Using marine reserves to estimate fishing mortality

The proportion of a fish stock that is killed by fishing activity is often calculated as the catch divided by the estimated stock biomass. However, stock biomass is notoriously difficult to estimate reliably, and moreover, the catch may be uncertain or misreported and does not include losses due to discarding. In all too many fisheries, these difficulties have lead to underestimates of total fishing mortality and the commercial demise of the fishery. No‐take marine reserves eliminate fishing mortality from within their boundaries and, for species that exhibit seasonal migratory behaviour, comparison of reserves with fished areas can provide direct estimates of the proportion killed by fishing. For an important exploited species in New Zealand, seasonal changes in density of sub‐legal fish at three marine reserves were similar in both reserve and adjacent non‐reserve areas. However, this result did not hold for legal‐size fish, and the difference in seasonal change between reserved and non‐reserved areas was used to obtain direct estimates of the total localized fishing mortality in the non‐reserve area over 6‐month periods. Estimates of the percentage of legal‐size fish killed by fishing ranged from 70 to 96%. These results demonstrate an unanticipated practical benefit from marine reserves that goes beyond their ecological role.     

Improvement of fish length estimates for underwater visual census of reef fish biomass

Accuracy and precision are of great importance in the assessment of reef fish biomass when conducting an underwater visual census (UVC). Quantification and subsequent correction of the bias is required in order to standardize the estimates and correct for underwater distortion. To optimize the UVC, the observer should receive length‐measurement training in order to obtain in situ‐measurements that are as accurate and precise as possible. The objective of this study was to quantify the bias of fish length measurements made by divers with and without training in order to enhance reef fish biomass estimates. Adaptation of the diver to estimate fish lengths was analysed as a part of reef fish biomass monitoring in the Karimunjawa National Park, a national marine sanctuary in the Java Sea, Indonesia. Two divers practiced estimating a variety of fish in a natural environment by using styrofoam models attached to strings and sinkers. Analyses showed that by training the diver, his/her accuracy and precision improved substantially. Proving its reliability, an underwater visual census (UVC) becomes a useful and reliable method to assess the biomass of reef fishes.     

Habitat complexity and fish size affect the detection of Indo-Pacific lionfish on invaded coral reefs

A standard approach to improving the accuracy of reef fish population estimates derived from underwater visual censuses (UVCs) is the application of species-specific correction factors, which assumes that a species’ detectability is constant under all conditions. To test this assumption, we quantified detection rates for invasive Indo-Pacific lionfish (Pterois volitans and P. miles), which are now a primary threat to coral reef conservation throughout the Caribbean. Estimates of lionfish population density and distribution, which are essential for managing the invasion, are currently obtained through standard UVCs. Using two conventional UVC methods, the belt transect and stationary visual census (SVC), we assessed how lionfish detection rates vary with lionfish body size and habitat complexity (measured as rugosity) on invaded continuous and patch reefs off Cape Eleuthera, the Bahamas. Belt transect and SVC surveys performed equally poorly, with both methods failing to detect the presence of lionfish in >50 % of surveys where thorough, lionfish-focussed searches yielded one or more individuals. Conventional methods underestimated lionfish biomass by ~200 %. Crucially, detection rate varied significantly with both lionfish size and reef rugosity, indicating that the application of a single correction factor across habitats and stages of invasion is unlikely to accurately characterize local populations. Applying variable correction factors that account for site-specific lionfish size and rugosity to conventional survey data increased estimates of lionfish biomass, but these remained significantly lower than actual biomass. To increase the accuracy and reliability of estimates of lionfish density and distribution, monitoring programs should use detailed area searches rather than standard visual survey methods. Our study highlights the importance of accounting for sources of spatial and temporal variation in detection to increase the accuracy of survey data from coral reef systems.     

Biases associated with the use of underwater visual census techniques to quantify the density and size-structure of fish populations

