Brian J. Rothschild

Coherence of Atlantic Cod Stock Dynamics in the Northwest Atlantic Ocean

Abstract The stocks of Atlantic cod Gadus morhua in the Northwest Atlantic Ocean declined in abundance from 1965 to 2003; the declines in spawning stock biomass (SSB) have been temporally coherent. A coherent, sharp increase in SSBs from 1975 to 1985 and a subsequent decrease from 1985 to 1992 are superimposed on the general decline. The coherence suggests that cod stock variability in the Northwest Atlantic Ocean is driven by a common set of causes or that the linkages among the nominal stocks are stronger than was previously thought. The coherent increases in cod SSB from the mid-1970s to 1985 occurred under relatively low fishing mortalities. The declines in SSB beginning in 1985 began during a period of low fishing mortalities. During the 1985-1992 period the declines in Atlantic cod abundance were coupled with greatly reduced growth rates, increased natural mortality rates, and a lack of response to reduced fishing mortality. This suggests that the 1985-1992 decreases were driven by a strong negative...

Impact of high-frequency nonlinear internal waves on plankton dynamics in Massachusetts Bay

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Interdisciplinary ocean science is evolving and a systems approach is essential

The earth’s surface fluids comprise a dynamic and highly variable coupled physical, biological and chemical system. The oceanic component of this system is controlled by ecosystem–biogeochemical– physical processes interacting over a vast range of scales in space and time. This is a propitious time in ocean science. The large-scale global studies of physical, biogeochemical, and biological ocean phenomena and processes now underway are setting the stage for a new generation of interdisciplinary ocean studies. Moreover, an opportunity exists to set future ocean studies efficiently on wide-ranging temporal and spatial scales in the context of satellite and in situ sensing systems that are now being planned and developed. Two decades of progress in describing, modeling and forecasting of the physical synopticmesoscale features in the ocean now makes it feasible to provide mesoscale fields over large-scale domains suitable for realistic physical–biological interactive process studies. Thus, the substantial fundamental interdisciplinary problems of ocean science, long recognized as of great importance, are now tractable. As the scope of interdisciplinary ocean science expands, its complexity enlarges and its re-

The Structure of Complex Biological Reference Points and the Theory of Replacement

Abstract The percent spawning-stock biomass per recruit (PSSBR) is a commonly used biological reference point (BRP). Justification for using specific values of PSSBR as BRPs has largely been based upon graphical interrelations of (1) the dependence of recruitment on stock, (2) the dependence of stock on recruitment (i.e., replacement), and (3) the dependence of spawning-stock biomass on fishing mortality. This paper provides analytic solutions of these interdependencies. Analytic solutions are provided for equilibrium and nonequilibrium cases: (1) fixed recruitment and fixed mortality, (2) fixed recruitment and variable mortality, and (3) variable recruitment and variable mortality. The analysis is used to develop equations for both the “replacement line” and a full recruitment→stock→recruitment cycle. The analysis clarifies the connection between iteroparous and semelparous populations in both equilibrium and nonequilibrium settings. The analytic derivations provide the opportunity to conduct research on...

Characterizing Uncertainty in Fish Stock Assessments: the Case of the Southern New England–Mid-Atlantic Winter Flounder

Abstract The reauthorization of the Magnuson–Stevens Act requires specification of scientific uncertainty associated with stock assessments. The scientific uncertainty associated with stock assessments of southern New England–mid-Atlantic winter flounder Pseudopleuronectes americanus is considered as a case study. Focus is placed upon the uncertainties associated with the assumptions, assertions, and choices (AACs) made in the stock assessment analysis. Two classes of AACs are discussed. The first class involves AACs that characterize the population dynamics of the stock; these AACs include the unit stock assumption, the problem of dealing with retrospective patterns, the method of averaging fishing mortality across cohorts to yield an annual value for fishing mortality, and the equilibrium structure of the stock. The second class of AACs is related to the choice of methods used to determine whether the stock is overfished; these AACs involve focusing on the maximum sustainable yield (MSY) proxy rather th...

A Critique of the Scale Method for Determining the Age of the Alewife, Alosa pseudoharengus (Wilson)

Abstract A critical evaluation of the annulus as an index of age in freshwater populations of the ale-wife has been made. Features of the alewife scale are described. Measurements of annulus position and agreement of ages estimated by the scale method with ages estimated from length-frequency distributions suggest the validity of the annulus as a criterion for estimating alewife age.

