Spawning Gravels of Salmon and Trout

Salmonids (salmon and trout) require freshwater stream gravels for spawning. The female digs a depression in the gravel stream bed by abruptly jerking her tail upwards from lying flat on the stream bed, thereby pulling gravel particles into the current, which carries them downstream, and which washes away some of the interstitial fine sediment within the gravel. The female deposits the eggs within the depression (pitt) and the attending male releases milt over them. The female then loosens gravel immediately upstream, which the currents carry downstream to cover the eggs. The eggs remain in the completed redd (nest) for a period of weeks or months, depending on water temperature. The embryos depend upon a flow of water through the gravel to supply them with oxygen and to remove metabolic wastes. After hatching, the alevins (fry) continue to live in the gravel for a period of time, then wiggle through the gravel to the surface, where they emerge to begin their lives as free-swimming fish.

In general, the literature suggests that interstitial sediments finer than about 1 mm reduce the permeability of the gravel and can impair the intergravel water flow needed to provide oxygen and remove metabolic wastes from embryos, while sediments in the 1 to 10 mm size range have been implicated in blocking intergravel pores; the embryos can successfully hatch into alevins (fry), but they are unable to migrate upwards through the gravel to emerge because the intergravel pores are blocked. In the course of spawning, interstitial fine sediment is removed from the gravel. As the fish tail lifts the gravel above the bed, fine sediment is exposed to the current and washed downstream, leaving a better-sorted gravel in the tailspill. Comparisons of the percentage fine sediment before and after spawning (or in between redd gravels and adjacent, unspawned gravels) shows that the amount of cleansing effected by spawning depends mostly on how much fine sediment was present initially (Kondolf et al. 1993).

For a gravel deposit to be useable for spawning by fish, the fish must be capable of lifting the gravel from the bed to create a redd, a requirement that imposes an upper limit on the size of the framework grains of the gravel. While salmonids use a wide range of gravel sizes for spawning, it is possible to define an envelope curve relating median size of spawning gravel used to the fish length. In general, fish can spawn in gravels with a median diameter up to about 10% of their body length.

Much of the literature on spawning gravels has been oriented to finding a single index that can capture all characteristics relevant to salmon spawning success, such as the geometric mean diameter and the fredle index. However, assessment of gravel quality could be more profitably approached by realizing that the appropriate measures depend on the questions being asked. If the question is whether gravels are too coarse to be mobilized by the spawning female, the framework size is of interest and the D50 (median) or D84 could be used. If the question is whether fine sediment content is excessive, a measure of the fine tail (such as D16 or the percentage finer than a given size) is more appropriate.

SELECTED RELEVANT PUBLICATIONS

Kondolf, G.M. 2000. Assessing salmonid spawning gravels. Transactions of the American Fisheries Society 129:262-281.

Kondolf, G.M., and A. Adhikari. 2000. Weibull vs. lognormal distributions for fluvial gravels. Journal of Sedimentary Research 70(3):456-460.

Kondolf, G.M. 1997. Application of the pebble count: Reflections on purpose, method, and variants. Journal of the American Water Resources Association (formerly Water Resources Bulletin) 33(1):79-87.

Kondolf, G.M., J.C. Vick, and T.M. Ramirez. 1996. Salmon spawning habitat rehabilitation on the Merced River, California: An evaluation of project planning and performance. Transactions of the American Fisheries Society 125:899-912.

Kondolf, G.M. 1995. Discussion: Use of pebble counts to evaluate fine sediment increase in stream channels by John P. Potyondy and Terry Hardy. Water Resources Bulletin 31(3):537-538.

Kondolf, G.M., and M.G. Wolman. 1993. The sizes of salmonid spawning gravels. Water Resources Research 29:2275-2285.

Kondolf, G.M., M.J. Sale and M.G. Wolman. 1993. Modification of gravel size by spawning salmonids. Water Resources Research 29:2265-2274.

Kondolf, G.M., G.F. Cada, M.J. Sale and T. Felando. 1991. Distribution of potential salmonid spawning gravels in steep, boulder-bed streams of the eastern Sierra Nevada. Transactions of the American Fisheries Society 120:177-186.

Kondolf, G.M., S.S. Cook, H.R. Maddux and W.R. Persons. 1989. Spawning gravels of rainbow trout in the Grand Canyon, Arizona. Journal of the Arizona-Nevada Academy of Science 23:19-28.

Kondolf, G.M. 1988. Salmonid spawning gravels: A geomorphic perspective on their distribution, size modification by spawning fish, and application of criteria for gravel quality. PhD thesis, The Johns Hopkins University. Available at the libraries of Johns Hopkins University and University of California, via interlibrary loan, and from University Microfilms, Ann Arbor, Michigan.