Dory

Abstract: Abundances of warm-water fish species (e.g. red mullet, john dory, triggerfish) have increased in UK waters during recent decades, while many coldwater species have experienced declines. There has been a massive influx of snake pipefish to UK waters since 2004, but unusual fish occurrences or sudden proliferations of species cannot definitively be attributed to climate change. A number of commercial and non-commercial fish species are suggested to have exhibited shifts in mean latitude over the past 25 years. Poor ‘recruitment’ in traditional fishery target species such as cod, plaice and herring may be related to a shift in the composition of zooplankton, which are a key prey for developing larvae. In some parts of the southern North Sea, cold-water species, such as cod and eelpout, have been shown to experience metabolic stress during warm years, as evidenced by slower growth rates and difficulties in supplying oxygen to body tissues. Climate change will have far-reaching impacts on the dynamics of fish populations, however knowledge of underlying mechanisms is rather limited, especially in non-commercial species. Excessive fishing pressure has caused fish populations to become more vulnerable to short-term natural climate variability by removing the oldest individuals, and making such populations less able to ‘buffer’ against occasional poor year classes. In the short term, climate change will have little influence on fish stock recovery, which depends instead upon reducing fishing effort to allow existing year classes to survive to maturity. Climate-related shifts in species distribution, behavior and depth preference may affect the ‘catchability’ of certain stocks to fishing fleets. Long-term climate change may affect the overall productivity of fish stocks in a given area. Some species may be adversely affected leading to reductions in sustainable yield whilst others, for example seabass, red mullet and John Dory, may be positively affected leading to enhanced fishing opportunities.

Abstract: The feeding habits of John Dory (Zeus faber) were studied, based on the analysis of stomach contents from fish sampled in five groundfish surveys. These surveys were carried out off the Portuguese coast during different seasons between 1990 and 1992. The main aspects of feeding biology analysed in this paper are: ontogenetic diet changes, temporal variations in food composition and feeding intensity. Multivariate methods were used to investigate ontogenetic diet shifts. Two main length groups were identified: 8.0–24.9 cm fish, feeding mainly on dragonets and silvery pout, and 25.0–55.9 cm fish whose diet was mainly composed of blue whiting and snipefish. A transitional phase (24.0–30.9 cm fish) with a mixed food composition was observed. This ontogenetic diet shift does not seem to correspond to any important change in body morphology but it does coincide with the onset of sexual maturity in the species.John Dory switched from a diet of small prey species with more pronounced benthic behaviour to a diet of larger schooling pelagic species. This suggests parallel evolution to more pelagic foraging behaviour. However, John Dory feeding habits appear to be largely controlled by the availability and accessibility of prey species: (i) the diet of adult John Dory is dominated by very abundant species; (ii) shifts in the main prey items between different times of the year and between different areas seem to be related both with their absolute and relative abundance in the environment and with the overlap between the depth distribution of predator and prey.

Abstract: Spatial distribution patterns of John Dory ( Zeus faber , L.) were examined in relation to sea bottom temperature, bathymetry, locational covariates and season. Data were collected during a 2-year period (1996–1997) of seasonal sampling using demersal trawl surveys in the Aegean Sea (eastern Mediterranean). The ecological preferences of the species throughout four seasons were elucidated and the degree in which these environmental relationships might be modulated by the different hydrographic and topographic regime was also considered. Key determinants of the species' spatial aggregation in all four seasons were the water depth, the sea bottom temperature and the latitude. The sea bottom temperature had a significant effect on seasonal John Dory abundance both directly, as a main effect, and indirectly, through its interactive effect with the water depth. John Dory seasonal abundance was consistently greater in the shallower regions of the area having warmer bottom waters. Results indicated a distinct southward shift as progressing through the year, with peak abundances being observed in gradually lower latitudes of shallow areas as seasons evolved. John Dory appeared to avoid the deeper waters regardless of their bottom temperature and geographic position. The present results also suggested a seasonal pattern of habitat associations for specific locations characterised by weak hydrographic activity. John Dory preferences for certain water depth zones, sea bottom temperatures and substrate types are hypothesized to modulate the seasonal spatial aggregation of the species to preferred grounds.