Daniel C. Gwinn

Exploring Population-Level Effects of Fishery Closures during Spawning: An Example Using Largemouth Bass

Abstract We used an age-structured model to evaluate the impacts of recreational angling during spawning on populations of largemouth bass Micropterus salmoides and the potential benefits of seasonal fishing closures. We simulated fisheries with average and high capture rates (i.e., fractions of the stock caught by anglers). We manipulated mortality rates to mimic the effects of various regulations, including (1) an open fishery with no closures, (2) a full fishery closure during the spawning season, (3) a catch-and-release fishery during the spawning season, and (4) a year-round catch-and-release fishery. We simulated two hypothetical populations: a high-productivity, low-latitude population and a low-productivity, high-latitude population. Spawning season closures produced the largest relative increases in adult abundance when applied to low-productivity populations and when capture rates were high (e.g., 70%) and harvest rates were relatively low (20%). This resulted because very high capture rates imp...

Spatial and temporal hypoxia dynamics in dense emergent macrophytes in a Florida Lake.

Many aquatic systems worldwide have experienced significant changes in littoral macrophyte communities from altered hydrology in the form of water controls structures (e.g., dams). Water level stabilization for flood control can cause persistent occurrence of dense stands of emergent macrophytes, which can affect the physicochemical environment for fishes. We evaluated dissolved oxygen (DO) concentrations in five emergent macrophyte species (cattail Typha spp., pickerelweed Pontedaria cordata, smartweed Polygonum spp., torpedograss Panicum repens, and water primrose Ludwigia spp.) at three levels of macrophyte coverage (i.e., 50–64, 65–79, and 80–95%) at Lake Istokpoga, Florida during July–August (summer) and October–November (fall) 2007. Dissolved oxygen exhibited substantial spatial and temporal variability at small scales (i.e., meters and hours), with the lowest DO and highest probability of hypoxia occurring in smartweed and water primrose habitats relative to other macrophyte types. The probability of hypoxia increased with macrophyte coverage for all macrophyte species tested. Dissolved oxygen was influenced by the structural differences (e.g., stem density and size) and spatial orientation (e.g., proximity to open water) associated with the individual macrophyte habitats. Restoration efforts that create open water pathways and maximize edge areas might improve DO concentrations and habitat quality and quantity for freshwater fishes.

Using abiotic drivers of fish spawning to inform environmental flow management

Summary 1. Environmental flows are a key restoration technique for conserving ecological function in flow-degraded rivers. Species-specific, flow–biota relationships are increasingly being used to determine environmental flow needs and manage their use; however, many of these relationships are poorly described. 2. We evaluate relationships between environmental variables and spawning intensity for a fish assemblage from the Murray River, Australia, over a ten-year period. We developed a hierarchical multispecies model that accounted for incomplete detection to compare spawning outcomes of native and non-native species using realistic, alternative, water management scenarios. 3. Temperature was an important predictor of spawning intensity for all seven species studied, while both concurrent and antecedent flow conditions were important for many species. Our water management scenario testing accounted for these relationships and indicated that increasing the magnitude of smaller floods following lower antecedent flow conditions, at water temperatures of 18–20C, achieves the greatest spawning outcome for native fish. 4. Synthesis and applications. Our results indicate that principally temperature, and flow as a secondary variable, influence the timing and strength of fish spawning. The synthesis of these spawning relationships predicts that managers will achieve the greatest spawning return per unit of environmental water when flows are applied on top of an existing flow pulse. This study highlights the importance of considering a range of abiotic factors and the use of modelling scenarios to improve environmental flow outcomes.

Evaluation of Age–Length Key Sample Sizes Required to Estimate Fish Total Mortality and Growth

Abstract Fisheries assessments often rely on age composition data to infer information about growth, mortality, and the outcome of various management policies. To reduce the costs of estimating the age of all fish collected (i.e., via direct aging), an age–length key (ALK) is often developed by directly aging only a subsample of fish and estimating the ages of the remaining fish based on observed lengths. Although this approach is common, there is little guidance in the literature for determining the appropriate total sample size and numbers of fish needed to directly age for each length bin. We developed a stochastic simulation to evaluate the sample-size requirement for using ALKs to estimate von Bertalanffy growth parameters and the instantaneous rate of total mortality (Z). The simulations accounted for variation in life history characteristics of stocks and exploitation rates of fisheries. Our simulations suggested that for overfished populations, aging more fish per bin provides negligible benefits ...

Occupancy Models for Monitoring Marine Fish: A Bayesian Hierarchical Approach to Model Imperfect Detection with a Novel Gear Combination

Occupancy models using incidence data collected repeatedly at sites across the range of a population are increasingly employed to infer patterns and processes influencing population distribution and dynamics. While such work is common in terrestrial systems, fewer examples exist in marine applications. This disparity likely exists because the replicate samples required by these models to account for imperfect detection are often impractical to obtain when surveying aquatic organisms, particularly fishes. We employ simultaneous sampling using fish traps and novel underwater camera observations to generate the requisite replicate samples for occupancy models of red snapper, a reef fish species. Since the replicate samples are collected simultaneously by multiple sampling devices, many typical problems encountered when obtaining replicate observations are avoided. Our results suggest that augmenting traditional fish trap sampling with camera observations not only doubled the probability of detecting red snapper in reef habitats off the Southeast coast of the United States, but supplied the necessary observations to infer factors influencing population distribution and abundance while accounting for imperfect detection. We found that detection probabilities tended to be higher for camera traps than traditional fish traps. Furthermore, camera trap detections were influenced by the current direction and turbidity of the water, indicating that collecting data on these variables is important for future monitoring. These models indicate that the distribution and abundance of this species is more heavily influenced by latitude and depth than by micro-scale reef characteristics lending credence to previous characterizations of red snapper as a reef habitat generalist. This study demonstrates the utility of simultaneous sampling devices, including camera traps, in aquatic environments to inform occupancy models and account for imperfect detection when describing factors influencing fish population distribution and dynamics.

