Monitoring Relative Abundance of American Shad in Virginia\u27s Rivers Annual Report 1998

A moratorium on the taking of American shad in the Chesapeake Bay and its tributaries was established by the Virginia Marine Resources Commission (VMRC) beginning 1 January 1994. The prohibition applied to both recreational and commercial fishers. The moratorium was imposed at a time when commercial catch rates of American shad in Virginia\u27s rivers were experiencing declines. Data from the commercial fishery were the best available for assessing the status of individual stocks. Catch-per-unit-effort (CPUE) data were compiled from logbooks that recorded landings by commercial fishermen using staked gill nets at various locations throughout the middle reaches of the three rivers. The logbooks were voluntarily provided to the Virginia Institute of Marine Science (VIMS) during the period 1980-1993, and subsequently used in an assessment of the status of American shad stocks along the Atlantic coast by the Atlantic States Marine Fisheries Commission (ASMFC) (ASMFC 1999). Since the moratorium, there have been no monitoring programs that provided direct assessment of stock recovery. The ban on in-river fishing in Virginia remained in effect, creating a dilemma for managers who needed reliable information in order to make a rational decision on when the in-river ban could safely be lifted. To address this deficiency, we proposed a method of scientific monitoring to estimate catch rates relative to those recorded before the prohibition of in-river fishing in 1994. This monitoring program consisted of sampling techniques and locations that were consistent with, and directly comparable to, those that generated historical logbook data collected by VIMS during the period 1980-1993 in the York, James and Rappahannock rivers. The results of the first year\u27s sampling program are reported in this document

Design of a Recreational Fishing Survey and Mark-Recapture Study for the Blue Crab, Callinectes sapidus, in Chesapeake Bay

The development of bay wide estimates of recreational harvest has been identified as a high priority by the Chesapeake Bay Scientific Advisory Committee (CBSAC) and by the Chesapeake Bay Program as reflected in the Chesapeake Bay Blue Crab Fishery Management Plan (Chesapeake Bay Program 1996). In addition, the BiState Blue Crab Commission (BBCAC), formed in 1996 by mandate from the legislatures of Maryland and Virginia to advise on crab management, has also recognized the importance of estimating the levels and trends in catches in the recreational fishery. Recently, the BBCAC has adopted limit and target biological reference points. These analyses have been predicated on assumptions regarding the relative magnitude of the recreational and commercial catch. The reference points depend on determination of the total number of crabs removed from the population. In essence, the number removed by the various fishery sectors, represents a minimum estimate of the population size. If a major fishery sector is not represented, the total population will be accordingly underestimated. If the relative contribution of the unrepresented sector is constant over time and harvests the same components of the population as the other sectors, it may be argued that the population estimate derived from the other sectors is biased but still adequately represents trends in population size over time. If either of the two constraints mentioned above is not met, the validity of relative trends over time is suspect. With the recent increases in the human population in the Chesapeake Bay watershed, there is reason to be concerned that the recreational catch may not have been a constant proportion of the total harvest over time. It is important to assess the catch characteristics and the magnitude of the recreational fishery to evaluate this potential bias. (PDF contains 70 pages

Monitoring Relative Abundance of American Shad in Virginia’s Rivers 2000 Annual Report

Since the moratorium, there have been no monitoring programs that provided direct assessment of stock recovery until this project began in 1998. The ban on in-river fishing in Virginia remained in effect, creating a dilemma for managers who needed reliable information in order to make a rational decision on when the in-river ban could safely be lifted. To address this deficiency, we proposed a method of scientific monitoring to estimate catch rates relative to those recorded before the prohibition of in-river fishing in 1994. This monitoring program began in 1998 and consisted of sampling techniques and locations that were consistent with, and directly comparable to, those that generated historical logbook data collected by VIMS during the period 1980-1992 in the York, James and Rappahannock rivers. The results of the third year in the sampling program (2000) are reported in this document. The results of the first two years of sampling (1998 and 1999) are reported in previous annual reports (Olney and Hoenig 2000a, 2000b)

Monitoring Relative Abundance of American Shad in Virginia\u27s Rivers Annual Report 1999

A moratorium on the taking of American shad (Alosa sapidissima) in the Chesapeake Bay and its tributaries was established by the Virginia Marine Resources Commission (VMRC) beginning 1 January 1994. The prohibition applied to both recreational and commercial fishers. The moratorium was imposed at a time when commercial catch rates of American shad in Virginia\u27s rivers were experiencing declines. Data from the commercial fishery were the best available for assessing the status of individual stocks. Catch-per-unit-effort (CPUE) data were compiled from logbooks that recorded landings by commercial fishermen using staked gill nets at various locations throughout the middle reaches of the James, York and Rappahannock rivers. The logbooks were voluntarily provided to the Virginia Institute of Marine Science (VIMS) during the period 1980-1993, and subsequently used in an assessment ofthe status of American shad stocks along the Atlantic coast by the Atlantic States Marine Fisheries Commission (ASMFC) (ASMFC 1999). Since the moratorium, there have been no monitoring programs that provided direct assessment of stock recovery. The ban on in-river fishing in Virginia remained in effect, creating a dilemma for managers who needed reliable information in order to make a rational decision on when the in-river ban could safely be lifted. To address this deficiency, we proposed a method of scientific monitoring to estimate catch rates relative to those recorded before the prohibition of in-river fishing in 1994. This monitoring program began in 1998 and consisted of sampling techniques and locations that were consistent with, and directly comparable to, those that generated historical logbook data collected by VIMS during the period 1980-1993 in the York, James and Rappahannock rivers. The results of the second year in the sampling program (1999) are reported in this document. The results of the first year of sampling ( 1998) are reported in Olney and Hoenig (2000)

