Minimum Length Limit Research Articles

An assessment of recreational fishery harvest policies for Murray cod in southeast Australia

Murray cod Maccullochella peelii peelii is one of the world’s largest freshwater fish and supports popular fisheries in southeast Australia, but no previous modelling efforts have evaluated the effects of fisheries regulations or attempted to develop sustainable harvest policies. We compiled existing population metrics and constructed an age-structured model to evaluate the effects of minimum length limits (MLLs) and fishing mortality rates on Murray cod fisheries. The model incorporated a Beverton and Holt stock recruit curve, age-specific survivorship and vulnerability schedules, and discard (catch and release) mortality for fish caught and released. Output metrics included yield (kg), spawning potential ratio (SPR), total angler catch, total harvest, and the proportion of angler trips that would be influenced by each regulation based on recent creel survey data. The model suggested that annual exploitation ( U) should be held to less than 0.15 under the current MLL of 500 mm total length to achieve an SPR > 0.3, a target usually considered to prevent recruitment overfishing. Exploitation rates at or exceeding 0.3 would cause SPR values to drop below typical management targets unless the MLL was set at or above 700 mm. Regulations that protected Murray cod from overfishing created higher angler catches and higher catch of trophy fish, but at a cost of reducing the proportion of angler trips resulting in a harvested fish. Expressing model output on a per-angler trip basis may help fishery managers explain regulation trade offs to anglers.

Migrations and Biological Characteristics of Adfluvial Coaster Brook Trout in a South Shore Lake Superior Tributary

AbstractWe used fish traps and electrofishing surveys to characterize the biology, life history traits, and potential biotic interactors important to the rehabilitation of native, adfluvial coaster brook trout Salvelinus fontinalis in Lake Superior. This study focused on the Salmon Trout River, Michigan, which is the site of the last known remnant population of adfluvial brook trout on the south shore of Lake Superior. The brook trout captured in passive traps (weirs) in the river ranged from 56 to 554 mm in total length (TL) and from 0 (young of the year) to 6 years of age. This population displayed a protracted 5‐month migration into the river but one that included relatively few fish. Coaster brook trout in this population appear to initiate adfluvial migrations near age 3 and 300 mm TL. Relative weight values increased with fish length and therefore reproductive life stage, possibly indicating the shift from river to lake habitats. This population of coaster brook trout is small and subject to potential biotic interactions with exotic species and population limitation because of an active fishery in the lake and river. Migratory runs of brook trout coincided with large runs of coho salmon Oncorhynchus kisutch, and the instream juvenile salmonid composition included proportionately high densities of coho salmon and rainbow trout O. mykiss. In addition, Floy tag return data indicated that exploitation of reproductive‐age Salmon Trout River coaster brook trout was at least 12% and may have been as high as 50% in the open waters of Lake Superior. A recent increase in the legal minimum length limit and reduction of the daily bag limit for recreational harvest may slow or reverse this trend.

Evaluating the potential effectiveness of harvest restrictions on riverine sunfish populations in Georgia, USA

Abstract The effects of minimum length limits (MLLs) on bluegill, Lepomis macrochirus Rafinesque, redbreast sunfish, Lepomis auritus (Linnaeus), and redear sunfish, L. microlophus (Günther), fisheries in 13 river sections located across Georgia were evaluated. A Beverton–Holt equilibrium yield model was used to predict and compare the response of these fisheries to three MLL scenarios: 100 (i.e. no size limit), 125 and 150 mm. Model results indicated that natural mortality rates of all sunfish species examined in this study were low enough to allow harvest restrictions to be effective if growth was fast. If sunfish reached 203 mm in <4.0 years, then MLLs were generally effective in increasing size structure without decreasing yield or causing unacceptable declines in the number of fish available for anglers to harvest. Harvest restrictions resulted in minimal increases in size structure, declines in yield and a large decrease in the number of fish harvested by anglers if time to reach 203 mm was ≥ 8.0 years. Response to MLLs was more variable when fish grew to 203 mm in 4.0–6.5 years. Examination of mortality caps largely supported the results from the model, but there was indication that empirical rates of total annual mortality may have been biased high in some instances.

