Low Harvest Rates Research Articles

Exploring Population‐Level Effects of Fishery Closures during Spawning: An Example Using Largemouth Bass

AbstractWe used an age‐structured model to evaluate the impacts of recreational angling during spawning on populations of largemouth bass Micropterus salmoides and the potential benefits of seasonal fishing closures. We simulated fisheries with average and high capture rates (i.e., fractions of the stock caught by anglers). We manipulated mortality rates to mimic the effects of various regulations, including (1) an open fishery with no closures, (2) a full fishery closure during the spawning season, (3) a catch‐and‐release fishery during the spawning season, and (4) a year‐round catch‐and‐release fishery. We simulated two hypothetical populations: a high‐productivity, low‐latitude population and a low‐productivity, high‐latitude population. Spawning season closures produced the largest relative increases in adult abundance when applied to low‐productivity populations and when capture rates were high (e.g., 70%) and harvest rates were relatively low (20%). This resulted because very high capture rates imply that most spawning fish will be caught by anglers and potentially lose their broods to nest predators. The existing evidence suggests that these conditions (very high capture rates combined with low harvest rates) are the exception rather than the rule across largemouth bass fisheries. Understanding capture and harvest rates is critical to the use of seasonal closures in recreational fisheries.

Using Statistical Population Reconstruction to Estimate Demographic Trends in Small Game Populations

ABSTRACT Statistical population reconstruction offers a robust approach to demographic assessment for harvested populations, but current methods are restricted to big‐game species with multiple age classes. We extended this approach to small game and analyzed 14 years of age‐at‐harvest data for greater sage‐grouse (Centrocercus urophasianus) in Oregon, USA, in conjunction with radiotelemetry data to reconstruct annual abundance levels, recruitment, and natural survival probabilities. Abundance estimates ranged from a low of 26,236 in 1995 to a high of 39,492 in 2004. Annual abundance estimates for adult males were correlated with a spring lek count index (r= 0.849,P< 0.029). We estimated the average annual harvest mortality for the population to be 0.028, ranging from 0.021 to 0.031 across years. We estimated the probability of natural survival of adult females to be 0.818 (= 0.052), somewhat higher than that of adult males (Ŝ= 0.609,= 0.163). Our precision in reconstructing the population was hampered by low harvest rates and the few birds tagged in the radiotelemetry investigations. Despite these issues, our analysis illustrates how modern statistical reconstruction procedures offer a flexible framework for demographic assessment using commonly collected data. This approach offers a useful alternative to small‐game indices and would be most appropriate for species with 5 or more years of age‐at‐harvest data and moderate‐to‐heavy harvest rates.

Models of the effects of marine-derived nutrients on salmon (Oncorhynchus spp.) population dynamics

Pacific salmon ( Oncorhynchus spp.) populations transfer large quantities of nutrients from their marine to their freshwater habitats. These nutrients have been shown to affect salmon populations in fresh water, including increasing basal food resources and elevating juvenile salmon growth rates and condition. The broader effects on recruitment and commercial harvests, however, are not clear. I developed and explored mathematical models of the effects of these nutrients on stock–recruitment relationships and used these models to investigate management implications. Populations strongly dependent on nutrients had lower sustained yields than those not dependent on nutrients. When nutrients strongly affected the stock–recruitment relationship, relatively low harvest rates and high escapement levels were necessary to maintain the population’s productivity. However, in some scenarios, the highest yields were obtained from small, nutrient-depleted populations. In other scenarios, the nutrient dependence had few management implications.

Deer Hunting on Pennsylvania's Public and Private Lands: A Two-Tiered System of Hunters?

