Rethinking length‐based fisheries regulations: the value of protecting old and large fish with harvest slots

Managing fisheries using length‐based harvest regulations is common, but such policies often create trade‐offs among conservation (e.g. maintaining natural age‐structure or spawning stock biomass) and fishery objectives (e.g. maximizing yield or harvest numbers). By focusing harvest on the larger (older) fish, minimum‐length limits are thought to maximize biomass yield, but at the potential cost of severe age and size truncation at high fishing mortality. Harvest‐slot‐length limits (harvest slots) restrict harvest to intermediate lengths (ages), which may contribute to maintaining high harvest numbers and a more natural age‐structure. However, an evaluation of minimum‐length limits vs. harvest slots for jointly meeting fisheries and conservation objectives across a range of fish life‐history strategies is currently lacking. We present a general age‐ and size‐structured population model calibrated to several recreationally important fish species. Harvest slots and minimum‐length limits were both effective at compromising between yield, numbers harvested and catch of trophy fish while conserving reproductive biomass. However, harvest slots consistently produced greater numbers of fish harvested and greater catches of trophy fish while conserving reproductive biomass and a more natural population age‐structure. Additionally, harvest slots resulted in less waste in the presence of hooking mortality. Our results held across a range of exploitation rates, life‐history strategies and fisheries objectives. Overall, we found harvest slots to represent a valuable option to meet both conservation and recreational fisheries objectives. Given the ubiquitous benefits of harvest slots across all life histories modelled, rethinking the widespread use of minimum‐length limits is warranted.     

Saving large fish through harvest slots outperforms the classical minimum‐length limit when the aim is to achieve multiple harvest and catch‐related fisheries objectives

We address the problem of optimal size‐selective exploitation in an age‐structured fish population model by systematically examining how density and size dependency in growth, mortality and fecundity affect optimal harvesting patterns when judged against a set of fisheries objectives. The study offers five key insights. First, while minimum‐length limits often maximize the biomass yield, exploitation using harvest slots (i.e. regulations that protect both immature and very large individuals) can generate within 95% of maximum yield; harvest slots also generally maximize the number of fish that are harvested. Second, density dependence in growth and size‐dependent mortality predict more liberal optimal size limits than those derived under assumptions of no density and size dependence. Third, strong density dependence in growth maximizes the catch of trophy fish only when modest harvest is introduced; the same holds for numbers harvested, when the stock–recruitment function follows the Ricker type. Fourth, the inclusion of size‐dependent maternal effects on fecundity or egg viability has only limited effects on optimal size limits, unless the increase in fecundity with mass (“hyperallometry”) is very large. However, large hyperallometry in fecundity shifts the optimal size limit for biomass yield from the traditional minimum‐length limit to a harvest slot. Fifth, harvest slots generally provide the best compromises among multiple objectives. We conclude that harvest slots, or more generally dome‐shaped selectivity to harvest, can outperform the standard minimum‐length selectivity. The exact configuration of optimal size limits crucially depends on objectives, local fishing pressure, the stock–recruitment function, and the density and size dependency of growth, mortality and fecundity.     

Balancing conservation and recreational fishery objectives for a threatened fish species,the Murray cod,Maccullochella peelii

Murray cod Maccullochella peelii (Mitchell) is a large, iconic Australian fish species targeted by anglers but also listed as nationally threatened. A consultative process that included conservation and fishery interests helped to develop a population model for this species and agree on management scenarios to be tested. The modelled scenarios illustrated that threats to populations (risk of decline) can be substantially reduced and catch rates increased through harvest slot length limits (HSLL) rather than minimum legal limits (MLL). A 600‐ to 1000‐mm HSLL provided lower risk of decline and greater catch rates than the existing 500‐mm MLL, but better results were achieved with a 400‐ to 600‐mm HSLL. Importantly, a range of other impacts (fish kills, stocking, thermal impacts, larval mortalities, habitat changes) were recognised and incorporated. This study provides an example of the utility of a population model to improve management decision‐making for both conservation and fishery objectives.     

