Minimum Size Limit Research Articles

Demography, fishery yield and potential management strategies of painted spiny lobster (Panulirus versicolor) at Northwest Island, Great Barrier Reef, Australia

Quantification of demographic parameters is important for understanding how populations interact with their environment and for developing sustainable harvest strategies. Our aims were to quantify demographic parameters and yield of painted spiny lobster (Panulirus versicolor) at Northwest Island, Great Barrier Reef and review fishery management regulations in view of the findings. Using catch-curve analysis and an intensive mark–recapture program, the local lobster population was found to be sparsely distributed (~1.3 ha–1), with low rates of natural mortality (0.25–0.28 year–1) and immigration (0.05 ha–1 year–1). Under current management regulations (no minimum size limit; MSL), yield-per-recruit peaked at 0.38 kg when fishing mortality was 0.4 year–1. Implementation of a 100-mm (carapace length) MSL is recommended because it provides the best compromise between yield and stock size and has minimal impact on existing fishing practices. Revocation of the prohibition on possessing egg-bearing females is also recommended because it would reduce wastage of lobsters that are inadvertently killed by spear-fishers and because there appears to be little chance of localised recruitment overfishing. This study provides the first estimate of mortality rate for P. versicolor and highlights the importance of this parameter for understanding demographic processes and optimising management regulations.

Gentle cold welding between gold nano objects driven by electrical field and atomic rearrangement

The combination of a top-down and a bottom-up fabrication approach will be a key technology to overcome the minimum size limit of the top-down approach. Gentle cold welding is a promising method to connect nano objects without appreciable damages to the nano device in a bottom-up approach. We developed the gentle cold welding method using the combination of an electrical field and an atomic rearrangement. The formulated process was visualized between gold tips using a transmission electron microscope. The initial connection mechanism was divided into three modes depending on a bias voltage between tips; (1) physical contact, (2) atom extraction, and (3) field evaporation, depending on the electrical fields. All the nano connections grew by the atomic rearrangement driven by gold surface self-diffusion. The grown connections were single crystalline structures. These results suggested that this welding will be a useful method to build up future nano devices and systems.

Conflicting objectives in trophy trout recreational fisheries: evaluating trade-offs using an individual-based model

Standard fisheries models, based on average population metrics, are inadequate for analyzing recreational fisheries where fishing is size-selective and management objectives are related to preserving population size structure. We developed a framework for policy analysis of size-based harvest strategies in recreational fisheries. The framework combines a mixed-effects body growth model and an individual-based harvest model to describe the relationship of growth, mortality, and size structure. Fishery performance is quantified with indicators directly associated to catch-related components of anglers’ satisfaction: yield (kg), population size, and availability of trophy-size fish. We applied our analyses to the steelhead ( Oncorhynchus mykiss ) fishery in the Santa Cruz River (Patagonia, Argentina). Large declines in trophy-size fish are to be expected at fishing mortalities much too low to cause a sizeable decline in recruitment from virgin values. When somatic growth is density-independent, harvest occurs at the expense of other indicators associated with the quality of fishing experienced by individual anglers. Size limits provide a tool to better accommodate harvest without compromising fishing quality. When preserving population size is favored over preserving trophy-size fish, minimum size limits constitute the best policy overall, whereas maximum size limits are best when the emphasis is on preserving trophy-size fish.

Ecologically and evolutionarily sustainable fishing of the pikeperch Sander lucioperca: Lake Oulujärvi as an example

