Todd Gedamke

Using Demographic Models to Determine Intrinsic Rate of Increase and Sustainable Fishing for Elasmobranchs: Pitfalls, Advances, and Applications

Abstract Leslie matrices and life tables are demographic models commonly used to evaluate the ability of specific elasmobranch life history strategies to sustain given levels and patterns of fishing pressure. These models are generally density independent and provide an instantaneous rate of population growth for a specified set of life history traits that correspond to a specific population size. Many investigators are using these models to compute rates of population growth that they claim are estimates of the maximum population growth rate (r intrinsic); they then use these estimates to compute purported estimates of maximum sustainable fishing mortality. However, neither a Leslie matrix nor a life table can be used to estimate r intrinsic without additional information, except in the special case where a severely depleted population is modeled. Only in a severely depleted population will competition for resources be at a minimum and both density-dependent compensation and the rate of population growth...

Estimating mortality from mean length data in nonequilibrium situations, with application to the assessment of goosefish

Abstract The Beverton–Holt length-based mortality estimator has received widespread use primarily due to its applicability in data-limited situations. The mean length of animals that are fully vulnerable to the sampling gear can be used to estimate total mortality from basic growth parameters and a known length at first capture. This method requires equilibrium conditions because the mean length of a population will change only gradually after a change in mortality. In this study, we derive the transitional behavior of the mean length statistic for use in nonequilibrium conditions. We investigate conditions affecting the reliability of the Beverton–Holt results and then develop a new procedure that allows a series of mortality rates to be estimated from mean length data representing nonequilibrium conditions in multiple years. We then examine an assessment of goosefish Lophius americanus that was criticized for its use of the Beverton–Holt estimator under nonequilibrium conditions. Using data from the 196...

Stock–Recruitment Dynamics and the Maximum Population Growth Rate of the Barndoor Skate on Georges Bank

Abstract In 1998, the barndoor skate Dipturus laevis was reported to have been locally extirpated in parts of its northern range and to be potentially on the brink of extinction. Managers were faced with assessing the species with virtually no information other than a limited number of individuals observed in annual groundfish surveys. Since that time, a number of the primary life history parameters have been estimated, but the population dynamics of the species remain largely unexplored. In this study, we use information from the National Marine Fisheries Service (NMFS) annual groundfish surveys to investigate two critical components of barndoor skate population dynamics: the relationship of recruitment to spawner abundance and the maximum population growth rate. A strong stock–recruitment relationship was found in the fall survey data, suggesting that recruitment is closely tied to spawner abundance. The Ricker and Beverton–Holt stock–recruit models were fitted to the survey data, and estimates of the s...

Index-Removal Estimates of Dredge Efficiency for Sea Scallops on Georges Bank

Abstract In June of 1999, fishermen were allowed access to the southern section of Georges Bank Closed Area II in the North Atlantic to harvest the large biomass of sea scallops Placopecten magellanicus that had accumulated during a 5-year multispecies fishing ban. Prior to the opening, managers conducted a fine-scale survey of the area and a catch quota was established as a fixed percentage of the estimated biomass. The estimate of biomass was uncertain because it was based on estimates of dredge efficiency that ranged from 16% to 40%. Because survey stations were reoccupied at the end of the fishery and significant removals had occurred, it was possible to use the index-removal method to obtain an estimate of gear efficiency. The estimate was 54% and ranged from 41% to 54% depending on model inputs. The 54% efficiency is believed to represent a maximal efficiency estimate.

A Spatially Explicit Open-Ocean DeLury Analysis to Estimate Gear Efficiency in the Dredge Fishery for Sea Scallop Placopecten magellanicus

Abstract The translation of survey data into estimates of absolute abundance hinges on the availability of an accurate estimate of gear efficiency. In many fisheries, however, a wide range of estimates exists and research directed at quantifying this critical parameter has focused on relatively small, fishery-independent data sets. In the present study, a technique was developed to utilize the copious amounts of data available from an open-ocean fishery for sea scallops Placopecten magellanicus in a spatially explicit depletion model. In June 1999, Georges Bank Closed Area II was opened to the commercial scallop fleet after a 5-year multispecies fishing ban. During the 5-month opening, the spatial distribution and magnitude of fishing effort was tracked through vessel monitoring systems, and a relatively small but still substantial number of catch observations were made aboard vessels from the commercial fleet. A spatial analysis of both catch and effort data was utilized to select areas consistent with t...

