47
IFM - GEOMAR Report 2002-2004
Many exploited stocks of fish and squid un-
dergo large fluctuations or extended trends
in population size on different time scales, re-
lated to environmental changes as well as to
fishing pressure. Therefore, one major aim of
fishery biology is to understand the mecha-
nisms regulating these stock fluctuations and
to develop methods to predict the effects of
fishery on exploited stocks and ecosystems in
total as well as the effects of natural environ-
mental fluctuations and climate trends on the
exploitable production. The complexity of the
task is increased by the fact that natural and
manmade effects are strongly interlinked and
cannot be considered separately. One aspect
of crucial importance is the reproductive strat-
egy of exploited stocks which is in many spe-
cies adapted to a highly variable environment,
based on extremely high fecundities and diver-
sities in stock structures, which includes the
chances for population survival over periods of
unfavourable conditions as well as the devel-
opment of large stock sizes under favourable
conditions. The regional stock structure, the
size and age structure of the spawning stocks,
both influenced by fishery, the nutritional con-
dition of adults and thus the quality of gametes
and the survival of the early life stages related
to stock structure, abiotic environment and
presence of prey and predators are the key
factors determining the reproductive success
of a species. One focus of the Research Unit
Fishery Biology is to improve the knowledge
on the most relevant factors governing the re-
cruitment processes in fish and squid stocks by
the development of new methodical approach-
es, which allow to elucidate the characteristics
of survivors during early development and to
address also behavioural patterns that provide
best likelihood for survival. For this purpose
the growthhistory and the chronology of im-
portant events during the early development
of individual fish have to be considered. This