Size Structure Data Research Articles

Growth patterns and population dynamics of Chiton articulatus (Mollusca: Polyplacophora): A multi-decade study

The edible chiton Chiton articulatus is a commercially important mollusk found in the rocky intertidal zones of the Mexican tropical Pacific. Despite the intense harvesting in Acapulco Bay, Mexico, knowledge of its growth patterns is limited, hindering the development of effective management strategies. This study investigated the growth dynamics of C. articulatus using a multi-model inference approach based on size structure data collected in four sampling periods covering four decades. Results revealed continuous recruitment throughout the year, contributing to population resilience. The species exhibited growth plasticity, highlighting its adaptive potential. We found complex temporal patterns influenced mainly by climatic events. The El Niño event sowed higher growth rates and lower asymptotic length, while La Niña events showed the opposite pattern. This research provides insights into the growth dynamics of C. articulatus, highlighting the need for holistic management strategies for this commercially important species in the face of environmental change.

Six years of demography data for 11 reef coral species.

Scleractinian corals are colonial animals with a range of life-history strategies, making up diverse species assemblages that define coral reefs. We tagged and tracked ~30 colonies from each of 11 species during seven trips spanning 6 years (2009-2015) to measure their vital rates and competitive interactions on the reef crest at Trimodal Reef, Lizard Island, Australia. Pairs of species were chosen from five growth forms in which one species of the pair was locally rare (R) and the other common (C). The sampled growth forms were massive (Goniastrea pectinata [R] and G. retiformis [C]), digitate (Acropora humilis [R] and A. cf. digitifera [C]), corymbose (A. millepora [R] and A. nasuta [C]), tabular (A. cytherea [R] and A. hyacinthus [C]) and arborescent (A. robusta [R] and A. intermedia [C]). An extra corymbose species with intermediate abundance, A. spathulata was included when it became apparent that A. millepora was too rare on the reef crest, making the 11 species in total. The tagged colonies were visited each year in the weeks prior to spawning. During visits, two or more observers each took two or three photographs of each tagged colony from directly above and on the horizontal plane with a scale plate to track planar area. Dead or missing colonies were recorded and new colonies tagged to maintain ~30 colonies per species throughout the 6 years of the study. In addition to tracking tagged corals, 30 fragments were collected from neighboring untagged colonies of each species for counting numbers of eggs per polyp (fecundity); and fragments of untagged colonies were brought into the laboratory where spawned eggs were collected for biomass and energy measurements. We also conducted surveys at the study site to generate size structure data for each species in several of the years. Each tagged colony photograph was digitized by at least two people. Therefore, we could examine sources of error in planar area for both photographers and outliners. Competitive interactions were recorded for a subset of species by measuring the margins of tagged colony outlines interacting with neighboring corals. The study was abruptly ended by Tropical Cyclone Nathan (Category 4) that killed all but nine of the more than 300 tagged colonies in early 2015. Nonetheless, these data will be of use to other researchers interested in coral demography and coexistence, functional ecology, and parametrizing population, community, and ecosystem models. The data set is not copyright restricted, and users should cite this paper when using the data.

Length-based assessment of an exploited coastal pike (Esox lucius) stock (Rügen, southern Baltic Sea) underscores the crucial relevance of growth and natural mortality for assessment outcomes

Many capture fisheries are considered data-poor, including the northern pike (Esox lucius) fishery in brackish lagoons in the southern Baltic Sea of Germany. The objective of our work was to assess the exploitation status of this stock, which is perceived by stakeholders to be in decline. Size structure data collected via rod-and-reel-angling, fyke nets, and gill nets, and empirical estimates of growth, maturation, and fecundity from the lagoon stock were used to fit the Length-Based Spawning Potential Ratio (LB-SPR) model. Parameter uncertainty in von Bertalanffy growth estimates and natural mortality in the Baltic Sea pike stock was considered in sensitivity analyses. Assessment outcomes were sensitive to estimates of growth rate, particularly asymptotic length L∞, instantaneous natural mortality M, and gear selectivity. Under-aging of old fish in scale-based age estimates overestimated terminal length and generated negative bias in the estimated stock status. Despite the sensitivity of assessment outcomes to life-history parameter choice, the stock status for the Baltic Sea consistently indicated a fully exploited situation with SPRs robustly above 0.4 and current fishing mortality rates between 0.2 and 0.4 yr−1. This result agreed with previous assessments using catch-only models. Our work serves as a reminder, that when using length-based methods, unbiased growth, and natural mortality estimates are critical for robust assessment outcomes.