Estimates of reef fish densities made by divers visually censusing 5 m wide strip transects were compared with capture-resight estimates calculated independently using data on the resighting frequencies of fish marked with colour-coded tags. The difference in density estimates between methods varied between species but with patterns consistent at the three eastern Tasmanian sites studied. Densities of the two most abundant species, the wrasses Notolabrus tetricus and Notolabrus fucicola, showed good agreement between methods. By contrast, populations of the two monacanthids Meuschenia australis and Meuschenia freycineti were underestimated by an order of magnitude in strip transects relative to capture-resight, while populations of the open-water latrid Latridopsis forsteri were overestimated.For all common fish species observed in strip transects, variation in density estimates between divers was extremely low compared to variation between sites and between months. Variation in density estimates between different days within a week and between 50 m blocks located 50 m apart was also negligible; however, residual error associated with variation by a single diver within a block and day was high. Biases associated with changing detectability of fishes in open versus densely vegetated habitats were assessed by algal removal experiments, and were found to be negligible or non-significantly low for five of the six species examined. The exceptional species—N. tetricus—exhibited a 70% rise in fish sighted by diver following algal clearance despite trapping data indicating no increase in fish numbers.Diver estimates of fish length were on average 7% greater than measured lengths. Divers possessed a clear tendency to make increasingly inaccurate size estimates as fish length deviated in either direction from 300 mm. Lengths of 175-mm animals were underestimated by ≈20% and 400-mm fishes were overestimated by ≈10%. These changes in diver bias with fish size were largely independent of fish species, site and diver. The precision of size estimates, as indicated by the standard deviation of bias, also varied with fish size, with values varying from ≈13% at 200 mm fish length to ≈8% at 400 mm length. The decline in precision at small body size largely reflected size intervals used by divers to bin data. Divers appear capable of making more precise size estimates than the 25-mm interval used at small fish sizes.     

Effects of marine reserves versus nursery habitat availability on structure of reef fish communities

No-take marine fishery reserves sustain commercial stocks by acting as buffers against overexploitation and enhancing fishery catches in adjacent areas through spillover. Likewise, nursery habitats such as mangroves enhance populations of some species in adjacent habitats. However, there is lack of understanding of the magnitude of stock enhancement and the effects on community structure when both protection from fishing and access to nurseries concurrently act as drivers of fish population dynamics. In this study we test the separate as well as interactive effects of marine reserves and nursery habitat proximity on structure and abundance of coral reef fish communities. Reserves had no effect on fish community composition, while proximity to nursery habitat only had a significant effect on community structure of species that use mangroves or seagrass beds as nurseries. In terms of reef fish biomass, proximity to nursery habitat by far outweighed (biomass 249% higher than that in areas with no nursery access) the effects of protection from fishing in reserves (biomass 21% lower than non-reserve areas) for small nursery fish (≤ 25 cm total length). For large-bodied individuals of nursery species (>25 cm total length), an additive effect was present for these two factors, although fish benefited more from fishing protection (203% higher biomass) than from proximity to nurseries (139% higher). The magnitude of elevated biomass for small fish on coral reefs due to proximity to nurseries was such that nursery habitats seem able to overrule the usually positive effects on fish biomass by reef reserves. As a result, conservation of nursery habitats gains importance and more consideration should be given to the ecological processes that occur along nursery-reef boundaries that connect neighboring ecosystems.     

Monitoring vegetation change caused by trampling: a study in the Cairngorms,Scotland

Summary

A study was made in the Cairngorms, Scotland to make recommendations for a monitoring scheme capable of detecting changes in the vegetation caused by recreational pressure following the development of a funicular railway. Four methods were used in field trials to assess percentage cover of plant species and gravel, rock and bare ground, where appropriate, in two vegetation types (open and closed). The methods used were visual estimates in 50 × 40 cm quadrats (Q), the mean of visual estimates in twenty 10 × 10 cm sub-quadrats of the 50 × 40 cm quadrats (Q₂₀), a modified point intercept method (RL) and photography. Variances between observers and between-quadrats were estimated for the different methods. The sampling design for detecting change was based on a model of variance, constructed from field trial data.

Between-observer and between-quadrat variances were related to mean percentage cover and approximated to a binomial distribution. The between-quadrat variance was larger than observer variance. The Q₂₀ method achieved appreciably better precision than the other methods. Analysis of half of the 10 × 10 cmsub-quadrats (1/2Q₂₀) selected in a checker board design achieved a relative efficiency of 78% compared with the Q₂₀. This result suggests that comparable precision to the Q₂₀ method could be achieved by choosing about 14 sub-quadrats in a larger quadrat, thus saving some time. Variation between quadrats also suggested that the Q₂₀ method was the one of choice for maximising precision. The precision of the photographic method was based on fewer data points, so is less accurate than other estimates.

Minimum sample sizes were estimated for detecting a 10% relative change of a species in open vegetation with 30% cover (i.e. a change from 30% to <27 or to >33% cover). With a 10 % Type II error rate and 5 % Type I error rate the minimum sample sizes were 47 quadrats for Q, 18 for Q ₂₀, 43 for RL, and 23 for the means of ten 10 × 10 cm sub-quadrats in open vegetation.

