Reconstruction of walleye exploitation based on angler diary records and a model of predicted catches

The walleye population in Canadarago Lake, New York, was 81-95% exploited in the 1988 fishing season, the year in which a previous restriction on the length and number of legally harvestable fish was liberalized. Using diary records from a subset of fishermen, growth estimates, and an estimate of the walleye population in the following year, a method is developed to reconstruct the fish population back to the spring of 1988 and thus determine the exploitation rate. The method is based on a model of diary catches that partitions time and fish length into a set of cells and relates predicted catches and population sizes in these cells. The method's sensitivity to the partitioning scheme, the growth estimates, and the diary data is analyzed. The method could be employed in other fish exploitation analyses and demonstrates the use of inexpensive angler-collected data in fisheries management.     

Population dynamics and potential of fisheries stock enhancement: practical theory for assessment and policy analysis

The population dynamics of fisheries stock enhancement, and its potential for generating benefits over and above those obtainable from optimal exploitation of wild stocks alone are poorly understood and highly controversial. I review pertinent knowledge of fish population biology, and extend the dynamic pool theory of fishing to stock enhancement by unpacking recruitment, incorporating regulation in the recruited stock, and accounting for biological differences between wild and hatchery fish. I then analyse the dynamics of stock enhancement and its potential role in fisheries management, using the candidate stock of North Sea sole as an example and considering economic as well as biological criteria. Enhancement through release of recruits or advanced juveniles is predicted to increase total yield and stock abundance, but reduce abundance of the naturally recruited stock component through compensatory responses or overfishing. Economic feasibility of enhancement is subject to strong constraints, including trade-offs between the costs of fishing and hatchery releases. Costs of hatchery fish strongly influence optimal policy, which may range from no enhancement at high cost to high levels of stocking and fishing effort at low cost. Release of genetically maladapted fish reduces the effectiveness of enhancement, and is most detrimental overall if fitness of hatchery fish is only moderately compromised. As a temporary measure for the rebuilding of depleted stocks, enhancement cannot substitute for effort limitation, and is advantageous as an auxiliary measure only if the population has been reduced to a very low proportion of its unexploited biomass. Quantitative analysis of population dynamics is central to the responsible use of stock enhancement in fisheries management, and the necessary tools are available.     

Parasite recruitment by stocked walleye, Stizostedion vitreum vitreum (Mitchill), fry in a small boreal lake in central Canada

Six species of parasites were recovered 4 mo after walleye fry were stocked in Heming Lake, Manitoba. The species of parasites acquired most rapidly were those that were non-host-specific and common to the indigenous populations of both walleye and yellow perch (Perca flavescens). Parasite species overlap (Jaccard's indices) was greatest within age groups of walleye and yellow perch, but was also high between older walleye and yellow perch. The higher numbers of parasites recruited by stocked walleye, particularly ones known to induce pathology, raises questions on the success of walleye introductions to aquatic systems with a diverse indigenous parasite fauna and a fish population with a large proportion of yellow perch.     

Fort Peck paddlefish population survival and abundance in the Missouri River

Excessive fishing pressure can induce population declines or complete collapse of fisheries. Unless commercial and recreational fisheries for K-selected fishes, or those with slow growth and late maturation, are carefully managed, declines in abundance or fishery collapse is probable. Paddlefish Polyodon spathula,are a K-selected species that experienced historical declines in abundance as a result of habitat degradation and overfishing. Mark-recapture studies are well-suited for long-lived fishes by providing information on population density and vital rates. For sustainable commercial or recreational fisheries targeting species such as the paddlefish, managers require accurate estimates of population vital rates including survival, abundance, and exploitation. We used a Montana Fish, Wildlife & Parks (MFWP) mark-recapture dataset and modified Jolly-Seber (POPAN) models to estimate survival, recapture, probability of entry, and abundance of 8,518 tagged paddlefish over a 25-year period. With many supporting estimates including stable survival (0.92 for females, mean of 0.82 for males), low exploitation rates (means of 2.6% for females and 2.9% for males), and stable abundance estimates (25-year mean of 12,309 individuals for both sexes), the Fort Peck paddlefish population appears to be stable and well-managed over the past 25 years. Presently, this is the only study focused on paddlefish in North America that has estimated survival and abundance for both male and female paddlefish using contemporary analyses. This research provided a unique opportunity to highlight that the effort exerted by management agencies to collect long-term field data is extremely useful to our understanding of fish populations and management.     