Ice fishing reveals size structure but not abundance of Eurasian perch in small boreal lakes

AbstractManagement of inland recreational fisheries would benefit from stock abundance and size structure data. Feasibly standardised angling methods such as ice fishing could produce representative catch‐per‐unit‐effort (CPUE) information on the abundance of different‐sized fish in small lakes. Here, we first used standard Nordic multimesh gillnets to obtain number‐per‐unit‐effort (NPUE), biomass‐per‐unit‐effort (BPUE) and size structure data on Eurasian perch (Perca fluviatilis) stocks in 11 small boreal lakes in summer. Second, the same lakes were ice‐fished by voluntary anglers using a pre‐defined, loosely standardised protocol to obtain angling‐based NPUE, BPUE, and length frequency distributions. Effects of environmental variables such as water oxygen concentration and light penetration on angling catch rates were controlled statistically. Neither perch Nordic gillnet NPUE nor BPUE corresponded to ice‐fishing CPUEs. However, the length distribution of the catch did not differ between methods. Our results imply that traditional ice fishing applying natural baits is relatively unselective for fish size and could produce valid length‐based indicators for management purposes while angling CPUE was poorly related to Nordic gillnet CPUE.

Patterns of biogeographic and regional life-history trait variation in four large-bodied tropical wrasses

Ectotherms display substantial demographic variation across latitudinal gradients of temperature. Higher temperatures are often associated with smaller size, rapid initial growth rates, and early maturation, generally described as the Temperature-Size Rule (TSR). The longevity of most ectotherms also declines at warmer, lower latitudes. However, these patterns may be modified by increases in food resources that can flow on to continuous growth and large adult size. The present study estimates age-based demographic parameters of large-bodied tropical wrasses (Hemigymnus melapterus, H. fasciatus, Cheilinus fasciatus, and Oxycheilinus digramma) collected from Philippine fish markets (9–11°N) and sampled from Palm (18.53–18.70°S) and Whitsunday (20.05–20.21°S) reefs on the Great Barrier Reef, Australia (GBR). Differences in longevity, initial growth rates, and the age at sexual maturation at a biogeographic scale, between the Philippines and GBR, conformed to predictions of the TSR. However, Philippine specimens exhibited greater relative body condition and sustained periods of growth beyond sexual maturity resulting in larger adult size than GBR samples. Size-structure data from Philippine marine reserves and fished sites indicated that these differences were not confounded by fishery-dependent sampling. Moreover, latitudinal length–weight relationships could not be explained by lower densities of the focal wrasses in the Philippines or by relative gonad size. Less pronounced patterns of demographic variation that differed across species were evident at a regional scale, among Palm and Whitsunday reefs. Patterns of demographic variation between the Philippines and GBR strongly suggest that differences in food resource levels will be important in explaining the observed geographic variation.

Scientific and budgetary trade-offs between morphological and molecular methods for deep-sea biodiversity assessment.

Deep-sea biodiversity, a source of critical ecological functions and ecosystem services, is increasingly subject to the threat of disturbance from existing practices (e.g., fishing, waste disposal, oil and gas extraction) as well as emerging industries such as deep-seabed mining. Current scientific tools may not be adequate for monitoring and assessing subsequent changes to biodiversity. In this paper, we evaluate the scientific and budgetary trade-offs associated with morphology-based taxonomy and metabarcoding approaches to biodiversity surveys in the context of nascent deep-seabed mining for polymetallic nodules in the Clarion-Clipperton Zone, the area of most intense interest. For the dominant taxa of benthic meiofauna, we discuss the types of information produced by these methods and use cost-effectiveness analysis to compare their abilities to yield biological and ecological data for use in environmental assessment and management. On the basis of our evaluation, morphology-based taxonomy is less cost-effective than metabarcoding but offers scientific advantages, such as the generation of density, biomass, and size structure data. Approaches that combine the two methods during the environmental assessment phase of commercial activities may facilitate future biodiversity monitoring and assessment for deep-seabed mining and for other activities in remote deep-sea habitats, for which taxonomic data and expertise are limited. Integr Environ Assess Manag 2022;18:655-663. © 2021 SETAC.