The most time-efficient field recording appeared to be the use of Q despite the required sample size being 2.6 times higher than that of Q₂₀. The far lower time requirement per quadrat, however, compensated for the higher numbers. The number of quadrats would depend on the specified change in percentage cover and on the statistical significance level used. For example, to detect a 10% absolute change in cover (i.e. from 30% to either <20 % or >40 % cover) at 95 % probability the net effective recording time is estimated at 5 h per vegetation type while to detect a 5 % change at 99 % probability would require c. 25 h. Larger samples may be required for other species or for species with a low initial cover.     

Juvenile salmonid production in a Lake Erie nursery stream

Normandale Creek (2531m²) provides spawning and nursery grounds for lake-run rainbow trout, Salmo gairdneri Richardson, brown trout, Salmo frutta L. , and coho salmon, Oncorhynchus kisutch (Walbaum). Upstream movements are significantly correlated with peak stream discharge (P < 0.05). In 1973–1974, 59 adult salmonids constructed 86 nests of which 60% were disturbed by re-use or sand deposition. From a calculated deposition of 90 400 ova, 483 juveniles, 6 cm in fork length ( f.l. ), were resident in the stream when estimates by electrofishing first became valid. Estimated within-stream (i.e. all sections combined) monthly densities of wild juveniles >6 cm f.l. ranged from six to 22 fish (100 m²)⁻¹, and biomass from 1.3 to 3.1 gm⁻². The highest within-section biomass was 8.3 gm⁻² in November. Additions of hatchery-reared rainbow trout temporarily increased monthly within-stream juvenile density up to 23 fish (100 m²)⁻¹, and biomass to 8.2 g m⁻². Density and biomass were positively correlated (P < 0.05) with both instream and bank cover, and biomass negatively correlated (P < 0.05) with gradient. The importance of substratum and flow characteristics on juvenile density and biomass was strongly indicated although not statistically significant. Annual within-section production of juveniles > 6 cm f.l. ranged from 4.79 to 5.93 g m⁻² year⁻¹ in Sections III and IV, respectively. Inclusion of calculated probable numbers of fish < 6 cm f.l. increased within-section production from 5.93 to 18.65 g m⁻² year^−l in upstream Section IV.     

Ichthyofauna associated to Thalassia testudinum at Lobos reef, Veracruz, Mexico: diversity and abundance

The diversity and abundance of ichthyofauna associated with Thalassia testudinum in the Lobos coral reef, Veracruz, Mexico, were studied in September and October 2002. Thirty six visual censuses in four sample sites were made using a 50 x 2 m transect belt. On each census, fish species, abundance and size were recorded. Leaf size and cover of T. testudinum were estimated. The similarity of fish groups was calculated with the Gower coefficient. The most abundant coral reef fishes were: Scarus iseri, Halichoeres bivittatus, Sparisoma radians, Stegastes adustus and Stegastes leucostictus. The highest density (0.04078 ind/m2) and biomass (0.72408 g/m2) of fish species were recorded in site II, where leaf size was greater (30.8 cm). The analysis of variance showed significant differences between sites in leaf size (F = 18.30856; p = 0.00001) and cover (H = 33.8119; p = 0.00001). These differences suggest a relationship between fish diversity and abundance, and T. testudinum leaf size and cover. The Gower similarity index produced two groups of fishes; one of them (site II) showed the highest abundance. In this reef, the fishes associated to sea grasses seem to reflect the characteristics of T. testudinum.     

Drawbacks of successive electrofishing in a lowland stream

This study is based on a 23‐year sampling of fish and compares raw population abundance and biomass obtained from three to six consecutive electrofishing passes in a small lowland stream in Poland, with values of calculated density and a biomass from the Zippin model. Three density estimates and four biomass estimates obtained with three passes were higher than those estimates based on six catches. This discrepancy occurred because fewer fish were collected in the first pass than in the second pass. However, when the six consecutive passes were considered, the higher raw abundance and biomass in the second pass did not produce any significant error in the density calculation. A catchability decline resulted from the low number of juvenile fish captured when there was a high number of fish that had successfully spawned in a given year. Nevertheless, fish density assessment on the basis of three runs in lowland streams is recommended for quantitative sampling, and the problems that sometimes emerge will be explained in this study.     

