Estimates Of Natural Mortality Rate Research Articles

Impact of fishing activities on the population dynamics of European whitefish in four peri-alpine lakes

Inland fisheries play a crucial role in socio-economic activities and food production. However, these fisheries are often less intensively monitored and studied than marine fisheries. In the past two decades, important fluctuations in European whitefish (Coregonus lavaretus) landings have been observed in four peri-alpine lakes in France and Switzerland. To better gauge the impact of fishing removal, a multi-lakes Delay-Difference model was developed and adapted to these fisheries. A hierarchical state-space Bayesian framework was used to integrate stochasticity for each process and incorporate diverse data. The model gave estimates of time series for biomass, recruitment, stage-structured landings, and fishing mortality, as well as estimates of natural mortality rate, Brody growth coefficient and mean recruitment productivity. Differences in the estimates of variables predicted by the model were observed among lakes. Negative impacts of exploitation have been highlighted for Lakes Geneva, Bourget, and Annecy. Low levels of biomass under high fishing mortality rates were observed, while landings tended to increase under lower mortality rates. Lakes Geneva and Bourget outputs suggest stocks were impacted by fishing activities and that fishing effort reduction could ultimately increase yields. Lake Annecy abundance and landings trends increased after 2010, suggesting management strategies that focused on effort reduction were successful. Lake Neuchâtel stock abundance was low after 2017 and probably more fragile due to low biomass levels and recruitment. The observed changes in European whitefish biomass cannot be attributed solely to fishing, suggesting that environmental factors may also influence recruitment success alongside the impacts of fishing pressure.

Growth and natural mortality of Maurolicus muelleri and Benthosema glaciale in the Northeast Atlantic Ocean

Mesopelagic fish are considered a possible future fisheries resource, but the biological sustainability of their potential exploitation has not yet been assessed. Sustainability should be evaluated at the population level, for which accurate stock-specific life-history parameters are required. Here, we use a length-based model to estimate life-history parameters related to growth and natural mortality, and their uncertainty, for the assessment of Northeast Atlantic populations of Maurolicus muelleri (Gmelin, 1789, Mueller’s Pearlside) and Benthosema glaciale (Reinhardt, 1837, glacier lantern fish). We compare three different approaches to estimate natural mortality rates and provide recommendations for future data collection and monitoring programs. For M. muelleri, we estimated an asymptotic length of 57.98 mm and a von Bertalanffy growth constant of 1.28 year−1, while for B. glaciale, we estimated an asymptotic length of 78.93 mm and a von Bertalanffy growth constant of 0.41 year−1. Estimates of natural mortality rates for M. muelleri were around 1.29 year−1 and 1.80 year−1 based on empirical formulae with the estimated growth parameters and maximum age, respectively, and around 1.51 year−1 with the length-converted catch curve method for B. glaciale estimates ranged between 0.5, 0.68, and 0.75 year−1, with the three respective methods. Due to limited data availability, the estimated uncertainty of the provided life-history parameters is large and should be considered in the evaluation of the sustainability of potential mesopelagic exploitation following the precautionary approach.

Should Natural Mortality Estimators Based on Maximum Age Also Consider Sample Size?

AbstractNatural mortality rate can be difficult to estimate; consequently, indirect methods based on life history correlations are frequently used to infer values for stock assessments. One common class of predictors is based on the relationship between natural mortality rate and maximum age in unexploited or lightly exploited populations. The oldest age in a sample depends on the sample size, but there is confusion in the literature over the correct expression. Those attributable to Sarhan (1954) and Holt (1965) are correct. Kenchington's (2014) estimators that incorporate sample size are mathematically flawed and should not be used. Expressions exist for the expected value and variance of order statistics from an exponential distribution (including the maximum age), and these account for sample size. However, the resulting estimators are not recommended without evaluation of their performance because (1) they may not be robust to failures of the parametric assumptions, (2) it is often difficult or impossible to determine the effective sample size, and (3) new estimators may not be needed because of the insensitivity of the maximum age to sample size. Empirical estimators based on regressing estimates of natural mortality rate from the literature on corresponding values of maximum age continue to be a viable approach. They do not use information on sample size, but fortunately the maximum observed age is insensitive to sample size provided the sample size is reasonably large. Sometimes, the maximum age from an exploited population is used with an empirical regression estimator. This can give an estimate close to the natural mortality rate if relict, old fish are largely protected by virtue of their habitat or their low vulnerability to capture. However, in general, use of maximum age from an exploited population tends to give an estimate somewhere between the natural mortality rate and the maximum total mortality that the stock has experienced.Received May 18, 2016; accepted October 12, 2016 Published online December 12, 2016

