185 research outputs found
Maturation Dynamics of Arcto-Norwegian Cod
No full textMany commercially important fish stocks are harvested with very high exploitation rates with the consequence of substantial changes in stock parameters as well as reduced harvest rates and, in the worst case, stock depletion. On a long-term the Arcto-Norwegian cod is the most productive cod stock in the world. It has a history of exploitation many hundred of years long and has been heavily exploited since about 1950. The chosen exploitation strategy might have substantial effects on the short-term production of the stock, and a serious question is if a long-term and heavy exploitation may change genetic properties of the stock and hence its reproductive and production potential.
In this paper, age and size data of spawning cod, going back to 1932, are analyzed with respect to long-term changes caused by exploitation. During the studied period, average age and size at first spawning have been reduced by about three years and 20 cm. Immature growth has also increased substantially. It is shown that a lot of the variability in age and size at first spawning can be explained by the exploitation itself and, even better, by stock biomass (density dependency). Other factors, like food availability (capelin abundance) and year class strength, also seem to play a role. Due to more or less continuous trends in the data from World War II onwards, it is difficult to disentangle temporal effects (environmental forcing) from other causal agents
Fluctuation in stock properties of north-east Arctic cod related to long-term environmental changes
No full textFrom a historic perspective, the north-east Arctic cod stock, which is found in the Barents SeaSvalbard region, has been the most productive gadoid stock in the Atlantic. Variation in catch has always been large, but during the last 1015 years catch and stock abundance have reached the lowest level on record. Three major causes of variation have been discussed: (i) stock reduction through exploitation; (ii) environmental influences on recruitment; and (iii) species interaction effects on maturation, growth and mortality. In addition, interactions among these three sources might be important. The influence of each specific factor is difficult to evaluate from incidental observations and short-term time series. In this respect, the time series on catches and on biological and environmental information of this stock, which partly extend back to the 19th century, occupy a unique position in comparison to data on most other stocks.
In this paper, fluctuations in catch and stock abundance are compared with changes in recruitment, size/age and growth. This information is discussed in view of historic variation in ecology and environment. The stock has been under particularly high exploitation pressure since the mid-1970s. Further, large changes in growth rates and poor recruitment to the commercially exploited stock are characteristic of late 1980s and throughout the 1990s. The analysis shows that substantial long-term variation might underlie short-term variability, and more importantly, that long-term changes roughly coincide with similar fluctuations in the environment. Such factors might substantially affect the relationship between spawning stock and recruitment, which is also apparent from the difference in conclusions reached by various published studies. Consequently, it is suggested that using a steady-state perspective for the population dynamics may lead to mismanagement and to a reduction of the long-term yield from this stock
Fluctuation in Stock Properties of Arcto-Norwegian Cod Related to Long-term Environmental Changes
No full textFrom a historic perspective the cod in the Barents Sea-Svalbard region has been the most productive gadoid stock in the Atlantic. Variation in catches has always been large, but during the last 10-15 years catches and stock abundance have reached the lowest level on record. Three major causes of variation have been discussed; stock reduction through exploitation, environmental influences on recruitment, and species interaction effects on maturation, growth and mortality. In addition, interactions between these three sources might be important. The influence of each specific factor is difficult to evaluate from incidental observations and short-term time series. In that respect, the time series on catches and on biological and environmental information of this stock, which partly goes back to the 19th century assumes a unique position in comparison with data on most other stocks.
In this paper fluctuations in catches and stock abundance will be compared with changes in recruitment, size/age composition and growth. This information is discussed in view of historic variation in ecological and environmental parameters. The stock has been under particularly high exploitation pressure since the mid-seventies. Further, large changes in growth rates and poor recruitment to the commercially exploited stock have characteristic for the end of the 1980s and the 1990s. The analysis here shows that substantial long-term variation might underlie short-term variability, and, more importantly, that long-term changes roughly coincide with similar fluctuations in the environment. Consequently, it is suggested that inserting on a steady-state perspective on the population dynamics of this stock may lead to mismanagement and to a reduction of long-term yield
Maturation Dynamics of Arcto-Norwegian Cod.
No full textMany commercially important fish stocks are harvested with very high exploitation rates with the consequence of substantial changes in stock parameters as well as reduced harvest rates and, in the worst case, stock depletion. On a long-term the Arcto-Norwegian cod is the most productive cod stock in the world. It has a history of exploitation many hundred of years long and has been heavily exploited since about 1950. The chosen exploitation strategy might have substantial effects on the short-term production of the stock, and a serious question is if a long-term and heavy exploitation may change genetic properties of the stock and hence its reproductive and production potential. In this paper, age and size data of spawning cod, going back to 1932, are analyzed with respect to long-term changes caused by exploitation. During the studied period, average age and size at first spawning have been reduced by about three years and 20 cm. Immature growth has also increased substantially. It is shown that a lot of the variability in age and size at first spawning can be explained by the exploitation itself and, even better, by stock biomass (density dependency). Other factors, like food availability (capelin abundance) and year class strength, also seem to play a role. Due to more or less continuous trends in the data from World War II onwards, it is difficult to disentangle temporal effects (environmental forcing) from other causal agents.
Fluctuation in Stock Properties of Arcto-Norwegian Cod Related to Long-term Environmental Changes.
