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Population viability of coho salmon, Oncorhynchus kisutch, in Oregon coastal basins: application of a habitat-based life cycle model Thomas E. Nickelson and Peter W. Lawson
Abstract: To assess extinction risk for Oregon coastal coho salmon, Oncorhynchus kisutch, we developed a life cycle model based on habitat quality of individual stream reaches estimated from survey data. Reach-specific smolt output was a function of spawner abundance, demographic stochasticity, genetic effects, and density- and habitat-driven survival rates. After natural mortality and ocean harvest, spawners returned to their natal reaches. Populations in reaches with poor habitat became extinct during periods of low marine survival. With favorable marine survival, high productivity reaches served as sources for recolonization of lower quality reaches through straying of spawners. Consequently, both population size and distribution expanded and contracted through time. Within a reach, populations lost resilience at low numbers when demographic risk factors became more important than density-dependent compensation. Population viability was modeled for three coastal basins having good, moderate, and poor habitat. With constant habitat conditions, extinction risk in 99 years was negligible in basins with good and moderate habitat and 5–10% in the basin with poor habitat. Reductions in habitat quality up to 60% in 99 years resulted in reduced coho salmon populations in all basins and significantly increased extinction risk in the basin with poor habitat. Résumé : Pour évaluer le risque d’extinction du coho, Oncorhynchus kisutch, des eaux côtières de l’Oregon, nous avons élaboré un modèle du cycle biologique basé sur la qualité de l’habitat de divers tronçons de cours d’eau, estimée d’après des données obtenues par relevé. La production de smolts par tronçon était fonction de l’abondance des géniteurs, de la stochasticité de la démographie, des effets génétiques et des taux de survie liés à l’habitat et à la densité. Après avoir été soumis à la mortalité naturelle et aux captures en mer, les géniteurs reviennent à leur cours d’eau de naissance. Les populations des zones où l’habitat est dégradé ont disparu après des périodes de faible survie en mer. Quand la survie en mer était favorable, les zones à forte productivité ont servi de source pour la recolonisation des tronçons de faible qualité par des géniteurs égarés. C’est ainsi que l’effectif de la population et sa distribution se sont élargis et rétrécis avec le temps. Sur un même tronçon, les populations ont perdu leur résilience à faible effectif lorsque les facteurs de risque d’ordre démographique sont devenus plus importants que la compensation dépendante de la densité. Nous avons modélisé la viabilité de la population pour trois bassins côtiers présentant des habitats de qualité bonne, modérée et faible. Si les conditions de l’habitat demeuraient constantes, le risque d’extinction en 99 ans était négligeable dans les bassins où l’habitat était bon et modéré, et de 5–10% dans les bassins où l’habitat était de faible qualité. Une réduction de 60% de la qualité de l’habitat en 99 ans causait une baisse de la population de coho dans tous les bassins, et accroissait de façon notable le risque d’extinction dans le bassin où l’habitat était de faible qualité. [Traduit par la Rédaction]
Nickelson and Lawson
The probability that a population will become extinct over a specified period of time has been suggested as a standard by which to classify relative vulnerability to extinction (Thompson 1991; Allendorf et al. 1997). This standard has commonly been expressed as a 5% probability of extinction Received August 19, 1997. Accepted July 30, 1998. J14172 T.E. Nickelson1 and P.W. Lawson.2 Oregon Department of Fish and Wildlife, 28655 Highway 34, Corvallis, OR 97333, U.S.A. 1
Author to whom all correspondence should be addressed. e-mail:
[email protected] 2 Present address: National Marine Fisheries Service, 2030 S Marine Science Drive, Newport, OR 97365, U.S.A. Can. J. Fish. Aquat. Sci. 55: 2383–2392 (1998)
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(Allendorf et al. 1997; Goodman 1999) or, conversely, a 95% probability of persistence (Thompson 1991) over the next 100 years. Population viability analysis (PVA) is the structured process of assessing this vulnerability (Shaffer 1990). PVA has been applied to a variety of species, from butterflies (Murphy et al. 1990) to grizzly bears, Ursus arctos (Shaffer 1981). Recent concern over threats to Pacific salmon have led to analyses of chinook salmon, Oncorhynchus tshawytscha, in the Snake River, Idaho (Emlen 1995), the Umpqua River, Oregon (Ratner et al. 1997), and the Sacramento River, California (Botsford and Brittnacher 1998). Coho salmon, Oncorhynchus kisutch, populations on the Oregon coast have been fragmented as a result of habitat destruction, with the result being relatively high densities remaining in only a few streams (Cooney and Jacobs 1995; Nickelson 1998). Murphy et al. (1990) suggested that PVA for species with fragmented habitats “must focus on the en© 1998 NRC Canada