Due to the multitude of participants and a diverse range of fishing gear used freshwater fisheries are often managed using minimum size limits (MSL) rather than regulations of total fishing effort. However, a concern has arisen whether attempts to improve ecological sustainability of fisheries by increasing MSLs would induce undesired adaptations to selective fishing. We examined the ecological and evolutionary impacts of varying fishing mortality rates under varying MSLs, with and without stockings, in an age-, size-, and maturity-structured evolutionary model which was parameterized for the Lake Oulujärvi pikeperch, Sander lucioperca. We found that at the current level of harvesting (fishing mortality rate, F = 0.7) and stockings (430 000 year −1), and under the assumption of strongly density-dependent growth, the nation-wide MSL of 370 mm maximizes theoretical biomass yield in a deterministic model but does not prevent severe recruitment overfishing under further increased fishing pressures or stochasticity in recruitment success. The recently imposed, local MSL of 450 mm better ensures stable yields, and even increases them if individual growth is density-independent, but further increase of MSL to 500 mm would already reduce yield especially if there was discard mortality for undersized fish. Given density-dependent growth, equal survival between wild and stocked fish, and sustainable fishing mortality rate, stockings do not increase yield or significantly improve the stability of yields. Evolutionarily stable size at maturation decreases under strong fishing mortality, but increased MSLs reduce the magnitude of this undesired effect. Negatively size-dependent natural mortality was found to have a positive effect on the otherwise negative selection for length-at-age. Increased MSLs also reduce the total selection for decreased length-at-age. Our results support the intentions to increase MSLs in order to improve both ecological and evolutionary sustainability of recreational fisheries.

Utilizing Spatial Demographic and Life History Variation to Optimize Sustainable Yield of a Temperate Sex-Changing Fish

Fish populations vary geographically in demography and life history due to environmental and ecological processes and in response to exploitation. However, population dynamic models and stock assessments, used to manage fisheries, rarely explicitly incorporate spatial variation to inform management decisions. Here, we describe extensive geographic variation in several demographic and life history characteristics (e.g., size structure, growth, survivorship, maturation, and sex change) of California sheephead (Semicossyphus pulcher), a temperate rocky reef fish targeted by recreational and commercial fisheries. Fish were sampled from nine locations throughout southern California in 2007–2008. We developed a dynamic size and age-structured model, parameterized separately for each location, to assess the potential cost or benefit in terms of fisheries yield and conservation objectives of changing minimum size limits and/or fishing mortality rates (compared to the status quo). Results indicate that managing populations individually, with location-specific regulations, could increase yield by over 26% while maintaining conservative levels of spawning biomass. While this local management approach would be challenging to implement in practice, we found statistically similar increases in yield could be achieved by dividing southern California into two separate management regions, reflecting geographic similarities in demography. To maximize yield, size limits should be increased by 90 mm in the northern region and held at current levels in the south. We also found that managing the fishery as one single stock (the status quo), but with a size limit 50 mm greater than the current regulations, could increase overall fishery yield by 15%. Increases in size limits are predicted to enhance fishery yield and may also have important ecological consequences for the predatory role of sheephead in kelp forests. This framework for incorporating demographic variation into fisheries models can be exported generally to other species and may aid in identifying the appropriate spatial scales for fisheries management.

Achieving Fairness Between Uplink and Downlink Flows in Error-Prone WLANs

This letter considers an unfairness problem between uplink and downlink in 802.11 WLANs. One of existing solutions for this problem is giving a larger transmission opportunity (TXOP) limit to an AP than stations. This TXOP differentiation scheme, however, does not work well in error-prone environments since a packet bursting during a TXOP duration is terminated when a data transmission fails due to channel errors. To overcome this, we propose a new scheme in which an AP controls its minimum contention window size and TXOP limit dynamically according to the packet error rate and the number of stations. Our simulation results show that the proposed scheme provides fair channel accesses between uplink and downlink in error-prone 802.11 WLANs.

The elasticity of fishing effort response and harvest outcomes to altered regulatory policies in eel ( Anguilla anguilla) recreational angling