Spatial and temporal dynamics of lake whitefish ( Coregonus clupeaformis) health indicators: Linking individual-based indicators to a management-relevant endpoint

We examined the spatial and temporal dynamics of health indicators in four lake whitefish ( Coregonus clupeaformis) stocks located in northern lakes Michigan and Huron from 2003 to 2006. The specific objectives were to (1) quantify spatial and temporal variability in health indicators; (2) examine relationships among nutritional indicators and stock-specific spatial and temporal dynamics of pathogen prevalence and intensity of infection; and (3) examine relationships between indicators measured on individual fish and stock-specific estimates of natural mortality. The percent of the total variation attributed to spatial and temporal sources varied greatly depending on the health indicator examined. The most notable pattern was a downward trend in the concentration of highly unsaturated fatty acids (HUFAs), observed in all stocks, in the polar lipid fraction of lake whitefish dorsal muscle tissue over the three study years. Variation among stocks and years for some indicators were correlated with the prevalence and intensity of the swimbladder nematode Cystidicola farionis, suggesting that our measures of fish health were related, at some level, with disease dynamics. We did not find relationships between spatial patterns in fish health indicators and estimates of natural mortality rates for the stocks. Our research highlights the complexity of the interactions between fish nutritional status, disease dynamics, and natural mortality in wild fish populations. Additional research that identifies thresholds of health indicators, below (or above) which survival may be reduced, will greatly help in understanding the relationship between indicators measured on individual fish and potential population-level effects.

CONTINUED DECLINE OF AN ATLANTIC COD POPULATION: HOW IMPORTANT IS GRAY SEAL PREDATION?

The continental shelf ecosystem on the Eastern Scotian Shelf (ESS) has experienced drastic changes. Once common top predators are a small fraction of their historical abundance, and much of the current community structure is now dominated by pelagic fishes and invertebrates. Embedded within this food web, Atlantic cod and gray seal populations have recently exhibited nearly opposite trends. Since 1984, cod populations have decreased exponentially at a rate averaging 17% per year, whereas gray seals have continued to increase exponentially at a rate of 12%. We reexamined the impact of gray seals on Atlantic cod dynamics using more than 30 years of data on the population trends of cod and gray seals while incorporating new information on seal diet and seasonal distribution. The closure of the cod fishery over 10 years ago allowed for a better estimation of natural mortality rates. We quantified the impact of seals on ESS cod by (1) estimating trends in seal and cod abundance, (2) estimating the total energy needed for seal growth and maintenance from an energetics model, (3) using estimates of the percentage of cod in the total diet derived from quantitative fatty acid signature analysis (QFASA) and of the size-specific selectivity of cod consumed (derived from otoliths collected from fecal samples), and (4) assuming a gray seal functional response. Uncertainties of the model estimates were calculated using the Hessian approximation of the variance-covariance matrix. Between 1993 and 2000, cod comprised, on average, < 5% of a gray seal's diet. Our model shows that, since the closure of the fishery, gray seals have imposed a significant level of instantaneous mortality (0.21), and along with other unknown sources of natural mortality (0.62), are contributing to the failure of this cod stock to recover.

A tag-attrition model as a means to estimate the efficiency of two types of tags used in tropical tuna fisheries

Abstract Conventional “spaghetti” tags and tags originally designed for sport fishing, called Betyp tags, were used during a tuna tagging program conducted on board Dakar bait boats in 1999. With the aim of quantifying a difference in tag losses between the conventional and the Betyp tags, we integrated the tag type into the tag-attrition model used to fit the recoveries per unit of time. To account for possible over-dispersion in mark-recapture data, the model selection was based on a quasi-likelihood information criterion. The main findings indicate that the immediate mortality on bigeye tuna due to the Betyp tags is around 23%. In contrast, because an additional type-2 tag losses effect was not supported by the data, we cannot conclude there is a difference in continuous tag shedding and continuous mortality between the two types of tags. The attrition rate for juveniles of bigeye tuna (average FL = 57.7 cm) provided by this model (0.58 per year) is consistent with previous estimates of natural mortality rate. One interest of combining different data sets, which have processes in common, is to provide information for the shared parameters, increasing the precision of their estimates. In addition, advantages and disadvantages of each type of tags are discussed. Based on these results and the difficulties to implement Betyp tags, we highlight the fact that Betyp tags are not well suited for intensive tropical tuna tagging programs.