Ecology and management of northern pike in Wisconsin

Northern pike (pike) Esox lucius Linnaeus are broadly distributed across Wisconsin and provide an important sport fishery for anglers. Historical management has included use of angling regulations, stocking, and protection of critical habitat areas. Pike management is often a lower priority in comparison to more intensively managed species such as muskellunge Esox masquinongy Mitchell and walleye Sander vitreous. Pike provide angling opportunities year around, with higher catch rates and live release during open water, and higher harvest rates during winter. While length of pike caught during the two seasons is generally similar, there is some evidence that larger fish are caught during winter months. Results from minimum length limit changes implemented in 1995 have been equivocal and warrant further monitoring using standardized protocol to better determine effects on populations. If natural recruitment is not limited, pike populations can reach high densities and stunting can result. Loss of nearshore habitat has continued at an increased rate as more lake homes are built and shorelines graded, and altered with riprap, sand blankets, or sea walls. If natural recruitment is lacking, fry or fingerling stocking sustains or rehabilitates populations. Most pike stocking occurs in the southeastern portion of the state where habitat loss has been extensive. Stocking in other areas is largely to re-establish populations following a chemical reclamation or from winterkill. Future management needs to establish sampling protocols to ensure comparable data on both a temporal and a spatial scale. Long-term monitoring with these methods is essential to help determine effects of various factors (e.g., regulations, habitat loss) on populations.

Effects and Utility of Minimum Length Limits and Mortality Caps for Flathead Catfish in Discrete Reaches of a Large Prairie River

AbstractPopulations of flathead catfish Pylodictis olivaris in four reaches of the Kansas River, Kansas, were examined to evaluate the effects of three minimum length limits (305 (the minimum size that anglers are willing to harvest), 610, and 762 mm total length (TL)) on population size structure and number harvested over a 30‐year simulation. We used electrofishing and hoopnetting to capture and tag flathead catfish throughout the Kansas River from 2005–2006. Current exploitation (u) based on tag returns was probably less than 10%, and total annual mortality A ranged from 14% to 28% across all reaches. Increased river access and promotion of this fishery may increase u in the future, so model simulations were conducted with conditional natural mortality (cm) and u ranging from 10% to 40%. Proportional stock density (PSD) and the relative stock density of preferred‐length fish (RSD‐P) substantially declined (by >25% and >15%, respectively) as u increased with the 305‐mm TL limit over the 30‐year simulation. The PSD showed similar trends across all reaches, indicating that use of different regulations among reaches was not necessary and that differences in growth among reaches were less influential than u and cm. No substantial differences were observed in size structure under the 610‐ and 762‐mm TL limits among reaches, but under a 762‐mm TL limit anglers would have to sacrifice about 42% of the number harvested under the current cm level. Mortality caps revealed that each reach could sustain an A of about 60% and 55% to maintain current PSD and RSD‐P levels, respectively; this result suggests that quality flathead catfish size structure can be preserved. Our results show that the effects (or lack thereof) on flathead catfish and other sport fishes should be evaluated before harvest restrictions are implemented.

Effects of cryptic mortality and the hidden costs of using length limits in fishery management

AbstractFishery collapses cause substantial economic and ecological harm, but common management actions often fail to prevent overfishing. Minimum length limits are perhaps the most common fishing regulation used in both commercial and recreational fisheries, but their conservation benefits can be influenced by discard mortality of fish caught and released below the legal length. We constructed a computer model to evaluate how discard mortality could influence the conservation utility of minimum length regulations. We evaluated policy performance across two disparate fish life‐history types: short‐lived high‐productivity (SLHP) and long‐lived low‐productivity (LLLP) species. For the life‐history types, fishing mortality rates and minimum length limits that we examined, length limits alone generally failed to achieve sustainability when discard mortality rate exceeded about 0.2 for SLHP species and 0.05 for LLLP species. At these levels of discard mortality, reductions in overall fishing mortality (e.g. lower fishing effort) were required to prevent recruitment overfishing if fishing mortality was high. Similarly, relatively low discard mortality rates (>0.05) rendered maximum yield unobtainable and caused a substantial shift in the shape of the yield response surfaces. An analysis of fishery efficiency showed that length limits caused the simulated fisheries to be much less efficient, potentially exposing the target species and ecosystem to increased negative effects of the fishing process. Our findings suggest that for overexploited fisheries with moderate‐to‐high discard mortality rates, reductions in fishing mortality will be required to meet management goals. Resource managers should carefully consider impacts of cryptic mortality sources (e.g. discard mortality) on fishery sustainability, especially in recreational fisheries where release rates are high and effort is increasing in many areas of the world.