Understanding how fishing effort responds to management interventions is important for conserving threatened fisheries resources such as the European eel ( Anguilla anguilla). In this paper, we use a discrete choice survey to predict the allocation of recreational angling days directed at eel versus potential substitute fishing opportunities in northern Germany as a function of eel angling regulations, catch attributes and hypothetical eel fishing costs. We found the allocation model to accurately predict current eel effort allocation patterns. Using the validated statistical model as a forecasting tool, we found eel angling effort to be largely resilient to changes in individual eel angling regulations, including daily bag limits, daily rod limits and fishery closures for up to two weeks each month. An inelastic effort response to the most commonly discussed policy interventions suggests that managers cannot expect to substantially reduce eel fishing effort, and thus mortality exerted by anglers on eel, using moderate management interventions. However, when severe regulations, including a two week closure per month, with remaining days limited to a harvest of 1 eel, 60 cm or larger, per angler using a single rod, would be implemented, angling effort devoted to eel can be expected to be reduced by about 42% relative to current conditions at unaltered expected catches. This would reduce landings of eel by anglers by 73%. This reduction in landings has unknown effects on the future recruitment of eel while at the same time substantially reducing angler welfare. Angler welfare can be largely maintained by increases in minimum-size limits and reductions in daily bag limits, while at the same time reducing eel landings by anglers substantially. Such actions are therefore preferred from an angler welfare perspective.

Variation of dimensional properties of particulate materials during grinding and their non-fractal nature

Dimensional properties of a particulate material are considered to be the mass, the surface area, the length and the number of its particles. The paper examines the variation of the distribution of these properties versus size as a function of the energy consumption during grinding. The data obtained show that the distributions of these properties have one maximum and one minimum size limit. Additionally, the presentation of the cumulative distribution of each of these properties, versus size between these two limits, can be approximated by an exponential curve, which if plotted in a log–log diagram gives an almost linear relationship. However, only ∼50% of the data can be described by this linear relationship. The question that rises is whether these properties can be considered to be fractal.On considère que la masse, la superficie, la longueur et le nombre de particules constituent les propriétés dimensionnelles d’un matériau particulaire. L’article examine la variation dans la distribution de la valeur de ces propriétés en fonction de la consommation d’énergie lors du broyage. Les données obtenues montrent que la distribution de ces propriétés a une valeur limite maximale et une valeur minimale. De plus, la présentation de la distribution cumulative de la valeur de chacune de ces propriétés entre ces deux limites peut être approximée par une courbe exponentielle qui, lorsque tracée sur un diagramme logarithmique, donne une relation presque linéaire. Cependant, on peut seulement décrire environ 50% des données par cette relation linéaire. La question est de savoir si l’on peut considérer ces propriétés comme étant fractales.

Modeling population dynamics and conservation of arapaima in the Amazon

To promote understanding of fish population dynamics in tropical river-floodplains, we have synthesized existing information by developing a largely empirical population model for arapaima (Arapaima sp.). Arapaima are characterized by very large bodies, relatively late sexual maturity, small clutches, and large parental investment per offspring, and their populations are overexploited and even declining due to overfishing. We used unparalleled time series data on growth, reproduction, catch-at-age, and size-class abundance estimates for a population that has increased several-fold and undergone drastic changes in fishing practices in the Amazon, Brazil. Model population numbers were close to observed numbers, with generally low mean absolute percentage errors for juveniles (16%), adults (30%), and catch (18%). In using the model to test ecological hypotheses and to investigate management strategies, we found the following: (1) Annual recruitment is directly and positively related to spawner abundance, and it appears to be density-compensatory following a Beverton–Holt relation (R 2 = 0.85). (2) Fishing-selectivity of arapaima caused by use of harpoons and gillnets can lower yield potentials dramatically through removal of the faster-growing individuals of the population. That is in part because fewer individuals live long enough to reproduce and survivors take longer to reach reproductive age. (3) Arapaima populations can sustain annual catches of up to 25% of the number of adults in the population the previous year if minimum size (1.5 m) and closed season (December–May) limits are met. (4) When 25% of the number of adults in the population the previous year is harvested under a 1.6 m minimum size limit of catch, catches are slightly smaller but abundance of adults in the population is considerably greater than under a 1.5 m limit. These findings can be used in ongoing management initiatives, but caution is needed because of present biological and ecological uncertainty about these fishes.

