| Abstract |
The early-summer ichthyoplankton of the N.E. Aegean Sea (eastern Mediterranean) was studied using samples collected during June 1993, 1994, 1995 and 1996. We subsequently focused on anchovy (Engraulis encrasicolus) egg and larval production. Sampling was conducted over a grid of stations covering the N.E. Aegean Sea continental shelf. Bongo-net oblique tows and vertical profiles of temperature and salinity were performed at each station. In 1993 and 1995 concurrent sampling of adult anchovy was carried out on board the commercial purse seiners, or by means of a pelagic trawl operated from the R/V "PHILIA". The adult samples were used to estimate parameters of the Daily Egg Production Method, such as batch fecundity and spawning frequency. Significant interannual differences were found in the environmental conditions. In June 1993 and 1996 waters were colder, less saline and richer in zooplankton than in 1994 and 1995. Existing meteorological and sea surface temperature data showed that that the observed differences were associated with colder winter/spring conditions during 1993 and 1996. The larvae of the small-sized anchovy and mesopelagic fishes were more abundant in 1993 and 1996. In contrast, the larvae of the middle-sized pelagic fishes, such as Sardinella aurita, Trachurus.mediterraneus, Scomber japonicus and Auxis rochei were most abundant in 1994 and 1995. Larvae of several summer spawning demersal taxa, such as Mullus spp., Serranus cabrilla, Scorpaena spp., Coris julis and Chromis chromis were similarly more abundant in 1994 and 1995. The results of the multivariate analysis of the station-taxa data indicated that the ichthyoplankton assemblages were characterized by significant interannual variability, which reflected the interannual variability in the distribution and abundance of the component taxa. However, the mean structure and distribution of the larval assemblages were clearly depth-related and reflected the bathymetric distribution of adults. The anchovy spawning intensity was associated with environmental conditions (temperature, salinity, zooplankton displacement volumes), which presented significantly higher among- than within-year variability. The abundance of eggs was higher when the zooplankton volumes were higher. In June 1993 and 1995 (the years that adults samples were collected), adult condition, batch fecundity, spawning frequency and egg size were all much higher in 1993 than in 1995, i.e., when the index of zooplankton production was significantly higher. Eggs were bigger in 1993 and 1994 when compared to 1995 and 1996. Higher anchovy larval numbers were associated with areas of high zooplankton displacement volumes. Furthermore, there was a trend of increasing abundance at lower latitudes (deeper waters) with increasing larval size. However, the mortality of larvae was significantly higher in 1996 when both anchovy egg and zooplankton production were higher. Growth and condition (estimated from measurements of fluctuating asymmetry) were higher in 1994 than in 1995 and 1996. Specifically, in 1994, the larvae, as well as their otoliths, grew at a much higher rate after the stage of notochord flexion. The flexion stage is characterized by changes in allometric growth, spatial distribution patterns and behavior. In conclusion, the timing of the spring plankton bloom, which takes place during the period of winter-to-summer transition, is characterized by high interannual variability in the N.E. Aegean Sea. Species differ in their response to increased zooplankton production. Anchovy spawning intensity, as well as the spawning intensity of other small-sized pelagics (mesopelagics), seem to respond quickly to the ambient zooplankton quantities, i.e., to adult prey availability. When trophic conditions are favorable anchovy increases egg production by adjusting batch fecundity and spawning frequency. The close match of egg production with zooplankton production does not ensure increased larval growth, condition and survival.
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Ιχθυοπλαγκτό του Βορειο-ανατολικού Αιγαίου με έμφαση στο Γαύρο, Engraulis encrasicolus(Linnaeus, 1758)(ΙΟΥΝΙΟΣ 1993,1994,1995,1996)
