Reproductive Success of Captive-Bred Steelhead Trout in the Wild: Evaluation of Three Hatchery Programs in the Hood River

Abstract:  Population supplementation programs that release captive-bred offspring into the wild to boost the size of endangered populations are now in place for many species. The use of hatcheries for supplementing salmonid populations has become particularly popular. Nevertheless, whether such programs actually increase the size of wild populations remains unclear, and predictions that supplementation fish drag down the fitness of wild fish remain untested. To address these issues, we performed DNA-based parentage analyses on almost complete samples of anadromous steelhead ( Oncorhynchus mykiss ) in the Hood River in Oregon (U.S.A.). Steelhead from a supplementation hatchery (reared in a supplementation hatchery and then allowed to spawn naturally in the wild) had reproductive success indistinguishable from that of wild fish. In contrast, fish from a traditional hatchery (nonlocal origin, multiple generations in hatcheries) breeding in the same river showed significantly lower fitness than wild fish. In addition, crosses between wild fish and supplementation fish were as reproductively successful as those between wild parents. Thus, there was no sign that supplementation fish drag down the fitness of wild fish by breeding with them for a single generation. On the other hand, crosses between hatchery fish of either type (traditional or supplementation) were less fit than expected, suggesting a possible interaction effect. These are the first data to show that a supplementation program with native brood stock can provide a single-generation boost to the size of a natural steelhead population without obvious short-term fitness costs. The long-term effects of population supplementation remain untested.     

Nonrandom, Size- and Timing-Biased Breeding in a Hatchery Population of Steelhead Trout

Abstract:  Hatcheries have been built and operated to buffer salmon and trout populations from overfishing and to compensate for habitat lost or degraded by human activities. These facilities are now so prevalent that in some cases hatchery-produced salmon outnumber salmon produced in the wild. By default, this makes them an important component in the current ecology and evolution of salmonids. Hatcheries differ from natural environments in many ways, and among the most fundamental is the necessity that humans select fish for breeding instead of allowing natural processes of mate choice and competition. We examined the mating system for steelhead trout ( Oncorhynchus mykiss ) at Forks Creek Hatchery in southwest Washington and investigated factors affecting selection of individual steelhead for spawning by the hatchery staff. Despite efforts by the staff to not spawn selectively, data on steelhead spawned over 7 years revealed selection for large adult body size and early reproductive timing and a tendency for size-assortative mating (i.e., large with large). Selection on size was related to selection on reproductive timing because early returning fish tended to be larger than those returning later. To improve the fitness of both hatchery fish destined to spawn in the wild and hatchery fish designated to be spawned in the hatchery, a better understanding of factors associated with the range of reproductive success and mate-choice mechanisms in the wild is vital. This knowledge may then be applied to artificial propagation programs designed for conservation or enhancement.     

Evaluating Alternative Strategies for Minimizing Unintended Fitness Consequences of Cultured Individuals on Wild Populations

Artificial propagation strategies often incur selection in captivity that leads to traits that are maladaptive in the wild. For propagation programs focused on production rather than demographic contribution to wild populations, effects on wild populations can occur through unintentional escapement or the need to release individuals into natural environments for part of their life cycle. In this case, 2 alternative management strategies might reduce unintended fitness consequences on natural populations: (1) reduce selection in captivity as much as possible to reduce fitness load (keep them similar), or (2) breed a separate population to reduce captive‐wild interactions as much as possible (make them different). We quantitatively evaluate these 2 strategies with a coupled demographic–genetic model based on Pacific salmon hatcheries that incorporates a variety of relevant processes and dynamics: selection in the hatchery relative to the wild, assortative mating based on the trait under selection, and different life cycle arrangements in terms of hatchery release, density dependence, natural selection, and reproduction. Model results indicate that, if natural selection only occurs between reproduction and captive release, the similar strategy performs better. However, if natural selection occurs between captive release and reproduction, the different and similar strategies present viable alternatives to reducing unintended fitness consequences because of the greater opportunity to purge maladaptive individuals. In this case, the appropriate approach depends on the feasibility of each strategy and the demographic goal (e.g., increasing natural abundance, or ensuring that a high proportion of natural spawners are naturally produced). In addition, the fitness effects of hatchery release are much greater if hatchery release occurs before (vs. after) density‐dependent interactions. Given the logistical challenges to achieving both the similar and different strategies, evaluation of not just the preferred strategy but also the consequences of failing to achieve the desired target is critical. Evaluación de Estrategias Alternativas para Minimizar las Consecuencias No Inesperadas en la Adecuación de Individuos Criados en Cautiverio sobre Poblaciones Silvestres     