Population structure and ontogenetic habitat use of Micropogonias furnieri in the Southwestern Atlantic Ocean inferred by otolith chemistry

The whitemouth croaker Micropogonias furnieri is a demersal sciaenid fish widely distributed in Atlantic coasts and estuaries from Mexico to Argentina. In the South Atlantic (33°- 41 °S), there are two stocks: Río de la Plata (RDP) and El Rincón (ER), respectively. Studies based on genetic, age and size structure data suggested that RDP stock would be composed of two different populations, one estuarine-dependent (estuarine type) and the other with oceanic spawning sites (oceanic type). The hypothesis of segregation between the estuarine and oceanic types, and between these and the ER stock was tested by using the otolith chemistry in the edge (adult stage) and core (early stage) of fish. Li/Ca, Na/Ca, Mn/Ca, Cu/Ca, Ba/Ca, and Pb/Ca ratios were determined by LA-ICP-MS. Several ratios were significantly different between sampling sites for edge and core (p < 0.05). For edge, PERMANOVA showed significant differences (p < 0.05) between all sampling sites, while the quadratic discriminant analyses showed a mean jackknifed classification of 83 % (87 % for Uruguay, 76 % for RDP, and 86 % for ER). This supports the hypothesis of differentiation of the RDP stock into oceanic and estuarine-dependent groups. The non-metric multidimensional scaling and cluster analysis based on the core chemistry suggested that there would be at least two large chemical signatures in the early stage. The elemental ratio time series showed a potential endogenous effect on the incorporation of various elements (e.g. Ba, Mn, Li) in the early stages, while Sr/Ca time series suggested a general migratory behavior between estuarine and marine habitats throughout ontogeny.

The relationship between eDNA particle concentration and organism abundance in nature is strengthened by allometric scaling.

Organism abundance is a critical parameter in ecology, but its estimation is often challenging. Approaches utilizing eDNA to indirectly estimate abundance have recently generated substantial interest. However, preliminary correlations observed between eDNA concentration and abundance in nature are typically moderate in strength with significant unexplained variation. Here, we apply a novel approach to integrate allometric scaling coefficients into models of eDNA concentration and organism abundance. We hypothesize that eDNA particle production scales nonlinearly with mass, with scaling coefficients <1. Wild populations often exhibit substantial variation in individual body size distributions; we therefore predict that the distribution of mass across individuals within a population will influence population-level eDNA production rates. To test our hypothesis, we collected standardized body size distribution and mark-recapture abundance data using whole-lake experiments involving nine populations of brook trout. We correlated eDNA concentration with three metrics of abundance: density (individuals/ha), biomass (kg/ha) and allometrically scaled mass (ASM) (∑(individual mass0.73 )/ha). Density and biomass were both significantly positively correlated with eDNA concentration (adj. r2 =0.59 and 0.63, respectively), but ASM exhibited improved model fit (adj. r2 =0.78). We also demonstrate how estimates of ASM derived from eDNA samples in "unknown" systems can be converted to biomass or density estimates with additional size-structure data. Future experiments should empirically validate allometric scaling coefficients for eDNA production, particularly where substantial intraspecific size distribution variation exists. Incorporating allometric scaling may improve predictive models to the extent that eDNA concentration may become a reliable indicator of abundance in nature.

Effect of Sample Duration on Catch Rate and Size Structure Data for Blue Catfish Collected by Low‐Frequency Electrofishing