The population density, growth rate and production of bream, Abramis brama, in Tjeukemeer, the Netherlands

A method of estimating the population density of bream in Tjeukemeer (21.3 km²) using 16 690 introduced fish (fin-clipped and opercular tagged) is described. Gill nets of the winter fishery proved to be a more effective method of sampling the population for marked fish than fyke nets. The population density of bream (⋝25 cm) was estimated to be 180 000. There was no significant difference between the estimates derived from fin-clipped and opercular tagged fish. The growth rate of bream in Tjeukemeer (L_∞=41 cm) is poor compared with that of bream in other waters, due to its high density and the scarcity of zoobenthos available to it.
The production of bream (I–XV) is estimated to be 34 kg ha⁻¹ of which 25% is contributed by two (1959, 1963) strong year classes. Of a total biomass of 37.5 kg ha⁻¹ available to the fishery in 1969 only 2.7 kg ha^−l was removed as yield. The average P/B ratio for the population was low (0.39). It is concluded that a major flow of energy to bream is through zooplankton.     

Diet composition and biomass consumption of harbour seals in Telemark and Aust-Agder,Norwegian Skagerrak

ABSTRACT

To explore ecosystem dynamics and functions it is vital to obtain knowledge on predator–prey relations. Harbour seals are piscivorous predators that can come into conflict with fisheries. Recently, as the Skagerrak and Kattegat population of harbour seals has increased, claims have emerged that seals are depleting coastal cod populations. The diet of harbour seals in Norwegian Skagerrak was investigated based on otolith identification from scats. The overall seal diet included 20 different fish species/groups. The most important prey (combined index Q _i ) were haddock/pollack/saithe (32.7%), genus Trisopterus (Norway pout/poor cod/bib, 12.5%), plaice (12.4%) and herring (10.0%). Plaice also had the largest biomass (24.1%). Gadoids and pleuronectids comprised 88.6% of the diet (combined index Q _i ) and 87.1% in terms of biomass. Cod constituted 0.7% (combined index Q _i ) of the overall diet and 2.3% in terms of biomass. Fish length estimates showed that seals generally prefer small fish below minimum allowed landing size. Estimated total amount of fish consumed was 315 tons per year and was dominated by non-commercial species. Annual cod consumption was an estimated 7.1 tons, representing 5% of annual cod landings, suggesting that competition between local fisheries and harbour seals is limited.     

Cascading trophic effects in pampean shallow lakes: results of a mesocosm experiment using two coexisting fish species with different feeding strategies

Planktivorous fish, both visual predators and filter feeders, enhance eutrophication processes in lakes. In pampean shallow lakes several planktivorous species may coexist but often two species dominate: silverside (Odontesthes bonariensis), a visual planktivorous fish when young adult, and sabalito (Cyphocharax voga), an omnivorous filter feeder. To assess the relative strength of the cascading trophic effects of the two species, a mesocosm experiment was conducted using different proportions of both species as treatments. Differences were found in water transparency, phytoplankton biomass, crustacean abundance, individual size and biomass. Our results suggest that visual predators intensify eutrophication effects more strongly than filter feeders do.     

Growth and production of the dominant pelagic fish, Acanthobrama terraesanctae, in subtropical Lake Kinneret, Israel

Acanthobrama terraesanctae (local name lavnun), an endemic planktivorous cyprinid, dominates total fish numbers (>80%) in Lake Kinneret, and may have a significant top-down impact on the lake ecosystem. The length of young-of-the-year fish calculated from the von Bertalanffy equation agreed with field observations of juvenile growth. An unusual bi-modal length-frequency distribution observed in May 1993 provided additional help in age identification. Males grew more slowly than females and reached a lower maximum length. Total mortality coefficients (exponents) of males and females >12 cm (minimal legal size of fish in the catch) were similar ( c. 1·52). An average cohort reaches maximum biomass during its second year. Maximum production is created at the end of the second year. The production: biomass ratio of the population was 1·16, and 36% of total lavnun standing stock was taken by fishing. From the late 1980s to early 1990s, when standing stock and population structure were stable, the average harvest of 1000 t was consistent with a total lavnun biomass of 2800 t, which constitutes 50–70% of the total fish stock measured acoustically in the lake. Such a biomass could be sustained by the known production of zooplankton. Absence of verified growth data for lavnun contributed to the collapse of the fishery in 1993, because it hampered timely revision of fishery policy in response to the drastic changes in the lavnun stock in 1992.     

