424 research outputs found
Keep it simple: three indicators to deal with overfishing
Three simple fisheries indicators are presented: (i) percentage of mature fish in catch, with 100% as target; (ii) percent of specimens with optimum length in catch, with 100% as target; and (iii) percentage of âmega-spawnersâ in catch, with 0% as target, and 30â40% as representative of reasonable stock structure if no upper size limit exists. Application of these indicators to stocks of Gadus morhua, Sardinella aurita and Epinephelus aeneus demonstrate their usefulness. It is argued that such simple indicators have the potential to allow more stakeholders such as fishers, fish dealers, supermarket managers, consumers and politicians to participate in fisheries management and eventually hold and reverse the global pattern of convenience overfishing, which is defined here as deliberate overfishing sanctioned by official bodies who find it more convenient to risk eventual collapse of fish stocks than to risk social and political conflicts
"We Knew no North, no South": U.S.-Mexican War Veterans and the Construction of Public Memory in the Post-Civil War United States, 1874-1897
In 1874, American veterans of the U.S.âMexican War 1846â1848 formed the National Association of Veterans of the Mexican War (NAVMW). Until the organizationâs demise in 1897, NAVMW members crafted and celebrated a vision of their war with Mexico as a national triumph which had united Americans from all sections of the Union in a common cause. This article examines how, by promoting this particular memory of the war to the American public, NAVMW members sought to remind their countrymen of their shared national history, and so aid the process of reconciliation between North and South in the post-Civil War era
Sudden change in long-term ocean climate fluctuations corresponds with ecosystem alterations and reduced recruitment in Norwegian spring-spawning herring (Clupea harengus, Clupeidae)
Fish stocks vary in abundance. The causes behind the fluctuations may be difficult to determine, especially ones caused by natural fluctuations, but longâterm data series may provide indications of the mechanisms. Assessments show that the recruitment to the Norwegian springâspawning herring (Clupea harengus, Clupeidae) has remained low since 2004, a year which produced the last really rich yearâclass. Long timeâseries of estimated recruitment and mean winter temperature in the ocean showed a significant positive correlation for the period 1921â2004. Here, we show that this positive correlation did not continue from 2005 onwards as the winter temperature increased to high levels while herring recruitment decreased and has remained low. The density of zooplankton in the drift route of the herring larvae dropped significantly after 2004, and their centre of gravity shifted northwards. There may currently be heavy predation on the larvae by Atlanic mackerel (Scomber scombrus, Scombridae), and topâdown regulation is suggested to hamper successful recruitment. Our analysis indicates that the presence of food and overlap with high food concentrations are likely important regulators of survival in herring larvae. The findings may be important for future management and planning of fisheries of this stock because recruitment failure may continue if temperature remains high and food abundance remains low.publishedVersio
Modelling the population size and dynamics of the British grey seal
Funding: part-funded by the UK Natural Environment Research Council to SMRU (Grant no. SMRU1001).1. Grey seals (Halichoerus grypus) were the first mammals to be protected by an Act of Parliament in the UK and are currently protected under UK, Scottish, and EU conservation legislation. Reporting requirements under each of these statutes requires accurate and timely population estimates. Monitoring is principally conducted by aerial surveys of the breeding colonies; these are used to produce estimates of annual pup production. Translating these data to estimates of adult population size requires information about demographic parameters such as fecundity and sex ratio. 2. An ageâstructured population dynamics model is presented, which includes density dependence in pup survival, with separate carrying capacities in each of the four breeding regions considered (North Sea, Inner Hebrides, Outer Hebrides, and Orkney). This model is embedded within a Bayesian stateâspace modelling framework, allowing the population model to be linked to available data and the use of informative prior distributions on demographic parameters. A computerâintensive fitting algorithm is presented based on particle filtering methods. 3. The model is fitted to regionâlevel pup production estimates from 1984 to 2010 and an independent estimate of adult population size, derived from aerial surveys of hauledâout seals in 2008. The fitted model is used to estimate total population size from 1984 to 2010. 4. The population in the North Sea region has increased at a nearâconstant rate; growth in the other three regions began to slow in the midâ1990s and these populations appear to have reached carrying capacity. The total population size of seals aged 1 year or older in 2010 was estimated to be 116 100 (95% CI 98 400â138 600), an increase of <1% on the previous year. 5. The modelling and fitting methods are widely applicable to other wildlife populations where diverse sources of information are available and inference is required for the underlying population dynamics.PostprintPeer reviewe
Growth and mortality estimates of Sardinella brasiliensis in the southeastern Brazilian Bight
Saturation Behavior: a general relationship described by a simple second-order differential equation