Development and utilization of a population growth history of Red Lake walleye, Stizostedion vitreum

Synopsis An environmental growth history of commercially harvested walleye, Stizostedion vitreum, in Red Lakes, Minnesota, was constructed for the years 1944–1992. This was accomplished using a linear model which was fitted to annular scale increment measurements. Increment size was separated into one component due to a combination of environmental factors, an environmental growth coefficient, and one due to the age of the fish. Our hypothesis was that variables such as air temperature, walleye year-class strength, catch-per-unit-effort (CPUE) of walleye, and CPUE of yellow perch, Perca flavescens, affect walleye growth, and therefore a historical series of these variables would show coherence to the series of walleye growth coefficients. Multiple regression techniques were used to test these hypotheses. Significant predictors of the growth of walleye after age one were average February temperature, cumulative degree days in July, walleye year-class strength, and growth of young-of-the-year (YOY) walleye. We hypothesized that YOY walleye respond to a different set of factors than walleye after age one, thus, a series of YOY growth measurements would show coherence to a different set of environmental factors. Significant predictors of YOY walleye growth were May, June, and August cumulative degree days, as well as the growth of older walleye. We expected the set of factors which affect freshwater drum, Aplodinotus grunniens, to include factors that affect walleye, such as temperature, shared prey availability, and abundance of both walleye and drum. As a test, environmental growth coefficients computed by Pereira (1992) for freshwater drum were compared to walleye growth coefficients. The growth coefficients of drum were significantly positively correlated with the walleye coefficients, and the significance increased if the poorest walleye growth years were excluded.     

Contribution to the dynamics of mean-size statistics in a changing fishery: I. Family citharinidae in kainji lake,Nigeria

A model on The dynamics of mean-size statistics in a changing fishery proposed by Henderson (FAO) was tested using the Kainji Lake experimental gill-net data collected between 1969 and 1977. The results revealed distinct trends in mean weight and population abundance (catch/effort) for members of the family Citharinidae. Simultaneous plots of mean weight against log₁₀ of population abundance allowed the diagnosis of the causes of change. Changes were attributed to the effects of fishing mortality, intensive recruitment into the fishery and change in selectivity coupled with the effect of growth. The pattern of change depicted for Citharinus citharus Geoffroy St. Hilaire revealed a three yearly cycle of intensive recruitment. The regression of mean weight on mean length for C. citharus gave a linear relationship thus facilitating the extrapolation of the mean weight values to the mean lengths of fish caught in the different meshes of gill-nets. The conversions permitted the selection of suitable mesh nets for capturing fish at appropriate levels of mean weights in order to safe-guard the survival of sexually mature adults for an intensive spawning cycle.     

Influence of population decline, fishing, and spawner variability on the recovery of marine fishes

Based on an analysis of 90 marine fish populations, collapses (the greatest proportional reduction in spawner biomass over 15 years) are predicated typically by dramatic increases in fishing mortality and recoveries are more likely to occur when exploitation is reduced. However, among populations for which fishing mortality declined after collapse, recovery was independent of exploitation rate, even when fishing mortality (F) post-collapse was expressed as a function of each population's maximum growth rate (r). After a period of 15 years, many populations that experienced 15 year declines >60% exhibited little or no recovery, despite considerable reductions in fishing mortality. This suggests that factors other than fishing may be considerably more important to recovery, and fishing less important, than previously thought. Furthermore, among populations for which fishing mortality decreased post-collapse, rate of population decline was a reliable predictor of recovery. With the possible exception of clupeids, variation in marine fish breeding population size was found to differ little from that of other vertebrates, and such variability appears to have no effect on rate of recovery. In addition to providing an empirical framework for the study of population collapse and recovery, the analyses presented here provide a means of assessing the precautionary nature of various population-decline thresholds used to assign extinction risks to marine fish.     

