Kyle W. Shertzer

When can we reliably estimate the productivity of fish stocks

In modern fishery stock assessments, the productivity of exploited stocks is frequently summarized by a scale-invariant “steepness” parameter. This parameter, which describes the slope of the spawner–recruit curve, determines resilience of a stock to exploitation and is highly influential when estimating maximum sustainable yield. In this study, we examined conditions under which steepness can be estimated reliably. We applied a statistical catch-age model to data that were simulated over a broad range of stock characteristics and exploitation patterns and found that steepness is often estimated at its upper bound regardless of underlying productivity. The ability to estimate steepness reliably was most dependent on the true value of steepness, the exploitation history of the stock, natural mortality, duration of the time series, and quality of an index of abundance; this ability was relatively unaffected by levels of stochasticity in recruitment and sampling intensity of age compositions. We further expl...

Targets and Limits for Management of Fisheries: A Simple Probability-Based Approach

Abstract Precautionary fishery management requires that a distinction be made between target and limit reference points. We present a simple probability framework for deriving a target reference point for the fishing mortality rate (F) or biomass (B) from the corresponding limit reference point. Our framework is a generalization of one devised previously by Caddy and McGarvey (1996). Both methods require an a priori management decision on the allowable probability of exceeding the limit reference point; our method removes a major assumption by accounting for the uncertainty in the limit reference point. We present the theory underlying the method, an algorithm for solution, and examples of its application. The new procedure, like the old, requires an estimate of the implementation uncertainty expected in the following years management, an estimate that might be obtained by a review of the effectiveness of past management actions. Either method can be implemented easily on a modern desktop computer. Our g...

Integrated Population Modeling of Black Bears in Minnesota: Implications for Monitoring and Management

Background Wildlife populations are difficult to monitor directly because of costs and logistical challenges associated with collecting informative abundance data from live animals. By contrast, data on harvested individuals (e.g., age and sex) are often readily available. Increasingly, integrated population models are used for natural resource management because they synthesize various relevant data into a single analysis. Methodology/Principal Findings We investigated the performance of integrated population models applied to black bears (Ursus americanus) in Minnesota, USA. Models were constructed using sex-specific age-at-harvest matrices (1980–2008), data on hunting effort and natural food supplies (which affects hunting success), and statewide mark–recapture estimates of abundance (1991, 1997, 2002). We compared this approach to Downing reconstruction, a commonly used population monitoring method that utilizes only age-at-harvest data. We first conducted a large-scale simulation study, in which our integrated models provided more accurate estimates of population trends than did Downing reconstruction. Estimates of trends were robust to various forms of model misspecification, including incorrectly specified cub and yearling survival parameters, age-related reporting biases in harvest data, and unmodeled temporal variability in survival and harvest rates. When applied to actual data on Minnesota black bears, the model predicted that harvest rates were negatively correlated with food availability and positively correlated with hunting effort, consistent with independent telemetry data. With no direct data on fertility, the model also correctly predicted 2-point cycles in cub production. Model-derived estimates of abundance for the most recent years provided a reasonable match to an empirical population estimate obtained after modeling efforts were completed. Conclusions/Significance Integrated population modeling provided a reasonable framework for synthesizing age-at-harvest data, periodic large-scale abundance estimates, and measured covariates thought to affect harvest rates of black bears in Minnesota. Collection and analysis of these data appear to form the basis of a robust and viable population monitoring program.

State-dependent energy allocation in variable environments: Life history evolution of a rotifer

We present a framework for studying the evolution of state-dependent life history strategies in unpredictable environments. A dynamic energy budget model tracks the flow of energy through individuals and is parameterized using the rotifer species Brachionus calyciflorous. Ingested energy is allocated between new growth, energy reserves, and reproductive effort, in proportions determined by the organisms physiological state, leading to a three-way trade-off when food supplies are unpredictable. The model generates testable predictions regarding how variability in food affects optimal allocation strategies. There is selection for indeterminate growth and for norms of reaction in body size, storage, and age at maturity. Harsher environments select for smaller sizes and increased storage and, consequently, greater ability to withstand starvation. The models predictions about body size, age at maturity, and effects of food deprivation are consistent with experimental results for B. calyciflorous, indicating that these life history traits have all been shaped by evolution of energy allocation and storage strategies in response to environmental variability.