AbstractAlthough low‐frequency, pulsed‐DC electrofishing is considered the most effective method for sampling Blue Catfish Ictalurus furcatus in reservoirs, efforts to improve sampling efficiency have not been fully explored. Optimizing sampling duration can reduce cost and effort if shorter sample times still produce precision similar to that of longer samples. We compared several catch rate and size structure metrics from the first and second 5‐min intervals of 10‐min low‐frequency electrofishing samples for Blue Catfish in three Oklahoma reservoirs (N = 40 sites total). The total Blue Catfish CPUE (CPUETotal), CPUE of preferred‐length fish (&gt;760 mm; CPUE760), mean TL, proportional size distribution (PSD), and PSD of preferred‐length fish did not significantly differ between sampling intervals. One reservoir had a higher proportion of 200–299‐mm fish in the second 5‐min interval; however, no other differences in length frequency or other size metrics were detected between the first and second 5‐min intervals in any reservoir. Sampling precision met the target level (relative standard error = 20) for CPUETotal but not CPUE760 during both the first 5 min and the full 10‐min sample at all reservoirs. Monte Carlo simulation indicated that 10 samples would be needed to achieve the target precision (relative standard error ≤ 20) for CPUETotal and 38–65 samples would be needed for precise CPUE760 metrics regardless of sample duration. In fisheries with moderate catch rates like those we observed (150–500 fish/h), we recommend short‐duration (i.e., 5‐min) sampling because data of the same quality can be collected with reduced cost and effort. For situations in which sampling precision is low (e.g., CPUE760), we recommend increasing the number of replicate 5‐min samples. Using shorter sample durations provides a method for managers to obtain quality population data more efficiently, allowing time for other worthwhile management activities that would otherwise not be possible.

Ecological analyses to inform management targets for the culling of crown-of-thorns starfish to prevent coral decline

The crown-of-thorns starfish (COTS), Acanthaster cf. solaris, is one of the main contributors to declines in coral cover on the Great Barrier Reef (GBR) and remains one of the major acute disturbances on coral reefs throughout much of the Indo-Pacific. Extensive control programs on the GBR involve manual culling of COTS in the field, and research is needed to inform these management efforts. Data from the Great Barrier Reef Marine Park Authority’s (GBRMPA) COTS control program provide near-real-time CPUE (Catch-Per-Unit-Effort, COTS culled per minute) data ideal for operational decision-making but these must be converted to density estimates before they can be related to ecological status of reefs or incorporated into ecological models. We developed conversions between common COTS field survey methods (i.e. manta tow, SCUBA transect searches) and COTS control program CPUE data using estimates of sightability and detectability. We used a population model and COTS size-structure data from COTS control program culling efforts to estimate that, on average, only 19% of 1-yr-old COTS (1–15 cm) are available to be culled. Finally, we developed a CPUE-COTS density relationship to estimate the threshold levels of COTS that prevent net growth of hard corals. Culling programs should therefore aim to achieve CPUEs below these ecological thresholds in order to effectively promote coral growth and recovery. These ecologically sustainable thresholds of COTS density varied depending on hard coral cover. For example, for 35% fast-growing coral cover, COTS culling needs to continue until CPUE decreases to below 0.05 COTS/min (1 COTS per 20 min) in order to prevent coral decline, whereas if coral cover is higher (80%), then a higher target threshold CPUE of ca. 0.08 COTS/min (ca. 3 COTS per 40 min) may be ecologically sustainable. These estimates underpin the current pest management rules being implemented by the GBRMPA in its COTS control program.

Integrating Multiple Data Types to Connect Ecological Theory and Data Among Levels

Ecological theories often encompass multiple levels of biological organization, such as genes, individuals, populations, and communities. Despite substantial progress toward ecological theory spanning multiple levels, ecological data rarely are connected in this way. This is unfortunate because different types of ecological data often emerge from the same underlying processes and, therefore, are naturally connected among levels. Here, we describe an approach to integrate data collected at multiple levels (e.g., individuals, populations) in a single statistical analysis. The resulting integrated models make full use of existing data and might strengthen links between statistical ecology and ecological models and theories that span multiple levels of organization. Integrated models are increasingly feasible due to recent advances in computational statistics, which allow fast calculations of multiple likelihoods that depend on complex mechanistic models. We discuss recently developed integrated models and outline a simple application using data on freshwater fishes in south-eastern Australia. Available data on freshwater fishes include population survey data, mark-recapture data, and individual growth trajectories. We use these data to estimate age-specific survival and reproduction from size-structured data, accounting for imperfect detection of individuals. Given that such parameter estimates would be infeasible without an integrated model, we argue that integrated models will strengthen ecological theory by connecting theoretical and mathematical models directly to empirical data. Although integrated models remain conceptually and computationally challenging, integrating ecological data among levels is likely to be an important step toward unifying ecology among levels.