Incorporating covariates into fisheries stock assessment models with application to Pacific herring.

We present a framework for evaluating the cause of fishery declines by integrating covariates into a fisheries stock assessment model. This allows the evaluation of fisheries' effects vs. natural and other human impacts. The analyses presented are based on integrating ecological science and statistics and form the basis for environmental decision-making advice. Hypothesis tests are described to rank hypotheses and determine the size of a multiple covariate model. We extend recent developments in integrated analysis and use novel methods to produce effect size estimates that are relevant to policy makers and include estimates of uncertainty. Results can be directly applied to evaluate trade-offs among alternative management decisions. The methods and results are also broadly applicable outside fisheries stock assessment. We show that multiple factors influence populations and that analysis of factors in isolation can be misleading. We illustrate the framework by applying it to Pacific herring of Prince William Sound, Alaska (USA). The Pacific herring stock that spawns in Prince William Sound is a stock that has collapsed, but there are several competing or alternative hypotheses to account for the initial collapse and subsequent lack of recovery. Factors failing the initial screening tests for statistical significance included indicators of the 1989 Exxon Valdez oil spill, coho salmon predation, sea lion predation, Pacific Decadal Oscillation, Northern Oscillation Index, and effects of containment in the herring egg-on-kelp pound fishery. The overall results indicate that the most statistically significant factors related to the lack of recovery of the herring stock involve competition or predation by juvenile hatchery pink salmon on herring juveniles. Secondary factors identified in the analysis were poor nutrition in the winter, ocean (Gulf of Alaska) temperature in the winter, the viral hemorrhagic septicemia virus, and the pathogen Ichthyophonus hoferi. The implication of this result to fisheries management in Prince William Sound is that it may well be difficult to simultaneously increase the production of pink salmon and maintain a viable Pacific herring fishery. The impact can be extended to other commercially important fisheries, and a whole ecosystem approach may be needed to evaluate the costs and benefits of salmon hatcheries.     

Management and Recovery Options for Ural River Beluga Sturgeon

Abstract: Management of declining fisheries of anadromous species sometimes relies heavily on supplementation of populations with captive breeding, despite evidence that captive breeding can have negative consequences and may not address the root cause of decline. The beluga sturgeon (Huso huso), a species threatened by the market for black caviar and reductions in habitat quality, is managed through harvest control and hatchery supplementation, with an emphasis on the latter. We used yield per recruit and elasticity analyses to evaluate the population status and current levels of fishing and to identify the life‐history stages that are the best targets for conservation of beluga of the Ural River. Harvest rates in recent years were four to five times higher than rates that would sustain population abundance. Sustainable rates of fishing mortality are similar to those for other long‐lived marine species such as sharks and mammals. Yield per recruit, which is maximized if fish are first harvested at age 31 years, would be greatly enhanced by raising minimum size limits or reducing illegal take of subadults. Improving the survival of subadult and adult females would increase population productivity by 10 times that achieved by improving fecundity and survival from egg to age 1 year (i.e., hatchery supplementation). These results suggest that reducing mortality of subadults and adult wild fish is a more effective conservation strategy than hatchery supplementation. Because genetics is not factored into hatchery management practices, supplementation may even reduce the viability of the beluga sturgeon.     

Interacting Effects of Translocation,Artificial Propagation,and Environmental Conditions on the Marine Survival of Chinook Salmon from the Columbia River,Washington, U.S.A.