Spearfishing to depletion: evidence from temperate reef fishes in Chile

Unreliable and data-poor marine fishery landings can lead to a lack of regulatory action in fisheries management. Here we use official Chilean landing reports and non-conventional indicators, such as fishers' perceptions and spearfishing competition results, to provide evidence of reef fishes depletions caused by unregulated spearfishing. Results show that the three largest and most emblematic reef fishes targeted mainly by spearfishers (> 98% of landings) [Graus nigra (vieja negra), Semicossyphus darwini (sheephead or pejeperro), and Medialuna ancietae (acha)] show signs of depletion in terms of abundance and size and that overall the catches of reef fishes have shifted from large carnivore species toward smaller-sized omnivore and herbivore species. Information from two snorkeling speargun world championships (1971 and 2004, Iquique, Chile) and from fishers' perceptions shows the mean size of reef fish to be declining. Although the ecological consequences of reef fish depletion are not fully understood in Chile, evidence of spearfishing depleting temperate reef fishes must be explicitly included in policy debates. This would involve bans or strong restrictions on the use of SCUBA and hookah diving gear for spearfishing, and minimum size limits. It may also involve academic and policy discussions regarding conservation and fisheries management synergies within networks of no-take and territorial user-rights fisheries areas, as a strategy for the sustainable management of temperate and tropical reef fisheries.

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/SAbstract – 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.

Beyond abundance and biomass: effects of marine protected areas on the demography of a highly exploited reef fish

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 411:243-258 (2010) - DOI: https://doi.org/10.3354/meps08672 Beyond abundance and biomass: effects of marine protected areas on the demography of a highly exploited reef fish Brett M. Taylor*, Jennifer L. McIlwain University of Guam Marine Laboratory, Mangilao, Guam 96923 *Email: brettmtaylor@gmail.com ABSTRACT: The success of marine protected areas (MPAs) in facilitating recovery of over-exploited stocks has been well documented. Few studies, however, have explored the effects of MPAs on the demographic profiles of reef fish populations. We tested the assumption that areas closed to fishing for >7 yr accumulate older individuals of a heavily targeted species, Lethrinus harak. Our survey design included 2 protected sites and 2 comparative fished sites. Using an age-based lengthconversion method, a novel approach in tropical fisheries, otolith-derived demographic information was supplemented with underwater visual census data. This approach provided valuable insights into the population dynamics of this species at small spatial scales (<2 km2). We found considerable site-specific differences in the age structure of L. harak, with protected sites having greater mean ages and lower total mortality. There was indication of density-dependent growth as greater size-at-age occurred in fished sites where biomass had been depleted. Although female L. harak reach 50% maturity at 208 mm fork length (FL) or age 3.8 yr, it is the largest and oldest females (>260 mm FL and >9 yr) who make a disproportionate contribution to overall reproductive output, as ovary weight increases exponentially with both length and age. Currently size (Lc) and age (tc) at first capture is ~100 mm and 2 yr below the size (L50) and age (t50) at first maturity. Numerous management scenarios based on minimum size limits were evaluated to determine which scenario would increase spawning biomass without compromising yield estimates. Our results demonstrate that effective implementation of MPAs allows a larger and older population to accrue, thus yielding considerable reproductive benefits. KEY WORDS: MPA · Fishing effects · Demography · Lethrinus harak · Guam · Otoliths · Density-dependent growth Full text in pdf format PreviousNextCite this article as: Taylor BM, McIlwain JL (2010) Beyond abundance and biomass: effects of marine protected areas on the demography of a highly exploited reef fish. Mar Ecol Prog Ser 411:243-258. https://doi.org/10.3354/meps08672 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 411. Online publication date: July 29, 2010 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2010 Inter-Research.

