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

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.

Bibliographic Details

Main Authors:	Raab, Kristina autora, Rijnsdorp, Adriaan D. promotor, Dickey-Collas, Mark co-promotor, Nagelkerke, Leo A. J. co-promotor
Format:	Texto biblioteca
Language:	eng
Published:	Wageningen, Amsterdam, The Netherlands Wageningen University. Institute of Animal Sciences. Graduate School of Wageningen 2013
Subjects:	Engraulis encrasicolus, Hábitos alimentarios, Crecimiento demográfico, Variables ambientales, Peces pelágicos, Ecología de peces,
Online Access:	http://library.wur.nl/WebQuery/wurpubs/442853
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