Maximum Sustained Yields from Fluctuating Environments and Mixed Stocks

1958 ◽  
Vol 15 (5) ◽  
pp. 991-1006 ◽  
Author(s):  
W. E. Ricker
Keyword(s):  
Environmental Variability ◽  
Numerical Models ◽  
Fishing Effort ◽  
Optimum Level ◽  
Fish Stocks ◽  
Total Potential ◽  
Fluctuating Environments ◽  
Low Levels ◽  
Average Catch

Using numerical models, effects of environmental variability upon yield were tested for six single-age fish stocks characterized by different kinds and degrees of density-dependent reproduction potential. The two levels of variability examined had extremes of yield standing in the ratios 7:1 and 18:1, respectively. Close regulation of fishing to the optimum percentage for each year's stock improves the long-term average catch taken, the improvement being the greater, the more variable the environment. With the higher level of variability, improvement in average catch among five of the stocks ranged from 26% to 79% increase. However this increase in mean catch is achieved at the expense of increased variability in catch from year to year—in fact, for some kinds of stocks there must be complete cessation of fishing in some years in order to get the long-term maximum. The yield of stocks, in which reproduction per spawner declines at low levels of abundance, is particularly improved by a close adaptation of fishing effort to the supply of fish available.When two or more populations of a species, characterized by different reproduction potentials, are fished in common, total potential catch is less than when each can be fished separately at its optimum level. If a common fishery cannot be avoided, the achievement of maximum average yield may find one of two originally-equal stocks as abundant or even more abundant than before the fishery began, while the other may persist only at a low level or even be exterminated completely.

scholarly journals North Sea cod recovery?

2006 ◽  
Vol 63 (6) ◽  
pp. 961-968 ◽  
Author(s):  
Joe Horwood ◽  
Carl O'Brien ◽  
Chris Darby
Keyword(s):  
North Sea ◽  
Gadus Morhua ◽  
Mortality Rates ◽  
Fishing Effort ◽  
Current Status ◽  
Fishing Mortality ◽  
Commercial Fish ◽  
Fish Stocks ◽  
Stock Assessments ◽  
Low Levels

AbstractRecovery of depleted marine, demersal, commercial fish stocks has proved elusive worldwide. As yet, just a few shared or highly migratory stocks have been restored. Here we review the current status of the depleted North Sea cod (Gadus morhua), the scientific advice to managers, and the recovery measures in place. Monitoring the progress of North Sea cod recovery is now hampered by considerable uncertainties in stock assessments associated with low stock size, variable survey indices, and inaccurate catch data. In addition, questions arise as to whether recovery targets are achievable in a changing natural environment. We show that current targets are achievable with fishing mortality rates that are compatible with international agreements even if recruitment levels remain at the current low levels. Furthermore, recent collations of data on international fishing effort have allowed estimation of the cuts in fishing mortality achieved by restrictions on North Sea effort. By the beginning of 2005, these restrictions are estimated to have reduced fishing mortality rates by about 37%. This is insufficient to ensure recovery of North Sea cod within the next decade.

Population fluctuations and recruitment in marine populations

1982 ◽  
Vol 297 (1087) ◽  
pp. 353-368 ◽  
Keyword(s):  
Oil Pollution ◽  
Fishing Effort ◽  
Background Information ◽  
Population Fluctuations ◽  
Japanese Sardine ◽  
Factors Affecting ◽  
Fish Stocks ◽  
The North ◽  
Natural Variations