Management and Recovery Options for Ural River Beluga Sturgeon

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

Maturation and sexual ontogeny in the spangled emperor Lethrinus nebulosus

The reproductive development and sexual ontogeny of spangled emperor Lethrinus nebulosus populations in the Ningaloo Marine Park (NMP) were investigated to obtain an improved understanding of its evolved reproductive strategy and data for fisheries management. Evidence derived from (1) analyses of histological data and sampled sex ratios with size and age, (2) the identification of residual previtellogenic oocytes in immature and mature testes sampled during the spawning season and (3) observed changes in testis internal structure with increasing fish size and age, demonstrated a non-functional protogynous hermaphroditic strategy (or functional gonochorism). All the smallest and youngest fish sampled were female until they either changed sex to male at a mean 277.5 mm total length (L(T)) and 2.3 years old or remained female and matured at a larger mean L(T) (392.1 mm) and older age (3.5 years). Gonad masses were similar for males and females over the size range sampled and throughout long reproductive lives (up to a maximum estimated age of c. 31 years), which was another correlate of functional gonochorism. That the mean L(T) at sex change and female maturity were below the current minimum legal size (MLS) limit (410 mm) demonstrated that the current MLS limit is effective for preventing recreational fishers in the NMP retaining at least half of the juvenile males and females in their landed catches.

Winners and losers of conservation policies for European eel, Anguilla anguilla: an economic welfare analysis for differently specialised eel anglers

Abstract Recreational specialisation theory was coupled with a discrete choice experiment to understand eel, Anguilla anguilla L., angler’s heterogeneity in their reaction to regulatory changes and the associated welfare changes. Differently specialised eel anglers exhibited distinct preferences for catch variables and eel angling regulations. All anglers preferred slightly to moderately stricter regulations than are currently in place; however, such policies particularly benefited casual eel anglers. In contrast, advanced eel anglers would be most penalised by highly restrictive regulations as indicated by substantial reductions in economic welfare. Aversions to stricter regulations found for advanced anglers contradicted predictions from specialisation theory. From an eel management perspective, the implementation of some simple tools such as increased minimum‐size limits will reduce angling mortality on eel and simultaneously increase the welfare of anglers. By contrast, highly restrictive eel angling regulations will result in considerable economic welfare losses of several million € per year for northern Germany alone.

Time trends in angler compliance with harvest regulations in stream fisheries

Abstract Harvest regulations in stream fisheries are used to improve fisheries and protect vulnerable fish stocks, but objectives are often not fulfilled. Roving interviews of grayling, Thymallus thymallus L., and brown trout, Salmo trutta L., anglers in north Swedish streams were performed during a 15‐year‐period to reveal the effects of regulation changes and levels of compliance. Harvest of fish below minimum size limits declined with time in river sections with 25‐ and 30‐cm minimum size limits. No clear time trend was found for the 35‐cm size limit. In addition, the proportion of voluntary released legal‐sized fish increased over time, indicating a change in angler attitudes. The overall effect of increasing the minimum size limit was lowering of angler‐induced fishing mortality. Interview data revealed perception, shared by many anglers, of an acceptable fish size to take home for food. From a management perspective, the realised effect of angler behaviour on the fish populations needs to be considered.

On the influence of the sputtering in determining the resolution of a scanning ion microscope

The determination of the quality of an imaging system is not an easy task for, in general, at least three parameters, strictly interdependent, concur in defining it: resolution, contrast, and signal-to-noise ratio. The definition of resolution itself in scanning microscopy is elusive and the case of scanning ion microscopy is complicated by the damage of the sample under the ion beam, which, especially for small features, can be the limiting factor. This is indeed the case for most focused ion beam systems, which exploit beams of Ga+. The only way to overcome this limit is to exploit sources of low mass ions, such as H+ and He+. In this article the authors analyze the way the sputtering may affect the resolution, defined as smallest detectable feature in an image, of a scanning ion microscope, for heavy and light ions, in the case of spherical features. It appears that the fundamental limit to the resolution in scanning microscopy is not given by the spot size, but by the dynamics of the interaction of the beam with the sample and the consequent modification of the sample’s geometry, even for beams of light ions. For example, in the case of Sn nanospheres under a He+ beam, the authors found a minimum theoretical detectable particle size limit of ∼1 nm and an experimental limit of ∼5 nm.