Flooding effects on abundance of an exploited,long-lived fish population in river-floodplains of the Amazon

Flooding regulates the amount and quality of habitat available for fish populations in river-floodplains. Although previous studies assessed fish population responses to river hydrology, the processes by which flooding affects fish abundance and catch remain unclear. Here, we investigated whether degree of flooding affects abundance and catch of Colossoma macropomum, a long-lived, overexploited fish population of the central Amazon Basin. We computed the degree of flooding corresponding to the feeding area of young-of-the-year C. macropomum as the annual magnitude of the moving littoral zone (ML). We estimated abundance of age classes one, two, and three of C. macropomum using a modeling program based on catch, fishing effort, and fish length frequency data from the principal commercial fishery. We found that flooding positively and non-linearly affected abundance of the age-one cohort but not of older age classes. ML data corresponding to a late rising water phase in which zooplankton, seeds, and fruits dominated the diet provided the strongest effect on age-one abundance. However, flooding effects on total catch were not found, likely due to catches comprising several age classes. These results provide support to existing evidence that the magnitude of the moving littoral zone regulates abundance of juvenile fish. Because the ML quantifies food and habitat availability for various other fish species, it may constitute an important control of fish abundance in these systems. Management of these fisheries may be improved by adjusting fishing effort based on hydrology. More generally, the information also serves to assess the impacts of hydrological alterations (e.g., dams) on fish recruitment.

     

Historic changes in length distributions of three Baltic cod (Gadus morhua) stocks: Evidence of growth retardation

Understanding how combinations of fishing effort and selectivity affect productivity is central to fisheries research. We investigate the roles of fishing regulation in comparison with ecosystem status for Baltic Sea cod stock productivity, growth performance, and population stability. This case study is interesting because three cod populations with different exploitation patterns and stock status are located in three adjacent but partially, ecologically different areas. In assessing stock status, growth, and productivity, we use survey information and rather basic stock parameters without relying on age readings. Because there is an urgent interest of better understanding of the current development of the Eastern Baltic cod stock, we argue that our approach represents partly a novel way of interpreting monitoring information together with catch data in a simplified yet more informative way. Our study reports how the Eastern and Western Baltic cod have gone toward more truncated size structures between 1991 and 2016, in particular for the Eastern Baltic cod, whereas the Öresund cod show no trend. We suggest that selective fishing may disrupt fish population dynamic stability and that lower natural productivity might amplify the effects of selective fishing. In support of earlier findings on a density‐dependent growth of Eastern Baltic cod, management is advised to acknowledge that sustainable exploitation levels for Eastern Baltic cod are much more limited than perceived in regular assessments. Of more general importance, our results emphasize the need to embrace a more realistic view on what ecosystems can produce regarding tractable fish biomass to facilitate a more ecosystem‐based fisheries management.     

World whale stocks

The history of whaling is very largely one of repeated over-exploitation of the various whale stocks which became available through discovery or technological advance. Modern whaling has similarly caused considerable reductions in the numbers of some species in the major whaling grounds. Stock assessment methods are based on catch and effort statistics, biological information including age and reproductive status, marking and sightings records. Catch effort data have to be used with caution, because of changes in species preference, shifts in the whaling grounds and national fleet variations. With allowance made for these factors, cumulative catches adjusted for recruitment can be used to estimate the initial stock number. Changes in stock density after known catches also lead to abundance estimates. Logarithmic regression of age composition data are used to find the total mortality rates. The natural mortality can be estimated from early season catches in a fishery or pre-fishery year classes caught more recently; fishing mortality is found by subtraction, which again leads to abundance estimates. Mathematical approaches incorporating recruitment estimates from actual age composition data and theoretical population models have been employed. Additional estimates come from mark release-recapture experiments and direct sightings counts from whaling vessels and research ships. The latter are the only means of estimating the protected species. The yields which the various stocks can sustain are calculated from direct observations and theoretical considerations of the changes in recruitment, largely due to increased pregnancy rates and the lower ages at sexual maturity which occur in exploited stocks. The results of all the available analyses have been compared and combined to produce the population estimates and yields tabulated. The object of whale management is to bring all stocks to the levels providing the maximum or optimum sustainable yields. These are defined in terms of numbers at the moment, but may be expressed as biomass in the future.     

Effects of access restrictions and stocking on small water body fisheries in Laos

Four different management regimes were identified in small water bodies in Laos: open-access fisheries, both with and without stocking of exotics (mainly Nile tilapia Oreochromis niloticus ); community fisheries with restricted access and regular stocking; and fisheries rented out to corporate entities, based on indigenous stocks only. These regimes represent all possible combinations of the two management measures, access (open/restricted) and stocking of exotic species (no/yes) and a test fishing experiment assessed their effects on stock abundance, richness and diversity. The combination of access restrictions and stocking had a strong positive effect on total standing stocks. Stocks of indigenous fish were significantly increased by access restrictions, while stocking of exotics had no effect on indigenous standing stocks. Community fisheries targeted large sizes of exotic species while reducing the exploitation of smaller size groups, which accounted for much of the indigenous stocks. This suggests that stocking can promote active effort regulation and reduce the exploitation of natural stocks. Data on yields and effort were too limited to allow the use of inferential statistics, but indicated that community fisheries were exploited with much lower effort and gave lower yields than open access fisheries, while providing higher returns to fishing effort. This suggests that active management is effective in increasing standing stocks and the efficiency of exploitation, but does not necessarily increase yields unless optimal management regimes can be identified and implemented by the management institutions. No significant effects on wild stock richness or diversity were detected in the test fishing experiment, but wide confidence limits indicated a low statistical power of the test and therefore no definitive conclusions could be drawn.     