No full textFrom a historic perspective the cod in the Barents Sea-Svalbard region has been the most productive gadoid stock in the Atlantic. Variation in catches has always been large, but during the last 10-15 years catches and stock abundance have reached the lowest level on record. Three major causes of variation have been discussed; stock reduction through exploitation, environmental influences on recruitment, and species interaction effects on maturation, growth and mortality. In addition, interactions between these three sources might be important. The influence of each specific factor is difficult to evaluate from incidental observations and short-term time series. In that respect, the time series on catches and on biological and environmental information of this stock, which partly goes back to the 19th century assumes a unique position in comparison with data on most other stocks. In this paper fluctuations in catches and stock abundance will be compared with changes in recruitment, size/age composition and growth. This information is discussed in view of historic variation in ecological and environmental parameters. The stock has been under particularly high exploitation pressure since the mid-seventies. Further, large changes in growth rates and poor recruitment to the commercially exploited stock have characteristic for the end of the 1980s and the 1990s. The analysis here shows that substantial long-term variation might underlie short-term variability, and, more importantly, that long-term changes roughly coincide with similar fluctuations in the environment. Consequently, it is suggested that inserting on a steady-state perspective on the population dynamics of this stock may lead to mismanagement and to a reduction of long-term yield.
Fisheries-induced selection pressures in the context of sustainable fisheries
No full textMan has a major impact on marine environments through exploitation of fish resources. Fisheries can induce different selective pressures, either directly, i.e., through elevated mortality (which is often highly selective) or through ecosystem-level responses, as exploitation affects food availability and predation risk in both target and nontarget species. Responses to selection can be observed at two levels. First, at the community level, some species may suffer more from effects of harvesting than others; some may even increase in abundance. Responses by species to exploitation are associated with their life histories. In particular, species with late maturation at large size and with low population growth rate tend to undergo more pronounced declines than early-maturing species with rapid growth. Second, the phenotypic composition within species may also change. If phenotypic variability has a genetic basis, then fisheries-induced selection can result in evolutionary change in life-history traits influencing sustainable yields, behavioral traits (e.g., gear-avoidance behavior), and morphological traits. We discuss the possible implications of fisheries-induced adaptive changes for sustainable fisheries management
Measuring probabilistic reaction norms for age and size and maturation
No full textWe present a new probabilistic concept of reaction norms for age and size at maturation that is applicable when observations are carried out at discrete time intervals. This approach can also be used to estimate reaction norms for age and size at metamorphosis or at other ontogenetic transitions. Such estimations are critical for understanding phenotypic plasticity and life-history changes in variable environments, assessing genetic changes in the presence of phenotypic plasticity, and calibrating size- and age-structured population models. We show that previous approaches to this problem, based on regressing size against age at maturation, give results that are systematically biased when compared to the probabilistic reaction norms. The bias can be substantial and is likely to lead to qualitatively incorrect conclusions; it is caused by failing to account for the probabilistic nature of the maturation process. We explain why, instead, robust estimations of maturation reaction norms should be based on logistic regression or on other statistical models that treat the probability of maturing as a dependent variable. We demonstrate the utility of our approach with two examples. First, the analysis of data generated for a known reaction norm highlights some crucial limitations of previous approaches. Second, application to the northeast arctic cod (Gadus morhua) illustrates how our approach can be used to shed new light on existing real-world data
Age at maturation predicted from routine scale measurements in Norwegian spring-spawning herring (Clupea harengus) using discriminant and neural network analyses
No full textWe evaluate two methods allowing the prediction of age at maturation from the widths of the annual growth layers in scales (or otoliths) in a case study on Norwegian spring-spawning herring. For this stock, scale measurements have been made routinely for many decades. We compare the performance in classifying age at maturation (at 3, 4, . . . , 9 years) between conventional discriminant analysis (DA) and the new methodology of artificial neural networks (NN) trained by back-propagation against a 'control' of historical estimates of age at maturation obtained by visual examination of scales. Both methods show encouraging, and about equally high, classification success. The marginal differences in performance are in favour of DA, if the proportion of correctly classified cases is used as criterion (DA 68.0%, NN 66.6%), but in favour of NN if other criteria are used, including prediction error (error >1 year: DA 5.2%, NN 2.9%), and the average degree of under- or overestimation (underestimation 1.1% of mean with DA; overestimation 0.2% of mean with NN). Evidence is provided that both methods approach the a priori limits to maximal classification success, limits set by overlapping combinations of predictor variables between maturation groups. The methods allow studies on age at maturation in this stock over a very long timespan, including periods well before, during, and after its collapse to commercial extinction. Similar techniques might well be applicable to any other fish stock with long-term data on scale or otolith growth layers
Measuring Probabilistic Reaction Norms for Age and Size at Maturation
No full textWe present a new probabilistic concept of reaction norms for age and size at maturation that is applicable when observations are carried out at discreet time intervals. This approach can also be used to estimate reaction norms for age and size at metamorphosis or at other ontogenetic transitions. Such estimations are critical for understanding phenotypic plasticity and life-history changes in variable environments, for assessing genetic changes in the presence of phenotypic plasticity, and calibrating size- and age-structured population models. We show that previous approaches to this problem, based on regressing size against age at maturation, give results that are systematically biased when compared to the probabilistic reaction norms. The bias can be substantial and is likely to lead to qualitatively incorrect conclusions; it is by failing to account for the probabilistic nature of the maturation process. We explain why, instead, robust estimation of maturation reaction norms ought to be based on logistic regression, or on other statistical models that treat the probability of maturing as a dependent variable. We demonstrate the utility of our approach with two examples. First, the analysis of data generated for a known reaction norm highlights some crucial limitations of previous approaches. Second, application to the Northeast Arctic cod ("Gadus morhua") illustrates how our approach can be used to shed new light on existing real-world data
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