Recreational hunting is crucial for controlling white-tailed deer populations. Public land is increasingly important as access to private lands declines. However, differences between public and private land hunters remain unknown. Our study of Pennsylvania hunters revealed differences between private and public land hunters that may pose problems for management. Hunters who only hunted public land had lower harvest rates, especially of antlerless deer, spent less time hunting, were less committed to hunting, were more likely to hunt alone, less likely to belong to a hunting camp, and more likely to live in urban areas. They were less likely to believe that high deer populations could damage forest ecosystems, and less willing to harvest antlerless deer. The implications of these findings, in the context of already-declining hunter capacity to keep deer populations in check, and concomitant declining access to private land, are discussed

Ecosystem management based on large-scale disturbance pulses: A case study from sub-boreal forests of western Quebec (Canada)

The northern Témiscamingue region (western Québec) sustained regional-scale pulses of natural disturbances during the 1850–2000 period, such as severe fires during the 1908–1926 period, two severe spruce budworm outbreaks that occurred in 1909–1918 and 1974–1984, and two birch dieback episodes around 1940 and 1980. These disturbances produced synchronous fluctuations in forest characteristics over large spatial scales. In this paper, we review possible responses of flora and fauna to pulsed large-scale disturbance events and speculate on whether they should be emulated to reduce the impacts of forest management on non-timber resources. The importance of large-scale disturbance pulses for biodiversity and forest ecosystem integrity is potentially great, but this aspect has been poorly investigated by previous research, and thus there is little information available to guide forest management. Large-scale, synchronous disturbances could be emulated by clustering harvesting activities in time, for example by creating “harvest pulses” of 10–20 years, separated by periods of 50–100 years or so with low harvest rates. A potential disadvantage of this strategy is that when our capacity to predict future natural disturbances is low, there is a higher probability of accidentally taking the forest ecosystem outside of the range of natural variability compared with a status quo forest management scenario. From a socio-economic perspective, another potential disadvantage is in creating irregular wood flows to the forest transformation industry. Nonetheless, in a context where the forest has been over-disturbed in the recent past, a forest management strategy involving fluctuating harvest rates could provide the means for faster ecosystem recovery compared with a status quo strategy. We recommend that the potential importance of disturbance pulses for boreal and sub-boreal ecosystems be more thoroughly investigated by future research to inform management and conservation policies.

Overwintering Distribution, Behavior, and Survival of Adult Summer Steelhead: Variability among Columbia River Populations

AbstractUnlike most anadromous salmonids, summer steelhead Oncorhynchus mykiss overwinter in rivers rather than the ocean for 6–10 months prior to spring spawning. Overwintering in rivers may make summer steelhead more vulnerable to harvest and other mortality sources than are other anadromous populations, but there has been little systematic study of this life history strategy. Here, we used a large‐scale radiotelemetry study to examine the overwintering behaviors and distributions of 26 summer steelhead stocks within the regulated lower Columbia–Snake River hydrosystem. Over 6 years, we monitored 5,939 fish, of which 3,399 successfully reached spawning tributaries or the upper Columbia River basin and were assigned to specific populations. An estimated 12.4% of fish that reached spawning areas overwintered at least partially within the hydrosystem (annual estimates = 6.8–19.6%), while the remainder overwintered in tributaries. Across all populations, later‐arriving fish were more likely to overwinter in the hydrosystem; overwintering percentages ranged from less than 1% for fish tagged in June to over 40% for those tagged in October. Proportionately more interior‐basin steelhead (Clearwater, Salmon, and Snake River metapopulations) overwintered in the hydrosystem than did fish from lower‐river populations. Steelhead were distributed in mixed‐stock assemblages throughout the hydrosystem during winter, usually in reservoirs closest to their home rivers but also in nonnatal tributaries. Overwintering fish moved upstream and downstream between reaches in all months; a nadir occurred in early January and peak egress into spawning tributaries was in March. The estimated survival to tributaries was higher for fish that overwintered in the hydrosystem (82%) than for fish that did not (62%); this difference was largely attributable to low winter harvest rates. Our results suggest that large main‐stem habitats, including reservoirs, may be widely used by overwintering summer steelhead. The complex migration behaviors of steelhead indicate both the potential for adaptation and possible susceptibility to future river environment changes.