Walleye, Stizostedion vitreum, and northern pike, Esox lucius, populations in three Alberta lakes

Results are presented from a survey of walleye and northern pike in Ethel, Marie and Wolf Lakes, Alberta conducted May-September, 1983. Three types of experimental gill net were fished, mainly inshore. Northern pike were abundant in all three lakes but white sucker and yellow perch also were common. Walleye were caught in largest numbers in Wolf Lake. Age and growth of both walleye and northern pike were determined from examination of the opercular bones. The walleye in the three lakes were represented by a few strong year classes but there was no synchronism between the lakes. The strong 1975 year class in Ethel and Wolf Lakes may have been correlated with high spring precipitation and heavy run-off. Although there were dominant year classes in the northern pike populations (especially the 1979 year class) the variability was not so marked as in the walleye. There were no significant differences between length-at-age for year classes of walleye in Ethel and Marie or northern pike in all three lakes. In Wolf Lake the 1978 and 1979 year classes grew faster than in previous years. Walleye and northern pike growth was well described by the von Bertalanffy growth model. Females of both species grew to a greater ultimate length and had a longer lifespan than the males. Fecundity data are presented for walleye from Wolf Lake (log₁₀ absolute fecundity = 0.856 + 2.441 log₁₀ length) and northern pike from Marie Lake (log₁₀ absolute fecundity =?2.671+4.052 log₁₀ length). The stomach contents of both species were examined during May and June. The majority of walleye had empty stomachs. Walleye and northern pike in Ethel and Wolf Lakes fed on a variety of fish and invertebrate species but both fed mainly on fish (in particular whitefish) in Marie Lake. The similarity of diet suggests competition for prey species. A walleye yield to commercial fishermen of c. 0.69 kg ha^?1 year^?1 in Wolf Lake has made up 8% of the total catch. Since 1970/71 when mesh size of nets was increased, the yield has been reduced to c. 0.30 kg ha^?1 year^?1. Anglers remove an additional c. 0.48 kg ha^?1 year^?1 walleye from Wolf Lake. Northern pike yields to commercial fishermen have been c. 2.00 kg ha^?1 year^?1 in Wolf Lake. Ethel and Marie Lakes have yielded only poor catches of walleye and northern pike. A yield model was used to illustrate that faster-growing northern pike have higher potential yields than walleye. Walleye produce higher yields in Wolf Lake than in Marie Lake, the reverse being true for northern pike. It is suggested that northern pike could be cropped at a higher rate in Marie and Wolf Lakes with a possible improvement in walleye stocks.     

Shifting dynamic forces in fish stock fluctuations triggered by age truncation?

Accumulating evidence shows that environmental fluctuations and exploitation jointly affect marine fish populations, and understanding their interaction is a key issue for fisheries ecology. In particular, it has been proposed that age truncation induced by fisheries exploitation may increase the population's sensitivity to climate. In this study, we use unique long‐term abundance data for the Northeast Arctic stock of cod (Gadus morhua) and the Norwegian Spring‐Spawning stock of herring (Clupea harengus), which we analyze using techniques based on age‐structured population matrices. After identifying time periods with different age distributions in the spawning stock, we use linear models to quantify the relative effect of exploitation and temperature on the population growth rates. For the two populations, age truncation was found to be associated with an increasing importance of temperature and a relatively decreasing importance of exploitation, while the population growth rate became increasingly sensitive to recruitment variations. The results suggested that the removal of older age classes reduced the buffering capacity of the population, thereby making the population growth rate more dependent on recruitment than adult survival and increasing the effect of environmental fluctuations. Age structure appeared as a key characteristic that can affect the response of fish stocks to climate variations and its consequences may be of key importance for conservation and management.     

