scholarly journals Estimation of the age-specific rate of natural mortality for Shetland sandeels

2004 ◽  
Vol 61 (2) ◽  
pp. 159-164 ◽  
Author(s):  
R.M. Cook
Keyword(s):  
Survey Data ◽  
Population Analysis ◽  
Mean Value ◽  
Specific Rate ◽  
Natural Mortality ◽  
The North ◽  
Virtual Population ◽  
Stock Assessments ◽  
The North Sea ◽  
The Mean

Abstract It is generally difficult to obtain reliable direct estimates of natural mortality, M, from conventional fisheries data and stock assessments. However, as a result of the closure of the Shetland sandeel (Ammodytes marinus) fishery from 1991 to 1994 and in the absence of any significant fishery in other years, research vessel survey data offer a rare opportunity to obtain estimates of M directly. A model is described that assumes that M can be decomposed into an age effect and year effects. Application of the model to the survey data produces values of M that decline from 2.1 for 0-group fish to 0.6 at age 2. There is some indication of an increase for ages 4 and older. Although there does not appear to be an overall trend in the mean value of M for the period 1985–1999, the annual values change by up to 50%. The values calculated from the model are in line with estimates obtained for the North Sea from multispecies virtual population analysis (MSVPA).

Monthly-mean residual flows through the Dover Strait, 1949–1972

1978 ◽  
Vol 58 (4) ◽  
pp. 965-973 ◽  
Author(s):  
D. Prandle
Keyword(s):  
Time Series ◽  
North Sea ◽  
Marine Biology ◽  
Small Variation ◽  
Mean Value ◽  
Residual Flow ◽  
Mean Flow ◽  
The North ◽  
The North Sea ◽  
The Mean

An estimate is made of the mean value of residual flow through the Dover Strait for each month over the 24–year period from 1949 to 1972. The estimates are based on results from a modelling investigation by Prandle (1978) where it was shown that the residual flow consists of three components, (a) a tidal residual, (b), a wind-driven residual and (c) a flow due to a long-term gradient in mean sea level. The components (a) and (c) are assumed to be constant and the value of (b) is deduced using wind data recorded by Dutch Light Vessels located in the southern North Sea.The mean flow over the whole period amounts to 155 × 103 m3 s–1 into the North Sea with a maximum value of 364 x 103 m3 s–1 and a minimum of – 15 × 103 m3 s–1 (out of the North Sea). One notable feature of the complete time series is the surprisingly small variation in the annual mean flows; perhaps this stability in the annual flow is of significance to the marine biology of the area.The validity of the computed time series is established by reference to comparable data including a 9–year record, from cross-channel submarine cables, of the potential induced by the flow of water through the Earth's magnetic field. Additional comparisons are also made with the results of a previous study of daily-mean flows.

Virtual Population Analysis (VPA) Equations for Nonhomogeneous Populations, and a Family of Approximations Including Improvements on Pope's Cohort Analysis

1986 ◽  
Vol 43 (12) ◽  
pp. 2406-2409 ◽  
Author(s):  
Alec D. MacCall
Keyword(s):  
Mortality Rate ◽  
Population Analysis ◽  
Seasonal Pattern ◽  
Cohort Analysis ◽  
Natural Mortality ◽  
Close Approximation ◽  
Virtual Population Analysis ◽  
Virtual Population ◽  
Special Case

A set of "backward" virtual population analysis (VPA) equations relates catch (Ct) from continuous fishing between times t and t + 1 to population n size (Nt, Nt+1) when a portion of the stock is unavailable to fishing. The usual VPA equations become a special case where the entire stock is available (i.e. the stock is homogeneous). A close approximation to the VPA equations is Nt = Nt+1 exp(M) + CtM/(1 − exp(−M)), which has properties similar to Pope's "cohort analysis" and is somewhat more accurate in the case of a continuous fishery, especially if the natural mortality rate (M) is large. Much closer simple approximations are possible if the seasonal pattern of catches is known.