Detecting mortality variation to enhance forage fish population assessments

Abstract Contemporary stock assessment models used by fisheries management often assume that natural mortality rates are constant over time for exploited fish stocks. This assumption results in biased estimates of fishing mortality and reference points when mortality changes over time. However, it is difficult to distinguish changes in natural mortality from changes in fishing mortality, selectivity, and recruitment. Because changes in size structure can be indicate changes in mortality, one potential solution is to use population size-structure and fisheries catch data to simultaneously estimate time-varying natural and fishing mortality. Here we test that hypothesis, using a simulation experiment to test performance for four alternative estimation models that estimate natural and fishing mortality from size structure and catch data. We show that it is possible to estimate time-varying natural mortality in a size-based model, even when fishing mortality, recruitment, and selectivity are changing over time. Finally, we apply the model to North Sea sprat, and show that estimates of recruitment and natural mortality are similar to estimates from an alternative multispecies population model fitted to additional data sources. We recommend exploring potential trends in natural mortality in forage fish assessments using tools such as the one presented here.

Effects of unequal capture probability on stock assessment abundance and mortality estimates: an example using the US Atlantic sea scallop fishery

Most stock assessment models assume that the probability of capture for all individuals of the same size or age in the stock area is equal. However, this assumption is rarely, if ever, satisfied. We used spatially referenced simulations, based on the US Atlantic sea scallop (Placopecten magellanicus) fishery, to generate catch, survey index, fishing effort, and size structure data that we input into a (nonspatial) catch-at-size stock assessment model to estimate abundance and mortality rates. We show that spatial patterns in fishing mortality degrade model performance for sessile stocks. Fishing mortality tended to be overestimated and abundance underestimated because trends in fishing mortality were exaggerated and the model misestimated the numbers of larger individuals due to spatial fishing patterns. These results are particularly relevant to sedentary species such as scallops, but are applicable wherever strong spatial patterns exist in fishing mortality.

Effectiveness of Continuous versus Point Electrofishing for Fish Assemblage Assessment in Shallow, Turbid Aquatic Habitats

AbstractWe evaluated the relative effectiveness of continuous versus point electrofishing methods for describing fish assemblage metrics in the lower Atchafalaya River Floodway system, Louisiana. Continuous data were collected by boat electrofishing opposing shores of a 100‐m reach for a total of 480 s. Point data were collected in an adjacent 100‐m reach by boat electrofishing for 60 s at four points (240 s total) at 25‐m intervals on alternating shorelines. We tested for significant differences between the methods with a linear model for species richness and CPUE (fish/min) and a generalized linear model for species diversity. We also compared the length frequencies of Largemouth Bass Micropterus salmoides between methods. We found no difference in diversity between methods but significantly higher species richness (mean = 1.46 species) and significantly lower CPUE (mean = −24.63 fish/min) for continuous electrofishing. We found no difference in Largemouth Bass length frequency between methods, but point electrofishing captured more individuals between 300 and 400 mm. Our results indicate that point electrofishing may be an efficient, effective technique for capturing target species and larger individuals that may otherwise avoid capture. When coupled with continuous electrofishing, point electrofishing may bolster assemblage and population size structure data, particularly in shallow, turbid environments.Received July 9, 2014; accepted December 8, 2015 Published online March 31, 2016

Population dynamics of endangered blanding's turtles in a restored area

Habitat protection and restoration are used to promote recovery in endangered reptile species, yet the long-term consequences of these efforts can be difficult to assess. Measuring the effects of habitat restoration on turtles can be especially difficult because of long generation times and consequent time lags between restoration activities and population-level responses to these activities. We collected 24 years of mark-recapture, size structure, and nest survival data on a population of International Union for Conservation of Nature (IUCN)-Endangered Blanding's turtles (Emydoidea blandingii) inhabiting a large restored wetland-upland complex in Wisconsin, USA. We used these data to examine survival and recruitment in the population and the response of individual aggregations of nesting females to recent restoration of an extensive upland prairie-oak barrens nesting habitat. The population exhibited positive growth over the 24-year period (λ = 1.03) based on temporal asymmetry mark-recapture models. Population growth rates were greatest in a recently restored prairie-barrens nesting area because of relatively high recruitment of nesting females. Females nesting in the recently restored prairie-barrens habitat were smaller and decreased in size over time compared to females using 2 older nesting areas, suggesting that recruitment was relatively high because of an influx of young breeding females. Nest depredation probability decreased with increasing distance from forest edges, suggesting that the recent clearing of woody vegetation in recently restored habitat increased reproductive success. Collectively, our results highlight the potential benefits of habitat restoration to turtles and identify upland habitats as a key to promoting population growth in turtles with complex habitat requirements. © 2015 The Wildlife Society.

