Comparison Between Maximum Sustained Yield Proxies and Maximum Sustained Yield
by Brian J. Rothschild and Yue Jiao
in 2013, The Bentham Journal: Open Fish Science Journal
Abstract: Attaining maximum sustained yield (MSY) is a central goal in U.S. fisheries management. To attain MSY, fishing mortality is maintained at FMSY and biomass at BMSY. Replacing FMSY and BMSY by “proxies” for FMSY and BMSY is commonplace. However, these proxies are not equivalent to FMSY and BMSY. The lack of equivalency is an important issue with regard to whether MSY is attained or whether biomass production is wasted. In this paper we study the magnitude of the equivalency. We compare FMSY/BMSY (calculated using the ASPIC toolbox) with the proxy estimates, F40%/B40%, published in GARM III. Our calculations confirm that in general the FMSY/BMSY calculations differ from the GARM III proxy estimates. The proxy estimates generally indicate that the stocks are overfished and are at relatively low biomasses, while the ASPIC estimates generally reflect the opposite: the stocks are not overfished and are at relatively high levels of abundance. In comparing the two approaches, the ASPIC estimates appeared favorable over the proxy estimates because 1) the ASPIC estimates involve only a few parameters in contrast to the many parameters estimated in the proxy approach, 2) “real variance” estimates for the proxy are not available so that it is difficult to evaluate the statistical adequacy of the proxy approach relative to the ASPIC approach, and 3) the proxy approach is based on many components (e.g., growth, stock and recruitment, etc.) that are subject to considerable uncertainty.
to read the full article click here Comparison Between Maximum Sustained Yield Proxies and Maximum Sustained Yield – Brian J. Rothschild and Yue Jiao.
Failure to eliminate overfishing and attain optimum yield in the New England groundfish fishery
by Brian J. Rothschild, Emily F. Keiley, and Yue Jiao
in 2013, in ICES Journal of Marine Science
Abstract: Under US law, fishery management is required to eliminate overfishing and attain optimum yield (OY). In New England, many groundfish stocks continue to be overfished, and the fishery continues to harvest less than OY. The reasons for the shortfalls are rooted in the socioeconomic structure of the management regime, and technical and scientific issues that constrain the management system. The most recent change in the management regime (days-at-sea to catch shares) and performance relative to OY and the prevention of overfishing are analyzed along with metrics used to gauge performance. The commonly used age based production model gives a problematic perception of stock abundance. Structural issues that seem to impair achieving OY are the adherence to the single-species interpretation of multiple species yield and the use of the Fx% proxy. Simpler approaches to stock assessment are discussed. A management system that creates feasible goals and uses improved and simpler metrics to measure performance is needed to facilitate attainment of management goals.
to read the full article click here Failure to Eliminate Overfishing and Attain Optimum Yield in the New England Groundfish Fishery – by Brian J. Rothschild, Emily Keiley, and Yue Jiao
Simulation Study of Biological Reference Points for Summer Flounder
by Brian J. Rothschild, Yue Jiao, ad Saang-Yoon Hyun
in March 16, 2012, Transactions of the American Fisheries Society
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 reduces yield and that smaller values of x produce greater yields for high-β stocks. The analysis focuses attention on the fact that the choice of Fx % as a management tool places extreme reliance on the least known and understood component of fish population dynamics: the S–R relationship. Our conclusion (to use a value of x considerably less than 40 to obtain MSY) was supported by (1) our simulation results, (2) averaging in a decision-theoretic approach, (3) the correspondence of the traditionally computed biomass at the maximum sustainable yield with high values of β, and (4) the values of x reported in the literature.
to read the full article click here Simulation Study of Biological Reference Points for Summer Flounder – Brian J. Rothschild, Yue Jiao, Saang-Yoon Hyun.
Characterizing variation in Northwest Atlantic fish-stock abundance
by Brian J. Rothschild and Yue Jiao
in January 19, 2012, ICES Journal of Marine Science
Abstract: Catch-per-tow indices obtained by research vessels for the years 1963–2009 from NAFO statistical areas 4W, 4X, 5Y, and 5Z were studied to determine how fish “apparent abundance” in the decade 2000–2009 differed from the long-term time-series. Cluster analysis of normalized catch-per-tow data indicated that the abundance and species composition of stocks in each statistical area changed dramatically over the 50-year period. There were decreases in thorny skate, ocean pout, cusk, witch flounder, and monkfish and increases in herring, haddock, northern shrimp, and spiny dogfish. Cluster analysis suggested that these decreases and increases were not gradual, but abrupt, and that these abrupt decreases and increases were concentrated in the decade of the 1980s. Observations of abrupt change were supported by regression-tree analysis of individual stocks. Examination of the interrelationship among abundance indices from different stocks by Bonferroni-adjusted correlation coefficients showed that the abundance trajectories of most stocks were uncorrelated. It appears that the set of population transitions during the decade of the 1980s was a dominant event in the statistical time-series.
to read the full article click here Characterizing variation in Northwest Atlantic fish-stock abundance – Brian J. Rothschild and Yue Jiao.