Demographic variation and population viability in a threatened Himalayan medicinal and aromatic herb Nardostachys grandiflora: matrix modelling of harvesting effects in two contrasting habitats

Summary Despite significant interest in the sustainable use of medicinal plants, the population ecology of many medicinal plant species remains unstudied. Also, few demographic studies have compared effects of harvesting across habitats. We studied the demography of a highly threatened perennial Himalayan medicinal herb, Nardostachys grandiflora, in two habitats and examined effects of indigenous harvesting regimes on its population dynamics. In a rocky‐outcrop population in 1998, different levels of rhizome harvesting were applied directly by the local users. In meadows, we sampled an unharvested population and another population that was commercially harvested. Data on recruitment, growth and survival of individuals classified by size were gathered over 4 years. Population growth rate (λ) was significantly higher in the unharvested meadow population than in control subplots of the outcrop population. Harvesting significantly reduced λ. Matrix model projections revealed that the size of outcrop populations would return to initial values within ≈5 years, only after 10% rhizome harvesting. In other treatments, recovery time varied from 17 (25% harvesting) to 33 (75%) years. In contrast, in the commercially harvested meadow population (with harvesting levels >25%), projected recovery time was 6 years. Higher growth rates and faster recovery in meadow populations appear to be due to higher recruitment and faster vegetative growth. In outcrops, slow growth and low fecundity slow down recovery after harvesting. Synthesis and application. Nardostachys grandiflora is extremely sensitive to harvesting of rhizomes, but this sensitivity is higher in outcrop than in meadow habitats. Given the constraints on its population growth and its high sensitivity to harvesting, N. grandiflora should be strictly managed, with low harvest rates (<10% in outcrop and <25% in meadow) and fairly long rotations (at least 5 years) between successive harvests. This work shows the importance of considering demographic variation across habitats when formulating specific management plans for threatened medicinal plants. It also demonstrates the importance of integrating local harvesting practices in ecological research for informing management. A large proportion of high‐altitude perennial medicinal plants may be expected to be similarly sensitive to exploitation, requiring careful management to achieve sustainable harvesting.

Evolutionary responses to harvesting in ungulates

1. We investigate the evolutionary responses to harvesting in ungulates using a state-dependent, stochastic, density-dependent individual-based model of red deer Cervus elaphus (L.) females subject to different harvesting regimes. 2. The population's mean weight at first reproduction shifts towards light weights as harvesting increases, and its distribution changes from a single peak distribution under very low or high harvest rates, to a bimodal distribution under intermediate harvest rates. 3. These results suggest that, consistent with previous studies on aquatic species, harvesting-induced mortality may drive adaptive responses in ungulates by reducing the fitness benefits from adult survival and growth in favour of early and lightweight reproduction. 4. Selective harvesting for heavy animals has no additional effect on the evolutionarily stable strategy, suggesting that harvest rate is more important than the degree of selectivity in driving adaptive responses. However, selective harvesting of light females is positively associated with maturation weights even higher than those of a nonharvested population, probably due to the reduction in the fitness value of the offspring. 5. The average number of weight at maturation strategies in the population declines but the total number of strategies across all simulations increases with harvest rate, suggesting that harvesting-induced selection on weight at maturity overcomes the increase in strategy diversity expected from density-dependent release. 6. Yield initially increases with harvesting due to enhanced productivity of light females experiencing density-dependent release. However, it crashes under intense harvesting resulting in a population skewed to light, young and, therefore, less reproductive animals.