Yellow Perch in South Dakota: Population Variability and Predicted Effects of Creel Limit Reductions and Minimum Length Limits

AbstractWe collected annual gill‐net samples of yellow perch Perca flavescens in six South Dakota lakes over 4–5 years. We also simulated the effects of reductions in daily creel limits for yellow perch (i.e., from 25 fish/angler to 5, 10, or 15 fish/angler) and use of minimum total length limits (229 and 254 mm). Population indices varied widely among lakes and among years within lakes to the extent that indices from any individual year were largely uninformative. Creel surveys indicated that few anglers typically achieved a daily creel limit of 25 yellow perch. Except in Waubay Lake, lowering the creel limit from 25 to 5 fish/angler would be necessary to achieve harvest reductions of 25% or more within most of the fisheries we examined. Minimum length limits were projected to improve size and age structure, but harvest reductions often exceeded 50% and yield declined or only slightly increased (by ≤13%). Yellow perch in these lakes had achieved much of their full growth potential by the time they reached 229 and 254 mm. Consequently, although length limits were predicted to increase age and size structures, asymptotic growth prevented yellow perch from attaining substantially larger sizes. Length limits were predicted to be most beneficial when fishing mortality represented most of the total mortality in each population and when growth occurred at median or fast rates. A 229‐mm length limit was predicted to improve size and age structures with less‐severe reductions in harvest or yield than a 254‐mm limit. Population dynamics and the harvest‐oriented nature of yellow perch anglers may dictate that yield maximization is still a reasonable management goal for many South Dakota yellow perch fisheries. Increasing the number of yellow perch that attain 229 mm is possible for certain fisheries if angler harvest represents the dominant source of mortality.

Effects of harvest and length limits on shovelnose sturgeon in the upper Wabash River, Indiana

Summary Shovelnose sturgeon Scaphirhynchus platorynchus are one of the few sturgeon species that currently support sustainable commercial harvest. However, harvest closures for many Eurasian sturgeons have resulted in increased exploitation of this fishery, thereby raising concerns about the sustainability of shovelnose sturgeon resources. As a result, the maintenance of self-sustaining shovelnose sturgeon populations will require the estimation of appropriate harvest levels. This study used an age-structured population model to examine the effects of harvest (u = 0.15–0.75) and length restrictions on population abundance, mean length-at-harvest, biomass, yield, and reproductive potential of female shovelnose sturgeon in the upper Wabash River, Indiana. Model simulations for four hypothetical length-restriction scenarios (610- to 813-mm reverse slot limit, and a 610-, 635-, and 660-mm minimum length limit) were compared to outputs with no restriction. All population parameters within each length-restriction scenario declined with increases in harvest level. For each harvest level, all population parameters increased as length limits became more restrictive. The reverse slot limit and 610-mm minimum length limit provided adequate protection to allow population parameters to increase through an annual harvest level of 0.55. However, these length restrictions were not sufficiently conservative to warrant implementation due to their similarity to length-at-maturity of female shovelnose sturgeon. The implementation of a 635-mm minimum length limit would protect female shovelnose sturgeon from harvest rates >0.75, allow 92% of the females to remain available for harvest, and minimize short-term (<30 years) declines in yield. Further, sensitivity and robustness analyses suggested that the 635-mm minimum length limit would allow population parameters to increase even at the worst-case scenario. As a result, the 635-mm minimum length limit was recommended as the most appropriate regulation to promote conservation and sustainable harvest of shovelnose sturgeon in the upper Wabash River.

Harvest of Mississippi River sturgeon drives abundance and reproductive success: a harbinger of collapse?

