Survival Strategies, a study in contrasts between two salmonid species and their implications in land management

by Cate Moore on March 23, 2013

Cate Moore

All salmonids are not created equal; this is becoming more apparent as studies continue into the life cycles of coho salmon and steelhead trout in the Scott Creek watershed.  The participants in the northern field trip of the Redwood Symposium got a fascinating update of the latest observations of the life cycles and needs of these two fish as they live the freshwater portion of their lives in Scott Creek and its tributaries.

The Scott Creek watershed is composed of Scott Creek and its tributaries Mill Creek, Big Creek and Little Creek.  These creeks begin in steep canyons under redwood canopies, then join in a wider river valley where the stream is sheltered by deciduous trees such as alder and big leaf maple, then flow into an estuary which often becomes landlocked for several months of the year by sandbars, forming a deep warm water lagoon.

The coho and the steelhead utilize different features of this topography in their life cycles at different times of the year, and their survival prospects are affected by their patterns.

The winter spawning cycle begins after the sandbar has been broken, typically in November to mid or late December.  The coho arrive in mid-December and are done by February; the steelhead start arriving in late December and finish in April.  Spawning for both species takes place in the headwaters.

The coho fry spend a year in the upper reaches of the watershed under the redwood canopy.  The coho smolts from the Scott Creek watershed are typically 10 cm long when they leave to go to sea, which is 1 cm shorter than ideal.  The hardest part of their life cycle is reported to be the transition into saltwater, where they must change their feeding patterns and learn to avoid a whole new array of predators.

Steelhead also spend their first year in the upper watershed, often under the deciduous riparian section of the valley, also growing to 9 to 10 cm in length and weighing in at 10 grams.  About 10,000 smolts descend into the estuary in a year; most of them choose to move out to sea, however 500 to 3,000 smolts spend about five months in the warm waters of the estuary, growing to up to 20 cm and weighing up to 100 grams when they do go on to the ocean.  The estuary allows them to grow very quickly to a much larger size, but it is not a safe environment.  The young fish must avoid an array of predators while they are there, including kingfishers, egrets and mergansers, who eat 50 to 100 fish a day.  The fish that remain in the headwaters of the watershed do not face as many predators, but they are subject to the danger of reduced water flows and starvation.  Insufficient sunlight seems to be a contributing factor to the lack of food.  Measurements of tagged fish from the headwaters show that they lose weight over the summer.

In general, the headwaters topography lends itself best to spawning, the lower valley and the estuary to rearing the smolts to ocean readiness.  The valley’s deciduous riparian trees contribute less tannins and more insects to the stream, enhancing the available food supply.  Coho favor areas with large wood in the stream and steelhead favor riffles.  It has also been observed that the local bloodlines do not need the cool temperatures needed by fish in Oregon and Washington.

Once in the ocean, coho inhabit coastal waters all up and down the coast, while steelhead spend their ocean time in the Aleutians before returning back to spawn.

These differences in life needs between the fish raise questions for land managers about how best to accommodate both species.  This close examination of both fish makes it apparent that some of their needs are mutually incompatible.  To me, the greatest difference appears to lie in the fish’s need for ocean access.  The stream structure that steelhead favor, featuring a landlocked lagoon at the end for much of the year is a serious impediment to the coho.

Land managers know they can use their money and effort most effectively by following the direction nature wants to take.  In this case, it means management will be most effective when they examine their stream’s entire geomorphology to choose which fish to favor.  If a stream is to be managed to favor coho, it would be most effective to choose those streams that are open to the sea year round.  If the stream is to serve steelhead, choosing a stream that does get blocked by sandbars to form a lagoon will serve their needs best.

After a stream has been assigned a dominant salmonid specie, then the work of providing for the rest of its needs can commence.  Spawning beds need to exist, and adequate light must touch the stream to grow smolt food.  The riparian vegetation must be the right sort; alders, maples and other hardwoods appear to be better than redwoods and other conifers.  Mosquito abatement can adversely affect salmonid food supplies, which will force a value judgment to be made by the affected people of which is more important, fish or disease control.

Many of these factors are well outside the bounds and capabilities of individual land managers.  How useful is it to require someone who owns land in the headwaters of Soquel Creek to provide coho spawning habitat when Santa Cruz County has an active mosquito abatement program and Soquel Creek gets blocked by sand bars regularly?

We welcome further debate and refinement in the salmon restoration projects.  There are a great many interlocking factors at play and not all of the work can be accomplished by forest land managers.  Residential communities, parks, urban communities and other agricultural concerns all touch our streams and they each have their own impacts on the fish as they pass by.


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