From the drawing board to the field: an example for establishing an MPA in Penghu, Taiwan

1. In order to promote the establishment of the first marine protected area (MPA) in Taiwan, the conservation values of 12 reef sites in the Penghu Islands were evaluated. The results together with a 6-year surveillance programme revealed that the Chinwan Inner Bay (CIB) possesses the best coral community in the Penghu Islands with the highest coral cover, high species diversity and habitat types, as well as a high conservation value. 2. Utilization of biological resources and socio-economic factors of CIB were investigated by intensive field surveys and a complete visit-and-poll survey respectively. Most of the fishing at CIB is for recreational purposes and is characterized by low investment, low harvest rates, high dependence on weather conditions, and self-consumption. 3. The socio-economic data showed that there was basically no direct conflict of interests with local communities at two neighbouring villages, and most local residents indicated that they would support the MPA proposal. 4. A blueprint for a CIB MPA based on the results of these biological and socio-economic investigations is proposed. Environmental threats including anchor damage and the predation of Drupella snails need to be ameliorated and monitored through the implementation of appropriate management. The involvement of local communities is key to the success of this MPA and environmental education is recommended to promote public awareness. Copyright © 2007 John Wiley & Sons, Ltd.

Is Density-Dependent Resource Harvest A Reliable Habitat Indicator for Conservation and Management?

Adaptive foraging decisions emerge from environmental conditions that alter fitness. Under most circumstances fitness varies with population density. So it should be possible to use density-dependent foraging not only as a surrogate for fitness, but also as an assay for habitat quality. We tested this prediction by measuring the quitting-harvest rates of red-backed voles foraging in artificial resource patches located in three different habitats where we manipulated population density. Quitting-harvest rates were less in safe foraging trays located under cover than in risky trays in the open. Voles avoided risky trays in old field and alder field habitats, and foraged all trays in a dense alder forest habitat to similar low harvest rates. Quitting-harvest rates declined with density in experiments conducted in 2005 but not in similar experiments during 2006. We interpret these complex results as evidence that rapid food renewal during the 2006 experiments increased the voles' foraging costs, and their qui...

Effects of Falconry Harvest on Wild Raptor Populations in the United States: Theoretical Considerations and Management Recommendations

We used recent population data and a deterministic matrix model that accounted for important aspects of raptor population biology to evaluate the likely impact of falconry harvest (including take of different age classes) on wild raptor populations in the United States. The harvest rate at maximum sustainable yield (MSY) ranged from 0.03 to 0.41 for the species examined. At least for peregrine falcons (Falco peregrinus), harvest rate at MSY was greatest for nestlings and lowest for adults. The quality of demographic data for the species influenced MSY. For most species the state of current knowledge probably underestimates the capacity for allowed harvest because estimates of vital rates, particularly survival, are biased low, because emigration is not distinguished from survival. This is offset somewhat by biases that might overestimate sustainability inherent in MSY-based analyses and deterministic models. Taking these factors into consideration and recognizing the impracticality of monitoring raptor populations to determine actual effects of harvest, we recommend that falconry harvest rates for juvenile raptors in the United States not exceed one-half of the estimated MSY up to a maximum of 5%, depending on species-specific estimates of capacity to sustain harvest. Under this guideline, harvest rates of up to 5% of annual production are supported for northern goshawks (Accipter gentilis), Harris's hawks (Parabuteo unicinctus), peregrine falcons, and golden eagles (Aquila chrysaetos); lower harvest rates are recommended for other species until better estimates of vital rates confirm greater harvest potential.

Foraging in the fallows: Hunting patterns across a successional continuum in the Peruvian Amazon

Studies comparing hunting between secondary and mature forests are critical to understanding secondary forests’ potential as sustainable hunting grounds. I examined hunting across a successional continuum by surveying 67 households in three communities near the Cordillera Azul National Park, Peru, and analyzing the potential for sustainable hunting. Ninety-nine percent of households surveyed went hunting during the six-month study. Ninety-one species were recorded from seven vertebrate and invertebrate classes, with mammals the most hunted. Five percent of extraction events were species of concern on IUCN’s Red List. Households extracted significantly more, in terms of number of species, number of collection events, and biomass, from older forests (>20 y) than from young secondary (1–5 y) or old secondary forests (5–20 y). However, when extraction is measured per unit area (kg/km 2 or collection events/km 2), households extracted more wildlife from old secondary forest. Households consumed meat at rates below Amazon regional averages. However, because human population densities are well above carrying capacity, current low harvest rates are likely a relic of past overharvests and do not reflect sustainability. Even if management focuses on source-sink dynamics with buffer zone hunting resupplied by protected area populations, long-term sustainability seems doubtful. As more agricultural clearing converts older forests to fields and fallows, the role of secondary forests in resource management plans will increase. The impact of high human population densities in the region means conservation and development programs should focus more on supplying alternative sources of protein and income and limiting immigration into the area.

