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

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

A study of the river origin of American shad captured in the Atlantic Ocean intercept fishery in Virginia : Final report, 2001

The Virginia Department of Game and Inland Fisheries (VDGIF) has been releasing larval American shad into the James and York river systems since 1993 and these fish are given river-specific marks before release. Our in-river monitoring program had established that we could estimate the proportion of fish returning to spawn in the rivers that have hatchery marks. Furthermore, we were able to obtain a sample of 200 fish from the intercept fishery off Chincoteague Island, Virginia, in 2000 and screening of the otoliths by VDGIF personnel revealed the presence of one fish with a James River hatchery mark and one with a York River mark. No marks from any other river were found. Thus, it appeared that hatchery markings would allow the opportunity to estimate the proportion of Virginia stocks that were harvested in the offshore fishery. On the basis of these preliminary findings, we proposed the present study

Evaluating a possible new paradigm for recruitment dynamics: predicting poor recruitment for striped bass (Morone saxatilis) from an environmental variable

Understanding what causes large year classes and predicting them has been called the holy grail of fisheries science, one of the last great unanswered questions. Recruitment prediction, or forecasting, is an important component for setting fishery catch limits. We propose a new approach, called the “poor-recruitment paradigm”, for predicting recruitment using environmental variables. This approach hypothesizes that it is easier to predict poor recruitment rather than good recruitment because an environmental variable affects recruitment only when its value is extreme (lethal); otherwise, the variable may be benign and not influence recruitment. Thus, good recruitment necessitates all environmental conditions not be harmful and for some to be especially favorable; poor recruitment, however, requires only one environmental variable to be extreme. This idea was evaluated using recruitment and river discharge data for striped bass (Morone saxatilis) from seven major spawning tributaries of Chesapeake Bay. Low spring river discharge reliably resulted in poor recruitment of striped bass. Specifically, in all rivers, median recruitment and standard deviation of recruitment were lower when spring river discharge was low compared to when it was average or high; additionally, the proportion of years with poor recruitment was higher in years of low discharge than in years of average to high discharge. The consistent predictability of poor recruitment has the potential to improve stock projections, and therefore, has the potential to improve catch advice

Effectiveness of the Blue Crab Spawning Sanctuary in Chesapeake Bay

The blue crab spawning stock in Chesapeake Bay sustained a severe and persistent decline beginning in 1992. As part of the effort to enhance the spawning stock, the spawning sanctuary in lower Chesapeake Bay was enlarged to over 240,000 ha. This marine reserve and corridor prohibits exploitation of mature females en route to or in the spawning grounds during the summer spawning season (1 June-15 September). To assess the effectiveness of the sanctuary, we tagged terminally molted, mature females inside and outside the sanctuary during three sanctuary seasons (2002-2004). Crabs were captured throughout the Bay and its tributaries, measured, tagged, and released on site. Recaptures of tagged crabs were reported by commercial and recreational fishers. Probability of recapture for crabs released outside of the sanctuary was 6.3, 5.2, and 2.8 times the probability of recapture for crabs tagged inside the sanctuary for 2002, 2003 and 2004, respectively. Consequently, a significant proportion of adult female blue crabs remains in the sanctuary to spawn and is not captured by the fishery. Hence, the marine reserve and corridor for the blue crab spawning stock in Chesapeake Bay is an effective means of protecting females migrating to or residing in the spawning grounds. This investigation serves as one of the few empirical tests to date of the effectiveness of a marine reserve designed to protect spawning stock