Abstract: Captive rearing and translocation are often used concurrently for species conservation, yet the effects of these practices can interact and lead to unintended outcomes that may undermine species’ recovery efforts. Controls in translocation or artificial‐propagation programs are uncommon; thus, there have been few studies on the interacting effects of these actions and environmental conditions on survival. The Columbia River basin, which drains 668,000 km² of the western United States and Canada, has an extensive network of hydroelectric and other dams, which impede and slow migration of anadromous Pacific salmon (Oncorhynchus spp.) and can increase mortality rates. To mitigate for hydrosystem‐induced mortality during juvenile downriver migration, tens of millions of hatchery fish are released each year and a subset of wild‐ and hatchery‐origin juveniles are translocated downstream beyond the hydropower system. We considered how the results of these practices interact with marine environmental conditions to affect the marine survival of Chinook salmon (O. tshawytscha). We analyzed data from more than 1 million individually tagged fish from 1998 through 2006 to evaluate the probability of an individual fish returning as an adult relative to its rearing (hatchery vs. wild) and translocation histories (translocated vs. in‐river migrating fish that traveled downriver through the hydropower system) and a suite of environmental variables. Except during select periods of very low river flow, marine survival of wild translocated fish was approximately two‐thirds less than survival of wild in‐river migrating fish. For hatchery fish, however, survival was roughly two times higher for translocated fish than for in‐river migrants. Competition and predator aggregation negatively affected marine survival, and the magnitude of survival depended on rearing and translocation histories and biological and physical conditions encountered during their first few weeks of residence in the ocean. Our results highlight the importance of considering the interacting effects of translocation, artificial propagation, and environmental variables on the long‐term viability of species.     

Human Influence on the Spatial Structure of Threatened Pacific Salmon Metapopulations

Abstract: To remain viable, populations must be resilient to both natural and human‐caused environmental changes. We evaluated anthropogenic effects on spatial connections among populations of Chinook salmon (Oncorhynchus tshawytscha) and steelhead (O. mykiss) (designated as threatened under the U.S. Endangered Species Act) in the lower Columbia and Willamette rivers. For several anthropogenic‐effects scenarios, we used graph theory to characterize the spatial relation among populations. We plotted variance in population size against connectivity among populations. In our scenarios, reduced habitat quality decreased the size of populations and hydropower dams on rivers led to the extirpation of several populations, both of which decreased connectivity. Operation of fish hatcheries increased connectivity among populations and led to patchy or panmictic populations. On the basis of our results, we believe recolonization of the upper Cowlitz River by fall and spring Chinook and winter steelhead would best restore metapopulation structure to near‐historical conditions. Extant populations that would best conserve connectivity would be those inhabiting the Molalla (spring Chinook), lower Cowlitz, or Clackamas (fall Chinook) rivers and the south Santiam (winter steelhead) and north fork Lewis rivers (summer steelhead). Populations in these rivers were putative sources; however, they were not always the most abundant or centrally located populations. This result would not have been obvious if we had not considered relations among populations in a metapopulation context. Our results suggest that dispersal rate strongly controls interactions among the populations that comprise salmon metapopulations. Thus, monitoring efforts could lead to understanding of the true rates at which wild and hatchery fish disperse. Our application of graph theory allowed us to visualize how metapopulation structure might respond to human activity. The method could be easily extended to evaluations of anthropogenic effects on other stream‐dwelling populations and communities and could help prioritize among competing conservation measures.     