Decision support system for fisheries management

This paper presents a decision support system that is oriented toward fisheries policy and management decisions. The important current issues involve the development of an optimal harvesting plan for the fishing industry. A simulation optimization has been built to assist authorities in scheduling for a fleet of hundreds of vessels in terms of time and location of fishing, as well as amount and target species to be fished. Marine fisheries are highly complex and stochastic. A simulation model, therefore, is required. Simulation-based optimization utilizes the simulation model in obtaining the objective function values of a particular fishing schedule. A genetic algorithm is used as the optimization routine to determine the optimal fishing schedule, subject to fleet capacity and conservation requirements. The decision support system is then applied to the real situation in the Northeastern U.S.

Simulation Study of Biological Reference Points for Summer Flounder

Abstract The biological reference point Fx % (i.e., the fishing mortality rate that maintains the spawning stock biomass per recruit at x% of its unfished value [where x is usually set to 40]) is a commonly used proxy for F MSY (the fishing mortality rate that results in the maximum sustainable yield). However, Fx % is not in general equivalent to F MSY. To investigate the difference between Fx % and F MSY, we developed a simple simulation model capable of representing the relationship between yield and fishing mortality, maximum spawning potential (%MSP), and the curvature of the stock–recruitment (S–R) curve (parameterized as β) for a stock similar to summer flounder Paralichthys dentatus (a high-β species). The model demonstrates that the dynamic trajectories of the stock are heavily dependent on β. The model confirmed the dependence of equilibrium yield on β and produced a specific relationship between the magnitude of β and yield. A decision-theoretic approach was used to suggest that setting x to 40...

Simulation based optimization in fishery management

The sea scallop resource of Georges Bank supports one of the largest commercial fisheries in the United States. The objective of this research was to develop a technique to examine different management strategies for the sea scallop resource of Georges Bank and compare these strategies to the optimal. A simulation model followed the sea scallop population dynamics using information from recent photographic surveys and studies on spatial and temporal life history parameters, such as growth, natural mortality, spawning, and fishing activities. A stochastic simulation technique was used to describe the influence of the highly variable marine environment. A genetic algorithm technique was used to develop a harvest strategy in the area for optimal utilization by maximizing long term fishing yield. Simulation and a genetic algorithm are combined to solve the optimization problem. Simulation returns performance measures for a given policy and a genetic algorithm provides the search process to obtain the optimum policy.

Critical Issues for Circulation Modeling of Narragansett Bay and Mount Hope Bay

Narragansett Bay is a medium-sized estuary located along the coast of the northeast United States with shoreline in bothMassachusetts andRhode Island. The bay covers 380 km, has an average water depth of 7.8 m, and a maximum depth of 56 m. Narragansett Bay contains three large islands—Aquidneck, Conanicut, and Prudence, all of which are oriented roughly north–south, and divide the bay into three interconnected channels—the West Passage, the East Passage, and the Sakonnet River (Fig. 9.1). The narrow linkages between these waterways control the water exchange among the various sectors of the bay. The connection to the sea is found in the southern reaches of the bay, where it opens onto the inner New England Shelf via Rhode Island Sound. In the northeast corner of the bay lies a semi-isolated shallow estuary called Mount Hope Bay. It is connected to the greater portion of Narragansett Bay through a narrow, deep channel of about 800 m in width and 25 m in depth. In view of water exchange dynamics, Narragansett Bay and Mount Hope Bay are an integrated inter-bay complex. In recent decades, intensive shortand long-term field measurements have been made in Narragansett Bay. These observations show that regional warming has caused a dramatic increase in the stratification of the water column (Hicks, 1959; Nixon et al., 2004). Annual mean water temperatures in the Narragansett Bay–Mount Hope Bay system underwent an increase of 28C from 1985 to 2001, following a decrease during 1972–1984 (Fig. 9.2a, dashed line), with a net increase of 1.18C overall (Fig. 9.2a, solid line). The warming trend was also observed in Woods Hole, Massachusetts (Nixon et al., 2004), and in coastal waters of the northeast United States (Oviatt, 2004). The bay, which remained vertically well mixed throughout the year in 1954–1955 (Hicks, 1959), has been strongly stratified since the summer of 1990.