ANCHOVY LARVAE IN CONTRASTING ENVIRONMENTS (A.L.I.C.E.)
O. Giovanardi, ICRAM, Viale Stazione, 5 30015 Chioggia (Ve)
Key-words: Engraulis encrasicolus, vertical and horizontal distribution of eggs and larvae, larval mortality, larval rearing, Northern Adriatic
The aim of the A.L.I.C.E. programme was to investigate the survival of anchovy larvae in the northern Adriatic and specifically to test the hypothesis that wind mixing disrupts layers of food concentration in the water column and leads to poor survival of the larvae. This was carried out by means of a field sampling programme to relate environmental conditions and food availability for anchovy larvae to survival of the larvae. It was supported by a complimentary laboratory based experimental programme on the development, growth and feeding of the larvae. The main findings were as follows:
° The outflow region of the river Po was shown to be an important spawning and larval nursery area for anchovy. Nutrient enrichment from the river and consequent enhanced plankton productivity is deemed to be one aspect of the favouribility of the region. The diet of anchovy larvae was predominantly copepod nauplii and copepodite stages; these organisms were shown to be more abundant in the outflow region of the river Po.
° During the four week field sampling period of 1996, initially calm weather was replaced by unsettled conditions with strong north-easterly winds and high river outflow. Relatively calm weather returned gradually towards the end of the sampling period. The changing meteorological conditions were reflected in the hydrographic structure which evolved from well-stratified conditions and warm surface temperatures to a lower level of stratification and cooler surface temperatures.
° General levels of abundance of food particles for anchovy larvae changed in parallel with the evolving weather conditions. Food abundance was relatively high at the beginning of the 1996 sampling cruise following the period of stable weather, then there were somewhat lower levels and an altered specied composition during the extended period of poor weather and, as stable conditions returned at the end of the cruise, an increase in food abundance. However, it is likely that food levels were at no time severely limiting to larval survival.
° The generally adequate feeding conditions were reflected in larval mortality which was at a similar level following the period of stable weather as after the period of wind mixing. The observed mortality rates of around 44% per day are comparable with other estimates from the Adriatic and elsewhere.
° The potential survival of anchovy larvae in relation to food availability was also investigated using measures of larval nutritional condition. These methods were validated in rearing experiments an anchovy larvae carried out at a shore laboratory.
° The link between the nutritional condition of anchovy larvae and food availability was evidenced both by the positive relationship between larval condition and the amount of food in their guts and by the general observation that larvae in poorer condition were found further offshore where food availability was lower. Smaller, and probably less resilient larvae were at a generally lower level nutritional condition than larger larvae.
° However, these was no clear specific relationship between food availability on each sampling occasion and larval condition. This was attributed to the generally favourable levels of food abundance and the high degree of spatial and temporal variabibility encountered during the 1996 field sampling.
Thus, considering the above broad-scale results from the field surveys, some general relationships were shown between weather conditions and the feeding environment for anchovy larvae, but without any precise linkage being apparent. The results from the more detailed sampling at individual stations, in particular the pattern of vertical distribution, provided material for an examination of possible feeding relationships at a finer scale of resolutions:
° Both eggs and larvae of anchovy were concentrated in the upper 15m of the water column. Food particles were generally rather more dispersed through the water column, but the highest concentrations, some 1.5x - 2.5x the mean water column abundance, were invariability in the upper 10m of the water colums. It is suggested that the vertical mobility of the larvae enables them to locate and exploit these layers of aggregated food particles.
° Changes in stratification were generally reflected in similar changes in the vertical distributions of anchovy larvae and food particles; thus, when the water column was less structured, both components were more dispersed. This was shown for food particles specifically as a positive relationship between the ratio of maximum to mean food abundance (i.e. vertical concentration) and stratification in the top 15m of the water column.
° The wind mixing experienced during the 1996 sampling period tended to reduce water column stratification, and hence, at some stations, resulted in lower vertical concentrations of food particles. However, in the more inshore areas close to the river mouth, the enhanced superficial freshwater outflow which accompanied the higher winds reinforced water column stratification and maintained the vertical concentrations of food particles.
These observations lead to the conclusion that the outflow region of the river Po is also advantageous for the survival of anchovy larvae due to the contribution that freshwater flow makes to the maintenance of water column stability, and hence concentrations of food particles. That elevated mortality rates were not observed in the fairly extreme wind conditions encountered, does not discount the possibility that water column mixing can jeopardise the survival of fish larvae, but indicates more the value of stratification for their survival. There are, therefore, implications for the survival of anchovy larvae under conditions of reduced river flow occasioned, for example, by changes in climate or management regime.
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