Strong correspondence between gillnet catch per unit effort and hydroacoustically derived fish biomass in stratified lakes

1. Sampling of lake fish assemblages is a challenging task in fish science, and the information obtained strongly depends on the choice of sampling gear. The use of more than one sampling technique is generally preferred in order to achieve a comprehensive view on fish assemblage structure. Therefore, the knowledge of whether catches between fishing gears are comparable is crucial. 2. We compared catches in benthic multi‐mesh gillnets with fish biomass estimates obtained by vertical hydroacoustics in 18 European lakes strongly varying in morphometry and trophic status. Separate analyses were conducted for different depth strata and for several fish length thresholds to account for depth‐ and size‐selective gillnet catches. 3. Gillnet catches and hydroacoustically obtained fish biomass estimates were significantly correlated. The strength of correlations was independent of the fish length thresholds applied, but varied across different depth strata of the lakes, with the strongest correlations occurring in the shallow strata. 4. The results support the applicability of vertical hydroacoustics for the quantification of fish biomass in stratified lakes. Survey designs combining hydroacoustics with limited gillnetting at sampling dates shortly one after the other, the latter for the purpose of inventory sampling only, are a cost‐effective strategy for sampling fish assemblages in lakes. However, gillnet sampling does not provide reliable fish density estimates in very deep lakes with separate, pelagic‐dwelling fish assemblages.     

Measuring the long-term success of small-scale marine protected areas in a Philippine reef fishery

Tropical coral reefs are subject to multiple pressures from both natural and anthropogenic sources. These pressures have caused widespread declines in reef health, resulting in the increased use of spatial management tools such as marine protected areas (MPAs). MPAs have proven generally effective if well designed and enforced, but there are limited long-term studies investigating how the presence of small-scale MPAs affects fish populations and reef communities. Using a 12-year time series, we found that small-scale (10–50 ha) community-managed MPAs along the Danajon Bank of the Philippines preserved average fish biomass within their boundaries over time relative to surrounding fished reefs. Unprotected areas are, however, showing significant long-term biomass decline. MPAs were also found to preserve more key trophic groups and larger-bodied commercially targeted reef fish families. Fish biomass of piscivore, scavenger and invertivore trophic groups inside individual MPAs is, however, still declining at a similar rate as outside. Surprisingly, long-term benthic cover and growth form composition were not significantly affected overall by MPA presence, despite the sporadic use of highly destructive dynamite fishing in this region. Coral cover has remained historically low (21–28%) throughout the study, following widespread bleaching mortality. While management tempered overall abundance declines, we found that irrespective of MPA presence, there was a generalised decline of both large- and small-bodied fish size groups across the study region, most steeply within the 20–30 cm length fish, and a shift towards proportionally higher abundances of small (5–10 cm) fish. This indicates a combination of over-exploitation, inadequate MPA size and coverage for larger fish, and the lingering effects of the 1998 bleaching event. Generalised shifts in body size and trophic structure reported here could lead to future reductions in fishery productivity and stability and will be further exacerbated unless broader fishery regulations and enforcement is instated.

     

Spawning seasonality and body sizes at sexual maturity in the bluespine unicornfish,Naso unicornis (Acanthuridae)

We herein evaluate several reproductive metrics of Hawaiian Archipelagic populations of the bluespine unicornfish (Naso unicornis), an economically and ecologically important, broadly distributed tropical Pacific reef fish, based on multi-year, fishery-dependent and fishery-independent collections. Sex-specific spawning seasonality was characterized for fish collected mostly from Oahu (Main Hawaiian Islands, MHI) using a gonadosomatic index. Histological slides preparations were used to score gonad developmental phase and to classify individuals of either sex as immature or mature. Sex-specific median body lengths at maturity (L₅₀) were estimated by logistic fits of proportion mature versus length class. Spawning was highly seasonal in Hawaii, with a single brief (May–June) peak spawning period. Proportionate gonad-to-body weight values were relatively low, averaging only about 0.1 % and 0.6 % across all months of year and 0.16 % and 1.03 % during May–June for males and females, respectively. Median lengths at sexual maturity differed between the sexes. L₅₀ values for fish collected throughout all months of year were 30.1 ± 0.5 (standard error) cm Fork Length (FL) for males and 35.5 ± 0.7 cm FL for females. Spawning seasonality and L₅₀ estimates for bluespine unicornfish in Hawaii suggest that the species spawns several months earlier in the calendar year and matures at larger body lengths in Hawaii versus Guam, the Northern Mariana Islands, and Pohnpei in the Federated States of Micronesia. Estimated lengths at sexual maturity are compared to the minimum length (14 inches or 35.6 cm FL) mandated for this species in Hawaii: median size at maturity occurs at a length appreciably less than (males) or approximately equal to (females) minimum legal size. A likely disproportionately large contribution of old females to population replenishment is discussed relative to the minimum size limit.     