Summary Within harvested populations, relationships between harvest intensity and reproductive responses are typically unclear, rendering regulatory decisions difficult. Harvest of the commercially important shovelnose sturgeon (Scaphirhynchus platorynchus) is increasing in the upper Mississippi River; standardized seasonal sampling revealed that adult abundance is declining. Relative density of annual cohorts varied negatively with historical harvest intensity (r 2 ¼ 0.84), suggesting that removal of mature adults is reducing the contribution of cohorts to population density. The results of simulation modeling suggest that this currently unregulated fishery is experiencing both growth and recruitment overfishing. Further, the current proposed multi-state minimum length regulation was insufficient to maintain a sustainable stock. Only a more conservative minimum length limit (685 mm) produced yields that were sustainable at the current level of mortality and provided room for the fishery to grow. The annual mortality rate of the sympatric, federally endangered pallid sturgeon (S. albus) was similar to that of the shovelnose sturgeon population, raising concerns that harvest-induced mortality is affecting this congener’s vital rates.

Evaluating Walleye Length Limits in the Face of Population Variability: Case Histories from Western Minnesota

AbstractPopulation and fishery responses to the implementation of minimum length limits (356 and 381 mm total length) for walleyes Sander vitreus were evaluated for two Minnesota populations (Big Stone Lake and Lac Qui Parle) and were compared with population trends observed in a single reference lake (Lake Traverse) between 1991 and 2004. High variation in walleye recruitment (coefficient of variation (CV = 100 × SD/mean) in mean gill‐net catch per unit effort of age‐2 walleyes &gt; 98%) was observed in all study populations and also in recruitment indices from 20 populations across North America (mean CV = 112%), indicating that high variability in recruitment is common. Walleye growth rates were also variable; mean length at age 3 varied by more than 120 mm among year‐classes in all three study populations. Observed trends in population age‐ and size‐structure indices estimated from gill nets appeared to be largely related to growth and recruitment patterns both before and after the regulations were in place. No direct evidence existed to indicate that adult walleye abundance, size structure, or age structure was improved after implementation of length limits or that the regulations reduced annual variation in size structure. In light of recruitment variation, meaningful evaluation of walleye length limits will require long‐term annual sampling efforts designed to monitor the fate of multiple year‐classes of similar magnitudes during both preregulation and postregulation periods. Lastly, managers must select meaningful metrics to measure the effects of length limits. Observed improvements in fishery‐related metrics, such as size structure of harvested fish, may merely reflect changes in angler behavior rather than actual improvements in the population. Also, fishery‐based metrics may be difficult to measure given the budgetary and logistical constraints associated with creel surveys.

Historical Trends in Creel Limits, Length-based Limits, and Season Restrictions for Black Basses in the United States and Canada

Abstract We determined for largemouth bass ( Micropterus salmoides ), smallmouth bass) M. dolomeui ), and spotted bass ( M. punctulatus ) historical trends in state- and province-wide creel limits, length limits, and season closures along with the rationale justifying these regulations. Based on data gathered via mail surveys and the Internet, 55 jurisdictions had state- or province-wide creel limits, minimum length limits, or season closures, with each regulation type enacted as early as pre-1900. Most early regulations were established to protect spawning bass, but providing equitable distribution of harvest and increasing the quality of bass catch or harvest were the most common rationales for current regulations. Spatial and temporal trends in regulations were similar among species, were affected by geographic location, were not affected by angler preference except for season closures, and were frequently uninfluenced by advances in scientific knowledge of black bass biology.

Comparison of Native and Introduced Flathead Catfish Populations in Alabama and Georgia: Growth, Mortality, and Management

AbstractWe compared growth of flathead catfish Pylodictis olivaris from two native populations in Alabama (Coosa and Tallapoosa rivers) and two introduced populations in Georgia (Ocmulgee and Satilla rivers). We also compared mortality rates and potential outcomes of various management regimes (minimum length limits [MLLs]) among the populations. Total length–log10(age) regression slopes for introduced fish were higher than those for native fish, and von Bertalanffy growth coefficients (K) were greater for introduced fish (Ocmulgee: 0.195; Satilla: 0.201) than for native individuals (Coosa: 0.057; Tallapoosa: 0.059). Therefore, introduced flathead catfish grew more rapidly than those in their native range. Mortality (instantaneous mortality rate, Z) was higher in the Satilla River population (Z = −0.602) than in the Ocmulgee River (Z = −0.227) and Coosa River (Z = −0.156) populations. However, fish in the Satilla River population had been introduced for only 10 years and presumably did not reach their theoretical maximum age, potentially biasing the mortality estimate for that population. Simulation of management regimes in Fishery Analyses and Simulation Tools software predicted that maximum biomass of flathead catfish in the Ocmulgee (1,668 kg) and Satilla (1,137 kg) rivers was substantially larger than that in the Coosa (873 kg) and Tallapoosa (768 kg) populations. However, increased exploitation rates in the Ocmulgee and Satilla River populations resulted in dramatic declines in overall biomass, especially at lower MLLs (254 and 356 mm, respectively). Therefore, in systems where introduced flathead catfish represent an important recreational fishery but have dramatically reduced the abundance of native fishes through predation, minimal protection is recommended. We contend that rapid growth of introduced flathead catfish has major implications for their management and the conservation of native fishes.