Marine reserve effects on population density and size structure of commonly and rarely exploited limpets in South Africa

Marine reserves are commonly used to conserve living resources, but their effectiveness where policing is difficult is unclear. We compared monthly population density and size structure data collected over 20 months for two rarely and two commonly exploited intertidal limpets inside and outside reserves in South Africa. Densities were greater inside reserves, but significantly so in very few months. Unexpectedly, no significant reserve effect was detected for the territorial Scutellastra longicosta, shown by surveys to be favoured by harvesters. Reserve effects for the rarely exploited S. granularis probably result from indirect effects of higher barnacle cover in reserves where trampling is reduced, while interviews indicated that Cellana capensis is not targeted but large individuals are sometimes misidentified and hence are harvested outside the reserve. There were few reserve effects on mean or mean maximum size. The results indicate a gradient of exploitation among species and sites. One non-reserve site was more heavily exploited than the other, while one reserve experienced more poaching than the other. The effectiveness of marine reserves thus differed between reserves and among species. Exploited limpets were generally larger and at higher densities in reserves, but these effects were rarely statistically significant, indicating that reserve effects are weak compared to natural variability, probably reflecting ineffective policing.

Do symbiotic polychaetes migrate from host to host?

Britayev, T.A. and Mekhova, E.S. 2014. Do symbiotic polychaetes migrate from host to host? Memoirs of Museum Victoria 71: 21–25. It is generally considered that symbiotic animals colonise their hosts during their early stages of development. The main goal of the present study was to assess whether post-settled stages (juvenile and adult) of the symbiotic polychaete Paradyte crinoidicola are able to colonise their host comatulid crinoids. We also considered possible motives for symbiont migrations based on the intraspecific traumatism, size and sex structure data, and distribution pattern of P. crinoidicola. To this end, field sampling and experiments with depopulated hosts of the comatulid crinoid Himerometra robustipinna were carried out. The infestation prevalence was 62%, each infested host harbored from 1 to 7 polychaetes, and multiple infestations with 2 or 3 polychaetes per host were common. Mean intensity was 2.1 specimens per host. The dispersion coefficient was 1.7, greater than 1, indicating the tendency to contagious distribution pattern. Male/female ratio in P. crinoidicola was very close to the expected 1:1 ratio. About 33 % of P. crinoidicola had a traumatised posterior ends, and 31% damaged and regenerated parapodia, elytra and cirri, likely attributable to intra-specific fighting. In the field experiments depopulated crinoids were rapidly colonised by symbionts. The infestation characteristics of recolonised hosts didn’t differ significantly to that of the control. Mean length of polychaetes and the ratio of small polychaetes to large polychaetes were similar in the experimental series and in the control, indicating a colonisation of crinoids not only by settling larvae, but predominately by migrating post-settled juveniles and adults. The male/female ratio deviated significantly in favor of males in the experimental series, suggesting that males more than females actively migrate among hosts. Intraspecific competition and searching for mating partners are proposed as causes for host swapping in P. crinoidicola.

Effect of a size-selective biomanipulation on nutrient release by gizzard shad in Florida (USA) lakes