Exposure to variable spatial information in the early rearing environment generates asymmetries in social interactions in cod (Gadus morhua)

Many re-introduction programs used for conservation of populations and species threatened with extinction advocate the use of enriched rearing environments to train animals how to behave appropriately in the wild. Curiously, most of the current fish re-stocking programs have paid little attention to lessons previously learned in bird and mammal re-introductions. Many rehabilitation programs that use releases of hatchery fish observe higher mortality in released fish compared to wild, with most mortality arising shortly after release. One explanation for this mortality is based purely on selection processes; many hatchery fish normally selected out of the population thrive in the predator free, food-rich hatcheries. Alternatively, mortalities may be high because hatchery nursery environments fail to shape fish behaviour appropriately. Here, we empirically address the effect of enrichment in the early rearing environment in coastal cod (Gadus morhua). We find asymmetries in aggressive behaviour when fish reared in plain or enriched environments are allowed to interact. Furthermore, cod reared in standard, impoverished, hatchery environments spend less time in shelter, are more active, and show weaker anti-predator responses than fish reared with access to heterogeneous spatial cues. These results suggest that the constant, plain environments of fish farms may generate behavioural deficits that could reasonably be expected to be associated with lower survival in fish released into the wild.     

Does juvenile experience affect migration and spawning of adult Atlantic salmon?

Summary This paper documents differences in seasonal time of river ascent and descent, and instream behavior of adult wild and sea-ranched Atlantic salmon (Salmo salar) of the Norwegian River Imsa stock during the period 1981–1989. Wild fish use River Imsa as a nursery, and at an age of 2 years most of them migrate to the sea as smolts. The sea ranched fish are hatchery reared offspring of the River Imsa stock and are released as smolts at the mouth of the river. They are thus deprived of juvenile river life and a downstream smolt migration. Wild and sea ranched salmon feed for 1 or more years in the Norwegian Sea before homing as spawners. Both groups returned simultaneously to coastal Norway, but sea ranched fish ascended the river later and descended sooner after spawning than wild fish. All wild females and almost all wild males (96.2%) spawned in the river, whereas 13.5% and 36.7%, respectively, of the mature sea-ranched females and males left the river unspawned. The annual number, but not the proportion, of unspawned fish increased with increasing density of adult salmon in the river. Unspawned females were medium sized and small (45–70 cm); unspawned males were medium sized and large (50–90 cm). Independent of the density of spawners in the river, sea ranched fish moved up- and downstream the river more often than wild fish. More than 20% of the sea-ranched salmon and less than 1% of the wild salmon passed a trap 100 m above the river outlet more than once in each direction during the same spawning reason. Moreover, sea-ranched salmon were about twice as often seriously injured during spawning as wild fish. Lack of juvenile experience from the river may be the main reason for the behavioral differences between sea-ranched and wild fish. Offprint requests to: B. Jonsson     

Assessment of harbor seal predation on adult salmonids in a Pacific Northwest estuary.

The populations of many native species have increased or expanded in distribution in recent decades, sometimes with negative consequences to sympatric native species that are rarer or less adaptable to anthropogenic changes to the environment. An example of this phenomenon from the Pacific Northwest is predation by locally abundant pinnipeds (seals and sea lions) on threatened, endangered, or otherwise depleted salmonid (Oncorhynchus spp.) populations. We used survey sampling methodology, acoustic telemetry, and molecular genetics to quantify the amount of harbor seal (Phoca vitulina) predation on a depressed run of coho salmon (O. kisutch) and to determine whether some seals consumed a disproportionately higher number of salmonids than others. Based on a probability sample totaling 759.5 h of observation, we estimated that seals consumed 1161 adult salmonids (95% CI = 503-1818 salmonids) during daylight hours over an 18.9-km estuarine study area in Oregon during an 84-d period in fall 2002. Simultaneous tracking of 56 seals via an acoustic telemetry array indicated that a small proportion of marked seals (12.5%) exhibited behavior that was consistent with specialization on salmonids. These seals spent the majority of their time in the riverine portion of the study area and did so disproportionately more at night than day. Genetic analysis of 116 salmonid structures recovered from 11 seal fecal samples suggested that coho salmon accounted for approximately one-half of total salmonid consumption. Though subject to considerable uncertainty, the combined results lead us to infer that seals consumed 21% (range = 3-63%) of the estimated prespawning population of coho salmon. We speculate that the majority of the predation occurred upriver, at night, and was done by a relatively small proportion of the local seal population. Understanding the extent and nature of pinniped predation can provide important inputs into risk assessments and other modeling efforts designed to aid the conservation and recovery of salmonids in the Pacific Northwest. Such understanding may also help inform management actions designed to reduce the impact of pinniped predation on salmonids, which potentially range from short-term lethal removal programs to long-term ecosystem restoration and protection efforts.     