Exploitation and length limit evaluation of Alabama bass in a South‐eastern USA reservoir

Alabama bass, Micropterus henshalli Hubbs & Bailey, are the dominant sportfish of Allatoona Reservoir, Georgia, USA, but no population assessment has been conducted. Thus, growth and total annual mortality were estimated in spring 2005, and a tagging study was conducted in 2006 and 2007 to estimate angler exploitation. These data were used with an age‐structured model to assess performance of a 356‐mm minimum length limit (MLL), a 406‐mm MLL and a 330‐ to 406‐mm protected slot length limit (SLL) compared to the present harvest regulation of no MLL. Mean annual exploitation varied from 12 to 22% each year and was generally highest for fish > 330 mm; total annual mortality was 44%. Models predicted a 49–153% increase in numbers of Alabama bass reaching 432 mm, a 22–66% decline in numbers harvested and only moderate declines in yield (5–25%) with the alternative harvest regulations compared to current conditions. The SLL may be an acceptable compromise to allow Allatoona Reservoir anglers to still harvest fish while also improving Alabama bass size structure.     

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.     

Spawning distribution and abundance of a northern Chinook salmon population

Chinook salmon, Oncorhynchus tshawytscha (Walbaum), is an important biological and cultural resource in Alaska, but knowledge about Chinook salmon ecology is limited in many regions. From 2009 to 2012, spawning distribution and abundance of a northern Chinook salmon population on the Togiak River in south‐west Alaska were assessed. Chinook salmon preferred deeper mainstem channel spawning habitat, with 12% (14 of 118 tags in 2009) to 21% (22 of 106 tags in 2012) of radio‐tagged fish spawning in smaller order tributaries. Tributary spawners tended to have earlier run timing than mainstem spawners. Chinook salmon exhibited extended holding and backout (entering freshwater but returning to saltwater before completing anadromous migration) behaviours near the mouth of Togiak River, potentially prolonging their exposure to fishery harvest. Mark–recapture total annual run estimates (2010–2012) ranged from 11 240 (2011) to 18 299 (2012) fish. Exploitation of Chinook salmon ranged from 36% (2012) to 55% (2011) during the study period, with incidental fishery catches near the mouth of the river comprising the largest source of harvest.     

Population density,growth and reproduction of arapaima in an Amazonian river‐floodplain

Arantes CC, Castello L, Stewart DJ, Cetra M, Queiroz HL. Population density, growth and reproduction of arapaima in an Amazonian river‐floodplain. Ecology of Freshwater Fish 2010: 19: 455–465. © 2010 John Wiley & Sons A/S Abstract – Compensatory density effects are key features of fish population dynamics that remain poorly understood in tropical river‐floodplains. We investigated possible compensatory growth and reproductive processes for a river‐floodplain population of Arapaima sp., an extinction‐prone fish species of South America. Body growth was studied through analysis of ring patterns on the scales, and size and age at sexual maturity was studied through analysis of female gonads. Growth and maturity were compared for unmanaged conditions with relatively low population density (in 1990s) versus managed conditions with markedly higher density (in 2005–2006); between 1999 and 2005–2006, abundance increased 7.3 fold. Results contradict theoretical expectations for slower growth and delayed reproduction at higher population density. Total lengths of arapaima at low population density were significantly shorter for age classes 1–5 compared with lengths of those age classes at high population density (ancova , P < 0.0001 for both slopes and intercepts). Total length at 50% maturity (L₅₀) only declined about 4% with increasing density (e.g., 164 cm at low density vs. 157 cm at high density). Apparent faster growth at high density and only a slight change in size at maturity resulted in fishes spawning at an earlier age with high density conditions (age 3 vs. age 4–5). We hypothesise that these patterns reflect compliance with minimum size limits of catch during the high density (managed) situation, where there was no harvest of immature fishes. Compliance with minimum size limits, thus, may have led to faster average body growth rate and earlier reproduction, which has greatly promoted population recovery.     