Key-words: Dorosoma cepedianum, biomanipulation, size-structure, nutrient excretion Although fish removal for biomanipulation is often highly size-selective, our understanding of the nutrient cycling effects of this size-selection is poor. To better understand these effects, we measured nutrient excretion by gizzard shad (Dorosoma cepedianum )o f differing sizes from four central Florida (USA) lakes and combined these measures with gillnet biomass and size-structure data to compare lake-wide effects among lakes and years. Direct removal of P in fish tissue ranged from 0.16−1.00 kg·P·ha −1 ·yr −1 . The estimated reduction in P excretion due to harvest ranged from 30.8−202.5 g·P·ha −1 ·month −1 , with effects strongly tied to the biomass and size structure harvested. The amount of P release prevented per kg of fish removed was lower in previously unharvested lakes, due to the initial removal of larger fish with lower mass-specific excretion rates. Gill net mesh size impacted the size distribution of harvested fish, with smaller fish that excrete more P per gram being more vulnerable to smaller mesh sizes. In Lake Apopka, decreasing the mesh size by 1.3 cm yielded P excretion reductions that were 10.7−15.1% larger. Fish harvesting to reduce internal nutrient cycling can be most effective by increasing total harvest and by harvesting smaller size classes over multiple years.

Spatial variability of disease incidence and mortality in the sea fan &lt;i&gt;Gorgonia ventalina&lt;/i&gt; in Puerto Rico (Alcyonacea: Goorgoniidae)

Populations of the common sea fan (Gorgonia ventalina) were decimated by an aspergillosis outbreak throughout the Caribbean two decades ago. Since then, aspergillosis has been considered as the principal cause of mortality in sea fans. However, prevalence and presumably incidence of this disease have been declining in the Caribbean since the mid 1990s. Incidence indicates new cases of disease in previously healthy colonies, while prevalence indicates percent of diseased colonies at a given sample. Most coral disease studies use prevalence rather than incidence to assess the temporal dynamics of diseases. Nevertheless, conclusions based only on prevalence should be handled carefully to avoid misinterpretation. This study was carried out at six reefs in Eastern Puerto Rico. We monitored a total of 448 colonies to (1) obtain estimates of incidence and prevalence of disease and other types of lesions, and (2) to determine causes of sea fan mortality plus their spatial and temporal variation. Three transects (10x1m) were haphazardly placed at each study site. At each transect, every colony was numbered and photographed and its height measured to the nearest cm. Transects were monitored at six months intervals and health status of the colonies was recorded. Also, the colonies were divided into height classes (small, medium and large) for incidence, prevalence and mortality analyses. Incidence and prevalence of disease were low in all reefs, suggesting that disease currently plays a minor role in the regulation of sea fans populations. Detachment was the main cause of mortality, and size structure data suggest that recruitment may compensate for mortality rates in two of the reefs. Spatial differences in size structure and density may be related to environmental and physical characteristics at the reef scale that allow sea fans to reach a safe colony size.

Capture Efficiency and Size Selectivity of Channel Catfish and Blue Catfish Sampling Gears

AbstractWe used proportional recapture data from mark–recapture experiments to directly estimate capture efficiency (percentage of fish captured per standard level of effort) and size selectivity (for 50‐mm size‐groups) of catfish sampling gears. Hoop‐net series were evaluated for collecting channel catfish Ictalurus punctatus, and low‐frequency, pulsed DC electrofishing was evaluated for collecting blue catfish I. furcatus. To examine spatial and temporal variability, capture efficiency and size selectivity were estimated in river and reservoir habitats during June, July, and September 2005. Selectivity of experimental gill nets was also assessed. Capture efficiency of hoop nets for channel catfish was consistent across months and between river and reservoir habitats (2.7–14.3% per 12 hoop‐net series). Capture efficiency of low‐frequency electrofishing was low (frequently, ≤1% per 120 min) and variable; recapture rates were greatest in river habitat during July and September. Hoop‐net series collected representative samples of channel catfish between 250 and 556 mm total length. Likewise, blue catfish between 250 and 855 mm were fully vulnerable to low‐frequency electrofishing. Although fish smaller than 250 mm were captured, they were frequently underrepresented in the catch of both gears. Size selectivity was not affected by habitat or month sampled for either gear type or either species. Capture efficiency of experimental gill nets was very low for both channel catfish and blue catfish (about 0.2% in 46 gill‐net nights), and gill nets were more size selective than other gears. Based on our findings, hoop‐net series and low‐frequency electrofishing provide accurate size structure data for adult channel catfish and blue catfish, respectively, from both river and reservoir habitat types. Hoop‐net series also provide consistent estimates of channel catfish relative abundance; however, relative abundance data for blue catfish collected with low‐frequency electrofishing should be used with caution due to observed variability in capture efficiency.