Comparison of Two Length-Based Estimators of Total Mortality: A Simulation Approach

AbstractLength-based methods for estimating the total mortality rate, Z, are appealing due to their potential application in data-poor situations, particularly when assessing tropical and invertebrate fisheries where age composition data are lacking. We evaluated two length-based estimators attributed to Beverton and Holt (1956) and to Ehrhardt and Ault (1992) for precision and accuracy when applied to simulated length data generated under varying combinations of Z rates, growth rates, variability in length at age, and the degree of length truncation imposed by the data analyst. The Beverton–Holt method generally overestimated Z, with bias ranging from −5% to +40%, when the abundance of the oldest age-groups is less than that associated with a constant mortality rate. The bias in the Ehrhardt–Ault method ranged from −80% to +140%, depending on the combinations of Z and the von Bertalanffy growth coefficient K, the degree of imposed length truncation, and the method for mean length calculation. In general,...

A Simple Method for Estimating Survival Rate from Catch Rates from Multiple Years

Abstract Survival rates can be estimated from annual surveys by tracking the abundance of one or more cohorts, as measured by catch per unit of sampling effort, from one year to the next. However, it can be difficult to attain reasonable precision unless sampling effort is extensive. Indeed, estimates of survival exceeding 100% are not infrequently obtained. We show that data from several years can be analyzed simultaneously to obtain a single estimate of survival under the assumption that survival is constant over the period analyzed. The method requires that only a single cohort be identified and separated from the other age-groups. Thus there are minimal data requirements. Estimates of the survival of goosefish Lophius americanus obtained by this method compare favorably with estimates obtained by analyzing changes in mean length over time.

Ontogenetic and Sex-Specific Shifts in the Feeding Habits of the Barndoor Skate

Abstract Diet analysis is critical in understanding the flow of energy within marine food webs and is necessary for trophic ecosystem modeling and subsequent ecosystem-based management recommendations. This study represents the first comprehensive diet description for the Barndoor Skate Dipturus laevis, the largest rajid species found on the continental shelf in the northwestern Atlantic Ocean. Stomach contents were extracted from 273 individual skate caught as bycatch in the commercial scallop fishery on Georges Bank and a total of 31 prey species were identified. The Barndoor Skate feeds primarily upon sand shrimp Crangon septemspinosa, the rock crab Cancer irroratus, the Acadian hermit crab Pagurus acadianus, and teleost fish. Length-specific analysis revealed four significant feeding groups (ANOVA: P < 0.01). Skate < 35 cm TL were specialized feeders foraging solely on caridean shrimp, and as size increased (35–75 cm TL), they began to feed upon rock crab and then the Acadian hermit crab. At lengths ranging from 85 to 105 cm TL, no caridean shrimp were found in the skates diet and the prevalence of crustaceans decreased. Large skate (>105 cm TL) began to prey heavily upon teleost fish, yet also continued to consume larger crustaceans. Significant sex-specific differences in food habits were also observed in the biggest skate (>105 cm TL): males fed primarily on teleost fish (∼80%); however, females maintained a diet of approximately equal amounts of fish and crustaceans. These sex-specific feeding patterns and differential food niche utilization may be mitigated by sexually dimorphic dentition.

Indication of Density-Dependent Changes in Growth and Maturity of the Barndoor Skate on Georges Bank

Abstract Drastic increases or decreases in biomass often result in density-dependent changes in life history characteristics within a fish population. Acknowledging this phenomenon and in light of the recent biomass increase in Barndoor Skate Dipturus laevis, the current study re-evaluated the growth rate and sexual maturity of 244 specimens collected from 2009–2011 within closed areas I and II on Georges Bank, USA. Ages were estimated using vertebral band counts from skate that ranged from 21 to 129 cm TL. The von Bertalanffy growth function was applied to pooled age-at-length data. Parameter estimates from the current study of L∞ = 155 cm TL and k = 0.10 represent a significant decrease from previously reported parameters of L∞ = 167 cm TL and k = 0.14. In addition to changes in growth parameters, age at 50% maturity for both males (based on clasper length, testes mass, and percent mature spermatocytes) and females (based on data from shell gland mass, ovary mass, and follicle diameter) increased by 3 years and 4 years, respectively. Based on our results and the 10- to 12-year gap in the collection of samples, it is likely that Barndoor Skate within this region have exhibited pliability in life history parameters.