色林错渔业生产的现状与可持续利用的对策

色林错裸鲤（Gymnocypris selincuoensis)是藏北色林错湖泊中惟一的一种鱼类。本文介绍了色林错渔业的开发利用情况,并对色林错裸鲤最小捕捞年龄、捕捞强度、最小网目以及最佳年捕捞产量进行了探讨。从可持续发展的角度,我们认为对色林错鱼类资源的利用必须以保证现有湖泊生态系统的稳定为核心,在满足维持一个足够数量及年龄结构的繁殖群体的基础上,以获得最佳持续经济利益为目标。以往强调甚至采用的最大持续产量理论在高原极端环境条件下并不能作为鱼类资源利用的追求目标或确定渔产量的标准,而只能作为对其捕捞强度的参考。总的允许渔获量应当根据最适捕捞死亡率F0．2来确定。本文最后提出了色林错湖泊鱼类资源可持续利用的8条具体措施。     

Acoustic Telemetry Reveals Large-Scale Migration Patterns of Walleye in Lake Huron

Fish migration in large freshwater lacustrine systems such as the Laurentian Great Lakes is not well understood. The walleye (Sander vitreus) is an economically and ecologically important native fish species throughout the Great Lakes. In Lake Huron walleye has recently undergone a population expansion as a result of recovery of the primary stock, stemming from changing food web dynamics. During 2011 and 2012, we used acoustic telemetry to document the timing and spatial scale of walleye migration in Lake Huron and Saginaw Bay. Spawning walleye (n = 199) collected from a tributary of Saginaw Bay were implanted with acoustic tags and their migrations were documented using acoustic receivers (n = 140) deployed throughout U.S. nearshore waters of Lake Huron. Three migration pathways were described using multistate mark-recapture models. Models were evaluated using the Akaike Information Criterion. Fish sex did not influence migratory behavior but did affect migration rate and walleye were detected on all acoustic receiver lines. Most (95%) tagged fish migrated downstream from the riverine tagging and release location to Saginaw Bay, and 37% of these fish emigrated from Saginaw Bay into Lake Huron. Remarkably, 8% of walleye that emigrated from Saginaw Bay were detected at the acoustic receiver line located farthest from the release location more than 350 km away. Most (64%) walleye returned to the Saginaw River in 2012, presumably for spawning. Our findings reveal that fish from this stock use virtually the entirety of U.S. nearshore waters of Lake Huron.     

A 27-year study of brown trout population dynamics and exploitation in Lake Songsjøen, central Norway

From 1968–1984 (period I), a brown trout Salmo trutta , population in a 70-ha oligotrophic lake in central Norway was exploited using larger mesh gill-nets selectively removing the larger fish. From 1985–1994 (period II), intermediate sized fish were removed using smaller-mesh sizes gill-nets. Fishing mortality and CPUE were correlated positively with effort and numbers of fish >3 years old for period II. The gill-net catchability was correlated negatively with spawner biomass and number of trout >3 years old. The significant positive correlation between natural mortality and stock biomass and spawning stock biomass indicated density-dependent mortality. The significant correlation between spawning stock and recruitment described by the Ricker model, indicated density-dependent recruitment of 1-year-old trout. The fishing regimes in the two periods affected the population dynamics and density differently. Selective removal of smaller fish permitted the larger fish to survive, and was beneficial in reducing fish density and maintaining stocks at low levels, consequently, achieving the expected increase in fish growth rates.     

Fish stock assessment in lakes based on mass removal

The efficiency of mass removal of fishes can potentially be assessed using catch statistics collected during intensive fishing periods. The calculations are based on the general assumption that a decline in population size will produce a decline in catch per unit effort (cpue). When the removal is efficient it is possible to estimate the population size both at the beginning of the fishing period and after the removal.
Two examples are based on the winter seine net fishing of vendace ( Coregonus albula ) in Lake Pyhäjärvi, SW Finland in (1) 1983–1984 and (2) 1989–1990. The effects of the error in catch composition samples and random changes in probabilities of capture during the removal period on the final estimates of initial stock sizes are also examined.
The precision of the population estimates shown here was greatly influenced by the violation of the underlying assumption that the probability of capture is equal for all members of the target age group. In any case, particular attention should be paid to optimizing the sampling programme so that it will reveal the best information on the exploited stock with the resources in hand.     

