| Abstract |
European anchovy (Engraulis encrasicolus) is a small pelagic species
characterized by short life span, multiple spawning along protracted reproductive
periods that last from spring to autumn, and high interannual variability in population
size. The aim of this dissertation was to investigate certain biological and ecological
aspects of reproduction and growth of European anchovy in the North Aegean Sea,
which, despite their significance for the population size and its interannual
fluctuations, are poorly investigated. Specifically, I studied the egg production at the
individual level (by examining the dynamics of vitellogenesis in the ovaries), the egg
production at population level and its variability in the field and the oceanographic
characteristics of the spawning habitats. Additionally, the otolith microstructure,
growth and somatic condition of late larvae and juveniles collected at different times
of the year were analysed and the causes of their seasonal variability were
investigated.
In order to study egg production at the individual level, and more specifically
the dynamics of vitellogenesis in a fish with indeterminate fecundity and the timing of
batch recruitment a suite of methods and modern theories in fish reproductive biology
(oocyte packing density theory, stereology, ovarian allometry, autodiametric oocyte
size frequencies and postovulatory follicle ageing) was used. To study egg production
at population level and investigate the causes of its fluctuations I used data on the
adult population characteristics obtained from the Daily Egg Production Method
(DEPM) surveys carried out in the North Aegean Sea during June 2003-2006 and
2008. For the characterization of the potential spawning habitats of anchovy and
round sardinella (Sardinella aurita) in the N. Aegean Sea I used presence/absence egg
data from the ichthyoplankton DEPM surveys and their relationship with satellitederived
environmental variables was investigated with the aid of using generalized
additive models (GAMs). Finally, the seasonal changes in otolith and somatic growth
and condition of anchovy late larvae and juveniles sampled in July 2007, December
2007 and February 2009 in the N. Aegean Sea, were studied in combination with
simulations of a coupled 3D hydrodynamic-biogeochemical model (POM-ERSEM)
implemented in the North Aegean Sea. The simulations of the coupled oceanographic
model aimed at reconstructing the potential oceanographic conditions that the fish
experienced during the entire period of their development.
Concerning the egg production at individual level, the results indicated that
recruitment of vitellogenic oocytes in anchovy occurs in pulses of very short duration
that are triggered by the hydration of the spawning batch. In <24 hours after spawning,
the relative oocyte numbers at different developmental stages are restored
(equilibrium) and remain stable until the next hydration event. During the interspawning,
equilibrium phase and until the next hydration event, the total number of
vitellogenic oocytes in the ovary equals two times the batch fecundity. On the
reproduction at population level, results showed that mean relative batch fecundity,
i.e., the number of eggs released per spawning event per gram of female weight, was
positively related to somatic condition (energy reserves). Spawning frequency, i.e.,
how often do the fish spawn, was related to liver condition (presumably, direct food
intake). Daily specific fecundity, i.e., number of eggs produced daily per gram of
spawning population was significantly related to the ratio mesozooplankton biomass :
total anchovy biomass, indicating that egg production in anchovy depends on per
capita food availability. On the characteristics of anchovy and round sardinella
spawning habitats, the model results showed that both species spawn over continental
shelf areas with increased surface chlorophyll values. However, round sardinella
spawned closer to the coast than anchovy. Model-predicted potential spawning areas
for anchovy and round sardinella in un-sampled areas of the Greek Seas and the entire
Mediterranean and Black Sea were in good agreement with existing information on
the distribution and extent of the spawning grounds, especially for anchovy. Finally,
on the growth of young fish, the results showed that larvae caught in summer had high
growth rates, better somatic condition and bigger otoliths; according to the model
outputs, they had been developed in warmer and more stratified water masses with
significantly higher plankton productivity. The analysis of otolith increment widths
using generalized additive models (GAMs), showed that, taking into account the
autocorrelation in successive increments widths and the ontogenetic changes in otolith
growth (associated with metamorphosis), increment width was significantly related to
temperature (maximum positive effect at ~24 oC). Additionally, juvenile otolith
growth was also positively related to high mesozooplankton biomass.
The findings of the dissertation are particularly important for the
understanding of the mechanisms behind the fluctuations of anchovy populations.
Moreover, the results on anchovy reproduction and growth can be used for building
and validating bioenergetic models of the species. These models combined with
coupled hydrodynamic-biogeochemical models are being used increasingly as tools
for the study of population fluctuations in relation to climatic changes.
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