Long-term variability in North Sea zooplankton off the Northumberland coast: productivity of small copepods and analysis of trophic interactions

1988 ◽  
Vol 68 (1) ◽  
pp. 143-164 ◽  
Author(s):  
John C. Roff ◽  
Ken Middlebrook ◽  
Frank Evans
Keyword(s):  
North Sea ◽  
Trophic Interactions ◽  
Growth Rates ◽  
Larval Fish ◽  
The North ◽  
The North Sea ◽  
The Mean ◽  
June July

All groups of meso- and macro-zooplankton in the North Sea off Northumberland, at a depth of 53 m, were studied during a 15-year period (1969–83); copepod productivity was estimated from biomass and growth rates. Phytoplankton were seasonally bi-modal with peaks in April and August–October; copepods were uni-modal peaking in June–July. The predatory zooplankters: larval fish, decapods, ctenophores, medusae (the summer-autumn predators) peaked between May and September, while chaetognaths and euphausiids (the winter predators) peaked in December–January. Copepods and the summer-autumn predators were seasonally and inter-annually positively correlated, and declined in abundance from 1974 to 1980. Euphausiids and chaetognaths on the contrary increased in abundance during these years, and were seasonally and inter-annually negatively correlated to the copepods. The mean annual abundance of copepods was positively related to the previous winter's minimum, and inversely related to the abundance of chaetognaths and euphausiids. Annual copepod productivity averaged 1260 kJ m-2 year-1, and showed no relationship to other groups of plankton.

scholarly journals Evaluación de la pesquería de Lutjanus peru (Pisces: Lutjanidae) de . Guerrero, México

1969 ◽  
pp. 571-580
Author(s):  
Apolinar Santamaría ◽  
Ernesto A Chávez
Keyword(s):  
Growth Model ◽  
Population Analysis ◽  
Small Fish ◽  
Natural Mortality ◽  
Von Bertalanffy ◽  
Cohort Size ◽  
Virtual Population ◽  
Von Bertalanffy Growth Model ◽  
Parameter Values ◽  
Yield Per Recruit

Red snapper (Lutjanus peru) fishery was analyzed from landings and catch records. Stock age structure was reconstructed after the parameter values of the von Bertalanffy growth model, the length-weight relationship, ages and the natural mortality coefficient through each of nine years of cateh records. The Pisat software package was applied to assess population parameters, whose estimates are, for the von Bertalanffy growth model, K = 0. 1 442 to 0.38; lo = -0.2; L = 87 cm; W = 9.4 Kg, and the natural mortality coefficient (M) afier several methods (0. 14 to 0.38). Cohort size was assessed by the virtual population analysis (VPA), estlmating population size in 5.2* 1 06 fish with a biomass of 8 454 tonnes. Current fishing mortality P, ranges from 0.06 to 1 . 1 3, depending upon the chosen M value; according to this, when the M value used is low, the results suggest that the stock is nnderexploited, and vice versa. The yield per recruit model applied suggests improvements to the management strategy. The model indicates recruit overfishing because very small fish are the main target (te

scholarly journals Age and Growth in Phocoena Phocoena Linnaeus, 1758 (Cetacea, Odontoceti) from the North Sea

1978 ◽  
Vol 48 (1) ◽  
pp. 16-28 ◽  
Author(s):  
W.L. van Utrecht
Keyword(s):  
North Sea ◽  
Posterior Part ◽  
The Body ◽  
Age And Growth ◽  
Body Measurements ◽  
Lower Growth ◽  
The North ◽  
The North Sea ◽  
Lower Growth Rate ◽  
The Mean

Data and material are collected from 106 Harbour Porpoises (P. phocoena) from the southern part of the North Sea. All animals are accidentally caught or found stranded. The greatest length for males in the sample is 151 cm, for females 186 cm. For detailed analysis of body measurements, 30 males and 37 females are selected, while from 20 males and 34 females teeth are used for age analysis. The analysis of the body measurements shows sexual dimorphism in the anterior and posterior part of the back, in the flukes and flippers and in the position of the genital slit. The maximum number of dentinal layers found in the teeth is 12. Males attain sexual maturity after the deposition of 5 dentinal layers, females when 6 layers are formed, at a body length of about 135 cm and 150 cm, respectively. Some evidence is found that the population of P. phocoena from the North Sea has a lower growth rate than the population from Canadian waters. The gestation period is estimated to be eleven months, the peak of the birth period being in June. The animals are born at a length varying between 67 cm to 80 cm. Growth of the visceral organs is isogonic. The mean weight of the organs is greater in females than in males.