Short‐Term Responses of Birds to Forest Gaps and Understory: An Assessment of Reduced‐Impact Logging in a Lowland Amazon Forest1

ABSTRACTWe studied physiognomy‐specific (i.e., gaps vs. understory) responses of birds to low harvest (18.7 m3/ha), reduced‐impact logging by comparing 3500 mist net captures in control and cut blocks of an Amazonian terra firme forest in Brazil at 20–42 mo postharvest. Species richness did not differ significantly between control (92 species) and cut (85) forest based on rarefaction to 1200 captures. Fifty‐six percent of all species were shared between control and cut forest, compared to the 64 percent shared between control blocks. Higher captures of nectarivores and frugivores in cut forest likely occurred as a consequence of postharvest resource blooms. Higher captures of some insectivores in cut as compared to control forest were unexpected, attributable to increased wandering or shifts from association with midstory to understory as a consequence of habitat alteration. Logging influenced capture rates for 21 species, either consistently, or via positive interaction with physiognomy or time (13 species higher in cut forest and 8 species higher in control forest). Cut understory sites had lower diversity (H′) and scaled dominance than understory and gap sites in control forest. Temporal changes in captures may have resulted from successional dynamics in cut forest: two guilds and three species increased in abundance. Increases in abundances of guilds and particular species were more prevalent in control than in cut forest, suggesting that logging displaced birds to control forest. In general, the effects of logging were relatively minor; low harvest rates and reduced‐impact methods may help to retain aspects of avian biodiversity in Amazon forest understories.

Active Management of Walleye Fisheries in Alberta: Dilemmas of Managing Recovering Fisheries

Managers of the recreational fishery for walleyes Sander vitreus (formerly Stizostedion vitreum) in Alberta, Canada, face an unusual combination of very low productivity (related to the northern climate) and high fishing pressure. Passive management of the large recreational fishery and active management of the smaller commercial fishery failed to prevent declines and collapses of walleye stocks. During the 1990s, extensive consultations with the public resulted in the development of an active recreational fishery management system using set points to classify stocks. Catch and release and large, highly restrictive length limits were used to regulate the harvest. These restrictions on the recreational harvest resulted in a dramatic increase in the catch rates of growth-overfished stocks. Paradoxically, this recovery has created dilemmas and controversies in both the recreational and commercial fisheries. Anglers are now dissatisfied with the low harvest rates and absence of large fish attending the high catch rates of small fish. The total allowable catch, however, is being taken by hooking mortality and illegal harvest of undersize walleyes in the recreational harvest. In commercial gill-net fisheries for lake whitefish Coregonus clupeaformis, increasing bycatch of walleyes restricts the harvest of lake whitefish and has created uneconomical fisheries. Resolving these dilemmas will require dramatic changes to fisheries management techniques in Alberta.

Mobility of largemouth bass in a desert lake in Arizona

Largemouth bass in Mittry lake, Arizona >360 mm total length generally remained in the same area of the lake for extended periods but some fish seemed to continually move from area to area. Sedentary and mobile fish did not differ by age or sex. Largemouth bass in Mittry lake maintained their respective behavior pattern (resident or mobile) throughout the year and did not move off-shore in the fall. The shallow depth of the lake and the moderate year round temperatures may explain this lack of seasonal movement. Harvest rates appeared to be modest. Lack of vulnerability and or angler access may explain low harvest rates.