Quantitative threat analysis for management of an imperiled species: Chinook salmon (Oncorhynchus tshawytscha).

Chinook salmon (Oncorhynchus tshawytscha) have declined dramatically across the Pacific Northwest because of multiple human impacts colloquially characterized as the four "H's": habitat degradation, harvest, hydroelectric and other dams, and hatchery production. We use this conceptual framework to quantify the relative importance of major threats to the current status of 201 Chinook populations. Current status is characterized by two demographic indices: population density and trend. We employ path analytic models and information theoretic methods for multi-model inference. Our results indicate that dams most strongly affect variation in population density, while harvest and hatchery production most strongly affect variation in population trend. Comparable results arise when the sample size of the analysis is reduced to 22 Chinook populations within a smaller region typical of the scale at which salmon recovery planning is conducted. Results from these threat analyses suggest that recovery strategies targeting specific demographic indices, and those considering natural and human-mediated interdependencies of major threats, are most likely to succeed.     

A simulation model to explore the relative value of stock enhancement versus harvest regulations for fishery sustainability

Harvest restrictions and stock enhancement are commonly proposed management responses for sustaining degraded fisheries, but comparisons of their relative effectiveness have seldom been considered prior to making policy choices. We built a population model that incorporated both size-dependent harvest restrictions and stock enhancement contributions to explore trade-offs between minimum length limits and stock enhancement for improving population sustainability and fishery metrics (e.g., catch). We used a Murray cod Maccullochella peelii peelii population as a test case, and the model incorporated density-dependent recruitment processes for both hatchery and wild fish. We estimated the spawning potential ratio (SPR) and fishery metrics (e.g., angler catch) across a range of minimum length limits and stocking rates. Model estimates showed that increased minimum length limits were much more effective than stock enhancement for increasing SPR and angler catches in exploited populations, but length limits resulted in reduced harvest. Stocking was predicted to significantly increase total recruitment, population sustainability, and fishery metrics only in systems where natural reproduction had been greatly reduced via habitat loss, fishing mortality was high, or both. If angler fishing effort increased with increased fish abundance from stocking efforts, fishing mortality was predicted to increase and reduce the benefits realized from stocking. The model also indicated that benefits from stock enhancement would be reduced if reproductive efficiency of hatchery-origin fish was compromised. The simulations indicated that stock enhancement was a less effective method to improve fishery sustainability than measures designed to reduce fishing mortality (e.g., length limits).     

Forecasting relative impacts of land use on anadromous fish habitat to guide conservation planning.

Land use change can adversely affect water quality and freshwater ecosystems, yet our ability to predict how systems will respond to different land uses, particularly rural-residential development, is limited by data availability and our understanding of biophysical thresholds. In this study, we use spatially explicit parcel-level data to examine the influence of land use (including urban, rural-residential, and vineyard) on salmon spawning substrate quality in tributaries of the Russian River in California. We develop a land use change model to forecast the probability of losses in high-quality spawning habitat and recommend priority areas for incentive-based land conservation efforts. Ordinal logistic regression results indicate that all three land use types were negatively associated with spawning substrate quality, with urban development having the largest marginal impact. For two reasons, however, forecasted rural-residential and vineyard development have much larger influences on decreasing spawning substrate quality relative to urban development. First, the land use change model estimates 10 times greater land use conversion to both rural-residential and vineyard compared to urban. Second, forecasted urban development is concentrated in the most developed watersheds, which already have poor spawning substrate quality, such that the marginal response to future urban development is less significant. To meet the goals of protecting salmonid spawning habitat and optimizing investments in salmon recovery, we suggest investing in watersheds where future rural-residential development and vineyards threaten high-quality fish habitat, rather than the most developed watersheds, where land values are higher.     