History of muskellunge management in Nebraska

Muskellunge, Esox masquinongy, were first introduced to Nebraska waters in 1958. By 1967 anglers had caught the first muskellunge from Nebraska waters. Hybrid muskellunge, Esox masquinongy × E. lucius, culture and stocking started in 1976. Harvest regulations for muskellunge and hybrid muskellunge were first established in 1977 with a daily bag limit of one muskellunge or hybrid muskellunge of at least 762 mm (30 inches) in length. In 2000 the minimum length limit for muskellunge and hybrid muskellunge was increased to 1016 mm (40 inches). Hybrid muskellunge have been phased out, none have been stocked in Nebraska waters since 2003. Since 1998 muskellunge fingerlings have been held in Nebraska hatcheries until age-1 before stocking. Yearling muskellunge are raised on minnows and other small fish in Nebraska hatcheries and stocking success apparently is very good. Management and research of Nebraska’s muskellunge and hybrid muskellunge fisheries has been limited to routine sampling that has documented the presence of those fish in waters where they were stocked. Fishable populations of muskellunge can now be found in nine Nebraska waters across the state, but most Nebraska anglers are relatively uneducated in fishing techniques for muskellunge, identification, and proper handling.

Differences in Population Metrics between Bluegill and Redear Sunfish: Implications for the Effectiveness of Harvest Restrictions

AbstractBluegills Lepomis macrochirus and redear sunfish L. microlophus were collected and aged from three reservoirs in Georgia and Alabama in 2003–2004 to describe population metrics and investigate the potential of harvest restrictions to maintain or improve the size structure of these populations. Redear sunfish grew faster and lived longer than bluegills in all three reservoirs, and estimated natural mortality was greater for bluegills than for redear sunfish; however, total annual mortality was similar between the species in two of the three reservoirs. Population dynamics models did not predict any significant benefits to bluegill size structure and yield from minimum length or bag restrictions. In contrast, redear sunfish populations in each study reservoir showed significant increases to size structure and minimal decline in yield from various harvest restriction scenarios. Because of higher mortality and slower growth in bluegills, harvest restrictions probably will not positively affect their size structure without an unacceptable decline in yield. However, our results indicated that harvest restrictions may have greater potential in managing redear sunfish populations. For redear sunfish, yield was not drastically affected by minimum length limits, but after implementation of harvest restrictions the number of larger, preferred‐length (≥230‐mm) fish could increase by over an order of magnitude in moderately to highly exploitative fisheries. Finally, results of this study have demonstrated that the common practice of managing sunfish as a homogenous group may not be appropriate, particularly if the fishery is composed of a diverse array of sunfish species that are targeted by anglers.

Evaluation of a Modified Bag Limit for Walleyes in Wisconsin: Effects of Decreased Angler Effort and Lake Selection