Factors Influencing Short‐Term Hooking Mortality of Bluegills and the Implications for Restrictive Harvest Regulations

AbstractAngling can adversely affect populations of bluegill Lepomis macrochirus, and many state agencies have begun to explore restrictive creel and length limits. The fate of released fish can be influenced by a variety of factors, and the success of regulations could be reduced if the mortality of released bluegills is high. We conducted experiments using bluegills caught by the general angling public in Ridge Lake, Illinois, to quantify bluegill hooking mortality and test for the effects of bait type (live versus artificial), season (spring versus summer), retention time in live wells (1, 3, or 5 h), and retention gears (live wells versus fish baskets). Across all seasons and baits, the mortality of caught‐and‐released bluegills was low; the initial and short‐term mortality for fish immediately released was 4.4%. There were significant differences in hooking mortality with respect to bait type, retention gear, and retention time. Bluegills caught on live bait experienced higher mortality (6.6%) than those caught on artificial baits (0%), and wire fish baskets caused higher mortality (34.0%) than live wells (7.6%). The mortality of bluegills held in live wells was higher for the 3‐ and 5‐h retention times than for the 1‐h treatment. The probability of dying from catch and release decreased with increasing fish length. We applied our results to a bluegill population under a restrictive harvest regulation consisting of a 203‐mm minimum size limit and a 10‐fish daily bag limit. Under these restrictive regulations, fish lost from catch and release represented 27.4% of the total kill (harvest and hooking mortality). While catch‐and‐release angling can cause some mortality, it does not appear from our study that releasing fish will have a deleterious effect on bluegill populations.

Quantifying selection differentials caused by recreational fishing: development of modeling framework and application to reproductive investment in pike (Esox lucius).

Methods for quantifying selection pressures on adaptive traits affected by size-selective fishing are still scarce, and none have as yet been developed for recreational fishing. We present an ecologically realistic age-structured model specifically tailored to recreational fishing that allows estimating selection differentials on adaptive life-history traits. The model accounts for multiple ecological feedbacks, which result in density-dependent and frequency-dependent selection. We study selection differentials on annual reproductive investment under size-selective exploitation in a highly demanded freshwater recreational fish species, northern pike (Esox lucius L.). We find that recreational angling mortality exerts positive selection differentials on annual reproductive investment, in agreement with predictions from life-history theory. The strength of selection increases with the intensity of harvesting. We also find that selection on reproductive investment can be reduced by implementing simple harvest regulations such as minimum-size limits. The general, yet computationally simple, methods introduced here allow evaluating and comparing selection pressures on adaptive traits in other fish populations and species, and thus have the potential to become a tool for evolutionary impact assessment of harvesting.

Optimising recreational harvests of blue cod: The effects of catch-and-release mortality and size selectivity

A yield-per-recruit model is developed for the recreational fishery on blue cod ( Parapercis colias: Pinguipedidae) in the Marlborough Sounds, New Zealand. The model incorporates sex differences in growth rates, protogyny, minimum size limits, catch-and-release mortality and size-selective fishing. Mortality of released fish due to hooking damage and post-release predation is a potentially significant factor for blue cod management. High levels of catch-and-release mortality favour small minimum size limits and reduced fishing intensity. Even in the absence of size limits, hook and line fishing for blue cod is highly selective on larger fish and this greatly reduces the potential impact of catch-and-release mortality on yields and management choices. Catch-and-release mortality will also lower the spawning stock biomass-per-recruit and therefore has the potential to impact on stock sustainability as well as yields. In protogynous species the effects of fishing on spawning stock biomass will be different for males and females and it is important to model these separately, as well as understanding the cues for, or timing and extent of, sex change. Catch-and-release mortality can also slow the recovery of stocks when no-take rules still allow fishing for other species and therefore by-catch of the species targeted for protection. It is suggested that a small reduction in size limit and a small increase in daily bag limit relative to present regulations, would result in optimal yields from this fishery. Predation on fish released by recreational anglers is less widely reported than mortality due to hooking damage but both have the potential to negate the benefits of minimum size limits.