The European anchovy (Engraulis encrasicolus) increase in the North Sea / Kristina Raab

Thesis Doctor Wageningen University. Institute of Animal Sciences. Graduate School of Wageningen 2013

Incluye bibliografía

Contents.. Chapter 1 - General introduction.. Chapter 2 - Anchovy Engraulis encrasicolus diet in the North and Baltic Seas (Raab et al 2011.. Chapter 3 - Dietary overlap between the potential competitors herring, sprat and anchovy in the North Sea (Raab et al 2012.. Chapter 4 - Anchovy population expansion in the North Sea (Petitgas et al 2012.. Chapter 5 - Impact of temperature and food availability on juvenile European anchovy (Engraulis encrasicolus at its northern boundary (Raab et al 2013.. Chapter 6 - Depensation in small pelagic fish illustrated with the North Sea anchovy example (manuscript.. Chapter 7 - Synthesis and general discussion.. English Summary.. Dutch Summary (Samenvatting.. Acknowledgements.. About the author.. List of publications

Acceso en línea sin restricciones

Small pelagic fish populations are of high socio-economic importance in several marine systems of the world. They are known for their strong fluctuations in abundance, for which the mechanisms are not always understood. European anchovy (Engraulis encrasicolus) increased its population in the North Sea starting in the mid-1990s while previously it was found in more southern waters in Europe. The reasons for this increase were unknown. A few years earlier, major changes in the plankton community had been reported, and climate change was warming North Sea waters at the same time. Therefore changed food availability (resulting from plankton changes) or changed habitat availability (due to warmer waters) seemed likely candidate explanations for the increase and expansion into more northern areas by a European planktivorous fish species with southern affiliations (chapter 1). Due to its relative novelty in the North Sea, little was known about the ecology of the species in this system. Therefore basic information on diet composition and life cycle information had to be collected to complement the survey data which documented the population increase. In this thesis, the anchovy increase in the North Sea was investigated with, at first, an emphasis on trophic mechanisms but expanding into other areas as it advanced. In chapter 2, we first describe the food of North Sea anchovy, since it had not been previously analysed in this system. Although there was some spatial variation in stomach contents, the species consumed zooplankton just like in other parts of its range, and copepods formed a major part of the observed diet. In chapter 3, the stomach contents of anchovy were compared to its likely competitors sprat and herring, both small pelagic planktivorous species like anchovy. Previous diet information on herring and sprat was sparse in time and space so this study which first described the stomach contents of co-occurring individuals (i.e. from the same haul) of these three species is likely the most recently appropriate accurate estimate of dietary overlap. Anchovy was found to be more general in its diet than sprat than herring, as the latter focused on fewer prey items to form most of its observed diet. The dietary overlap of each species pair indicated that anchovy and herring had low dietary overlap, suggesting they are not the most likely trophic competitors. It nevertheless remains possible that under changing plankton communities anchovy has a competitive advantage purely because of its generalist diet. The anchovy increase was then approached by an interdisciplinary group of marine scientists including myself who in chapter 4 considered three possible pathways to the anchovy increase: expansion of a local population, or invasion by an external population via larval drift or by active adult migration. Information from the North Sea (empirical survey data and physical oceanographic model data) and the Bay of Biscay (larval drift models and more in depth ecological understanding) indicated that the former explanation was the most likely and that an expansion of thermal habitats allowed the North Sea anchovy to expand its population from a residual area of distribution located in the southern North Sea. This may have been due to increased overwinter survival, reproduction, or early life growth. This latter idea was further built upon in chapter 5, in which the focus lay on the first growing season of anchovy. Early life growth was modelled using an ecophysiological modelling approach and ecosystem model data on food availability and temperature, two major determinants of fish early life growth. eng

Temporal development of the growth potential of young anchovy was found to correspond well to the abundance increase in surveys, i.e. the better the modelled early life growth, the higher the abundance in survey catches in the next following season. Adult distribution as documented by the international bottom trawl survey was related to environmental variables representing food availability and temperature. Temperature seemed to better explain the occurrence and abundance of anchovy throughout the North Sea in past decades than various measures of food availability chosen based on insights from earlier chapters. The spatial spread of anchovy was further investigated in chapter 6 using a theoretical model that included so-called 'depensation', or positive densitydependence. Several mechanisms in small schooling fish may cause such a characteristic in its population dynamics and although none of these were empirically investigated in anchovy, we wanted to represent the consequences of using this assumption rather than the more commonly used assumption of logistic growth (where population growth is always positive and becomes null when maximum carrying capacity of the system is reached). We found that the spread of a localized anchovy population depended on the intensity of depensation and dispersal characteristics of the population. Therefore an investigation of the possible depensation-causing mechanisms for North Sea anchovy or in other small schooling fish populations would be highly relevant to understand the range changes in these populations. In conclusion, the North Sea anchovy expansion is likely to have been made possible through a combination of its generalist feeding behaviour, which may avoid limitation due to food scarcity, and by increased temperatures, making for improved population growth conditions for young fish, and possibly through positively influencing overwinter survival and reproduction. The possibility for depensatory dynamics in the population exists and remains a further topic of investigation. This and other topics of broader interest are discussed in chapter 7. This doctoral thesis thus contributes to an increased understanding of the ecology of anchovy in the North Sea, its diet (chapter 2) and potential trophic interactions (chapter 3), but also the likely origin of the expanded population (chapter 4), important environmental variables in the increase (chapter 5) and the population dynamic properties affecting this type of range expansion (chapter 6). These insights can support studies of small pelagic fish in other systems of the world and may support fisheries scientists who wish to integrate more ecology into their assessments and management practice. eng