This paper is not concerned with the effect of oil pollution as such, but sets out to consider the range of natural variation and the extent to which this might mask the effects of other factors such as possible pollutant effects. To detect the effect of a pollutant (or a change in fishing effort) it is necessary to allow for natural variations, both random and periodic. Some examples are given to illustrate the extent of natural variations, in a variety of marine populations. For Arcto-Norwegian cod, information is available on catches from about the middle of the last century to the present day. Large catches are about 3-6 times the size of small catches. The difference between large and small catches is about equal to the mean catch. Further, the time taken to change from a small to a large catch level is very variable. The Greenland cod provides an example of a stock that increased very considerably due to a northerly increase of the limits of distribution of the species. This increase was associated with a warm period in the North Atlantic and with increased catches. Many fish stocks and in particular many species of pelagic fishes, exhibit much larger fluctuations in stock size, e.g. the Hokkaido herring, the Japanese sardine, the Bohuslan herring, the Atlanto-Scandian herring and the Califonian anchovy and sardine. Fluctuations also occur in invertebrate species and evidence is given of changes that have occurred in North Sea phytoplankton and zooplankton. The Peruvian anchovy provides an example of a stock that decreased very considerably, due partly to fishing and partly to changes in the hydrographic regime that caused the fish to become more available for exploitation. Fluctuations in fish stocks are primarily due to fluctuations in recruitment. The factors affecting recruitment are not yet fully understood but are known to be determined during the first year of life and probably during the larval or early juvenile stages. Whatever the mechanism, however, it is the variations in recruitment that determine a very large part of the variations in adult fish stocks and hence it is variations in recruitment and the causes of these that are important. In conclusion, the examples show that natural communities can exhibit large natural fluctuations, of varying periodicity, in the long term. Apart from incidents where there is gross pollution, an effect of pollution can therefore only be convincingly demonstrated for those species for which background information is available for a long enough period to allow for long-term periodicity as well as for short-term and irregular variability.

scholarly journals MARINE CAPTURE FISHERIES POLICY FORMULATION AND THE ROLE OF MARINE PROTECTED AREAS AS TOOL FOR FISHERIES MANAGEMENT IN INDONESIA

2018 ◽  
Vol 30 ◽  
pp. 33-45
Author(s):  
D. G.R. Wiadnya ◽  
P. J. Mous ◽  
R. Djohani ◽  
M. V. Erdmann ◽  
A. Halim ◽  
...  
Keyword(s):  
Protected Areas ◽  
Marine Protected Areas ◽  
Scientific Evidence ◽  
Code Of Conduct ◽  
Fishing Effort ◽  
Policy Formulation ◽  
Maximum Sustainable Yield ◽  
Fish Stocks ◽  
Fisheries Policy

The FAO Code of Conduct for Responsible Fisheries states that conservation and management decisions for fisheries should be based on the best scientific evidence available. Studies show that most of Indonesia's capture fisheries are either full or over-exploited. However, the fishery sector is still expected to contribute to the increase of Indonesia's GNP through an increase in total catches. Furthermore, the current practice of using catch-effort data and Maximum Sustainable Yield models to inform Indonesia’s fisheries policies is flawed, putting sustainability and long-term profitability of Indonesia's fisheries at risk. In this paper, the authors argue that to ensure the survival of Indonesia's fish stocks and fisheries: fisheries policy must shift from development-oriented management towards management for sustainability. Furthermore, fisheries managers must accept that 'untapped resources' may not exist or cannot be exploited profitably, and that any transfer of fishing effort between fishing grounds may contribute to collapse of local fisheries. Also, fisheries managers should change the management paradigm from MSY models to eco-system based management, wherein Marine Protected Areas should play an important role.

Fishery ecology and oceanography of the prawn Penaeus merguiensis (de Man) in the Gulf of Papua: estimation of maximum sustainable yield and modelling of yield, effort and rainfall

1997 ◽  
Vol 48 (3) ◽  
pp. 219 ◽  
Author(s):  
C. R. Evans ◽  
L. J. Opnai ◽  
B. D. Kare
Keyword(s):  
Fishing Effort ◽  
Maximum Sustainable Yield ◽  
Sustainable Yield ◽  
Optimum Level ◽  
Total Rainfall ◽  
Real Effort ◽  
Low Levels ◽  
Gulf Of Papua ◽  
De Man ◽  
Penaeus Merguiensis

Annual fishing effort for banana prawns, Penaeus merguiensis (de Man), in the Gulf of Papua increased from low levels in 1974 to around the optimum level in 1978 and peaked at 95000 trawl-hours in 1989 before decreasing after the introduction of effort controls in 1988. Maximum sustainable yield of P. merguiensis was estimated at 559 ± s.e. 25 t (of tails) by linear regression of CPUE (U) on effort using the Schaeffer model. Optimum effort was estimated at 84·0 (± s.e. 11·8) 103 trawl-hours, and an equation for the yield (CE) for a given effort (E) was derived as CE = 13·5E – (8·31 10-2)E2. Application of the CLIMPROD program to Gulf of Papua yield and effort and seasonal rainfall inputs resulted in validated modelling of the fishery. CLIMPROD selected a Schaeffer model for the CPUE–effort relationship (U = a + bE). When CLIMPROD incorporated rainfall (V), the model used was = UaV(b+c) + dV(2b)E (linear–exponential–exponential), where rainfall influences abundance and catchability. For 1974–93 overall, effort was by far the chief determinant of yield. The model using regionally averaged total rainfall in the fourth quarter and unstandardized effort data explained only an extra 5% of the variation in the following year’s catch over the model using effort alone. Standardization of effort data by 3% probably approximates the real effort applied more accurately than does recorded effort.

