the salmon species. Southern chum populations in British Columbia spawn in coastal systems ranging from streams near the U.S. border in the south to the northern end of Vancouver Island. More than 400 populations originate from this area but 45 of them account for 85 % of the total production with the Fraser River stocks being the largest producers. Southern chum stocks are divided into two groups based on the timing of spawning migration. The summer run chum stocks migrate between June and August and spawn in September and early October. These are generally smaller populations the largest of which return to Knight and Bute inlets on the mainland coast. The fall run chum stocks migrate in September, October and November and spawn between October and January. The vast majority of southern chum salmon production comes from the fall run complex. Spawning migration timing for all southern chum stocks is later than for northern populations and age at maturity is younger with either three or four year old fish dominating the return. Early timing stocks tend to have a higher proportion of older fish than those that enter spawning streams later in the year.

Chum salmon spawn most commonly in the lower reaches of rivers from areas of tidal influence to about 200 km from the sea. Areas selected for redd construction are either in the mainstem of the river or in side channels and are generally immediately upstream of turbulent flows or where there is a source of upwelling water. When fish reach the spawning grounds, the female selects a nest site with the appropriate features including adequate depth, flow and nearby cover, and begins digging a pit referred to as a redd, where eggs will be deposited. 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 is partially dependent on water temperature. In general, the lower the water temperature, the longer the incubation period required. Upon hatching, the juvenile chum salmon (called alevins) move within the spaces between the gravel particles varying distances depending on gravel size. The newly hatched fish have an attached yolk sac that provides the required nutrition. Towards the end of incubation (late winter to early spring), alevins move up through the gravel to emerge as fry. This process occurs at night which helps to minimize predation and generally coincides with the complete absorption of the yolk sac. The survival of chum salmon eggs from spawning to emergence varies widely between systems and years and is influenced by stream flow, water temperature, dissolved oxygen, gravel composition, spawning timing and spawner density. Typically, around 10% of the eggs deposited by females will survive to the fry stage.

After emergence, chum salmon fry migrate quickly downstream to the estuary where they spend time rearing before making the transition to saltwater. The timing of this migration ranges from February to May. Chum fry do not school as strongly as either pink or sockeye salmon and predation rates can be significant during fry migration. One study found that predation mortality ranged from 20 to 85 % over a 2.6 km migration path.

Chum salmon are highly dependent on estuaries for rearing and preparing for life at sea. Entry into the estuary is timed to take advantage of plankton production in this nutrient rich habitat. Juvenile chum stay in the estuary longer than most other salmon species spending up to several months in tidal creeks, sloughs and side channels where their movements are often coordinated with the pulse of the tides. The period of residence in estuarine and near shore waters around river mouths appears to be the most critical phase in the life history of chum salmon and plays a major role in determining the size of the subsequent adult return. As chum salmon fry leave the estuary they generally remain close to the shoreline feeding in the productive, shallow, near shore marine habitat until large enough to move offshore. Food items consumed include various zooplankton species as well as adult and larval insects. As they grow, the juveniles begin to migrate northward along the coast where they are often found together with pink salmon juveniles relatively close to shore. During the fall or early winter, a general dispersion from the coastal belt occurs and southern chum mingle in off shore waters with other stocks from around the north Pacific basin. During the spring and early summer of their final year at sea, maturing chum salmon depart the open ocean areas of the Gulf of Alaska and begin the long migration back to their home streams to spawn.

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. Southern chum salmon populations require quality freshwater spawning and incubation habitats to thrive and remain productive. Since chum salmon are not significantly dependent on fresh water as a rearing habitat, it is the quantity of prime spawning and incubation areas that determine freshwater production potential. Estuary and near shore environments are also vital and the rate of survival through the rearing period spent in these habitats greatly influences the return run size. Healthy habitat is challenged by human competition for accessible land and fresh water, for ocean spaces and for the interconnecting estuarine and coastal areas. The loss or degradation of quality habitat in any of these important zones will have a negative impact on chum salmon populations.

Spawning

Adult chum salmon require 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 for long periods, waiting for conditions to improve. This delay 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. As a result, it is important to critically assess any activities that impact river flows or water temperatures when chum salmon are returning to spawn and to ensure that fish have unimpeded access to spawning grounds.

Chum salmon require spawning sites within the stream or river where water velocity, depth and gravel size are optimal for the incubation of developing eggs. The substrate must be small enough to be moved by the fish and large enough to allow good intragravel water flow to the incubating eggs and developing alevins. Chum salmon prefer moderately sized gravel with minimal fine sediments and some upwelling. Water depth must be sufficient to cover the redd at all times. A lack of prime spawning habitat can limit chum salmon production as later spawners may be forced to build redds in secondary locations or on top of previously constructed redds resulting in reduced overall production. When spawner densities are high, the digging up of redds by later spawners has resulted in up to 50% of the total eggs lost as they are exposed and washed downstream. This situation may be magnified during years when spawner of both pink and chum salmon are abundant as they often compete for the same spawning areas.