Evaluation of Electrofishing Catch per Unit Effort for Indexing Fish Abundance in Florida Lakes

Abstract Electrofishing CPUE data are commonly used to index temporal trends in abundance in fish monitoring programs, but the reliability of this index requires the assumption that the fraction of fish stock caught per unit effort (catchability, q) is relatively precise and constant through time. We evaluated how fish species, season, and lake affected electrofishing catchability in Florida lakes using a field study. We used the field study results to simulate how variable electrofishing q affects statistical power and type I error rate (i.e., the probability of detecting a difference when in fact no difference occurred). Model selection showed that electrofishing catchability varied by species, season, and lake, and submodels showed that catchability varied by lake for Largemouth Bass Micropterus salmoides and by season for Lake Chubsucker Erimyzon sucetta, and was constant across season and lake for Bluegill Lepomis macrochirus. Our results revealed that statistical power decreased and the type I error...

Evaluating mark–recapture sampling designs for fish in an open riverine system

Sampling designs for effective monitoring programs are often specific to individual systems and management needs. Failure to carefully evaluate sampling designs of monitoring programs can lead to data that are ineffective for informing management objectives. We demonstrated the use of an individual-based model to evaluate closed-population mark–recapture sampling designs for monitoring fish abundance in open systems, using Murray cod (Maccullochella peelii (Mitchell, 1838)) in the Murray–Darling River basin, Australia, as an example. The model used home-range, capture-probability and abundance estimates to evaluate the influence of the size of the sampling area and the number of sampling events on bias and precision of mark–recapture abundance estimates. Simulation results indicated a trade-off between the number of sampling events and the size of the sampling reach such that investigators could employ large sampling areas with relatively few sampling events, or smaller sampling areas with more sampling events to produce acceptably accurate and precise abundance estimates. The current paper presents a framework for evaluating parameter bias resulting from migration when applying closed-population mark–recapture models to open populations and demonstrates the use of simulation approaches for informing efficient and effective monitoring-program design.

Evaluating estimators of species richness: the importance of considering statistical error rates

Summary The performance of species richness estimators can be highly variable. Evaluating the accuracy and precision of different estimators for different assemblages is common in the ecological literature, but estimator performance is rarely measured in terms of research goals such as detecting patterns in diversity. We evaluated the efficacy of nonparametric richness estimators to detect changes (i.e. type-I and type-II error rates) in species richness using two experimental designs: a block design and a trend analysis. We also evaluated estimator efficacy across a variety of species-abundance distributions, species number and sample sizes. The evaluation was performed using simulated data that mimicked the qualities of real data to ensure real-world relevance. We found that the bias and precision of all estimators evaluated had high sensitivity to sample size and shifts in the species-abundance distribution. Importantly, all estimators demonstrated elevated type-I error rates when the species-abundance distribution varied. These inflated type-I error rates resulted in spurious conclusions about patterns in species richness. Results suggest that caution should be taken when using nonparametric estimators to detect pattern in species richness. Furthermore, estimator evaluations should always include measures of type-I and type-II error rates. These quantities can reveal the inference consequences of the dependency of estimator bias and precision on community and sampling characteristics.

Assessing the Potential for the Angling of Nesting Males to Influence Largemouth Bass Recruitment Should Consider the Quality of Nesting Individuals: Response to Comment

Assessing the Potential for the Angling of Nesting Males to Influence Largemouth Bass Recruitment Should Consider the Quality of Nesting Individuals: Response to Comment Michael S. Allen a , Mark W. Rogers a , Matthew J. Catalano a , Daniel C. Gwinn a & Stephen J. Walsh b a School of Forest Resources and Conservation, University of Florida, 7922 North West 71st Street, Gainesville, Florida, 32653, USA b U.S. Geological Survey, Southeast Ecological Science Center, 7920 North West 71st Street, Gainesville, Florida, 32653, USA Published online: 01 Feb 2013.

Use of Recovery Probabilities Can Improve Sampling Efficiency for Throw Traps in Vegetated Habitats

Abstract Throw traps are commonly used to assess small fish and macroinvertebrate (SFI) communities in aquatic habitats. Traditionally, numerous passes with a bar seine are used to remove all SFIs from within the throw trap, providing density estimates that are comparable across habitat types. However, similar information can be gathered with fewer seine passes than are traditionally used, thus allowing more sampling in a given field effort. The appropriate number of seine passes to meet study objectives can be determined if habitat-specific recovery probabilities are estimated. We investigated how SFI recovery probabilities within throw traps varied among five submerged aquatic vegetation habitat types and how these recovery probabilities can be used to determine the number of seine passes necessary to assess and detect differences in SFI relative abundance. Using a 1-m2 throw trap and bar seine, we found that estimates of SFI recovery probability were high (75–80%) and similar across habitats. Because o...