Growth, mortality, parasitism, and potential yields of two Priacanthus species in the South China Sea

In the northern part of the South China Sea the 'big-eye', Priacanthus tayenus , spawned once a year in June, had von Bertalanffy growth parameters of k = 0.8 and L ∞= 30 cm, and a mean total annual instantaneous mortality of Z = 2.0, calculated from adjusted catch curves and a mean length equation. The natural mortality rate M = 1.4, fishing mortality rate F = 0.6, and the exploitation rate (E) was 0.27. The maximum potential yield, calculated using Marten's method, was 0.06 kg/recruit when F = 5.4. The fish were heavily parasitised by the protozoan Pleistophora priacanthicola .
A second big-eye, P. macracanthus , spawned twice a year in May-June and September, had growth parameters of κ= 0.7 and L∞= 32, and population parameters of Z = 2.0, . F = 0.7, and E = 0.34. The maximum potential yield was 0.13 kg/recruit when F = 5.8.
A marked reduction in fishing mortality occurred for both species between 1965 and 1966, coinciding with the onset of the Chinese Cultural Revolution. Our estimates of maximum potential yield correspond to fishing mortalities eight times estimated levels, though such heavy exploitation could risk recruitment failure.     

Natural fishing experiments in marine reserves 1983 – 1993: roles of life history and fishing intensity in family responses

 This study examined the effect of fishing on the abundance and species richness of families of coral reef fish at two islands (Sumilon and Apo) in the Philippines from 1983 to 1993. Natural fishing experiments occurred in marine reserves at each island, where long term estimates of fishing intensity were available. Responses to fishing were interpreted in terms of life histories of fish. The intensity of fishing and fish life histories were generally good predictors of the differential rates of decline and recovery of abundance in response to fishing. Large predators had vulnerable life histories (low rates of natural mortality, growth and recruitment) and were subjected to high intensity fishing. They declined significantly in density when fished and increased significantly but slowly when protected from fishing. Caesionidae, a family with a life history resilient to fishing (high rates of natural mortality, growth and recruitment) but fished intensively also declined rapidly in abundance when fished. Thus, knowledge of life history alone was insufficient to predict response to fishing. Acanthuridae were fished relatively hard and had a life history of intermediate vulnerability but displayed weak responses to fishing. Thus level of fishing intensity alone was also not sufficient to predict response to fishing. For Chaetodontidae, effects of fishing conformed to expectations based on life history and fishing intensity at one island but not the other. Three families with intermediate vulnerability and subjected to intermediate to light fishing (F. Scaridae, Labridae and Mullidae) displayed predictably weak responses to fishing, or counter-intuitive responses (e.g., increasing in abundance following fishing). These counter-intuitive responses were unlikely to be secondary effects of increase in prey in response to declines of predators. Two lightly-fished families with resilient life histories (F. Pomacentridae, Sub F. Anthiinae) predictably displayed weak numerical responses to fishing except during a period of use of explosives and drive nets. Accepted: 30 June 1998     

A Mixed-Method Approach for Quantifying Illegal Fishing and Its Impact on an Endangered Fish Species

Illegal harvest is recognized as a widespread problem in natural resource management. The use of multiple methods for quantifying illegal harvest has been widely recommended yet infrequently applied. We used a mixed-method approach to evaluate the extent, character, and motivations of illegal gillnet fishing in Lake Hovsgol National Park, Mongolia and its impact on the lake’s fish populations, especially that of the endangered endemic Hovsgol grayling (Thymallus nigrescens). Surveys for derelict fishing gear indicate that gillnet fishing is widespread and increasing and that fishers generally use 3–4 cm mesh gillnet. Interviews with resident herders and park rangers suggest that many residents fish for subsistence during the spring grayling spawning migration and that some residents fish commercially year-round. Interviewed herders and rangers generally agree that fish population sizes are decreasing but are divided on the causes and solutions. Biological monitoring indicates that the gillnet mesh sizes used by fishers efficiently target Hovsgol grayling. Of the five species sampled in the monitoring program, only burbot (Lota lota) showed a significant decrease in population abundance from 2009–2013. However, grayling, burbot, and roach (Rutilus rutilus) all showed significant declines in average body size, suggesting a negative fishing impact. Data-poor stock assessment methods suggest that the fishing effort equivalent to each resident family fishing 50-m of gillnet 11–15 nights per year would be sufficient to overexploit the grayling population. Results from the derelict fishing gear survey and interviews suggest that this level of effort is not implausible. Overall, we demonstrate the ability for a mixed-method approach to effectively describe an illegal fishery and suggest that these methods be used to assess illegal fishing and its impacts in other protected areas.