Incubation

The survival of chum salmon eggs from spawning to emergence varies widely between systems and years and is influenced by stream flow, dissolved oxygen, gravel composition, water temperature, spawning timing and spawner density. Studies indicate that survival of chum salmon embryos and alevins is higher in more stable flow regimes. In one southern system, survival to emergence averaged 11.2 % before flow control structures were installed and averaged 24.9 % after the installation. Successful incubation requires flow rates that are adequate to supply the required level of oxygen but not high enough to cause gravel movement and streambed scour which could expose eggs to predators or wash them downstream. Data shows that the average flow rate preferred by chum salmon is approximately 50 cm/s.

The percentage of chum salmon eggs and alevins that survive depends to a large extent on stream and stream bed conditions. Studies have found that higher survivals were correlated with high gravel permeability which ensured that the developing eggs and alevins were supplied with a constant current of water that delivered oxygen and removed waste. Spawning areas with slightly larger gravel size and low rates of sedimentation consistently generate higher survival rates. In cases where large amounts of silt build up in spawning beds survival rates are greatly reduced. This situation can occur in areas where streamside activities such as logging, road building, or agricultural practices result in high sediment runoff into the river or where high flows move sediments from upstream areas into spawning beds.

Incubation temperatures outside the ideal range can cause hatching and emergent times that reduce survival. In extreme cases, freezing of redds can result in the loss of all eggs in the affected areas. Studies indicate that while chum salmon eggs and alevins can withstand a wide fluctuation in temperature, decreased survival and impaired development occurs at incubation temperatures below 4.5 0C and above 14 0C. The removal of streamside (riparian) vegetation can result in lower winter temperatures and higher summer temperatures and it is important that healthy, natural growth remains undisturbed along the banks of salmon bearing streams.

The water surrounding pink salmon redds must be non-toxic and of sufficient quality to provide the basic requirements of incubation. There are many types of pollution that can affect water quality including waste water, pesticides, toxic chemicals, petroleum products and organic compounds. All efforts must be made to minimize the introduction of such pollutants into salmon incubation area.

Juvenile Migration and Rearing

After emergence from the gravel, chum salmon juveniles typically begin an immediate migration towards the ocean. The period of emergence usually coincides with the high river runoff and most migrations are short with juveniles reaching the estuary on the first night of travel. Those fry not completing the journey on the first night typically seek out refuge areas as predation during seaward migration can be a major sources of mortality on chum fry. In one study, researchers found that almost 85 % of the emergent chum fry were lost to predators during migration. Although cover requirements are not well understood, the fry are known to hide in the spaces between bottom substrates during the day as they migrate downstream. As a result, an unimpeded migration path with cover from predators is important to the survival of chum fry traveling to the estuary.

Water temperature is an important environmental variable for seaward migrating chum salmon fry. Temperature can affect the timing of downstream migration, as well as the growth, survival and smolting of chum fry. The recommended temperature range for successful downstream migration is from 6.7 to 13.3 0C. Temperatures above 15 0C have a detrimental effect on fry behaviour and performance and may cause premature migration. As mentioned earlier, alterations to streamside vegetation can impact stream temperatures and it is important to maintain healthy riparian growth in all salmon bearing streams.

The period of residence in estuarine and near shore waters around river mouths is a critical phase in the life history of chum salmon and appears to play a major role in determining the size of the subsequent adult run back to freshwater. Coastal estuaries are important as they provide an environmental transition zone, early opportunities for feeding and growth, and refuge from predators. As environmental transition zones, brackish estuaries allow out-migrant chum salmon the opportunity to acclimate from freshwater to saltwater and between waters of differing temperatures. They provide the first substantial opportunities for feeding and growth for species like chum salmon that migrate to sea soon after hatching, and typically have higher food productivities than adjacent ocean or freshwater areas. Estuaries may thus offer the opportunity for enhanced growth and therefore, larger size at ocean entry which is known to correlate with higher marine survival. One final role of estuaries is to provide refuge from predators. The higher turbidity often associated with estuarine areas limits the ability of visual predators to key on salmon juveniles thus reducing their exposure to the fish and birds that feed on salmon fry. All of these factors point to the importance of estuaries in the life of chum salmon and emphasize the need to protect these fragile areas from any activities that may be detrimental.