Milo D. Adkison

The Decline of the Steller Sea Lion in the Northeast Pacific: Demography, Harvest or Environment?

When apparent declines in population numbers are observed, biologists are confronted with the challenge of determining if such behavior conforms to the expected dynamics of the population or if, on the contrary, the decline reflects some pathological change in the environment that requires management actions. Experimental investigations are often impractical for large free-ranging populations such as fish or large mammals. Consequently, biologists need to get the best possible answer from the limited data available. In this paper we combine deterministic and stochastic modeling, together with statistical estimation techniques, to explore the likelihood of various hypotheses for the decline of Steller sea lions in the North Pacific. Our results show that deterministic transient popu- lation behavior, historical pup harvesting, and short-term environmental stochasticity are unlikely causes for the decline. The elimination of these alternatives leads us to agree with previous authors, who suggest that some long-term change in the environment or a novel catastrophe is responsible for the decline.

Drawbacks of complex models in frequentist and Bayesian approaches to natural‐resource management

Previous studies have shown that, for managing harvest of natural resources, overly complex models perform poorly. Decision-analytic approaches treat uncertainly differently from the maximum-likelihood approaches these studies employed. By simulation using a simple fisheries model, I show that decision-analytic approaches to managing harvest also can suffer from using overly complex models. Managers using simpler models can outperform managers using more complex models, even if the more complex models are correct and even if their use allows the incorporation of additional relevant information. Decision-analytic approaches outperformed maximum-likelihood approaches in my simulations, even when Bayesian priors were uninformative.

Biophysical Factors Associated with the Marine Survival of Auke Creek, Alaska, Coho Salmon

Abstract Correlation analyses and stepwise regression models were run to examine relationships between the marine survival of Auke Creek coho salmon Oncorhynchus kisutch and a number of physical and biological covariates: Local sea surface temperature, local precipitation, local hatchery releases of pink salmon O. gorbuscha and chum salmon O. keta, regional and state coho and pink salmon catches, and smolt survival of three other regional coho salmon stocks. Auke Creek jack return rates and adult marine survival covaried, indicating that a significant amount of the variation in marine survival is due to conditions encountered in the first 4 to 5 months of marine life. Sea surface temperature was not significantly correlated with adult survival but was significantly associated with jack return rates. Precipitation had no relationship with marine survival. The number of locally released hatchery pink and chum salmon had the strongest correlation with marine survival (r = 0.71). Marine survival trends of coh...

Balancing Biological Sustainability with the Economic Needs of Alaska's Sockeye Salmon Fisheries

Abstract Total revenue in the Bristol Bay, Alaska, fishery for sockeye salmon Oncorhynchus nerka has declined by 80% over the past two decades. In contrast to other fisheries where declining revenues are a consequence of declining catches, Bristol Bay sockeye salmon landings have remained high and the revenue decline is a result of falling prices, which in turn are a consequence of competition from increased production of rainbow trout/steelhead O. mykiss (hereafter, rainbow trout) and coho salmon O. kisutch farmed in Chile. This paper explores possible changes to current management strategies that would continue to ensure biological sustainability while enhancing economic returns. We simulate three management strategies for Bristol Bay sockeye salmon: fixed escapement range, conditional fixed harvest, and conditional fixed harvest rate. Yields from these simulations are combined with a forecast of Chilean rainbow trout and coho salmon production and a model of international trade flows for Alaskan sockey...

Linking Alaskan Salmon Fisheries Management with Ecosystem-based Escapement Goals : A Review and Prospectus

Abstract A growing body of literature documents that spawning Pacific salmon have profound effects on terrestrial and aquatic ecosytems. This has led to calls for considering ecosystem effects in setting salmon escapement goals; most stocks, however, are still managed using single-species stock-recruit models. Our objective is to ascertain whether current knowledge is adequate for setting ecosystem-based escapement goals, with particular reference to Alaskan fisheries. We review the literature to determine how well the quantitative relationships between salmon and a given ecosystem component (e.g., the growth rate of a given species) have been identified. In most cases, quantitative relationships are not available for a wide enough range of physical and biological conditions to make accurate predictions of the response to a given level of salmon escapement. Thus, fisheries managers might curretly have difficulty in justifying ecosystem-based escapement goals. We discuss the potential costs and benefits of...

Variable Effects of Biological and Environmental Processes on Coho Salmon Marine Survival in Southeast Alaska

Abstract Correlation analyses, linear regression models, and multistock mixed effects models were used to examine the relationships between coho salmon Oncorhynchus kisutch marine survival and six biological and environmental covariates across 14 southeast Alaska (SEAK) stocks. A primary focus of the study was to investigate the influence of pink salmon O. gorbuscha and chum salmon O. keta fry abundances on coho salmon marine survival. The coho salmon stocks exhibited strong covariation; 88 of the 91 pairwise comparisons among the coho salmon stocks covaried positively and 54 of them were significant (P < 0.05). Only one of the covariates, the North Pacific index, which is a measure of the Aleutian low pressure zone, had consistent relationships (positive) across all 14 stocks. The other covariates, including freshwater discharge, the Pacific decadal oscillation, sea-surface temperature, and two indices of pink salmon and chum salmon fry abundances, all had inconsistent relationships with marine survival....