Application of a stochastic spawning stock biomass per recruit model for the horse clam fishery in British Columbia

A directed commercial fishery on horse (gaper) clams, Tresus capax and Tresus nuttallii (Bivalvia: Mactridae) is being developed in BC, Canada. We adapted a spawning stock biomass per recruit (SPR) analysis to evaluate the effect of different multi-year rotation fisheries and alternative exploitation rates on the stock reproduction potential. Our analysis also incorporated variation in the estimates of natural mortality rate and growth, converting uncertainties inherent in scientific knowledge into estimates of biological risk. The information produced will enable fishery managers to choose a fishing scheme based on the degree of precaution required. At the initial phase of developing this fishery, a highly precautionary biological reference point is recommended.

Theoretical model of a new fishery under a simple quota management system

Quota management systems (QMS) have become a popular method in fisheries management. Unfortunately, good data is rarely available for setting robust total allowable catch (TAC) limits. TACs are often set based on previous yields or estimates of natural mortality rates and may be adjusted in response to new data or even political pressures. This paper presents a discrete time, age structured, single species fishery model with dynamic TAC adjustment. The model can be used to explore a range of TAC adjustment strategies in the absence of fishery population data. The model was created in the STELLA modeling software. It is modular and may be extended to support the study of other fisheries, management systems and economic considerations. This paper applies the model to a hypothetical new fishery. Results suggest that TACs may be adjusted rather aggressively based on previous catches to achieve the maximum sustainable yield (MSY) without significant risk to the fishery.

Direct estimation of natural mortality rates for littoral marine fishes using populational data from a marine reserve

Visual censuses conducted in a marine reserve (Medas Islands) were used to estimate the natural mortality rates (M) for five common fish species in the NW Mediterranean Sea (Coris julis, Diplodus annularis, D. sargus, Serranus cabrilla and Symphodus roissali). Visual censuses of these same five species were also performed at three sites in unprotected areas of the coast where both commercial and sport fishing activity was normal. Censuses were conducted over a 3 year period. Estimates of M in the 3 years displayed scant seasonal or interannual variation, which may mean that the populations were in equilibrium during that period. The results of this study showed that the relationships between M and the growth parameters and maximum life span were unclear, and considerable caution is therefore recommended when using indirect methods of estimating M based on those parameters. For certain species the values of M were equal to or greater than the estimated total mortality in the exploited areas. The virtual absence of piscivorous predators in the unprotected area as a consequence of the high level of fishing in that area contrasts with the high abundance of such predators in the marine reserve. Since predation is the main contributor to M, estimated mortality in the unprotected areas is attributable nearly entirely to fishing. It is suggested that M may vary according to alterations taking place in conditions in the ecosystem inhabited by a species and thus that use of a value of M for a pristine population cannot be extrapolated to exploited areas.

A spatial population dynamics simulation model of tropical tunas using a habitat index based on environmental parameters

We are developing a spatial, multigear, multispecies population dynamics simulation model for tropical tunas in the Pacific Ocean. The model is age‐structured to account for growth and gear selectivity. It includes a tuna movement model based on a diffusion–advection equation in which the advective term is proportional to the gradient of a habitat index. The monthly geographical distribution of recruitment is defined by assuming that spawning occurs in areas where sea surface temperature is above 25°C. During the first 3 months of their life, simulated tunas are transported by oceanic currents, after which movement is conditioned by gradients in the habitat index. Independent estimates of natural mortality rates and population size from large‐scale tagging experiments carried out by the Secretariat of the Pacific Community are used in the simulations. The habitat index consists of components due to forage density and sea surface temperature, both of which are suspected to play major roles in determining tuna distribution. Because direct observations of forage are not available on a basin scale, we developed a submodel to simulate the surface tuna forage production (Lehodey et al., 1998). At present, only skipjack (Katsuwonus pelamis; a surface tuna species caught by purse seine and by pole‐and‐line) is considered, at a 1°‐square resolution and on a monthly climatological time series. Despite the simplicity of the model and the limitations of the data used, the simulation model is able to predict a distribution of skipjack catch rates, of the different fleets involved in the fishery, that is fairly consistent with observations.

