Clay E. Porch

Estimating the Dependence of Spawning Frequency on Size and Age in Gulf of Mexico Red Snapper

Abstract In 2011, a large multivessel survey was conducted to provide nearly synoptic sampling of Red Snapper Lutjanus campechanus throughout their reproductive season in the U.S. Gulf of Mexico. A total of 2,487 Red Snapper were caught with a female : male ratio that was approximately 1:1. The ovaries of 1,002 females were histologically examined. Females (n = 391) were found with spawning markers (postovulatory follicles and hydrated oocytes) throughout the study area, but primarily in outer shelf waters. Statistical models were developed to quantify and test the dependence of the proportion of females bearing spawning markers (spawning fraction) on female length and age, time of year, depth, gear type (vertical line or longline), or region (east or west of the Mississippi River). Most of the variance in spawning fraction was explained by the time of year; spawning fractions were generally low in spring, peaked in midsummer, and declined by fall. There was also strong statistical evidence of a positive relationship between spawning fraction and either age or length. The effects of region and gear type were not significant once time of year and size or age were accounted for. These results demonstrate the need to account for differences in the time of year and age structure of the population when the productivity of populations of Red Snapper are compared. For example, productivity has been hypothesized to be greater in the western Gulf than in the eastern Gulf, as evidenced by regional patterns of egg and larval abundance. Our results suggest that this regional difference is not due to any intrinsic difference in the biology of the fish, but simply a consequence of there being more large, old Red Snapper in the western Gulf. Recent stock assessments have indicated that Red Snapper are increasing in abundance and there is a need to continue monitoring to detect any possible compensation in reproduction.

Experimental Assessment of Circle Hook Performance and Selectivity in the Northern Gulf of Mexico Recreational Reef Fish Fishery

Abstract Circle hooks are required when targeting reef fishes in the U.S. federal waters of the Gulf of Mexico. However, limited data is available to evaluate circle hook performance (e.g., hooking location and catch rate) or selectivity in this fishery. Therefore, a fishing experiment was conducted to test the performance of a range of circle hook sizes (2/ 0 and 4/0 Mustad 39940BLN and 9/0, 12/0, and 15/0 Mustad 39960D) in the recreational reef fish fishery, as well as to estimate hook selectivity directly for Red Snapper Lutjanus campechanus, the most targeted reef fish in the northern Gulf of Mexico. Reef fish communities were surveyed with a micro remotely operated vehicle equipped with a laser scaler and then fished with one of five circle hook sizes. Hooking location typically was in the jaw for all hooks examined, with the mean percentage of jaw hooking being 94.1% for all reef fishes and 92.9% for Red Snapper. Fish size generally increased with hook size but at the cost of a reduced catch rate. The percentage of the catch constituted by Red Snapper decreased from 73% for 2/0 hooks to 60% for 9/0 hooks but then increased to 84% for 15/0 hooks. Dome-shaped (exponential logistic) selectivity functions resulted when fitting candidate models to hook-specific Red Snapper size at catch and remotely operated vehicle laser-scaled size distribution data. While Red Snapper median size at full selectivity increased with circle hook size, the difference in that parameter between the smallest and largest hooks was only 66 mm, or a difference of approximately one age-class. Results of this study suggest that mandating the use of large (e.g., ≥12/0) circle hooks would have relatively little effect on either Red Snapper catch rate or selectivity but would decrease the catch rate for other reef fishes, which would be problematic during closed Red Snapper seasons when fishermen attempt to target other species.

On Making Statistical Inferences Regarding the Relationship between Spawners and Recruits and the Irresolute Case of Western Atlantic Bluefin Tuna (Thunnus thynnus).

Forecasts of the future abundance of western Atlantic bluefin tuna (Thunnus thynnus) have, for nearly two decades, been based on two competing views of future recruitment potential: (1) a “low” recruitment scenario based on hockey-stick (two-line) curve where the expected level of recruitment is set equal to the geometric mean of the recruitment estimates for the years after a supposed regime-shift in 1975, and (2) a “high” recruitment scenario based on a Beverton-Holt curve fit to the time series of spawner-recruit pairs beginning in 1970. Several investigators inferred the relative plausibility of these two scenarios based on measures of their ability to fit estimates of spawning biomass and recruitment derived from stock assessment outputs. Typically, these comparisons have assumed the assessment estimates of spawning biomass are known without error. It is shown here that ignoring error in the spawning biomass estimates can predispose model-choice approaches to favor the regime-shift hypothesis over the Beverton-Holt curve with higher recruitment potential. When the variance of the observation error approaches that which is typically estimated for assessment outputs, the same model-choice approaches tend to favor the single Beverton-Holt curve. For this and other reasons, it is argued that standard model-choice approaches are insufficient to make the case for a regime shift in the recruitment dynamics of western Atlantic bluefin tuna. A more fruitful course of action may be to move away from the current high/low recruitment dichotomy and focus instead on adopting biological reference points and management procedures that are robust to these and other sources of uncertainty.

Multispecies Extensions to a Nonequilibrium Length-Based Mortality Estimator

AbstractRecent advances in methodology allow the history of the total mortality rate experienced by a population to be estimated from periodic (e.g., annual) observations on the mean length of the population. This approach is generalized to allow data on several species that are caught together to be analyzed simultaneously based on the theory that changes in fishing effort are likely to affect several species; thus, the estimation of times when the mortality rate changes for one species borrows strength from data on other, concurrently caught species. Information theory can be used to select among models describing the degree of synchrony (if any) in mortality changes for a suite of species. This approach is illustrated using data on Puerto Rican handline fishery catches of three snapper species: Silk Snapper Lutjanus vivanus, Blackfin Snapper L. buccanella, and Vermilion Snapper Rhomboplites aurorubens. We identified the best model as the one that provided for simultaneous decreases in mortality rate ar...

Estimating Total Mortality Rates from Mean Lengths and Catch Rates in Nonequilibrium Situations

AbstractA series of estimates of the total mortality rate (Z) can be obtained by using the Beverton–Holt nonequilibrium-based approach of Gedamke and Hoenig (2006) on observations of population mean length over time (ML model). In contrast, only relative mortality rates (not absolute values) can be obtained from a time series of catch rates. We derived the transitional behavior of the catch rate following a change in total mortality in the population. From this derivation, we developed a new method to estimate Z that utilizes both mean lengths and catch rates (MLCR model). Both the ML model and the MLCR model assume constant recruitment in the population. We used a simulation study to test performance when recruitment is variable. Simulations over various scenarios of Z and recruitment variability showed that there may be correlated residuals in the mean lengths and catch rates arising from fluctuations in recruitment. However, the root mean square errors of the Z estimates and the change point (i.e., the...

Experimental assessment of circle vs. J hook performance and selectivity in the northern Gulf of Mexico recreational reef fish fishery

Experimental assessment of circle vs. J hook performance and selectivity in the northern Gulf of Mexico recreational reef fish fishery Steven B. Garner*, William F. Patterson III, and Clay E. Porch Marine Sciences, University of South Alabama and Dauphin Island Sea Lab, 101 Bienville Boulevard, Dauphin Island, AL 36528, USA University of Florida, Fisheries and Aquatic Sciences, 7922 NW 71st Street, Gainesville, FL 32653, USA Sustainable Fisheries Division, National Marine Fisheries Service, Southeast Fisheries Science Center, 75 Virginia Beach Drive, Miami, FL 33149, USA *Corresponding author: tel: þ850-774-1936; e-mail: steve.garner.disl@gmail.com