A performance evaluation of surplus production models with time-varying intrinsic growth in dynamic ecosystems

We conducted a simulation study to evaluate performance of surplus production models (SPMs) with a time-varying intrinsic growth rate (SPMTVr) for stocks with predation-driven changes in productivity. Data sets were simulated using an age-structured, linked, predator-prey model of Atlantic menhaden (Brevoortia tyrannus), a forage fish native to the Northwest Atlantic, and Atlantic striped bass (Morone saxatilis), its primary predator, with differing time series of fishing mortality on both predator and prey. Simulations generated test data sets for Atlantic menhaden SPMs that included either a static or time-varying intrinsic growth rate parameter. The SPMTVr largely produced more accurate, less variable estimates of exploitation rate and biomass than models with static intrinsic growth. We also applied SPMTVr to empirical Atlantic menhaden catch and survey data for 1964-2016. The SPMTVr fit the survey data well, estimated an intrinsic growth rate time series that mirrored long-term juvenile survey trends, and produced biomass and exploitation rate trends that mirrored a statistical catch-at-age model. The SPMTVr estimated dynamic, MSY-based reference points that reflected changing stock productivity.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author

A Simulation-Based Evaluation of Commercial Port Sampling Programs for the Gulf and Atlantic Menhaden Fisheries

© 2020 American Fisheries Society. Biological data that are collected in commercial port sampling programs are a critical component of the assessment and management of Gulf Menhaden Brevoortia patronus and Atlantic Menhaden Brevoortia tyrannus. The menhaden port sampling program represents one of the longest continuous commercial sampling efforts in the United States; however, this sampling program has not been evaluated recently to determine whether the program adequately characterizes the size and age structure of the catch despite significant changes in the spatial extent and magnitude of the fisheries in the last three decades. We conducted a simulation study to evaluate current menhaden fishery sampling targets and to examine the relative performance of a suite of alternative targets. To simulate data collection, we conducted a bootstrap analysis of the observed port sampling data. These observations were resampled with replacement across a range of current and alternative combinations of number of trips and fish sampled per trip. At the current target for sampling intensity and allocation, the mean sample weight and proportions at age for ages 2 and 3 are well characterized in both the Gulf and Atlantic menhaden fisheries. The proportions of age‐1 fish in the catch differed by stock and region, with samples from the northern Atlantic regions displaying the greatest uncertainty overall. The proportions of age‐4 and older fish were poorly characterized in both fisheries, which is likely due to their rarity in the population (Gulf) and lack of spatial overlap between the fishery and the stratified distribution of menhaden by age along the coast (Atlantic). Our results indicate that reducing the number of fish that is sampled per trip from the current target of 10 to as few as four would have a minimal effect on estimates of mean size and proportions at age in the catch. Increasing the number of sampled trips will not greatly improve the characterization of catch size or age composition

Trends in Relative Abundance and Early Life Survival of Atlantic Menhaden during 1977–2013 from Long-Term Ichthyoplankton Programs

<p>The Atlantic Menhaden <i>Brevoortia tyrannus</i>, a commercially important clupeid, supports one of the oldest and largest commercial fisheries on the U.S. East Coast. Despite recent increases in adult biomass, juvenile indices have declined coastwide and have remained particularly low in Chesapeake Bay. In order to understand the underlying causes of this decline, knowledge of larval recruitment is essential. We developed an index of larval abundance by using larval data collected from two large-scale ichthyoplankton sampling programs that occurred from Nova Scotia, Canada, to Cape Hatteras, North Carolina, during 1977–1987 and 1999–2013. Larval abundance data were standardized to a day-0 age by applying an age–length key from a study of larval ingress into Chesapeake Bay; a delta-lognormal model was used to account for spatial and temporal changes in sampling. We found that Atlantic Menhaden larval abundance increased from 1977 to 2013 and was highest in the winter; most individuals were detected at nearshore stations. Over our time series, larval abundance corresponded closely to adult spawning stock biomass. Due to the lack of a direct relationship between our larval abundance estimates and a coastwide juvenile index, we examined several environmental factors (temperature, Chesapeake Bay discharge, Atlantic Multidecadal Oscillation [AMO], wind speed, and wind direction) for potential effects on the relative survival of larvae. Larval abundance had a significant negative effect on relative survival. Temperature and to a lesser extent wind speed and AMO appeared to exert the greatest effects on the early life survival of Atlantic Menhaden: cooler temperatures, intermediate wind speeds, and negative-phase AMO were the most favorable for survival. Our findings suggest that the observed reduction in recruitment is not a problem of larval supply but rather is limited by survival between the larval and juvenile stages.</p> <p>Received December 17, 2015; accepted May 31, 2016 Published online August 12, 2016</p