Alternative interpretation of trophic cascade
by Brian J. Rothschild
in November 3, 2011, Nature
“Transient dynamics of an altered marine ecosystem” portrays the spectacular collapse of “large benthic fish” (LBF) in the early 1990s in the waters of the Scotian Shelf as a “trophic cascade.” Frank et al. define the Scotian-Shelf trophic cascade as a fishing-driven, top-down, restructuring of an ecosystem where top predators of pelagic fish (PF) are removed by fishing; as a consequence, the PF populations explode; the LBF and the PF reverse trophic roles (“predator-prey reversal” – the PF prey on young stages of LBF); and increased predation by PF suppresses the ability of the LBF to increase to their pre-collapse abundance. The PF also feed on zooplankton. Eventually, feeding by PF on zooplankton reduces the abundance of zooplankton; this causes the PF to decline and the LBF begin to increase in abundance. Frank et al. imply that in the course of these trophodynamic changes, physical forcing was minimal.
to read the full article click here Alternative interpretation of trophic cascade – Brian J. Rothschild.
Characterizing Uncertainty in Fish Stock Assessments: the Case of the Southern New England-Mid-Atlantic Winter Flounder
by Brian J. Rothschild and Yue Jiao
in May 18, 2011, Transactions of the American Fisheries Society
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 than MSY per se to determine overfishing levels. The MSY proxy approach is discussed and compared with heuristic calculations of the MSY approach. Arbitrary choices of instantaneous natural mortality and percent maximum spawning potential in the analysis can lead to an arbitrary decision on whether or not the stock is overfished. We conclude that there is considerable scientific uncertainty on the status of the southern New England–mid-Atlantic winter flounder stock. The uncertainty identifies critical unknowns in winter flounder population dynamics.
to read the full article click here Characterizing Uncertainty in Fish Stock Assessments: the Case of the Southern New England-Mid-Atlantic Winter Flounder – Brian J. Rothschild and Yue Jiao
The Overfishing Metaphor
by Brian J. Rothschild
in January-February 2011, The American Institute of Fishery Research Biologists
The term “overfishing” seems to have first been used in 1854 at a meeting of the British Association for the Advancement of Science (Rozwadowski, 2002). The determination of whether or not a stock is overfished preoccupied fishery scientists for the next several decades. During this time no precise definition of overfishing could be developed, despite a prestigious inquiry by the International Council for Exploration of the Sea (ICES) (Petersen, 1903).
Progress seemed evident in the 1940s and 1950s with the development of quantitative theories of fishing. The theories created the belief that practical and concrete overfishing definitions could be developed from mathematical models linking fishing mortality and population abundance. These models could generate optimal values (maxima) of production, yield-per-recruit, and recruitment as functions of stock size or fishing mortality. Thus, if optimal values were exceeded—the stock size was too low, or fishing mortality was too high—the stock could be declared overfished. . . .
to read the full article click here The Overfishing Metaphor – Brian J. Rothschild
The Structure of Complex Biological Reference Points and the Theory of Replacement
by Brian J. Rothschild and Yue Jiao
in July 9, 2009, the American Fisheries Society
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 optimization of the equations and set the stage for field and laboratory studies of the dependence of stock on recruitment, complementing the much more intensive studies of the dependence of recruitment on stock.
to read the full article click here The Structure of Complex Biological Reference Points and the Theory of Replacement – Brian J. Rothschild and Yue Jiao
Coherence of Atlantic Cod Dyanmics in the Northwest Atlantic Ocean
by Brian J. Rothschild
in May 21, 2007, the American Fisheries Society
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 environmental signal, implying that the environment had a stronger role in affecting cod abundance than had been previously thought. It appears that the decline in SSB over most of the range of the cod was coupled with a major perturbation in the forage available to cod. Inasmuch as this perturbation involved seemingly disparate groups, such as capelin Mallotus villosus, euphausids, and Atlantic herring Clupea harengus, it appears that the change in the environment was associated with the dynamics of the plankton.
to read the full article click here Coherence of Atlantic Cod Stock Dynamics in the Northwest Atlantic Ocean – Brian J. Rothschild.