The benefits of large woody debris for breeding salmon

We may usually think of deadwood as something that benefits terrestrial species (bats, insects, fungi, mammals), though this is not the limit of what organisms may benefit from its presence. Many rivers and streams will, at some point in their existence, run through woodland and adjacent to hegderows, and beyond the beneficial cooling effects of a dappled shade courtesy of a canopy cover, the provisioning of deadwood (coarse woody debris) into the aquatic ecosystem may be of marked benefit for fish. Without question, the exact requirements of a woodland stream or river will vary depending upon constituent species, though by-and-large the presence of deadwood can be considered to be of benefit. The slowing-down of flow to create pools can aid with feeding, can certainly help salmon travelling upstream (in terms of enabling them to expend less energy on reaching egg-laying grounds), and can also reduce the build-up of silt on the stream bed by reducing bank erosion (‘clean’ gravel beds are critical for successful egg-laying). Benefits beyond this absolutely exist, and in this case we’ll be looking at the relationship between Chinook salmon redds (spawn sites) and large woody debris within the stream / river environment in a Californian river.

The study site chosen by the researchers was a 7.7km stretch of the lower Mokelumne River, where around 90% of all the river’s Chinook salmon redds are created. At its source, some 3,000m above sea level, its watershed is dressed with mountainous forests, before flowing westwards through oak woodlands and agricultural fields clad with tree belts and hedgerow. Riparian zones adjacent to the river are around 20m in width, and the constituent trees (alder, cottonwood, oak, walnut, willow, etc) reach heights of up to 25m. The river ends at its confluence with the San Joaquin River. The Mokelumne River is considered to be of medium size, with a channel width ranging from 15-83m (averaging at 31m). The below map gives more of a representation of the river’s location.

The location of the study site in the wider landscape.

Because of the historic management of the river (gravel bed extraction, salmon fishing, and so on), the river had actually been rather degraded up until more recently (management began in the 1960s to repair the degradation). Improvements to the river involved the addition of significant (over 100,000 tonnes) amounts of gravel and cobble (for improving spawning beds), and this has served to markedly increase Chinook salmon populations to the point that over 2,000 redds are created each year (up from only a couple of hundred). A great deal of the gravel augmentation took place in the 500m stretch of river west of the Camanche Dam (this dam sits just to the east of the study site), and was added in a way that made the river bed heterogenous in nature (providing riffles and pools, viable egg-laying sites, and even locations for adult salmon). Boulders were also added, as were large pieces of woody debris. This woody debris was buried in parts by gravel, so not to have the woody debris drift away.

The lower Mokelumne River. Source: Wikimedia Commons.

In this study, a total of 340 pieces (plus nearly 200 more where there we no redds) of large woody debris in the study area were surveyed and mapped (as redds were located nearby), where the average length was 6.9m (give or take 4m), and the average diameter was 23cm (give or take 12cm). The large majority (70%) of the large woody material was oriented in the direction of flow, and only 20% was oriented laterally (and never beyond 6m into the river). A total of 59% of the 340 deadwood pieces were located within the river but no more than 2.5m away from the river margin. The rest resided (at least partially) upon the bank. 65% of the deadwood was at least significantly decayed, and much of the identifiable deadwood was from alder.

In the first 3km of the study site, Chinook salmon utilised an area, 10m in radius, surrounding large woody material located in riffles (to a total of 68% of all redds). In the western 4.7km, this dropped to 44% of all redds. In many instances, these redds were situated downstream from large woody material, and were located in areas where there had been gravel augmentation. Furthermore, most of the redds were found within a 10m margin from the river bank, which is where 90% of all woody debris was also located. In this sense, one can identify how large woody material is certainly important for Chinook salmon redds, and the addition of such woody material to the river ecosystem is likely to have been a marked driver behind why Chinook salmon populations have increased. Reasons are because of, for example, altered flow velocities, increased levels of dissolved oxygen, cooler temperatures, and the shelter (against predators and other female salmon) provided by woody debris for egg-laying females (a larger list is given at the end of the journal article).

By a similar token, the addition of gravel would have provided improved conditions for redd creation by female Chinook salmon, though it is perhaps almost certain that the two, when combined, have the most beneficial impact. In fact, the researchers suggest that the much-increased habitat complexity associated with their presence is of great benefit and therefore, from a river restoration perspective, large woody material should be incorporated into management projects associated with ecosystem rejuvenation.

Source: Senter, A. & Pasternack, G. (2010) Large wood aids spawning Chinook salmon (Oncorhynchus tshawytscha) in marginal habitat on a regulated river in California. River Research and Applications. 27 (5). p550-565.

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The benefits of large woody debris for breeding salmon

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