An Estimation of the Mortality Rates in a Population of Tilapia esculenta Graham (Pisces, Cichlidae) in Lake Victoria, East Africa

1963 ◽  
Vol 20 (1) ◽  
pp. 195-227 ◽  
Author(s):  
D. J. Garrod
Keyword(s):  
Population Dynamics ◽  
Marked Effect ◽  
Lake Victoria ◽  
Central Africa ◽  
Mortality Rates ◽  
Fishing Effort ◽  
Tropical Fish ◽  
Commercial Fisheries ◽  
Fishing Mortality ◽  
Gill Net

In the commercial fisheries of the great lakes of Central Africa the nylon gill net is the most widespread type of gear in use at present. Recent increases in fishing effort are believed to have had a marked effect upon the abundance of some species and this paper describes a method of estimating the fishing mortality from a fishery of this type. The method is demonstrated in its application to a population of Tilapia esculenta in Lake Victoria. In particular the data from the gill net fishery are combined with the known selectivity of the gear to distinguish between fishing mortality and trends in natural loss from the population with increasing age of the fish. The study also shows that the population dynamics of a tropical fish species are amenable to conventional techniques of analysis.

scholarly journals Optimal fishing mortality assignment for southern hake Merluccius australis in Chile

2020 ◽  
Vol 48 (4) ◽  
pp. 613-625
Author(s):  
Felipe Lopez ◽  
Jorge Jimenez ◽  
Cristian Canales
Keyword(s):  
Mortality Rate ◽  
Economic Benefit ◽  
Mortality Rates ◽  
Fish Population ◽  
Fishing Effort ◽  
Economic Benefits ◽  
Historical Background ◽  
Total Catch ◽  
Fishing Mortality ◽  
Objective Functions

Since 1979, southern hake (Merluccius australis) has been exploited in Chile from the Bio Bio to the Magallanes regions, between the parallels 41°28.6'S and 57°S. There is evidence of a constant fishing effort and a sustained reduction of the fish population, consistent with a progressive decrease in total annual catches. Management strategies based on the maximum sustainable yield (MSY) and quota assignment/ distribution criteria have not been able to sustain acceptable biomass levels. A non-linear optimization model with two objective functions was proposed to determine an optimal total catch quota for more sustainable exploitation of this fishery. The first function maximizes the total catch over time in response to an optimal assignment of fishing mortality rates per fleet; the second function maximizes the total economic benefit associated with the total catch. The dynamics of the fish population were represented with the equations of a predictive age-structured model. Decision variables were fishing mortality rates and annual catch quotas per fleet, subject to constraints that guarantee a minimum level of biomass escape over a long-term period. The input parameters were obtained from the last stock evaluation report carried out by the Instituto de Fomento Pesquero (IFOP) of Chile. The historical background data of the fishery and the regulatory framework were relevant aspects of the methodology. Five scenarios were evaluated with the two objective functions, including a base scenario, which considered the referential mortality rate as input data as the average mortality rate per fleet from 2007 to 2012. Total economic benefits fluctuate between 102 and USD 442 million for total catches in the range of 108 to 421 thousand tons, which were obtained from maximizing the economic and biological objective functions. Economic benefit/catch ratios were reduced for scenarios with higher constraints on catch limits, and they were more efficient from a biological point of view. Situations with lighter constraints showed in general higher economic benefits and better performance ratios than those with stronger restrictions. The use of optimization models may provide a useful tool to evaluate the effect of regulations for adequate conservation and economical utilization of a limited resource.

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.

A Bayesian state–space mark–recapture model to estimate exploitation rates in mixed-stock fisheries

2006 ◽  
Vol 63 (2) ◽  
pp. 321-334 ◽  
Author(s):  
Catherine G.J Michielsens ◽  
Murdoch K McAllister ◽  
Sakari Kuikka ◽  
Tapani Pakarinen ◽  
Lars Karlsson ◽  
...  
Keyword(s):  
State Space ◽  
Expert Knowledge ◽  
Mortality Rates ◽  
Fishing Effort ◽  
Model Parameters ◽  
Fishing Mortality ◽  
Mark Recapture ◽  
Observation Process ◽  
Mixed Stock ◽  
Reporting Rates