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vironmental factors and metapopulation characteristics that determine local population persistence.” We have developed a model that takes this approach by incorporating the contribution of fish from individual stream reaches of differing qualities. The model mimics the life cycle of coho salmon and simulates population fluctuations and random dispersal over years by incorporating density-driven compensation and depensation, short-term stochastic variation in survival, long-term climatic cycles, reduced genetic fitness from small population size, and straying of spawners from their natal spawning areas. Habitat quality determines the number of coho salmon smolts that a stream can produce as well as the efficiency with which those smolts are produced (i.e., survival rate). Production within a basin was estimated for individual stream reaches, based on habitat quality data. The result is a model that incorporates population dynamics, environmental variability, genetic factors, and metapopulation structure. The model was used in a PVA to assess extinction probabilities of single cohorts of Oregon coastal coho salmon at the basin scale. We used three basins of varying habitat quality to represent the range of conditions on the Oregon coast. We tested the sensitivity of populations in these three basins to varying levels of marine survival and exploitation rates over 30 years (10 generations). We simulated the effects of a range of starting population sizes, including the 1995 actual spawner escapements, on median population size and probability of extinction in 99 years (33 generations). We also modeled the effects of changes in habitat quality on persistence and population size. To estimate extinction probabilities rigorously would require incorporating “everything that is known and everything that is not known about the dynamics of the population” (Goodman 1999). We have not achieved this ambitious goal; therefore, our results in this area must be viewed as exploratory.
Can. J. Fish. Aquat. Sci. Vol. 55, 1998 capement. Populations in a basin were linked through straying of spawners among reaches. Successive generations were simulated by using spawners from one generation to seed the next. Stochastic variation was included at several stages of the life cycle model. Monte Carlo simulations were used to produce a set of likely outcomes from a set of input parameters. Details of the modeling at each life stage are described below.
Spawners Spawners were the starting point for the simulations and the ending point for each generation. For the purpose of the model, spawners included only age 3 adults, thus a 3-year generation time. For simplification, jacks were not included in the calculations. Similarly, because age 4 adults are very rare, they were also excluded from the model. The absence of these two age-classes from the modeled populations could possibly result in an underestimation of the productive potential of the modeled populations (Botsford and Brittnacher 1998) because of the lost contribution to the reproductive capacity of successive broods. Wild coho salmon in coastal Oregon streams tend to spawn over a period of 2–3 months (Cooney and Jacobs 1995), preventing fish spawning early from interacting with fish spawning later. This usually is not a problem when populations are large; spawners should have little difficulty finding mates. However, when spawner populations are very small and some fish are present in a stream early and others late, finding a mate could become a matter of chance. A spawner not finding a mate is a depensatory effect of small spawner numbers. To simulate the effects of this depensation, time of spawning was split into three periods: early, middle, and late. Spawners in each reach were assigned randomly to the three periods. The number of female spawners in each time period was drawn from a binomial distribution where p = 0.5 and n is the number of spawners in the time period. There are no reliable data for the sex ratio of wild coho salmon in Oregon, and the data for hatchery coho salmon are conflicting. With 20% of males returning as jacks, it is likely that more than half of the returning age 3 adults are females. However, given the lack of consistent data, we modeled a 50:50 sex ratio.
Eggs Coho salmon life cycle We based our model on the life cycle of coho salmon in Oregon. Coho salmon in coastal streams typically spawn from early November through mid-January. Juveniles emerge from the gravel in spring and typically spend a summer and winter in freshwater (primarily in second- to fifth-order streams) before migrating to the ocean as smolts in their second spring. A very small percentage of juveniles (
