Reproductive Trade-Offs May Moderate the Impact of Gyrodactylus salaris in Warmer Climates

Gyrodactylus salaris is a notifiable freshwater ectoparasite of salmonids. Its primary host is Atlantic salmon (Salmo salar), upon which infections can cause death, and have led to massive declines in salmon numbers in Norway, where the parasite is widespread. Different strains of S. salar vary in their susceptibility, with Atlantic strains (such as those found in Norway) exhibiting no resistance to the parasite, and Baltic strains demonstrating an innate resistance sufficient to regulate parasite numbers on the host causing it to either die out or persist at a low level. In this study, Leslie matrix and compartmental models were used to generate data that demonstrated the population growth of G. salaris on an individual host is dependent on the total number of offspring per parasite, its longevity and the timing of its births. The data demonstrated that the key factor determining the rate of G. salaris population growth is the time at which the parasite first gives birth, with rapid birth rate giving rise to large population size. Furthermore, it was shown that though the parasite can give birth up to four times, only two births are required for the population to persist as long as the first birth occurs before a parasite is three days old. As temperature is known to influence the timing of the parasite's first birth, greater impact may be predicted if introduced to countries with warmer climates than Norway, such as the UK and Ireland which are currently recognised to be free of G. salaris. However, the outputs from the models developed in this study suggest that temperature induced trade-offs between the total number of offspring the parasite gives birth to and the first birth timing may prevent increased population growth rates over those observed in Norway

On the problem of identifying and assessing populations of falkland island squid loligo gahi

Patterns of catches of Loligo gahi around the Falkland Islands in the years 1982-1986 initially suggested a simple population structure with one stock peaking in the fishery in autumn and another peaking in spring. Management of two seasons was therefore imposed in 1987. This pattern has proved highly variable, with the result that, for some years, it seems as if up to four cohorts are present in two separated northern and southern fishing areas. In this paper, evidence from fishing vessel and scientific observer data is examined for these different population structures. The conclusion is that the simplest structure that fits the observed data is of two main cohorts recruiting during a year to a single fishing area, albeit with considerable geographic heterogeneity, with a smaller recruitment episode sometimes present at the end of the year. The recruitment of these cohorts to the fishery is not, however, coincident with current management seasons

Perspectives de modélisation du réseau trophique en étang. Applications potentielles à l'optimisation et au contrôle de la production piscicole

The possibilities and prospects of creating a model of the trophic system in fish culture ponds are analysed on the basis of actual examples from Polish fish culture and ecosystem models developed for lacustrine media. The authors show which kind of information can be obtained from this model both for understanding the mechanisms involved in the dynamics of the aquasystem and for optimizing the bioconversion through the food chain. An attempt is made to investigate the kind of problems occurring most likely in pond fish culture and the trend of research to be developed