Population Viability Analysis of Spring Chinook Salmon in the South Umpqua River, Oregon

Run sizes of spring chinook salmon in the South Umpqua River in Oregon have declined dramatically since the early part of this century. Habitat degradation is thought to be an important factor contributing to the decline of this stock, and qualitative assessment suggests the stock is at moderate risk of extinction. We use data from this and similar stocks to develop an age-structured, density-dependent model of the population dynamics that incorporates both demographic and environmental stochasticity. Under the assumption of no further habitat destruction, the population is predicted to have a greater than 95% probability of persistence for 200 years. However, sensitivity analysis for the density-dependence estimated from historical run-return data shows that substantially lower predicted viabilities are also statistically consistent with the data. A model that simulates continued habitat degradation results in almost certain extinction within 100 years.     

The effect of incubation temperature on hatchling quality in the olive ridley turtle, Lepidochelys olivacea, from Alas Purwo National Park, East Java, Indonesia: implications for hatchery management

Nest protection through egg relocation from natural nests into protected hatcheries is a common practice used at rookeries around the world to increase hatchling recruitment into sea turtle populations. However, rarely have the impacts of this practice on hatchling recruitment and quality been assessed. This study investigated the influences of the thermal nest environment of olive ridley turtles Lepidochelys olivacea on emergence success and quality of hatchlings of hatchery nests in Alas Purwo National Park, East Java, Indonesia (2009 and 2010 nesting seasons). Nest temperatures above 34?°C for at least 3 consecutive days during incubation in the hatchery resulted in decreases in emergence success and locomotor performance of hatchlings. The use of the hatchery is recommended due to extremely high predation rate of nests left on the beach; however, altering hatchery management practice by spacing nests one meter apart and providing shade should improve hatchery outcomes now and into the future.     

Cover Caption

Cover: Sockeye salmon (Oncorhynchus nerka) swim upriver to Kuril Lake on Russia's remote Kamchatka Peninsula, the most productive spawning area for the species in the western Pacific. On pages 1075—1079, Charles Conn reflects on the state and future of conservation science from his perspective as an environmental grant maker whose career began in high‐profile internet businesses. His work on wild salmon ecosystems highlights the effectiveness of aligning biological and economic values held by society. Conn suggests that implementation of national sustainability policies, international certification regimes, and changes in resource access and governance may change patterns of human consumption and achieve conservation objectives in the coming decades. Cover image © 2011 Igor Shpilenok. Photographer: Igor Shpilenok travels across Russia documenting its nature reserves, national parks, and wild places. Shpilenok drove the creation in 1987 of a strict nature reserve, the Bryansky Les Zapovednik, which conserves floodplains, forests, and nesting habitat of the rare Black Stork (Ciconia nigra). During the 11 years Shpilenok managed Bryansky Les Zapovednik, he and his colleagues led the designation process for another 12 nature reserves. Shpilenok is a regular contributor to National Wildlife, Ranger Rick, Russian Life, National Geographic–Russia, Canadian Wildlife, Geo–Germany, and BBC Wildlife. His photographs twice were awarded first prize in the BBC Wildlife Photographer of the Year competition, one in 2006 and one in 2009. In 2008 Shpilenok and his wife, nature writer Laura Williams, published their memoir Th e Storks' Nest.     

Twenty years of stream restoration in Finland: little response by benthic macroinvertebrate communities