Human,Oceanographic and Habitat Drivers of Central and Western Pacific Coral Reef Fish Assemblages

Coral reefs around US- and US-affiliated Pacific islands and atolls span wide oceanographic gradients and levels of human impact. Here we examine the relative influence of these factors on coral reef fish biomass, using data from a consistent large-scale ecosystem monitoring program conducted by scientific divers over the course of >2,000 hours of underwater observation at 1,934 sites, across ~40 islands and atolls. Consistent with previous smaller-scale studies, our results show sharp declines in reef fish biomass at relatively low human population density, followed by more gradual declines as human population density increased further. Adjusting for other factors, the highest levels of oceanic productivity among our study locations were associated with more than double the biomass of reef fishes (including ~4 times the biomass of planktivores and piscivores) compared to islands with lowest oceanic productivity. Our results emphasize that coral reef areas do not all have equal ability to sustain large reef fish stocks, and that what is natural varies significantly amongst locations. Comparisons of biomass estimates derived from visual surveys with predicted biomass in the absence of humans indicated that total reef fish biomass was depleted by 61% to 69% at populated islands in the Mariana Archipelago; by 20% to 78% in the Main Hawaiian islands; and by 21% to 56% in American Samoa.     

Estimating the stocking potential of fish in impoundments by modelling supply and steady‐state demand

1. Fish stocking is an increasingly common management tool for freshwater and marine environments and is often used to create and maintain fisheries in closed waters. The densities at which fish are stocked can have a large impact on a stocking programme’s success and sustainability. Stocking densities in impoundment sport‐fisheries, for example, are often based on social or practical factors, and ecologically based stocking models are needed to assist the selection of stocking densities that are appropriate for the environment. 2. In this study, stocking density is calculated with a numerical model that balances the supply of prey production with the energetic demand of stocked fish. The model aims to deliver outcomes over a range of potential management objectives, by providing specific consumption scenarios that represent the trade‐off between population abundance and individual body size in stocked fisheries. 3. The model uses a steady‐state population approach to calculate stocking density, which optimises population consumption by maintaining a consistent biomass distribution and encourages sustainable stocking by considering the energetic needs of all cohorts. Carrying capacity is represented by the steady‐state stocking density under a minimum consumption scenario (when fish meet only their minimum energetic needs). The comparison between a desired consumption rate and the existing level of production is used to assess the status or ‘health’ of the existing population and is used to determine whether stocking can occur and whether stocking densities can be sustainably increased. The probability of incorrectly assuming populations are achieving a given consumption level is also estimated, which is an ideal approach for interpreting multiple probability distributions. 4. A Monte‐Carlo analysis of uncertainty was used to provide a probability distribution of stocking density of Australian bass (Macquaria novemaculeata) in three Australian impoundments under various seasonal and consumption scenarios. The likely consumption rates of the existing populations were determined using historical stocking densities, which showed that the three populations were of reasonable health, although one impoundment may be overstocked. The steady‐state stocking densities depended on the desired consumption rate, and there was an eightfold difference in the stocking density aimed at providing large ‘trophy’ fish and the density required to reach carrying capacity. 5. Model outputs of existing abundance and biomass density agreed with empirical estimates of abundance and relative density in these impoundments, which provides support to the model’s accuracy. This supply–demand approach to estimating the range of appropriate stocking densities shows promise as a decision‐support tool for stocked impoundments and other closed fisheries.     

Potential consequences of climate change for primary production and fish production in large marine ecosystems

Existing methods to predict the effects of climate change on the biomass and production of marine communities are predicated on modelling the interactions and dynamics of individual species, a very challenging approach when interactions and distributions are changing and little is known about the ecological mechanisms driving the responses of many species. An informative parallel approach is to develop size-based methods. These capture the properties of food webs that describe energy flux and production at a particular size, independent of species'' ecology. We couple a physical–biogeochemical model with a dynamic, size-based food web model to predict the future effects of climate change on fish biomass and production in 11 large regional shelf seas, with and without fishing effects. Changes in potential fish production are shown to most strongly mirror changes in phytoplankton production. We project declines of 30–60% in potential fish production across some important areas of tropical shelf and upwelling seas, most notably in the eastern Indo-Pacific, the northern Humboldt and the North Canary Current. Conversely, in some areas of the high latitude shelf seas, the production of pelagic predators was projected to increase by 28–89%.