Ecological and Evolutionary Dynamics of Experimental Plankton Communities

Publisher Summary This chapter describes an attempt to reconcile the dynamics of a simple laboratory community consisting of algae and rotifers with the predictions of an even simpler mechanistic model. Initial goals were to establish a predator–prey system in laboratory culture and to develop a tractable model that would adequately describe the range of dynamic behaviors. The chapter presents the initial model formulation and the minor adjustments needed to obtain qualitative agreement with the dynamic patterns revealed by the experimental data. It introduces theoretical evidence that the cyclical algal-rotifer dynamics observed could not result from classical predator–prey interaction alone but that some additional biology is needed to explain the observed population oscillations. Several competing hypotheses about the missing biological mechanism have been mathematically tested and evolution of the algae as the most likely explanation has been identified. The chapter describes a successful experimental test of this hypothesis showing that evolution does indeed play the role suggested by the models. Evidence that rapid evolution of rotifers can influence the population dynamics has been provides.

Relationships between Larval and Juvenile Abundance of Winter-Spawned Fishes in North Carolina, USA

Abstract We analyzed the relationships between the larval and juvenile abundances of selected estuarine-dependent fishes that spawn during the winter in continental shelf waters of the U.S. Atlantic coast. Six species were included in the analysis based on their ecological and economic importance and relative abundance in available surveys: spot Leiostomus xanthurus, pinfish Lagodon rhomboides, southern flounder Paralichthys lethostigma, summer flounder Paralichthys dentatus, Atlantic croaker Micropogonias undulatus, and Atlantic menhaden Brevoortia tyrannus. Cross-correlation analysis was used to examine the relationships between the larval and juvenile abundances within species. Tests of synchrony across species were used to find similarities in recruitment dynamics for species with similar winter shelf-spawning life-history strategies. Positive correlations were found between the larval and juvenile abundances for three of the six selected species (spot, pinfish, and southern flounder). These three species have similar geographic ranges that primarily lie south of Cape Hatteras. There were no significant correlations between the larval and juvenile abundances for the other three species (summer flounder, Atlantic croaker, and Atlantic menhaden); we suggest several factors that could account for the lack of a relationship. Synchrony was found among the three southern species within both the larval and juvenile abundance time series. These results provide support for using larval ingress measures as indices of abundance for these and other species with similar geographic ranges and winter shelf-spawning life-history strategies.

An Introduction to Statistical Algorithms Useful in Stock Composition Analysis

Publisher Summary Stock composition analysis is specific to fishery science. In many fisheries, catches include fish that are conspecific but that originate in several spawning stocks. Because the population effects of fishing—and thus the choice of suitable management approaches—depend on which stock or stocks are harvested, estimates of stock composition of catches are needed. Data used for such analyses are observations on characteristics of individual specimens; typical characteristics may include morphometrics, meristics, genetic characters, or chemical signatures.. Analyzing characteristics of individual objects in a mixture to estimate the mixtures proportions is a general statistical problem known as finite mixture analysis. Constituent fish stocks in a mixed harvest are just one example of constituent classes or statistical populations of mixed objects; here, the term class is used when describing algorithms generally, and stock when describing fisheries applications. Estimating stock composition is therefore a special case of the general problem of estimating mixing proportions. This chapter introduces some algorithms useful for stock composition analysis. It also discusses issues involved in estimating performance of various algorithms, either in an absolute sense or relative to one another on a particular data set.