A Bayesian state–space mark–recapture model is developed to estimate the exploitation rates of fish stocks caught in mixed-stock fisheries. Expert knowledge and published results on biological parameters, reporting rates of tags and other key parameters, are incorporated into the mark–recapture analysis through elaborations in model structure and the use of informative prior probability distributions for model parameters. Information on related stocks is incorporated through the use of hierarchical structures and parameters that represent differences between the stock in question and related stocks. Fishing mortality rates are modelled using fishing effort data as covariates. A state–space formulation is adopted to account for uncertainties in system dynamics and the observation process. The methodology is applied to wild Atlantic salmon (Salmo salar) stocks from rivers located in the northeastern Baltic Sea that are exploited by a sequence of mixed- and single-stock fisheries. Estimated fishing mortality rates for wild salmon are influenced by prior knowledge about tag reporting rates and salmon biology and, to a limited extent, by prior assumptions about exploitation rates.

scholarly journals Estimating natural and fishing mortality and tag reporting rate of southern rock lobster (Jasus edwardsii) from a multiyear tagging model

2001 ◽  
Vol 58 (12) ◽  
pp. 2490-2501 ◽  
Author(s):  
S D Frusher ◽  
J M Hoenig
Keyword(s):  
Mortality Rates ◽  
Fishing Effort ◽  
Reporting Rate ◽  
Standard Errors ◽  
Natural Mortality ◽  
Fishing Mortality ◽  
Rock Lobster ◽  
Relative Standard ◽  
Jasus Edwardsii ◽  
Mortality Estimates

Fishing and natural mortality rates and tag reporting rate for rock lobsters (Jasus edwardsii) in northwest Tasmania, Australia, were estimated using multiyear tagging models. These estimates are necessary for assessment of the resource. Several models were examined that had either two or three tagging events each year, and either combined sexes or kept sexes separate. The model that best described the dynamics of the fishery utilized three tagging events within a year. The year was divided into discrete periods and, within each year, fishing effort and duration of period were used to apportion fishing and natural mortalities, respectively, to the periods. The separation of fishing mortalities by sex was not found to improve the models. Although high (1.0–1.2·year–1), the instantaneous fishing mortality estimates were comparable to estimates obtained from other methods and the relative standard errors were low. Reporting rate estimates were also precise and indicated a lack of participation by the fishing industry. Estimates of natural mortality were low (0.00–0.02·year–1) but imprecise.

Bioeconomics of the Allee effect in fisheries targeting sedentary resources

2018 ◽  
Vol 75 (4) ◽  
pp. 1362-1373 ◽  
Author(s):  
Enrique González-Durán ◽  
Alvaro Hernández-Flores ◽  
Juan Carlos Seijo ◽  
Alfonso Cuevas-Jiménez ◽  
Angélica Moreno-Enriquez
Keyword(s):  
Population Dynamics ◽  
Spatial Distribution ◽  
Allee Effect ◽  
Positive Relationship ◽  
Commercial Fisheries ◽  
Bioeconomic Model ◽  
Fishing Mortality ◽  
New Models ◽  
The Right

Abstract The improvement of survival in some species due to the close coexistence between individuals is a concept that has caught the attention of scientists over the years. This behaviour has been documented in many taxa, including species which support commercial fisheries. In recent years, a better understanding of the mechanisms associated with the Allee effect has shown its importance in population dynamics and probably plays a more important role than previously thought. In this article, the Allee effect was analysed by means of a spatial bioeconomic model. The Allee effect component was the positive relationship between the mating probability and the density of spawners. A depensation function with an offset to the right side of the origin represented the fraction of females that mated based on density. When the model did not include the Allee effect, the fishery reached the bioeconomic equilibrium; however, when the Allee effect was considered, the fishery did not reach equilibrium, but the stock collapsed in 15 years. Fishing mortality altered the spatial distribution of the stock, causing depensation in recruits per spawner. These results may contribute to a better understanding of the Allee effect on sedentary resources and can be used to explore new models.