ESTIMATING NATURAL MORTALITY RATE IN REPRODUCTIVELY ACTIVE FEMALE SOUTHERN RIGHT WHALES, EUBALAENA AUSTRALIS

AbstractCow‐calf pairs of southern right whales on the South African coast have been photographed in aerial surveys in October each year since 1979. In this paper 469 resightings of 177 individually identified cows photographed in the first six years of surveys have been analyzed in two ways to produce estimates of natural mortality rate. Both methods assume that all females calve either two, three, or four years after their previous calf. In Method A there is assumed to be no systematic trend with time in the probability of a female being photographed on each calving occasion. Natural logarithms of the numbers photographed 2‐4, 5‐7, 8‐10, etc. yr after being first seen are regressed against time, the slope of which provides an estimate of natural mortality rate of 0.0255 ± 0.0071. The intercept value for this regression provides an estimate of the detection probability, or 0.769 ± 0.011. In Method B it is assumed that if a female has not been photographed for at least nine years then it is dead. Annual mortality estimates are obtained from the proportional reduction in the numbers of females known to be alive at each three‐year interval after being first photographed, up to at least 6‐8 yr from the present. Method B produces an estimate of natural mortality of 0.0260 ± 0.0190 (corrected to 0.0227 ± 0.0192 using the average detection probability). Both methods may be subject to various biases that tend to inflate estimates of natural mortality.

Use of the bomb radiocarbon chronometer to determine age of southern bluefin tuna Thunnus maccoyii

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 143:1-8 (1996) - doi:10.3354/meps143001 Use of the bomb radiocarbon chronometer to determine age of southern bluefin tuna Thunnus maccoyii Kalish JM, Johnston JM, Gunn JS, Clear NP The growing otoliths of fish incorporate radiocarbon in concentrations that are equivalent to that found in ambient seawater dissolved inorganic carbon. Therefore, pulses of anthropogenic radiocarbon produced by the atmospheric detonation of nuclear weapons can ultimately be detected in otoliths. This study estimates the age of large southern bluefin tuna Thunnus maccoyii using an age estimation procedure based on the determination of levels of bomb-derived radiocarbon in their otoliths. Radiocarbon data from selected regions of southern bluefin tuna otoliths indicate that this species may reach ages in excess of 30 yr. Furthermore, individuals that approach the asymptotic length are likely to be 20 yr of age or older. The data agree generally with accepted models of southern bluefin growth, but show that these fish live longer than was believed previously. Comparisons between otolith section and bomb radiocarbon age estimates indicate that reading otolith sections is an effective method to estimate the age of larger southern bluefin. The presence of a significant number of individuals greater than 20 yr of age in the southern bluefin population may alter estimates of natural mortality rates currently used in Virtual Population Analysis models for stock assessment of this species. Age validation · Bomb radiocarbon chronometer · Carbon · Otoliths · Radiocarbon · Southern bluefin tuna · Thunnus maccoyii Full text in pdf format NextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 143. Publication date: November 14, 1996 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1996 Inter-Research.

Influence of Stream Flows and Stock Size on Recruitment of Arctic Grayling (Thymallus arcticus) in the Chena River, Alaska

The hypothesis that recruitment of Arctic grayling (Thymallus arcticus) in the Chena River is influenced by stream flows and stock size was tested using population data collected from 1976 through 1990. Recruitment may be influenced by stream flows during the initial weeks of life of Arctic grayling, namely during spawning, emergence, and the larval stage. Using correlation and regression analyses, stream flow during the time-frame was found to be a significant descriptor of variability in recruitment (r = −0.751, P = 0.005). Although stream flows were implicated in recruitment variation, creation of an environment-dependent, stock–recruitment model was not possible because estimates of measurement error were lacking, because of bias due to the relation between residuals and subsequent stock size, and because of the presumed autocorrelation of stock size. An alternative analysis was conducted to investigate the influence of stock size on recruitment when stream flows were thought to minimally affect recruitment. Using an estimate of natural mortality rate and assuming no fishing mortality, the theoretical contribution of recruits to the spawning stock exceeded the maximum observed stock size. I concluded that the maximum observed stock size failed to negatively influence recruitment, and the level of stock size that might influence recruitment is greater than the maximum observed stock size.