Growth of Stocker Channel Catfish to Large Market Size in Single-Batch Culture

Catfish farmers increasingly are producing fish larger than the traditional size of 0.45-0.57 kg/fish in order to meet processing plant requirements for larger fish. Production of larger channel catfish Ictalurus punctatus in multiple-batch culture has been investigated in a few studies, but the impact of understocked fingerlings on growth of carry-over fish is unknown. The present study was conducted to quantify growth, feed conversion ratio, net daily yield, and net and total yield of stocker channel catfish grown in single-batch, one-season culture to mean individual weights of 0.60, 0.72, 0.91, or 1.17 kg/fish. Channel catfish (mean weight = 0.26 kg/fish) were stocked into 12 0.1-ha ponds at 11,115 fish/ha. Fish were fed a 32% crude protein floating extruded feed once daily to apparent satiation. When the average weight of the fish population reached the target weight, three randomly selected ponds were harvested. Fish growth was linear in all treatments. Growth rates were similar for fish grown to 0.60, 0.72, and 0.91 kg/fish, and significantly lower ( P < 0.05) than for fish grown to 1.17 kg. Variation in individual fish weight increased linearly with increased duration of culture period. Feed conversion ratio averaged 1.9 and did not differ significantly among treatments. The percentage of the fish population at harvest that fell within the 0.57 to 2.04 kg-size range preferred by processing plants increased from 56.6 to 98-5% as the mean weight at harvest increased from 0.60 to 1.17 kg/fish.     

Efficacy of harvest and minimum size limit regulations for controlling short‐term harvest in recreational fisheries

It is important to consider the potential effectiveness of regulations for reducing total harvest levels when developing fishery management plans. A random forest (RF) modelling approach was used to examine how changing per‐angler harvest or minimum size limit regulations affected sport fishery harvest in US Atlantic coast recreational fisheries. Harvest limits per angler (i.e. bag limits) were typically high initially and subsequently reduced, whereas almost half of minimum length limits were initially below the length‐at‐maturity and subsequently increased. Across most fisheries examined, extreme reductions in harvest limits (e.g. from unlimited to catch‐and‐release) were largely ineffective at limiting total fishery harvest. Increasingly restrictive minimum length limits caused a greater average harvest reduction than per‐angler harvest limits. Some regulation changes were associated with higher angling effort and thus increased harvest, which suggests that when effort cannot be constrained, more direct harvest limitations should be considered.     

Exploitation of walleye in a Great Plains reservoir: harvest patterns and management scenarios

Abstract This study assessed exploitation and evaluated management options for walleye, Sander vitreus (Mitchill), in Glen Elder Reservoir, Kansas. A total of 2429 walleye varying from 240 to 687 mm was tagged with Carlin dangler tags during 2000–2003. After correcting for tag loss and non‐reporting, exploitation of walleye was estimated as 68.3%. More than 85% of the tagged walleye were harvested during April to June and 75% were harvested within 6 months after tagging. A Beverton‐Holt yield‐per‐recruit model was used to evaluate six minimum length limits varying from 250 to 610 mm. Given current exploitation rates, population demographics and harvest regulations (381‐mm minimum length limit), the walleye population is probably experiencing recruitment overfishing. Model results suggested that a 610‐mm minimum length limit would be required to prevent growth overfishing and a 533‐mm or longer minimum length limit would prevent recruitment overfishing.     

Changes in yellow perch length frequencies and sex ratios following closure of the commercial fishery and reduction in sport bag limits in southern Lake Michigan

Abstract  The closure of the Indiana commercial fishery in Lake Michigan in 1997 provided the opportunity to examine the effects of commercial exploitation on the yellow perch, Perca flavescens (Mitchill), population. Data from annual gillnet and trawl sampling, recreational angler creel surveys and commercial fishing harvest were used to determine whether changes in length–frequency distributions and sex ratios were altered following closure. The proportions of large, adult yellow perch were significantly greater after the closure of the commercial fishery. Overexploitation of fish >200 mm by commercial harvest likely truncated the size distribution, but recreational harvest was comparatively small and did not appear to be a factor influencing size distributions. This post-closure period also had a greater proportion of females in trawl catches, and the mean length of harvested fish increased in the recreational fishery. These findings suggest commercial exploitation influenced yellow perch population dynamics in far southern Lake Michigan.     