Optimal Effort Allocation Among Competing Mixed-Species Fisheries, Subject to Fishing Mortality Constraints

1986 ◽  
Vol 43 (1) ◽  
pp. 90-100 ◽  
Author(s):  
S. A. Murawski ◽  
J. T. Finn
Keyword(s):  
Optimal Allocation ◽  
Mortality Rates ◽  
Fishing Effort ◽  
Discrete Distributions ◽  
Sensitivity Analyses ◽  
Otter Trawl ◽  
Individual Species ◽  
Fishing Mortality ◽  
Effort Allocation ◽  
By Catch

A linear programming (LP) approach to effort allocation among two or more fisheries (fleets) exploiting several common species/stocks is described and applied to otter trawl fisheries exploiting demersal fish stocks on Georges Bank (northeastern United States). Total instantaneous fishing mortality on a particular species (i) is computed as the linear summation of fishing mortalities generated by each fishery (j):[Formula: see text]where fj is the amount of standardized fishing effort exerted in fishery j and qij is the catchability coefficient for species i taken in fishery j. Mortality on species i due to both directed fishing and by-catch can thus be accounted for in the qij's. Optimal allocation of effort among the j fisheries may be considered a minimization problem (minimize Σfj), subject to the constraints that fishing mortality rates on particular species are maintained at, above, or below certain predefined levels. Fishing mortality goals for individual species can be based on various biological and/or economic criteria: fishing mortality rates that prevent growth or recruitment overfishing, or that optimize productivity from predator–prey systems. Other constraints in the LP model may be included to modify optimal solutions based on various economic and social considerations (e.g. protection of certain fisheries). Sensitivity analyses indicate the general infeasibility of maintaining relatively high or low fishing mortality rates on ubiquitously distributed species, while moderately fishing species with more discrete distributions, due to by-catch considerations.

scholarly journals Partial fishing mortality per fishing trip: a useful indicator of effective fishing effort in mixed demersal fisheries

2006 ◽  
Vol 63 (3) ◽  
pp. 556-566 ◽  
Author(s):  
Adriaan D. Rijnsdorp ◽  
Niels Daan ◽  
Willem Dekker
Keyword(s):  
Fishing Effort ◽  
Fine Tuning ◽  
Positive Trend ◽  
Fishing Mortality ◽  
Auxiliary Equipment ◽  
Solea Solea ◽  
Seasonal And Spatial Variations ◽  
Sea Beam ◽  
Quota Management ◽  
Engine Power

Abstract Effort management has been proposed as an alternative for quota management in mixed demersal fisheries. It requires a metric to estimate the fishing mortality imposed by a given quantity of nominal fishing effort. Here, we estimate the partial fishing mortality rate imposed by one unit of fishing effort (Fpue) during individual fishing trips and explore the usefulness of this indicator for managing North Sea beam trawlers >300 hp targeting sole (Solea solea) and plaice (Pleuronectes platessa). Fpue is positively related to vessel engine power, and increased annually by 2.8% (sole) and 1.6% (plaice). The positive trend was due to an increase in skipper skills and investment in auxiliary equipment, the replacement of old vessels by new ones and, to a lesser extent, to upgrade engines. The average Fpue imposed per day at sea by a 2000 hp beam trawler was estimated to be 1.0 × 10−5 (sole) and 0.6 × 10−5 (plaice), and it showed substantial seasonal and spatial variations. The Fpue of sole and plaice were negatively related in summer and showed no relationship in winter. The existence of predictive seasonal and spatial patterns in Fpue opens up the possibility of fine-tuning management by directed effort restrictions and uncoupling management of plaice and sole.

scholarly journals The influence of microhabitat on the population dynamics of four herbaceous species in a semiarid area of northeastern Brazil

2016 ◽  
Vol 76 (1) ◽  
pp. 45-54 ◽  
Author(s):  
K. A. Silva ◽  
J. M. F. F. Santos ◽  
J. R. Andrade ◽  
E. N. Lima ◽  
U. P. Albuquerque ◽  
...  
Keyword(s):  
Population Dynamics ◽  
Population Size ◽  
Mortality Rates ◽  
Dry Forest ◽  
Northeastern Brazil ◽  
Annual Rainfall ◽  
Herbaceous Species ◽  
Forest Fragment ◽  
Experimental Station ◽  
Over Time