Effect of Changes in Dissolved Oxygen Concentrations on the Spatial Dynamics of the Gulf Menhaden Fishery in the Northern Gulf of Mexico

Abstract Declines in dissolved oxygen (DO) concentrations in aquatic environments can lead to conditions of hypoxia (DO ≤ 2 mg/L), which can directly and indirectly affect aquatic organisms. Direct effects include changes in growth and mortality; indirect effects include changes in distribution, movement, and interactions with other species. For mobile species, such as the pelagic filter-feeding Gulf Menhaden Brevoortia patronus, indirect effects are more prevalent than direct effects. The northern Gulf of Mexico experiences one of the largest areas of seasonal hypoxia in the world; this area overlaps spatially and temporally with the Gulf Menhaden commercial purse-seine fishery, which is among the largest fisheries by weight in the United States. Harvest records from the Gulf Menhaden fishery in 2006–2009 and fine-scale spatial and temporal predictions from a physical—biogeochemical model were used with spatially varying regression models to examine the effects of bottom DO concentration, spatial location, depth, week, and year on four response variables: probability of fishing, total Gulf Menhaden catch, total fishery effort, and CPUE. We found nearshore shifts in the probability of fishing as DO concentration declined, and we detected a general westward shift in all response variables. We also found increases in CPUE as DO concentration declined in the Louisiana Bight, an area that experiences chronic, severe hypoxia. The overall effects of environmental conditions on fishing response variables appeared to be moderate. Nevertheless, movement of either Gulf Menhaden or the purse-seine fishery in response to environmental conditions could potentially affect the susceptibility of Gulf Menhaden to harvest and could therefore influence assessment of the stock and associated stock status indicators.

Relationship between Gulf Menhaden Recruitment and Mississippi River Flow: Model Development and Potential Application for Management

Abstract The Gulf menhaden Brevoortia patronus is one of the most abundant pelagic fishes in the northern coastal Gulf of Mexico (hereafter, “Gulf”) and is the principal forage for various commercial and sport fishes, sea birds, and marine mammals. Part of the life history of Gulf menhaden is spent on the continental shelf and part is spent within estuaries. Adults spawn near the mouth of the Mississippi River, and larvae aggregate within the river plume front. Larval Gulf menhaden transit the continental shelf and enter estuaries of the northern Gulf as juveniles. Govoni (1997) demonstrated an association between the discharge of the Mississippi and Atchafalaya rivers and Gulf menhaden recruitment. In particular, he found an inverse association between Mississippi River discharge and estimated recruitment of half-year-old fish based on recruitment data from Vaughan et al. (1996). Vaughan et al. (2000) updated this relationship with a regression analysis. Here, we revisit the relationship with additional years of data through 2004. The inverse relationship continues to hold. In addition, we reframed this relationship to produce a 1-year-ahead prediction model for forecasting recruitment to age 1 from Mississippi River discharge; this model can be used in proactive fishery management. Finally, we revisited the stock assessment model of Vaughan et al. (2007) and demonstrated an improvement in model performance when information on annual river discharge was incorporated.

Probabilistic Approaches to Setting Acceptable Biological Catch and Annual Catch Targets for Multiple Years: Reconciling Methodology with National Standards Guidelines

Abstract In U.S. federal fishery management, acceptable biological catch (ABC) is set below (or equal to) the overfishing limit to account for scientific uncertainty, and annual catch targets (ACTs) are set below (or equal to) the ABC to account for implementation uncertainty (i.e., imperfect management control). In previous papers, we discussed probabilistic approaches to setting target and limit reference points for fishery management. Here, we explain how those approaches can be adapted to provide ABCs and ACTs over multiple years and otherwise made consist with recent revisions to the National Standards Guidelines, a part of the U.S. Code of Federal Regulations that describes implementation of the Magnuson–Stevens Reauthorization Act. Although described in terms of U.S. fishery management, our methods are sufficiently general for use by researchers in U.S. state agencies or elsewhere in the world. We demonstrate them via an example application to vermilion snapper Rhomboplites aurorubens in U.S. Atlantic waters.