Testing Predictions of Marine Fish and Shellfish Landings from Environmental Variables

1987 ◽  
Vol 44 (9) ◽  
pp. 1568-1573 ◽  
Author(s):  
K. F. Drinkwater ◽  
R. A. Myers
Keyword(s):  
Environmental Variables ◽  
Environmental Effect ◽  
Degrees Of Freedom ◽  
Gulf Of Maine ◽  
Fishing Effort ◽  
Fish Stocks ◽  
Environmental Models ◽  
The Mean ◽  
Fish And Shellfish

Previous studies by Sutcliffe and co-workers using exploratory analysis found correlations between environmental variables and lagged annual catch for several Gulf of St. Lawrence and Gulf of Maine fish and shellfish stocks. The present study tests these relationships using recent data. For 6 of the 13 stocks investigated, correlations between the 9–14 yr of new catch data and that predicted from the environmental models remained high (r > 0.5) and of the same sign; however, individually none was statistically significant (p > 0.05) after accounting for the loss of degrees of freedom due to the high autocorrelation in the data. The hypothesis of an overall environmental effect on the landings was considered. It could not be substantiated as the correlation coefficient for 5 of the 13 stocks reversed sign using the new data, but changes in fishing effort are believed to mask detection of environmentally induced variability in the landings of many stocks. The utility of environmentally based predictions was also tested. Overall, the mean deviations of the predicted catch based on environmental regressions were similar to predictions based on the long-term means but were higher than predictions using lagged catch. Environmentally based predictions of landings for invertebrate stocks were generally more accurate than those for fish stocks.

scholarly journals Fishing for MSY: using “pretty good yield” ranges without impairing recruitment

2016 ◽  
Vol 74 (2) ◽  
pp. 525-534 ◽  
Author(s):  
Anna Rindorf ◽  
Massimiliano Cardinale ◽  
Samuel Shephard ◽  
José A. A. De Oliveira ◽  
Einar Hjorleifsson ◽  
...  
Keyword(s):  
Good Yield ◽  
Maximum Sustainable Yield ◽  
Fishing Mortality ◽  
Fish Yield ◽  
Fish Stocks ◽  
Northern European ◽  
Stock Biomass ◽  
Spawning Stock ◽  
Average Catch

Pretty good yield (PGY) is a sustainable fish yield corresponding to obtaining no less than a specified large percentage of the maximum sustainable yield (MSY). We investigated 19 European fish stocks to test the hypothesis that the 95% PGY yield range is inherently precautionary with respect to impairing recruitment. An FMSY range was calculated for each stock as the range of fishing mortalities (F) that lead to an average catch of at least 95% of MSY in long-term simulations. Further, a precautionary reference point for each stock (FP.05) was defined as the F resulting in a 5% probability of the spawning-stock biomass falling below an agreed biomass limit below which recruitment is impaired (Blim) in long-term simulations. For the majority of the stocks analysed, the upper bound of the FMSY range exceeded the estimated FP.05. However, larger fish species had higher precautionary limits to fishing mortality, and species with larger asymptotic length were less likely to have FMSY ranges impairing recruitment. Our study shows that fishing at FMSY generally is precautionary with respect to impairing recruitment for highly exploited teleost species in northern European waters, whereas the upper part of the range providing 95% of MSY is not necessarily precautionary for small- and medium-sized teleosts.