Incorporating Uncertainty into Fishery Models

2002 ◽  
Keyword(s):  
Monte Carlo ◽  
Stock Assessment ◽  
Population Analysis ◽  
Current Method ◽  
Spanish Mackerel ◽  
Virtual Population ◽  
Stock Assessments ◽  
History Of ◽  
Management Advice ◽  
Bootstrap Algorithm

<em>Abstract.—</em> The stock assessment analyses of king and Spanish mackerel fisheries of the southeastern United States have a long history of incorporating uncertainty. The development of this philosophy resulted from a number of unique circumstances, both biological and historical, that encouraged the incorporation of stochastic approaches and risk evaluation to the assessment and management process. The progression from simple discrete decision tree analysis to delta methods to Monte Carlo/bootstrap methods was due not only to advances in assessment technology but also to changing requirements for management. The current method for mackerel stock assessment is a tuned virtual population analysis with uncertainty incorporated via a mixed Monte Carlo/bootstrap algorithm. Through this procedure, uncertainty in the tuning indices, catch-at-age and natural mortality rate are directly incorporated into the advice provided to management. The management advice is given in terms of probability statements, as opposed to point estimates, to reflect this uncertainty in the stock assessments. This approach is a result of the evolution of the assessment and management and provides a pragmatic alternative in the “frequentist versus Bayesian” debate.

scholarly journals Temporally varying natural mortality: Sensitivity of a virtual population analysis and an exploration of alternatives

2017 ◽  
Vol 185 ◽  
pp. 185-197 ◽  
Author(s):  
Shanae D. Allen ◽  
William H. Satterthwaite ◽  
David G. Hankin ◽  
Diana J. Cole ◽  
Michael S. Mohr
Keyword(s):  
Population Analysis ◽  
Natural Mortality ◽  
Virtual Population Analysis ◽  
Virtual Population

Consistency of Towing Speed and Sampling Depth for the Continuous Plankton Recorder

1993 ◽  
Vol 73 (4) ◽  
pp. 967-970 ◽  
Author(s):  
G. C. Hays ◽  
A. J. Warner
Keyword(s):  
Time Series ◽  
North Atlantic ◽  
Data Sets ◽  
Sampling Depth ◽  
The North ◽  
The North Sea ◽  
The Mean ◽  
Towing Speed ◽  
Term Data

The mean annual towing speed of the Continuous Plankton Recorder (CPR) varied systematically between 1946 and 1991. By mounting a pressure transducer on the CPR to record towing depth, it was shown, however, that changes in towing speed did not cause a significant change in towing depth, although the mean towing depth (6–7 m, SD=l-7 m, N=77) was shallower than the previously assumed towing depth of 10 m. Thus the observed changes in towing speed are unlikely to have caused discontinuities in the CPR time-series by affecting sampling depth.Long-term data sets play an important role in attempts to understand the causes of fluctua- tions in plankton abundance. The Continuous Plankton Recorder (CPR) survey provides multi- decadal information on plankton abundance in the North Sea and North Atlantic (McGowan, 1990), and is one of the longest standing marine plankton abundance time-series. However, while the CPR time-series has great potential, as with all other data sets spanning many years, questions may be asked regarding the consistency with which the data have been collected and hence the true continuity of the time-series.

Reliability analysis of foundation stability for gravity platforms in the North Sea

1989 ◽  
Vol 26 (3) ◽  
pp. 359-368 ◽  
Author(s):  
Tien H. Wu ◽  
Oddvar Kjekstad ◽  
In-Mo Lee ◽  
Suzanne Lacasse
Keyword(s):  
North Sea ◽  
Reliability Index ◽  
Site Investigation ◽  
Offshore Structures ◽  
Material Strength ◽  
Strength Measurement ◽  
The North ◽  
The North Sea ◽  
Coefficients Of Variations ◽  
The Mean

The uncertainties encountered in the evaluation of foundation stability for four offshore gravity structures in the North Sea are reviewed. The uncertainties include those about the load, the material type, the material strength, and the analytical method. The means and coefficients of variations of these are used as inputs to compute the mean and coefficient of variation of the safety factor and the reliability index. The latter are used to assess the effect of various options in site exploration and strength measurement on foundation reliability. Key words: foundation, gravity platforms, offshore structures, probability, reliability, shear strength, site investigation, stability.

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