Biomass Dynamics of Lake Superior Lake Herring (Coregonus artedii): Application of Schnute's Difference Model

Schnute's (1985. Can. J. Fish. Aquat. Sci. 42: 414–429) difference model was used to study the biomass dynamics of lake herring (Coregonus artedii) in Thunder Bay and Black Bay, Lake Superior, during 1948–79. Recruitment to both fisheries appeared to be independent of escapement during previous years. Abundance of lake herring in Black Bay varied without trend during the study period despite a strong peak in effective fishing effort during 1968–70 and a general trend of increasing effort throughout the study period. Abundance of Thunder Bay lake herring declined during 1948–50 and 1959–67 and increased when fishing effort declined. The estimated catchability coefficient for small-mesh gill nets (54–83 mm stretched measure) set in Black Bay for lake herring during November was 0.467/102 km net. The estimated catchability coefficient for small mesh gill nets set in Thunder Bay for lake herring during December was 0.132/102 km net. Estimates of natural mortality rates and recruitment levels for both fisheries were confounded and unreliable. This analysis demonstrates the utility of Schnute's model for management of Great Lakes herring stocks and some difficulties with its application.

Effects of delayed feeding and temperature on the age of irreversible starvation and on the rates of growth and mortality of Pacific herring larvae

The time periods from exhausion of the yolk to the age of irreversible starvation for Pacific herring Clupea harengus pallasi larvae were 8.5, 7.0 and 6.0 d at 6°, 8° and 10°C, respectively. These periods are within the range perviously measured for Atlantic herring larvae and other temperature zone fish species; they are long compared to the periods for tropical species. The variation in the length of this period is due almost entirely to temperature; the natural logarithm of the time period from fertilization to irreversible starvation is highly correlated (r=0.91) with the mean rearing temperature for 25 species of pelagic marine fish larvae. The rates of growth and mortality, measured for 26 experimental populations of Pacific herring larvae reared at 6°, 8° and 10°C and ten ages of delayed first feeding, decreased and increased, respectively with increasing age of first feeding and increasing temperature. These rates, adjusted for the effects of rearing conditions, were compared with the rates for natural populations of herring larvae. Growth is generally faster in the sea than in experimental enclosures. Two of the eleven estimates of natural mortality rate were high enough to indicate possible catastrophic mass starvation. This is consistent with Hjort's critical period concept of year class formation and it suggests that mass starvation occurs in 18 to 36% of the natural populations of first feeding herring larvae.

Application of a Multispecies Approach for Assessing the Population Abundance and the Age–Structure of Fish Stocks

Application of a multispecies approach for assessing the population abundance and the age-structure of fish stocks is proposed. The main objective of this approach is to eliminate a bias in the results of cohort analysis (the most frequently used single-species procedure for assessing the fish population age-structure) caused by uncertainties in the external estimates of natural mortality rates. The approach is applicable if fish predation is a major cause for fish natural mortality and all fish species which significantly interact among themselves through predation are taken into account. It is additionally assumed that these fish species do not leave the area under consideration. Compliance with these conditions allows the prediction, on the basis of ecological theory and required field and laboratory data, of a major component of natural mortality, predation mortality, within extended cohort analysis. The paper describes the proposed procedure (extended cohort analysis) and outlines possible ways of collecting the required input data.Key words: assessment, fish population abundance, age-structure, multispecies approach, natural and fishing mortality

Estimation of Mortality Rates in Fish Populations

Abstract The natural log of catch per unit of effort (CPUE) calculated for successive fish ages during the lifetime of a cohort, is related to the size of the cohort and to the fishing and natural mortality rates. After a simple linearization procedure, regressions of logeCPUE against effort and age can give estimates of natural mortality rate, catchability coefficient, and size of the cohort. The method is applied to Lake Opeongo trout data.

An Application of Computers to the Estimation of Exploited Populations

This paper describes a computer programme for the estimation of the size of exploited populations by methods described in Allen (1966, J. Fish. Res. Bd. Canada 23: 1553–1574). Although these methods were originally developed for use on whale populations, they are applicable to any other populations where suitable data are available. The essential data are the total catch, the catch for a known amount of effort, and the age distribution of the catch, all for a series of years. An estimate of natural mortality rate is also required but population estimates may be obtained for up to 10 values of this parameter in a single computer run.The entire programme incorporates six subroutines, as well as the main controlling programme. One subroutine reads in the catch and effort data, a second reads in the age distribution data for each year and, if necessary, converts it according to a predetermined transformation from age expressed in terms of whale ear-plug laminations to age in years. The third subroutine estimates the rate of recruitment as the proportion of newly recruited animals in the catch for each year, using the method of Allen (1966). The other three subroutines derive population estimates, using the "q," modified DeLury, and actual and expected catch methods (Allen, 1966). As many sets of data as desired may be processed in a single computer run.