Population extinction and optimal resource management

The optimal exploitation of a population is considered for three stochastic population models; these allow both demographic and environmental variability and the possibility of extinction. The dynamics are linear in the harvest rate; the optimal policy then recommends harvesting at the maximal rate above a critical level (the ‘threshold’) and at zero rate below. However, in all cases the optimal threshold differs radically according as to whether one maximizes the total return before extinction or the rate of return per unit time over the period before extinction. In the former case the optimal threshold is at the deterministic equilibrium level of the unexploited population, in the latter case it is approximately at the level of maximal sustainable production. Part of the explanation is that maximization of total yield turns out to be almost equivalent to maximization of time to extinction. Both average yield rate and the expected time to extinction vary with the policy, but the second much more powerfully. Both the criteria above are extreme: one obtains a balanced criterion (and an intermediate threshold) if one maximizes rate of return (before extinction) subject to the conservation requirement of a lower bound on the expected time to extinction. In the case when extinction is excluded because of a potential ‘rescue effect’ one comes to the same view by taking account of the relative time needed to restart an obliterated population. The practical implication is that more attention should be paid to extinction and restart times. For vulnerable populations it is likely that maximal utilization before an inevitable extinction will be achieved at low harvest rates. For large populations or metapopulations, with large times to extinction or quick recovery from a temporary extinction, classical resource models are appropriate.

Automated, in vitro harvest of somatic embryos

This work has demonstrated the aseptic, automated harvest of somatic embryos from a bioreactor suspension culture. Machine vision, emulating the selection criteria of an experienced biologist, classified embryos as harvestable or non-harvestable as they flowed through a 3 mm glass conduit. Embryos classified as harvestable were separated in a sealed harvest chamber. The system harvested 60% of the embryos at a rate of 2.4 embryos/h and incorrectly harvested less than 1% of the non-harvest objects. The low harvest rate precludes the applicability of this technique to research and commercial tissue culture laboratories. The suspension feed-rate, culture population density and culture homogeneity were identified as the most important factors influencing embryo harvest rate. The performance of this technique on more densely populated cultures was projected using anticipated improvements in suspension feedrate. It was concluded that, under the conditions of this analysis, the harvester would be of limited value in a commercial propagation environment but could be beneficial to many research labs working with plant somatic embryos.

Differential harvest of male and female mallards under conventional and point system regulations: 92Z/00019 J. K. Ringelman, Wildlife Society Bulletin, 19(3), 1991, pp 258–267

Since its initial testing during the late 1960's (Mikula et al. 1972, Geis and Crissey 1973, Hopper et al. 1975), the point system has been widely used to determine the bag limit for ducks. During the 1980's, species or sexes within species were assigned point values between 10 and 100 (always a multiple of 5). The daily bag limit was attained when the total point value of birds harvested, in the order in which they were taken, reached or exceeded 100 points. Point values were assigned based on the desired harvest levels for each species or sex; relatively low values. (i.e., 10 or 20) were assigned to species or sexes whose populations were believed able to withstand increased harvest pressure, whereas high point values (i.e., 70 or 100) were given to species or sexes where low harvest rates were desired. In contrast, the conventional method for determining bag limits placed a numerical limit on the number of ducks that could be harvested, and imposed additional restrictions on the number of ducks of a particular species or sex that could be taken. Compared to the conventional bag, the point system is thought to offer (1) an enhanced ability to direct harvest away from species or sexes needing protection, (2) a better means of increasing hunting opportunity by shifting harvest towards species or sexes that are believed able to withstand higher harvest rates, (3) a reduction in the incidence of unintentional violations caused by not identifying or misidentifying ducks in flight before shooting, and (4) a wider range of hunting opportunity to those

An assessment of the maximum sustainable yield of ivory from african elephant populations

A general, logistic population model is used to explore the dynamics of harvested elephant populations. The model includes two features peculiar to elephant populations and the harvesting of ivory. First, because of the shape of the growth curve of tusks with age, the conversion factor that relates the number of elephants killed to the ivory yield in weight is not constant, but a function of the population size. Second, tusks from animals that die from natural causes can be retrieved and included in the total yield of ivory. The implications of the relationship between tusk size and age of an animal on the maximum sustainable yield in terms of ivory tonnage and in terms of the number of tusks are explored. The nonequilibrium implications of the tusk growth curve on the population dynamics under different harvesting strategies are also investigated. Results indicate that the maximum sustainable yield is achieved at very low harvest rates with population levels close to the pristine equilibrium. When tusks from animals that die of natural causes are included in the harvest, the maximum yield may, depending on the mortality and recruitment parameters, occur when there is no direct harvest.