AbstractWe evaluated a modified daily bag limit for walleyes Sander vitreus to determine whether management goals were achieved. The modified bag limit consisted of no minimum length limit but allowed a daily harvest of only one walleye longer than 356 mm. The Wisconsin Department of Natural Resources implemented the single‐fish, 356‐mm regulation in 1997 on a number of lakes in northern Wisconsin that were previously subject to a 381‐mm minimum length limit. The intent of the regulation was to increase the harvest of smaller walleyes in order to increase growth, spawning stock abundance, and harvest rate in lakes characterized by high densities of slow‐growing walleyes. We examined the effects of the regulation in light of variables associated with walleye populations, the walleye fishery, and tribal harvest. All lakes included in our evaluation are located within the ceded territory of northern Wisconsin, where tribal harvest of walleyes, which is regulated differently than angler harvest, occurs annually. The daily bag limit for walleyes ranged between two and five, depending on the level of tribal harvest. Walleye fishery characteristics measured before 1997 (381‐mm limit) and after 1999 (single‐fish, 356‐mm limit) demonstrated an increased angler harvest rate and a decreased mean length of harvested walleyes. However, we found none of the desired changes in walleye population characteristics. In addition, we found no evidence that lakes that were changed to the single‐fish, 356‐mm regulation were characterized by slower growth than those that retained the 381‐mm regulation. The single‐fish, 356‐mm regulation requires increased harvest to achieve the desired changes to walleye population characteristics. Our results suggest that a significant decline in angler effort over time, in conjunction with the possibility that the lakes selected to receive the single‐fish, 356‐mm regulation did not exhibit particularly slow walleye growth, acted to offset increases in harvest rate and played a role in the general failure of this regulation to meet its goals.

The New River, Virginia, muskellunge fishery: population dynamics, harvest regulation modeling, and angler attitudes

Although muskellunge, Esox masquinongy, fisheries in northern US states and Canadian provinces are increasingly being managed by introduction of restrictive harvest regulations (e.g. 1370-mm (54′′) minimum length limits), many southern US muskellunge fisheries continue to be managed with comparatively liberal regulations (e.g. 762-mm (30′′) minimum length limits) that are implemented statewide. We studied the population dynamics of the New River, Virginia, muskellunge fishery and used predictive modeling to determine whether restrictive harvest regulations also might prove beneficial for this southern latitude fishery. A creel survey was also conducted to learn more about angler attitudes to the New River muskellunge fishery. Muskellunge grew quickly, with fish reaching harvestable lengths (762 mm, 30′′) in 2–3 years. Muskellunge fishing pressure, harvest rates, and voluntary release rates were low compared with reports for more northern areas. Most anglers, irrespective of how often they fished for muskellunge, defined “trophy” muskellunge to be approximately 1050–1100 mm (41–43′′) in length. Although angler support for restrictive harvest regulations was low, abundance of memorable-length (≥1070 mm, 42′′) muskellunge was predicted to increase under all evaluated length limits. Muskellunge yield would remain static at 914-mm (36′′) and 1016-mm (40′′) length limits, because of the rapid growth of fish, but yield would decline dramatically with a 1143-mm (45′′) length limit, because male muskellunge rarely exceeded 1100 mm (43′′). Because of rapid growth and low release rates, implementation of higher length limits (e.g. 965–1067 mm, 38–42′′) may indeed prove beneficial for augmenting “trophy” muskellunge production on the New River. Angler support for higher minimum length limits might be increased by educating anglers about the rapid growth rates of muskellunge and the expected size structure changes that will result from a length-limit increase. Size structure changes resulting from an increase in the minimum length limit may be difficult to detect because of potential increases in fishing pressure or reduced fish growth as a result of competition for food resources. Long-term monitoring of muskellunge growth and angling pressure may therefore be needed to ensure that new regulations are indeed benefitting the fishery.

Fisheries exploitation pattern of narrow-barred Spanish mackerel, Scomberomorus commerson, in Oman and potential management options

Summary A data base including length frequency distributions and catches of the Scomberomorus commerson in Oman according to fleet (gear/technique) and region has been established to carry out length cohort analyses, determine yield per recruit and simulate changes in fishing effort and/or increase in minimum length limit in catches. The analyse of data showed that: (i) The average fishing mortality rate is moderate (0.5–0.6), but acts in part on the juvenile fraction of the stock. The exploitation pattern differs, however, among fleets with some fleet components targeting largely immature kingfish and others largely the adult stock. (ii) An increase in total fishing effort would lead to long-term losses in total catch; the losses would be highest for fleets that target the larger specimens. A reduction of the effort would, in the long term, lead to an increase in yield and spawning stock biomass. (iii) An increase of minimum length limit in catches would, in the long term, lead to a substantial increase in yield and spawning stock biomass; the gain in catch would be largest for fleet components that target the adult fraction of the stock and (iv) an increase in minimum length limit in catches combined with an increased selectivity of the fisheries (i.e. favorising fleets targeting the adult fraction of the stock) would lead to the highest gain in sustainable catch. In this case, the sustainable catch could be increased by more than 50%. Therefore, this option represents the optimal management strategy obtained in the present study.

