Feeding of anchovy Engraulis encrasicolus larvae in the northwestern Adriatic Sea in response to changing hydrobiological conditions

Abstract

Results from depth integrated and vertically stratified plankton sampling in the northwestern Adriatic Sea were used for comparison of gut contents of larvae of European anchovy Engraulis encrasicolus with composition and concentration of potential prey in the plankton. Sampling was carried out over a grid of stations both before and after a period of increased wind mixing to investigate changes in food availability and larval feeding success. All larvae had empty guts soon after dusk, indicating daytime feeding and rapld gut clearance. With increasing larval length there was a greater percentage of specimens with empty guts, despite suitable food being ava~lable in the plankton for these larger larvae; this suggests differential gut evacuation during sampling-possibly related to the degree of gut development. Larval diet was principally the various developmental stages of copepods, especially calanoid and cyclopoid nauplii, which were preferentially selected by larvae, whereas selection was against harpacticoid nauplii. Lamellibranch larvae and Peridiniurn spp. were generally abundant in the plankton, but were only present in the gut contents in any number when the preferred dietary organisms were present in the plankton at low concentrations. The number of food organisms in the gut contents increased with concentration of the preferred food organisms in the plankton .up to a limit of -50 organisms 1-'. Within the upper 18 m of the water column, there was a reduct~on in the proportion of larvae with food in their guts with increasing depth, irrespective of the vertical profile of food concentration. Following a period of wind mixing the compos~tion of the plankton changed. This was reflected in the diet of anchovy larvae, which altered in parallel. There was also an overall 4 1 R:, decrease in concentration of the preferred food particles of larvae in the plankton following the period of wind mixing, but larvae were still able to maintain their food intake. These results show that anchovy larvae can successfully adapt their diet to a changing prey field and suggest that in the conditions observed in the northern Adriatic, quite radical changes in the feeding environment were probably insufficient to affect overall larval mortality.

Flexible diel vertical migration behaviour of zooplankton in the Irish Sea

“…However, nauplii and early copepodite stages are usually the main food source of fish larvae (e.g. Conway 1980, Conway et al 1998; they also suffer from invertebrate predation, and they often have higher mortality rates than adults (Ohman et al 2002). Therefore, one would expect them to enhance the behaviours aimed at reducing mortality.…”

“…However, in the literature, copepod nauplii and Oithona spp. are often reported to be the main prey of fish larvae (Conway 1980, Conway et al 1998) and copepods the main prey of chaetognaths (Duro & Saiz 2000), even when large numbers of alternative prey are abundant in the plankton (Saito & Kiør-boe 2001). Chaetognaths are likely to be mainly sizeselective predators (e.g.…”

Contribution of larval growth rate vairability to the recruitment of the Bay of Málaga anchovy (SW Mediterranean) during 2000-2001 spawning seasons

“…Considering that the Mediterranean anchovy larvae feed mainly on the developmental stages of copepods, mainly nauplii (Tudela and Palomera, 1995;Conway et al, 1998), and that the predominant groups of zooplankton in the Bay of Málaga are composed of copepods and cladocers (Pérez-Martin and Camiñas, 1995), whose relative proportion is 60/20 in favour of the former (Rodríguez, 1983), a great increase in zooplankton production could have led to a greater availability of feeding resources. Rilling and Houde (1999) observed in the Bay anchovy (Anchoa mitchilli) a positive relationship of larval growth rate with temperature and zooplankton abundance as a proxy for prey abundance.…”

Wind mixing, food availability and mortality of anchovy larvae Engraulis encrasicolus in the northern Adriatic Sea

“…Microzooplankton samples were analysed for potential food items for anchovy larvae (mostly nauplii and copepodite stages of copepods; Conway et al 1998, Tudela et al 2002) the numbers being converted to numbers l -1 using the computed volume of water filtered from the flowmeter readings. The values given by Hay et al (1988) and Thompson & Harrop (1991) were used for conversion of numerical abundance to dry weight.…”