Fishing the Nursery is Shrinking ChinookJanuary 24, 2012
The life history of Chinook salmon differs in important ways from those of the other salmon species. Compared to all other Pacific salmon, Chinook salmon are bigger, they reach sexual maturity at a variety of ages (three and older for females, two and older for males), and they can attain older ages, up to eight years. Chinook salmon are, of course, well known for their relatively large body size. Historically, Chinook commonly attained weights in excess of fifty pounds and occasionally exceeded one hundred pounds. This characteristic is the result of not just longevity, but also a unique pattern of adult growth.
Chinook salmon grow relatively slowly during the first two years in the ocean and progressively faster in later years. This pattern indicates that Chinook “traded” the increased risk of dying before reaching maturity in exchange for the increased benefits of larger body size. Those benefits include not only more eggs to deposit, but larger, better quality eggs that give newly emerged fry a better chance of surviving early life in freshwater. Large-bodied Chinook can spawn in deeper, faster water, and build nests in larger cobbles that better protect eggs from scour during high flow events. Thus, Chinook salmon are potentially able to exploit spawning habitat that is simply unavailable to other salmon and steelhead, such as the spawning habitat in and above the steeper canyon reaches of the Elwha River.
These inherent attributes of Chinook resulted in large populations (in excess of one million in some large river basins) of large fish, at least until the intense commercial fisheries began in the last quarter of the 19th century. Up to 1920 or so in the Sacramento/San Joaquin and the 1930s in the lower Columbia River, the average weight of Chinook caught by the various in-river commercial fisheries exceeded twenty pounds (McDonald 1894, Rich 1940, Yoshiyama & Moyle 1998). By the 1960s, average weights coastwide had dropped below twenty pounds, and were closer to fifteen pounds in most areas. By 1975, the average weights of many stocks had begun to approach ten pounds (for the entire BC coast and Georgia Strait; Ricker 1981).
The average ages of Chinook declined in a corresponding manner. For example, based on tagging data conducted off the west coast of Vancouver Island in the late 1920s, Ricker (1981) estimated the spawning-age composition of unfished Chinook populations in this area to be: 12% of the population was three-year-old fish; 29%, four-years-old; 31%, five-years-old; 23%, six-years-old; and 5%, seven-years-old, yielding an average age of 4.8 years. By the mid-1990s, the average age of most British Columbia and Washington Chinook populations was less than 3.1 years. Significantly, the most common age of the majority of Chinook populations today is four-years-old, in contrast to five-years-old prior to the 1950s. In many of today’s populations, the second most common age is three-years-old, whereas prior to the 1950s it was six-years-old. This constitutes a huge decrease in spawning potential and life history diversity, and thus, in the resilience of Chinook populations to environmental challenges.
What caused this decline in age and size? Numerous factors have probably contributed to the current drastic condition of Chinook, including harvest, competition in the ocean from large hatchery releases, and large-scale changes in the ocean environment. However, the last two most likely only make worse changes that are fundamentally caused by harvest. For thousands of years prior to European colonization, Native Americans harvested salmon with traditional methods in or near the rivers as mature salmon returned from the sea to their natal waters. Generally, the descendants of Europeans used in-river gillnet fisheries during the first fifty years of commercial salmon fishing starting in the 1870s and targeted large Chinook. By the 1920s, when motorized fishing boats enabled fisheries to extend to the estuaries and the near ocean, the average size and age of many Chinook stocks within large rivers like the Sacramento/San Joaquin, Columbia, and Fraser had already been reduced, despite average catch weights that still ranged twenty to thirty pounds.
The consequence of developing the ocean troll fishery meant that immature, growing Chinook that were still one to three years away from maturity were subject to constant harvest pressures. This highly favored those fish that would have naturally matured at a younger age. With an ocean fishery, the longer a fish stays at sea, the greater the likelihood it will be harvested. Over time, this effect will reduce the average age and size of the population, as the portion that would have matured at an older age will now be harvested. In the case of Chinook salmon, older (and consequently larger) seven- and eight-year-old fish are disappearing from runs. These fish are the most productive within the population and their absence indicates a serious stock failure as well as an overall reduction in productive capacity. If the more productive older/larger and female fish were not targeted and released, the quality of the escapement would increase.
The net result of all of this is that today’s Chinook salmon populations are significantly less complex and diverse in life history than they were a century ago. They are younger and smaller, and growth rates of the majority of populations have been changed in negative ways. Those populations are less likely to have the growth and maturity rates required to produce fish that are five-years-old and older that mature at large body sizes in excess of forty pounds.
Can this trend be reversed? Yes, but it will require changes in harvest rates and practices, and unless selective fishing methods that harvest only hatchery fish are mandated, it will also require reductions in hatchery releases of Chinook and other salmon species. Ocean harvest rates will need to continue to be reduced and fisheries that directly or indirectly encounter immature Chinook, particularly troll fisheries, must either be terminated or become selective. Fisheries that select for larger Chinook, particularly gillnet fisheries, must also be eliminated or become selective. Then the proportionately few remaining older, larger, natural-origin Chinook will return to spawn in the immediate future and become the foundation for the slower, longer process of recovering the proportions of five-year-old and older age classes that were typical of most historic Chinook salmon populations.
This perspective is especially relevant to the recovery of Elwha River Chinook following the removal of the Elwha dams. The legendary large body size of Elwha Chinook was probably essential to the ability of the population to colonize the middle and upper Elwha basins where they were required to pass through several rough canyon reaches and make use of large spawning substrates in the deep, fast waters of the upper basin. The science on how populations respond to harvest indicates that the large-bodied Elwha Chinook will not be restored simply through dam removal if no other measures are taken. Restoring the Elwha Chinook probably requires a closer examination of how harvest has and will continue to affect the stock.
McDonald, M. 1894. The salmon fisheries of the Columbia River Basin. Report of Commissioner of Fish and Fisheries.
Rich, W.H. 1940. Seasonal variation in weight of Columbia River Chinook salmon. Department of Research, Fish Commission of the State of Oregon, Contribution No. 5.
Ricker, W.E. 1981. Changes in the average size and average age of Pacific salmon. Canadian Journal of Fisheries and Aquatic Sciences v.38, pp.1636-1656.
Yoshiyama, R.M., F.W. Fisher, and P.B. Moyle. 1998. Historical abundance and decline of Chinook salmon in the Central Valley region of California. North American Journal of Fisheries Management, v.18: 487-521.
Nick Gayeski, Aquatic Ecologist, Wild Fish Conservancy.
Originally published in the 2011 Wild Fish Journal