Fishing and Natural Mortality of Bluegills and Redear Sunfish at Lake Panasoffkee, Florida: Implications for Size Limits

AbstractGrowth, annual exploitation (u), and natural mortality (v) of bluegills Lepomis macrochirus and redear sunfish L. microlophus were investigated on Lake Panasoffkee, Florida, during 1998 and 1999 to assess the potential benefits of several minimum size limits. The average u of 15.0‐cm total length (TL) or larger fish was 0.14 for bluegills and redear sunfish during the 2‐year period. For both species, exploitation increased up to threefold as fish size increased (e.g., u = 0.37 for bluegills ≥23.0 cm TL in 1999). Total annual mortality (A) was 0.64 for bluegills and 0.68 for redear sunfish. Natural mortality estimates were 0.50 (78% of A) for bluegills and 0.55 (81% of A) for redear sunfish. Simulations predicted that minimum size limits would decrease average total yield and harvest for both bluegills and redear sunfish, but that a 20.3‐cm minimum length limit could slightly increase the number of fish harvested over 20.0 cm TL. A 22.9‐cm minimum length limit was predicted to greatly reduce yield without creating a measurable increase in the harvest of fish over the size limit. Lake Panasoffkee sunfishes incurred record levels of estimated effort and harvest in 1998–1999, but size‐selective exploitation acted essentially as a minimum size limit, resulting in no predicted benefit for a 20.3‐cm limit. Separation of fishing mortality and natural mortality allowed us to conclude that natural mortality had a greater influence on Lake Panasoffkee sunfish population abundance and fishing quality than did fishing mortality, so a minimum size limit was not recommended. Estimates of exploitation, natural mortality, and growth rates are more valuable than measures of size structure and angler catch when considering or evaluating harvest restrictions.

Channel Catfish Maturation in Mississippi Streams

AbstractCommercial fisheries for wild channel catfish Ictalurus punctatus in Mississippi are subject to a single statewide minimum length limit of 300 mm. This assumes either that all populations of channel catfish in Mississippi are biologically similar or that known differences are not significant enough to override social issues, such as the enforcement of more specific regulations. We quantified sexual maturity of channel catfish from nine rivers in Mississippi to establish a biologically appropriate spatial resolution for application of length‐based harvest regulations. Age and length at 50% maturity differed among channel catfish populations from different rivers. Age and length at 50% maturity for channel catfish from all rivers combined differed among years but did not differ between sexes. Age at 50% maturity was negatively related to watershed soil fertility and positively related to mean watershed elevation, whereas length at 50% maturity was not related to any tested environmental factor (P &lt; 0.05). Our findings suggest that sexual maturity is reached sooner in systems that have greater fertility or a longer growing season. This effect is modified by annual climatic variability (e.g., rainfall) but indicates that length‐based regulations should be applied at a river‐ or watershed‐specific scale.

Relationships between Recapture Rates from Different Gears for Estimating Walleye Abundance in Northern Wisconsin Lakes

AbstractMaximizing sampling efficiency and reducing sampling costs are desirable goals for fisheries management agencies. Expensive and labor‐intensive methods (such as mark–recapture) are commonly used to estimate the population abundance of walleye Sander vitreus, but more efficient methods may be available. We compared recapture rates from surveys and harvests to evaluate the efficiency of currently used recapture gears and the potential for using gears that require less effort. To evaluate the usefulness of walleye harvest as mark–recapture samples, we used errors‐in‐variables models to determine whether recapture rates differed between fyke‐netting and spearing, electrofishing and spearing, and electrofishing and angling. We found no significant differences between fyke‐netting and adult walleye electrofishing recapture rates or between spearing and adult walleye electrofishing recapture rates. In contrast, we found that recapture rates from angling and electrofishing differed significantly in lakes with and without minimum length limits. We concluded that the lack of significant differences between the slopes of some harvest and survey recapture rates may allow the use of harvest recapture rates to estimate walleye abundance, but the biases associated with each gear should be considered. We also concluded that more attention should be given to understanding the biases of recapture gears.