The primary focus of many in-stream restoration projects is to enhance habitat diversity for salmonid fishes, yet the lack of properly designed monitoring studies, particularly ones with pre-restoration data, limits any attempts to assess whether restoration has succeeded in improving salmonid habitat. Even less is known about the impacts of fisheries-related restoration on other, non-target biota. We examined how restoration aiming at the enhancement of juvenile brown trout (Salmo trutta L.) affects benthic macroinvertebrates, using two separate data sets: (1) a before-after-control-impact (BACI) design with three years before and three after restoration in differently restored and control reaches of six streams; and (2) a space-time substitution design including channelized, restored, and near-natural streams with an almost 20-year perspective on the recovery of invertebrate communities. In the BACI design, total macroinvertebrate density differed significantly from before to after restoration. Following restoration, densities decreased in all treatments, but less so in the controls than in restored sections. Taxonomic richness also decreased from before to after restoration, but this happened similarly in all treatments. In the long-term comparative study, macroinvertebrate species richness showed no difference between the channel types. Community composition differed significantly between the restored and natural streams, but not between restored and channelized streams. Overall, the in-stream restoration measures used increased stream habitat diversity but did not enhance benthic biodiversity. While many macroinvertebrates may be dispersal limited, our study sites should not have been too distant to reach within almost two decades. A key explanation for the weak responses by macroinvertebrate communities may have been historical. When Fennoscandian streams were channelized for log floating, the loss of habitat heterogeneity was only partial. Therefore, habitat may not have been limiting the macroinvertebrate communities to begin with. Stream restoration to support trout fisheries has strong public acceptance in Finland and will likely continue to increase in the near future. Therefore, more effort should be placed on assessing restoration success from a biodiversity perspective using multiple organism groups in both stream and riparian ecosystems.     

Modeling the Effect of Population Dynamics on the Impact of Rabbit Hemorrhagic Disease

Abstract:  The European wild rabbit ( Oryctolagus cuniculus ) is a staple prey species in Mediterranean ecosystems. The arrival and subsequent spread of rabbit hemorrhagic disease throughout southwestern Europe, however, has caused a decline in rabbit numbers, leading to considerable efforts to enhance wild rabbit populations, especially through habitat management. Because rabbit population dynamics depend on habitat suitability and changes in habitat structure and composition subsequent to habitat management, I evaluated the effects of population dynamics on the long-term impact of rabbit hemorrhagic disease on rabbit populations. I used an age-structured model with varying degrees of population productivity and turnover and different habitat carrying capacities, and I assumed the existence of a unique, highly pathogenic virus. My results suggest that disease impact may be highly dependent on habitat carrying capacity and rabbit population dynamics, and the model provided some insight into the current abundance of wild rabbits in different locations in southwestern Europe. The highest disease impact was estimated for populations located in habitats with low to medium carrying capacity. In contrast, disease impact was lower in high-density populations in habitats with high carrying capacity, corresponding to a lower mean age of rabbit infection and a resulting lower mortality from rabbit hemorrhagic disease. The outcomes of the model suggest that management strategies to help rabbit populations recover should be based on improving habitats to their maximum carrying capacity and increasing rabbit population productivity. In contrast, the use of strategies based on temporary increases in rabbit density, including vaccination campaigns, translocations, and temporal habitat improvements at medium carrying capacities, may increase disease impact, resulting in short-term decreases in rabbit population density.     

Putting Longline Bycatch of Sea Turtles into Perspective

Abstract:  Although some sea turtle populations are showing encouraging signs of recovery, others continue to decline. Reversing population declines requires an understanding of the primary factor(s) that underlie this persistent demographic trend. The list of putative factors includes direct turtle and egg harvest, egg predation, loss or degradation of nesting beach habitat, fisheries bycatch, pollution, and large-scale changes in oceanographic conditions and nutrient availability. Recently, fisheries bycatch, in particular bycatch from longline fisheries, has received increased attention and has been proposed as a primary source of turtle mortality. We reviewed the existing data on the relative impact of longline bycatch on sea turtle populations. Although bycatch rates from individual longline vessels are extremely low, the amount of gear deployed by longline vessels suggests that cumulative bycatch of turtles from older age classes is substantial. Current estimates suggest that even if pelagic longlines are not the largest single source of fisheries-related mortality, longline bycatch is high enough to warrant management actions in all fleets that encounter sea turtles. Nevertheless, preliminary data also suggest that bycatch from gillnets and trawl fisheries is equally high or higher than longline bycatch with far higher mortality rates. Until gillnet and trawl fisheries are subject to the same level of scrutiny given to pelagic longlines, our understanding of the overall impact of fisheries bycatch on vulnerable sea turtle populations will be incomplete.