Increasing Yields of Channel Catfish Using a Combination of Cage and Open Pond Production Systems1

Channel catfish (lctalurus punctatus) fingerlings stocked at a rate of 450 fish/0.04 ha pond were simultaneously cultured with fingerlings stocked in 1.25 m³ cages (0, 250, 350, or 450 fishlcage; one cage/pond). The fish in the cages were cultured and harvested for a 90–330 g (whole fish) market. The fish in the open ponds were cultured and harvested for a 490–1,140 g market. Harvest weights of open pond fish in all treatments were similar indicating that the presence of the caged fish and the associated higher daily pond feeding rates did not affect open pond production. Ninety-five to 99% of the caged fish and 96 to 98% of the open pond fish were of marketable size at harvest. Survival and food conversion ratios were similar among treatments. Results of this study indicate that total pond production can be increased (in this case up to 19%) by using a combination of open pond and cage techniques and by simultaneously producing fish for two markets.     

Analysis of the trophy sport fishery for the speckled peacock bass in the Rio Negro River, Brazil

Abstract  The middle portion of the Rio Negro River in Brazil near the equator supports a popular recreational sport fishery for speckled peacock bass, Cichla temensis (Humboldt). The objective of this study was to determine the effect of fishing mortality on this population. Fish were collected from sport-fishing ( n  = 72) and commercial ( n  = 103) catches and otoliths were aged to estimate longevity, growth and natural mortality. Recreational anglers in this region seek to catch, then release, larger speckled peacock bass; and fish larger than 62 cm standard length (SL) (about 4.5 kg) served as a bench mark to assess the potential impact of subsistence and commercial harvest on the abundance of larger fish in the sport fishery. Time of opaque band formation on otoliths generally coincided with the dry season (November to April); these bands appeared to form once per year, but formation was highly variable. Speckled peacock bass grew to 62 cm SL on average in 6.4 years, but some fish obtained this size in 4–5 years. Maximum age was 9 years, but most fish were less than 7 years. Instantaneous annual natural mortality ( M ) estimated from maximum size, longevity and growth ranged from 0.19 to 0.44. Simulation modelling predicted that exploitation rates of fish >25 cm SL similar to the estimated natural mortality rates would reduce the abundance of fish >62 cm by 67–89% compared with no harvest. Even modest exploitation rates of 5% and 10% would result in approximately 30–50% reduction, respectively, of these larger fish. Abundance of large speckled peacock bass that sustains the sport fishery is susceptible to low rates of exploitation in this remote region of Brazil.     

Dynamic angling effort influences the value of minimum‐length limits to prevent recruitment overfishing

Recruitment overfishing occurs when stocks are fished to a level where recruitment declines proportionally with adult abundance. Although typically considered a commercial fishery problem, recruitment overfishing can also occur in freshwater recreational fisheries. This study developed an age‐structured model to determine if minimum‐length limits can prevent recruitment overfishing in black crappie, Pomoxis nigromaculatus (LeSueur), and walleye, Sander vitreus (Mitchill) fisheries considering angling effort response to changes in fish abundance. Simulations showed that minimum‐length limits prevented recruitment overfishing of black crappie and walleye, but larger minimum‐length limits were required if angler effort showed only weak responses to changes in fish abundance. Low angler‐effort responsiveness caused fishing mortality rates to remain high when stock abundance declined. By contrast, at high effort responsiveness, anglers left the fishery in response to stock declines and allowed stocks to recover. Angler effort for black crappie and walleye fisheries suggested that angler effort could be highly responsive for some fisheries and relatively stable for others, thereby increasing the risk of recruitment overfishing in real fisheries. Recruitment overfishing should be considered seriously in freshwater recreational fisheries, and more studies are needed to evaluate the responsiveness of angler effort to changes in fish abundance.     

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.     

Implications of using small meshed gillnets for the sustainability of fish populations: a theoretical exploration based on three case studies

The study explores the impacts of varying gillnet mesh size and fishing level on yield per recruit (Y/R), escapement spawning stock (ESS) and mega‐spawners (MS) of three widely distributed freshwater fishery target species. Y/R is maximised when the optimal length of capture (L_cap) is above the size of maturity (L₅₀). However, the unimodal shape of gillnet selectivity results in lower impacts to ESS and MS with both smaller and larger mesh sizes. Under conditions of moderate exploitation, the fraction of MS was significantly larger if small meshed gillnets were used. This is due to the relatively smaller cumulative vulnerability from small mesh sizes through time, as they target a smaller size range of fish, which also grow more quickly through the vulnerable window due to higher growth rates. Therefore, unlike trawls and beach seines, which select all size classes beyond the minimum length of capture (L_c), small meshed gillnets are not necessarily destructive and may rather promote sustained production by allowing a higher proportion of the spawning biomass to remain in the stock. The work also helps to explain the observation of sustained fish production in many developing countries despite the persistent use of gillnets of small mesh size that target small, under‐sized individuals.     