Results of evaluating the performance of empirical estimators of natural mortality rate

Natural mortality rate, M, of fish is a highly influential stock assessment parameter. The M parameter is also difficult to estimate directly and reliably. Various empirical estimators have been developed to estimate M indirectly, based on relationships established between M and predictor variables such as growth parameters, lifespan and water temperature (e.g., Beverton and Holt, 1959; Alverson and Carney, 1975; Pauly, 1980; Hoenig, 1983). Despite the importance of these estimators, there is no consensus in the literature on how well they work in terms of prediction error or how their performance may be ranked. Then et al. (2015) evaluated estimators based on various combinations of maximum age (tmax), von Bertalanffy growth parameters (K) and asymptotic length (L∞), and water temperature (T), by seeing how well they reproduce independent, direct estimates of M for more than 200 unique fish species. They also considered the possibility of combining different estimators using a weighting scheme to improve estimation of M. This report documents additional analyses and results to supplement the results in the journal article. The estimators, evaluation criteria, and other important details are given in the journal article

Simulated performance of catch curve methods for estimating total mortality rate

This document has been issued as VIMS Data Report 60 and provides additional simulation results for Smith et al. (2012) published in the North American Journal of Fisheries Management. Catch curve methods are a basic tool of population dynamics for estimating total mortality rate from age composition. There are a number of methodological issues which remain unresolved. Smith et al. (2012) attempts to provide guidelines on the use of these methods based on extensive Monte Carlo simulations. This report presents additional simulation results to supplement the results in the journal article. The estimators, evaluation criteria, simulation procedures, and conditions simulated are given in the journal article

Scallop Mark-Recapture to Estimate Density Dependent Natural Mortality and Growth : Final Report

Natural mortality, growth and movement are fundamental processes critical to understanding and describing population dynamics. These population characteristics not only inform and influence stock assessment models, but are also highly relevant with respect to the design and implementation of management strategies to meet fishery objectives. For many species, including sea scallops, these population parameters are difficult to measure due to the nature of the habitats inhabited and as a result, minimal information is often available. In many cases, what estimates do exist are highly uncertain as a result of both observation and process error. The uncertainty of these parameters is exacerbated for a species such as the sea scallop whose life history strategy is predicated on large, episodic recruitment events where natural mortality and growth may vary as a function of animal density. In 2015, resource surveys by the Virginia Institute of Marine Science (VIMS) and others observed what appeared to be an exceptionally large incoming year class of sea scallops throughout the Mid Atlantic Bight (MAB), with the locus of the event located in the Elephant Trunk Closed Area (ETCA). At the time of first encounter, these animals were roughly two years old and the scale of the event in terms of the spatial extent and magnitude was extensive. Multiple surveys delineated the distribution of the event and confirmed the enormous magnitude of scallops in the area, but critical questions remained. What is the survival rate of the unfished cohort (i.e. natural mortality rate)? What is their growth rate? Would the scallops survive? Would they grow at rates similar to what was expected? Would the animals move either inshore or offshore to a different habitat? To address these questions, we conducted a sea scallop mark-recapture study in the area of newly recruited scallops in the ETCA

Simulations to Compare the Performance of Two Length-based Estimators of Total Mortality Rate

Mean length-based methods to estimate instantaneous total mortality rates, Z, are important assessment tools for data-poor stocks. One commonly used method was developed by Beverton and Holt (1956). The behavior of this method, especially in relation to bias, has been fairly well characterized. Another method by Ehrhardt and Ault (1992) was proposed to correct the Beverton-Holt (BH) method for applications to length frequency distributions that are truncated at the upper end. The Ehrhardt-Ault (EA) method has zero bias at equilibrium when there is no variability in length at age but the reliability of the method has not been demonstrated under conditions of reasonable magnitude of growth variability. It is also unclear how one would determine the best input value for the upper length truncation parameter. This report presents additional simulation results to supplement the results in Then et al. (2015). The estimators, evaluation criteria, simulation procedures, and conditions simulated are given in Then et al. (2015)

A general theory of age-length keys: combining the forward and inverse keys to estimate age composition from incomplete data

There are two approaches to estimating age composition from a large number of length observations and a limited number of age determinations: the forward and the inverse age-length keys. The forward key looks at the distribution of age within each length bin while the inverse key looks at the distribution of length at each age. The former is more precise but has stringent requirements for the way data are collected. The latter approach is more widely applicable. We review the theory of the two keys with particular attention to necessary assumptions and the restrictions on when the methods are applicable. We show it is possible to combine the two approaches into a combined forward-inverse age-length key. This approach can be used to estimate age composition in several years simultaneously. It takes advantage of the efficiency of the forward key in years when that is appropriate, applies the inverse key to years with no age data, and uses a blending of the two approaches for years with moderate amounts of age data