Effects of marine-derived nutrients on population dynamics of sockeye salmon (Oncorhynchus nerka)

The effects of marine-derived nutrients (MDN) on the productivity of sockeye salmon (Oncorhynchus nerka) stocks in Alaska, USA, were examined through nitrogen stable isotope analysis of smolts and mathematical models of the sockeye stock–recruit relationship. Smolt δ15N was used to infer the degree to which smolts depend on MDN for their growth. We found that characteristics of sockeye nursery lakes and watersheds significantly affected the availability of MDN to juvenile sockeye. The magnitude of escapement and water residence time were the most important factors affecting the MDN availability to juvenile salmon. Analysis of stock–recruit models indicated that regional environmental fluctuations had a large effect on stock productivities. However, stock–recruitment data showed little evidence that increasing MDN input to nursery lakes increased stock productivities. Stock–recruitment data may be poorly suited to detection of the influence of MDN because of the multitude of factors that influence juvenile...

Benefits and risks of diversification for individual fishers

Significance Individuals who rely on natural resources for their livelihoods, such as fishers, farmers, and forestry workers, face high levels of income variability. For fishers, catching multiple species has been shown to reduce revenue variability at large scales (vessels and communities), but the individual-level consequences of maintaining catch diversity are unknown. Our work demonstrates that individuals in fisheries targeting a diversity of species and individuals who participate in multiple fisheries buffer income variability compared with less diverse individuals. However, large adjustments in diversification strategies from year to year are risky and usually increase revenue variability. The most effective option to reduce revenue variability via diversification—purchasing additional permits—is also expensive, often limited by regulations, and therefore unavailable to many. Individuals relying on natural resource extraction for their livelihood face high income variability driven by a mix of environmental, biological, management, and economic factors. Key to managing these industries is identifying how regulatory actions and individual behavior affect income variability, financial risk, and, by extension, the economic stability and the sustainable use of natural resources. In commercial fisheries, communities and vessels fishing a greater diversity of species have less revenue variability than those fishing fewer species. However, it is unclear whether these benefits extend to the actions of individual fishers and how year-to-year changes in diversification affect revenue and revenue variability. Here, we evaluate two axes by which fishers in Alaska can diversify fishing activities. We show that, despite increasing specialization over the last 30 years, fishing a set of permits with higher species diversity reduces individual revenue variability, and fishing an additional permit is associated with higher revenue and lower variability. However, increasing species diversity within the constraints of existing permits has a fishery-dependent effect on revenue and is usually (87% probability) associated with increased revenue uncertainty the following year. Our results demonstrate that the most effective option for individuals to decrease revenue variability is to participate in additional or more diverse fisheries. However, this option is expensive, often limited by regulations such as catch share programs, and consequently unavailable to many individuals. With increasing climatic variability, it will be particularly important that individuals relying on natural resources for their livelihood have effective strategies to reduce financial risk.

Evaluating signals of oil spill impacts, climate, and species interactions in Pacific herring and Pacific salmon populations in Prince William Sound and Copper River, Alaska

The Exxon Valdez oil spill occurred in March 1989 in Prince William Sound, Alaska, and was one of the worst environmental disasters on record in the United States. Despite long-term data collection over the nearly three decades since the spill, tremendous uncertainty remains as to how significantly the spill affected fishery resources. Pacific herring (Clupea pallasii) and some wild Pacific salmon populations (Oncorhynchus spp.) in Prince William Sound declined in the early 1990s, and have not returned to the population sizes observed in the 1980s. Discerning if, or how much of, this decline resulted from the oil spill has been difficult because a number of other physical and ecological drivers are confounded temporally with the spill; some of these drivers include environmental variability or changing climate regimes, increased production of hatchery salmon in the region, and increases in populations of potential predators. Using data pre- and post-spill, we applied time-series methods to evaluate support for whether and how herring and salmon productivity has been affected by each of five drivers: (1) density dependence, (2) the EVOS event, (3) changing environmental conditions, (4) interspecific competition on juvenile fish, and (5) predation and competition from adult fish or, in the case of herring, humpback whales. Our results showed support for intraspecific density-dependent effects in herring, sockeye, and Chinook salmon, with little overall support for an oil spill effect. Of the salmon species, the largest driver was the negative impact of adult pink salmon returns on sockeye salmon productivity. Herring productivity was most strongly affected by changing environmental conditions; specifically, freshwater discharge into the Gulf of Alaska was linked to a series of recruitment failures—before, during, and after EVOS. These results highlight the need to better understand long terms impacts of pink salmon on food webs, as well as the interactions between nearshore species and freshwater inputs, particularly as they relate to climate change and increasing water temperatures.

Population Trend and Elasticities of Vital Rates for Steller Sea Lions (Eumetopias jubatus) in the Eastern Gulf of Alaska: A New Life-History Table Analysis.

Steller sea lion (Eumetopias jubatus) numbers are beginning to recover across most of the western distinct population segment following catastrophic declines that began in the 1970s and ended around the turn of the century. This study makes use of contemporary vital rate estimates from a trend-site rookery in the eastern Gulf of Alaska (a sub-region of the western population) in a matrix population model to estimate the trend and strength of the recovery across this region between 2003 and 2013. The modeled population trend was projected into the future based on observed variation in vital rates and a prospective elasticity analysis was conducted to determine future trends and which vital rates pose the greatest threats to recovery. The modeled population grew at a mean rate of 3.5% per yr between 2003 and 2013 and was correlated with census count data from the local rookery and throughout the eastern Gulf of Alaska. If recent vital rate estimates continue with little change, the eastern Gulf of Alaska population could be fully recovered to pre-decline levels within 23 years. With density dependent growth, the population would need another 45 years to fully recover. Elasticity analysis showed that, as expected, population growth rate (λ) was most sensitive to changes in adult survival, less sensitive to changes in juvenile survival, and least sensitive to changes in fecundity. A population decline could be expected with only a 6% decrease in adult survival, whereas a 32% decrease in fecundity would be necessary to bring about a population decline. These results have important implications for population management and suggest current research priorities should be shifted to a greater emphasis on survival rates and causes of mortality.