Estimating Harvest Rates of Bears from Sex Ratio Changes

Using simulation modeling, we assessed the potential for estimating harvest rates for generalized bear (Ursus spp.) populations from changes in kill sample sex ratios (Paloheimo and Fraser 1981, Fraser 1984). Models underlying these techniques require assumptions that may be violated in the field. We found that both estimators were sensitive to violations of 4 critical assumptions, and that bias was generally most pronounced when applied to bear populations with low harvest rates, moderate differences in hunting vulnerability between sexes, and complex age structures. Additionally bears typically occur in low densities and yield small harvests, producing limited data from which harvest estimates are produced. Chance deviations from the expected patterns occurred with small (20-180) samples of harvested bears, producing substantial variability in year-to-year estimates. J. WILDL. MANAGE. 51(4):802-811 Data analyses based on mathematical models are now commonplace in wildlife studies. Models inevitably make simplifying assumptions that will be violated in nature to some degree. In addition, many models are deterministic, thereby providing no way to assess the variability of the technique when applied to field data. When possible, analytical methods are validated under controlled field conditions where the parameters of interest are known, quantifying bias and variability directly. Empirical assessments of this sort are now common in the literature; e.g., Robinette et al. 1974, Greenwood et al. 1985, Conner et al. 1986. However, some analyses are intended to help biologists estimate parameters in situations where controlled field conditions are difficult or impossible. In such cases a biologist must either apply a technique in which unmet assumptions produce unknown consequences, or dispense with the technique entirely. Many will proceed with the analysis, adding a note that assumptions may be violated. Managers and administrators often place credence on the results of such studies, ignoring accompanying qualifiers and disclaimers regarding assumption violations or insufficient sample size. Simulation modeling can identify some consequences of unmet model assumptions with realistic amounts of data. In this paper we use simulated grizzly (Ursus arctos) and black bear (U. americanus) populations to assess a harvest rate estimator developed by Fraser (1976, 1984). Obtaining precise population estimates for bears, especially grizzlies, is exceedingly difficult in practice, making field verification of harvest rates virtually impossible. The analysis, originally developed by Fraser (1976) for moose (Alces alces), considers a cohort that starts with equal numbers of males and females, in which males are consistently more vulnerable to harvest. As the cohort ages and th more vulnerable males are depleted by hunting, the standing age structure increasingly favors females. In more intensely harvested populations, females predominate more quickly. Fraser (1976, 1984) found that the reciprocal of the age at which females 1st predominate aproximated the composite harvest rate (T of M and F harvest rates) over a broad range of harvest rates and differential vulnerabilities. Consequently, harvest rate could be estimated by regressing percent males on age. Four assumptions underlie the model: (1) males and females are equally represented in the age class prior to the 1st age of harvest or, alternately, that sex ratios can be determined empirically; (2) no systematic changes occur with age in the relative male and female vulnerabilities to hunting; (3) both sexes experience equal non-harvest sources of mortality; and (4) harvest effort is constant during the period analyzed. Other than the last, these assumptions also underlie the more general model (Paloheimo and Fraser 1981) on which this analysis is based. These 4 assumptions may be violated in bear populations. Production of cub (age 0.5 year) grizzly bears in Yellowstone National Park was biased toward males (Craighead et al. 1974, Knight and Eberhardt 1985), violating Assumption 1. Conversely, sex ratios of preharvest age ' Present address: Alaska Fish and Wildlife Research Center, 101 12th Ave., Fairbanks, AK 99701.