Erik H. Williams

Forecasting the dynamics of a coastal fishery species using a coupled climate?population model

Marine fisheries management strives to maintain sustainable populations while allowing exploitation. However, well-intentioned management plans may not meet this balance as most do not include the effect of climate change. Ocean temperatures are expected to increase through the 21st century, which will have far-reaching and complex impacts on marine fisheries. To begin to quantify these impacts for one coastal fishery along the east coast of the United States, we develop a coupled climate-population model for Atlantic croaker (Micropogonias undulatus). The model is based on a mechanistic hypothesis: recruitment is determined by temperature-driven, overwinter mortality of juveniles in their estuarine habitats. Temperature forecasts were obtained from 14 general circulation models simulating three CO2 emission scenarios. An ensemble-based approach was used in which a multimodel average was calculated for a given CO2 emission scenario to forecast the response of the population. The coupled model indicates that both exploitation and climate change significantly affect abundance and distribution of Atlantic croaker. At current levels of fishing, the average (2010-2100) spawning biomass of the population is forecast to increase by 60-100%. Similarly, the center of the population is forecast to shift 50 100 km northward. A yield analysis, which is used to calculate benchmarks for fishery management, indicates that the maximum sustainable yield will increase by 30 100%. Our results demonstrate that climate effects on fisheries must be identified, understood, and incorporated into the scientific advice provided to managers if sustainable exploitation is to be achieved in a changing climate.

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...

Comparison of Reef Fish Catch per Unit Effort and Total Mortality between the 1970s and 2005-2006 in Onslow Bay, North Carolina

Abstract Stock assessments indicate many reef fish species have declined in size and abundance in the Atlantic Ocean off the southeastern coast of the United States. However, commercial fishers often state that stock assessments do not match their observations. We compared fishery-independent catch per unit effort (CPUE) and species composition data between the 1970s and 2005-2006 for reef fishes in the vicinity of Onslow Bay, North Carolina. Additionally, total mortality (Z) was estimated by means of a length-based catch-curve analysis. Effort (drops) by rod and reel focused on three sites, two inshore (30 m deep) and one offshore (125 m). The CPUE was compared between periods within each site and larger area (inshore, offshore). The CPUEs of red porgy Pagrus pagrus, vermilion snapper Rhomboplites aurorubens, black sea bass Centropristis striata, and gray triggerfish Balistes capriscus were greater in the 1970s than in 2005-2006 at specific capture sites. Conversely, the CPUEs of red grouper Epinephelus ...

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.