<p>Abstract</p> <p>Background</p> <p>The numerous natural phenomena that exhibit saturation behavior, <it>e.g</it>., ligand binding and enzyme kinetics, have been approached, to date, via empirical and particular analyses. This paper presents a mechanism-free, and assumption-free, second-order differential equation, designed only to describe a typical relationship between the variables governing these phenomena. It develops a mathematical model for this relation, based solely on the analysis of the typical experimental data plot and its saturation characteristics. Its utility complements the traditional empirical approaches.</p> <p>Results</p> <p>For the general saturation curve, described in terms of its independent (<it>x</it>) and dependent (<it>y</it>) variables, a second-order differential equation is obtained that applies to any saturation phenomena. It shows that the driving factor for the basic saturation behavior is the probability of the interactive site being free, which is described quantitatively. Solving the equation relates the variables in terms of the two empirical constants common to all these phenomena, the initial slope of the data plot and the limiting value at saturation. A first-order differential equation for the slope emerged that led to the concept of the effective binding rate at the active site and its dependence on the calculable probability the interactive site is free. These results are illustrated using specific cases, including ligand binding and enzyme kinetics. This leads to a revised understanding of how to interpret the empirical constants, in terms of the variables pertinent to the phenomenon under study.</p> <p>Conclusions</p> <p>The second-order differential equation revealed the basic underlying relations that describe these saturation phenomena, and the basic mathematical properties of the standard experimental data plot. It was shown how to integrate this differential equation, and define the common basic properties of these phenomena. The results regarding the importance of the slope and the new perspectives on the empirical constants governing the behavior of these phenomena led to an alternative perspective on saturation behavior kinetics. Their essential commonality was revealed by this analysis, based on the second-order differential equation.</p
Population regulation in lake whitefish, Coregonus clupeaformis (Mitchill)
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/73745/1/j.1095-8649.1981.tb03822.x.pd
Demographic Diversity and Sustainable Fisheries
Fish species are diverse. For example, some exhibit early maturation while others delay maturation, some adopt semelparous reproductive strategies while others are iteroparous, and some are long-lived and others short-lived. The diversity is likely to have profound effects on fish population dynamics, which in turn has implications for fisheries management. In this study, a simple density-dependent stage-structured population model was used to investigate the effect of life history traits on sustainable yield, population resilience, and the coefficient of variation (CV) of the adult abundance. The study showed that semelparous fish can produce very high sustainable yields, near or above 50% of the carrying capacity, whereas long-lived iteroparous fish can produce very low sustainable yields, which are often much less than 10% of the carrying capacity. The difference is not because of different levels of sustainable fishing mortality rate, but because of difference in the sensitivity of the equilibrium abundance to fishing mortality. On the other hand, the resilience of fish stocks increases from delayed maturation to early maturation strategies but remains almost unchanged from semelparous to long-lived iteroparous. The CV of the adult abundance increases with increased fishing mortality, not because more individuals are recruited into the adult stage (as previous speculated), but because the mean abundance is more sensitive to fishing mortality than its standard deviation. The magnitudes of these effects vary depending on the life history strategies of the fish species involved. It is evident that any past high yield of long-lived iteroparous fish is a transient yield level, and future commercial fisheries should focus more on fish that are short-lived (including semelparous species) with high compensatory capacity
Densityâ and sizeâdependent mortality in fish early life stages
The importance of survival and growth variations early in life for population dynamics depends on the degrees of compensatory density dependence and size dependence in survival at later life stages. Quantifying densityâ and sizeâdependent mortality at different juvenile stages is therefore important to understand and potentially predict the recruitment to the population. We applied a statistical stateâspace modelling approach to analyse time series of abundance and mean body size of larval and juvenile fish. The focus was to identify the importance of abundance and body size for growth and survival through successive larval and juvenile age intervals, and to quantify how the dynamics propagate through the early life to influence recruitment. We thus identified both relevant ages and mechanisms (i.e. density dependence and size dependence in survival and growth) linking recruitment variability to early life dynamics. The analysis was conducted on six economically and ecologically important fish populations from cold temperate and subâarctic marine ecosystems. Our results underscore the importance of size for survival early in life. The comparative analysis suggests that sizeâdependent mortality and densityâdependent growth frequently occur at a transition from pelagic to demersal habitats, which may be linked to competition for suitable habitat. The generality of this hypothesis warrants testing in future research.publishedVersio
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