Comparison of Constant Effort Harvest Policies for Fish Stocks with Variable Recruitment

1985 ◽  
Vol 42 (5) ◽  
pp. 982-988 ◽  
Author(s):  
Joseph E. Hightower ◽  
Gary D. Grossman
Keyword(s):  
Environmental Variability ◽  
Fishing Effort ◽  
Economic Benefits ◽  
Maximum Sustainable Yield ◽  
Atlantic Menhaden ◽  
Sustainable Yield ◽  
Simulation Studies ◽  
Brevoortia Tyrannus ◽  
Fish Stocks ◽  
Variable Recruitment

Environmental variability may have a substantial influence on marine fish stocks, primarily by affecting survival to the time of recruitment. Simulation studies at low, intermediate, and high levels of variability in recruitment were used to compare alternative constant effort policies for anchovy (Engraulis capensis), Atlantic menhaden (Brevoortia tyrannus), and Pacific ocean perch (Sebastes alutus) fisheries. These policies were either to maintain effort at the level producing maximum sustainable yield (fMSY), or to permit levels of effort 25–100% greater than fMSY. An increase in effort of 25% above fMSY typically did not reduce annual yield significantly; however, a significant reduction in yield was apparent in all cases when effort increased by 75–100%. When recruitment is highly variable, comparable yields may be obtained at several levels of fishing effort. In such cases, environmental variability provides the fishery manager with considerable flexibility to enhance social or economic benefits without decreasing yields significantly.

CRITICAL FACTORS OF WATER CONTENT FOR FISH STOCKS IN THE WATER RESERVOIRS OF KAZAKHSTAN

2017 ◽  
pp. 9-18
Author(s):  
Saule Zhangirovna Asylbekova ◽  
Kuanysh Baibulatovich Isbekov ◽  
Evgeniy Vyacheslavovich Kulikov
Keyword(s):  
Water Content ◽  
Water Level ◽  
Water Availability ◽  
Hydrological Regime ◽  
Fishing Effort ◽  
Water Reservoirs ◽  
The Caspian Sea ◽  
Annual Fish ◽  
Commercial Fish ◽  
Fish Stocks

The hydrological regime of water reservoirs in different years has a decisive impact on the abundance of commercial fish stocks and the quality of ichthyocenoses. In this connection in 2015-2016 there was conducted a retrospective analysis and ranking of hydrological regime impact on these factors. The paper gives evaluation of catches and fish stocks under different scenarios of water availability in the main fishing ponds of the Republic of Kazakhstan that give about 80% of the annual fish catch of the country (except the Caspian Sea). There were analyzed 2000 factors of hydrological regime (water level, annual discharge) and 1845 factors of fishing stocks (catches, abundance, fish biomass). The paper determines the critical characteristics of water availability for fish stocks. There have been proposed a number of administrative decisions and actions in case if water content would approach to the critical level. Among them: limitation of fish catches in the following year; widening zones restricted for fishing; intensification of safety measures of the fish young in residual ponds during arid periods; introduction of catch standards for a unit of fishing effort in low-water years, high-water years and years with normal water level in rivers.

Planning Micronutrient-Dense Menus in Ontario Long-Term Care Homes: Strategies and Challenges

2020 ◽  
Vol 81 (4) ◽  
pp. 198-203
Author(s):  
Lisa M. Duizer ◽  
Heather H. Keller
Keyword(s):  
Small Volume ◽  
Long Term Care ◽  
Term Care ◽  
Dietary Reference Intakes ◽  
Portion Sizes ◽  
Calcium Magnesium ◽  
Micronutrient Malnutrition ◽  
Low Levels ◽  
Food Consumed

Prevalence of micronutrient malnutrition is high in individuals living in long-term care (LTC) homes with many individuals consuming low levels of vitamins B6, D, and E; folate; calcium; magnesium; and zinc. The focus of this research was to identify strategies and challenges encountered during development of micronutrient-dense menus for use in Ontario LTC homes and to examine costs associated with development of a menu with acceptable micronutrients. Semi-structured open-ended interviews were conducted with 13 menu planners (7 dietitians, 6 nutrition managers) in diverse LTC homes in Ontario. Data were thematically analyzed. A 7-day hypothetical menu meeting all nutrient requirements was developed and costed. Analysis of the interview data showed that menus are planned according to the Canada’s Food Guide (2007) and focus placed on Dietary Reference Intakes of protein, fibre, calcium, and sodium. Little focus is placed on micronutrients. Flexibility in foods offered was important to accommodate the small volume of food consumed. Resident preferences were balanced against nutritional requirements. Challenges included planning for diverse populations, managing portion sizes, and balancing the budget. A hypothetical menu planned to contain adequate levels of all micronutrients is 49% higher in food costs than the amount currently provided to Ontario LTC homes.

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