Feasibility of Pond Production of Largemouth Bass,Micropterus salmoides,for a Filet Market

The effects of stocking density on food‐size largemouth bass (LMB), Micropterus salmoides, production (>0.5 kg) were evaluated in a 2‐yr study by stocking LMB fingerlings (mean weight = 57 g/fish) in 0.1‐ha earthen ponds at rates of 6175, 12,350, or 18,525 fish/ha. Gross yields increased from 3989 to 9096 kg/ha as stocking density increased. No significant differences were observed in survival rates (range of 65–74%) due to density. Maximum feed consumption occurred at water temperatures of 27–30 C. Feed conversion ratio (FCR) and mean harvest weight were significantly different (P < 0.05) among densities, with the lowest FCR and the lowest mean weight found at the highest density (18,525 fish/ha). At harvest, LMB were considered to be in good condition with relative weight (W_r) values of 123–124. Dressout yield percentages were 61–62% for whole‐dressed LMB and 34–35% for shank filets. LMB grew well and reached a size adequate for targeted shank filet sizes. However, the production costs of $7.26–$9.34/kg mean that LMB production for a filet market is unlikely to be feasible. Research to lower LMB fingerling and feed costs and improved FCR would contribute to improved economic feasibility.     

Status and management of fish populations of Rawal Dam reservoir, Pakistan

Abstract Studies were conducted to determine the composition and relative abundance of commercially desirable and less desirable fish populations of Rawal Dam reservoir. Fifteen fish species belonging to 11 genera were identified. The less desirable species were abundant, constituting 62.3%, 84.8% and 78% of the total number of fish sampled on three occasions. Carnivores were represented by two species with very low percentage composition by number or weight. The ratio of total weight of forage fish to that of carnivorous fish indicated an unbalanced population in the lake. The proportion of the fish population at harvestable size suggests a balanced population, but this index may have been overestimated because of a minimum harvestable size.     

Physiological Stress in Channel Catfish,Ictalurus punctatus,Harvested by Lift Net,Vacuum Pump,or Turbine Pump

Stress in freshwater fish results in an increase in serum cortisol and glucose and a decrease in serum electrolytes. These physiological indicators of stress were measured in fignerling and food-size channel catfish, "Ictalurus punctatus," before harvest, immediately after harvest, and for several days after the fish were harvested. Fingerlings were harvested in spring with either a turbine fish pump or a traditional boom-and-basket lift net. Food-size fish were harvested in winter, spring, and summer by a turbine fish pump, a vacuum fish pump, or a life net. Elctrolyte concentrations were very stable and did not show consistent effects of harvesting. Cortisol and glucose were the most sensitive to harvesting, and both increased during harvesting in all months tested. Recovery of the serum concentrations to pre-harvest levels was generally completed by the fifth day after harvest. Stress responses were similar among the different harvesting techniques, which suggests that the method of choice may be based on other factors, such as cost of available facilities.     

The effects of mechanically reducing northern pike density on the sport fish community of West Long Lake, Nebraska, USA

Abstract  Removal of piscivores may affect population size structure, abundance and growth rates of the remaining fish community. A total of 572 northern pike, Esox lucius L., was removed from 25-ha West Long Lake, Nebraska from 2003 to 2006. The northern pike population was reduced from 36 fish ha⁻¹ and 22 kg ha⁻¹ in 2002 to 20 fish ha⁻¹ and 9 kg ha⁻¹ in 2006. The northern pike population shifted to dominance by smaller individuals. Relative abundance increased for bluegill, Lepomis macrochirus Rafinesque, but remained similar between years for largemouth bass, Micropterus salmoides (Lacepède), and yellow perch, Perca flavescens (Mitchill). Bluegill and yellow perch size structure increased probably because of reduced predation by northern pike. The effects of reduced northern pike density on largemouth bass were less evident. The largemouth bass population may be returning to higher density with smaller size structure observed before pike were introduced. The sport fish community responded positively to the removal of northern pike in this small lake.