Abstract Variation in annual rainfall is considered the most important factor influencing population dynamics in dry environments. However, different factors may control population dynamics in different microhabitats. This study recognizes that microhabitat variation may attenuate the influence of climatic seasonality on the population dynamics of herbaceous species in dry forest (Caatinga) areas of Brazil. We evaluated the influence of three microhabitats (flat, rocky and riparian) on the population dynamics of four herbaceous species (Delilia biflora, Commelina obliqua, Phaseolus peduncularis and Euphorbia heterophylla) in a Caatinga (dry forest) fragment at the Experimental Station of the Agronomic Research Institute of Pernambuco in Brazil, over a period of three years. D. biflora, C. obliqua and P. peduncularis were found in all microhabitats, but they were present at low densities in the riparian microhabitat. There was no record of E. heterophylla in the riparian microhabitat. Population size, mortality rates and natality rates varied over time in each microhabitat. This study indicates that different establishment conditions influenced the population size and occurrence of the four species, and it confirms that microhabitat can attenuate the effect of drought stress on mortality during the dry season, but the strength of this attenuator role may vary with time and species.

Efficiency and selectivity of the Anomalocardia brasiliana (Mollusca: Veneridae) hand dredge used in southern Brazil

2010 ◽  
Vol 90 (7) ◽  
pp. 1455-1464 ◽  
Author(s):  
Paulo R. Pezzuto ◽  
Caroline Schio ◽  
Tito C.M. Almeida
Keyword(s):  
Management Plan ◽  
Fishing Effort ◽  
Management Tool ◽  
Small Scale ◽  
Commercial Fisheries ◽  
Effective Management ◽  
Southern Brazil ◽  
Fishing Rights ◽  
Before And After ◽  
Experimental Plots

In Florianópolis, southern Brazil, the venerid clam Anomalocardia brasiliana has supported subsistence and small-scale commercial fisheries for decades. The introduction of a hand dredge (gancho) since 1987 led to the development of a significant fishery supplying both local and regional shellfish markets. In 1992 one of the main fishing areas in the region was designated as the first Brazilian Marine Extractive Reserve (Pirajubaé RESEX), a federal form of governance intended to promote sustainable exploitation of natural resources by assigning exclusive fishing rights to traditional users. However, excessive fishing effort, institutional shortcomings and lack of a negotiated management plan have resulted in the overexploitation of the species since 2000. This study was aimed at evaluating the efficiency and selectivity of the hand dredge currently in use at the RESEX, through a field experiment conducted in October 2006. Quantitative samples of A. brasiliana were collected before and after dredging 15 experimental plots. Additional samples were obtained inside the dredge (catch) and respective cover cod-end (discard) for selectivity analysis. A single haul of the hand dredge can dislocate up to 76% of the individuals present in the sediment irrespective of their size, and retain up to 69% of the commercial-sized organisms. The gear has a knife-edge selection pattern, which enables the use of the minimum spacing between the iron bars of the dredge's basket as an effective management tool.

scholarly journals An integrated tagging model to estimate mortality rates of Albemarle Sound – Roanoke River striped bass

2017 ◽  
Vol 74 (7) ◽  
pp. 1061-1076 ◽  
Author(s):  
Julianne E. Harris ◽  
Joseph E. Hightower
Keyword(s):  
Striped Bass ◽  
Stock Assessment ◽  
Mortality Rates ◽  
Natural Mortality ◽  
Fishing Mortality ◽  
Albemarle Sound ◽  
High Reward ◽  
Accuracy And Precision ◽  
Summer Mortality ◽  
Roanoke River

We developed an integrated tagging model to estimate mortality rates and run sizes of Albemarle Sound – Roanoke River striped bass (Morone saxatilis), including (i) a multistate component for telemetered fish with a high reward external tag; (ii) tag return components for fish with a low reward external or PIT tag; and (iii) catch-at-age data. Total annual instantaneous mortality was 1.08 for resident (458–899 mm total length, TL) and 0.45 for anadromous (≥900 mm TL) individuals. Annual instantaneous natural mortality was higher for resident (0.70) than for anadromous (0.21) fish due to high summer mortality in Albemarle Sound. Natural mortality for residents was substantially higher than currently assumed for stock assessment. Monthly fishing mortality from multiple sectors (including catch-and-release) corresponded to seasonal periods of legal harvest. Run size estimates were 499 000–715 000. Results and simulation suggest increasing sample size for the multistate component increases accuracy and precision of annual estimates and low reward tags are valuable for estimating monthly fishing mortality rates among sectors. Our results suggest that integrated tagging models can produce seasonal and annual mortality estimates needed for stock assessment and management.

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