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Impact of winter oceanographic conditions on zooplankton abundance in northern Adriatic with implications on Adriatic anchovy stock prognosis
... The projected types of ocean winters were already associated with primary production in Supić et al. (2012), in which type A of circulation is characterized-due to increased advection of riverine nutrients towards the open NAd-with higher production rates. Kraus et al. (2015) extended the analysis to zooplanktons and anchovy, showing that zooplankton abundances are increased during type A of ocean winter circulation, in which ciliates are dominated. The consequence is an increase in anchovy abundance, since they are zooplanktivorous. ...
Reliability of climate change projections in coastal and shallow seas may be largely influenced by complex air-ocean-land interactions, like in the northernmost shelf of the Mediterranean, the Northern Adriatic (NAd). However, classification of winter characteristics in the NAd may be simplified following the observed cross-basin bottom density distributions. Two main types of winter circulation occur: (i) type A, when the basin is strongly affected by the freshwater load and (ii) type B, in which the dense water formation is occurring. Neutral winter circulation type with characteristics between types A and B is called type C. This paper evaluates a usage of such a classification in climate models, defines the best proxy index for the winter NAd characteristics and then quantifies winter regimes in the future climate. The latter is assessed by regional coupled atmosphere-ocean model CNRM-RCSM4 for three different scenarios. Acknowledging an offset in density differences, the historical climate run perfectly fits the observations. The number of type C circulation, i.e. with low cross-basin density differences , is projected to increase in the future climate, in particular in RCP4.5 scenario. The number of type A circulation is showing a decrease in all climate scenarios, while type B circulation is decreasing in RCP4.5 and RCP8.5 scenarios. As two main circulation types are conjoined with large differences in biogeochemical properties and fish stocks, these projected changes may have a substantial impact to the future productivity of this least oligotrophic basin in the Eastern Mediterranean.
... In the Adriatic, anchovies represent one of the most relevant fish stocks, and their populations over time have been subjected to many stressors such as climate change-driven shifts in biotic factors and overfishing (Grbec et al., 2002). Specific autumn and winter conditions were invoked in the explanation of Adriatic anchovy stock changes (Santojanni et al., 2006;Kraus & Supić, 2011;Kraus et al., 2015). Moreover, the northern Adriatic is considered a vital nursery and foraging area for anchovy and sardine (Sardina pilchardus), which together account for about 40% of the total marine catch in the basin (Morello & Arneri, 2009). ...
Blooms of invasive ctenophore Mnemiopsis leidyi can have massive consequences on fish stocks and marine food webs. The distribution, abundance and diet of this ctenophore were investigated in the northeastern (NE) Adriatic between 2016 and 2019. The abundance of M. leidyi was determined daily along the coast of Rovinj (Croatia), and its spatial distribution monitored by visual census from research vessels and by pelagic trawl during acoustic surveys in September of each year. Mesozooplankton samples were collected along the western coast of Istria by vertical tows from the bottom to the surface. Spatial distribution and abundance of anchovy (Engraulis encrasicolus) assemblages were determined by geo-referenced hydro-acoustic sampling using a scientific echosounder. Large swarms of M. leidyi covering several km2 were regularly reported between July and November extending up to 25 NM from the western coast of the Istrian peninsula and reaching maximum offshore densities of 270 individuals per m2. The abundance of anchovy in the areas where M. leidyi was present evidently decreased. The content of M. leidyi’s digestive tract and plankton samples consisted both mainly of cladocerans, copepods, pteropods, echinodermata and bivalvia larvae. Results indicate that M. leidyi may accumulate and increase its abundance in the stagnant and trophically rich areas of the northern Adriatic gyres, spreading to the surrounding larger areas with currents. We assume that M. leidyi presence correlates with a decrease in anchovy population due to competition for food (zooplankton). Our findings support the importance of implementing an international monitoring program throughout the Adriatic Sea and demonstrate the ability of current MEDIAS surveys to detect changes in the pelagic ecosystem throughout surveyed areas.
... Mixing period with recent freshwater inputs: Episodic events of high runoff at the same time as ENE wind can induce a "double gyre" circulation, characterized by a cyclonic gyre in the northern area of the basin, which is able to push the Po River plume up to the eastern coast [46]. This process causes the spreading of new freshwater over a rather homogeneous water column and a large dispersion of riverine nutrients in the NA, resulting in high production over the entire NA [38,47,48]. ...
The effects of changing hydrological and climatic conditions on the dynamics of phosphorus (P) were studied in the Northern Adriatic Sea (NA), a coastal system currently affected by these changes. P limitation is one of the key stresses in the NA and it is a globally important phenomenon in aquatic ecosystems. Therefore, the response to P stress by inducing alkaline phosphatase activity (APA) was studied in characteristic water types in the NA, formed by the changing freshwater input in different thermic conditions. APA was important in providing P for microbial growth in upper waters dominated by assimilation during the warmer part of the year in stratified conditions. Contrarily , APA was not important during mixing in the colder part of the year, as well as in waters dominated by regeneration. In waters influenced by freshwater, temperature had no effect on APA, while in high-salinity waters, temperature was an important factor for APA increase. The highest APA occurred during riverine nutrients supply, indicating that the alteration of anthropogenic nutrient loads might strongly change P status in this coastal system. Furthermore, predicted increases in winter sea temperature and summer Po River discharge could delay the water column mixing, prolonging periods of P limitation.
... Most data on feeding by larval fish are from gut content studies that are biased against delicate PMZ such as oligotrich ciliates (de Figueiredo et al. 2007;Montagnes et al. 2010;Bils et al. 2017;Hirai et al. 2017). Field and laboratory studies suggest that high densities of microplankton, such as non-thecate dinoflagellates, most of which are heterotrophic or mixotrophic, and non-loricate ciliates favor survival of fish larvae such as Atlantic herring, Pacific herring, Atlantic cod, northern anchovy and Adriatic anchovy (see references cited in Kraus et al. 2015;Bils et al. 2017;Hirai et al. 2017). ...
... In Italian coastal water, a baseline assessment on levels of microplastics in commercial organisms highlighted the MP presence in 45% of biota from the Adriatic Sea (Dehaut et al. 2016), and particularly in 95% of the benthic flatfish Solea solea (Pellini et al. 2018). For Adriatic fishery, small pelagic planktivores fishes, such as sardines and anchovies, are the most important commercial species with high economic and social value (Kraus et al. 2015). Sardina pilchardus is a marine, freshwater, brackish, pelagic-neritic, oceanodromous species (Riede 2004), living within a depth range of 10-100 m (Whitehead 1990) and feeding mainly on phytoplankton (Nikolioudakis et al. 2011). ...
Marine litter impacts oceans and affects marine organisms, representing a potential threat for natural stocks of pelagic fish species located at the first levels of the marine food webs. In 2013–2014, on a seasonal basis, marine litter and microplastics in stomach contents from Sardinia pilchardus and Engraulis encrasicolus were evaluated. Selected species are plankitivores of great ecological and commercial importance in the Adriatic Sea. Collected data were correlated to possible factors able to affect ingested levels as well as species, season of sampling, biometry and sex of animals. Almost all tested samples (80 organisms for each species) contained marine litter (over 90% of samples from both species) and also microplastics; while any meso- or macroplastics were recorded. On average, recorded items were as follows: 4.63 (S. plichardus) and 1.25 (E. encrasicolus) per individual. Sardines evidenced a higher number of microplastics characterised by a smaller size than those recorded in anchovies. For sardines, sex, Gastro Somatic Index and sampling season showed negligible effects on the number of ingested litter; conversely, anchovies showed differences related with both sex of animals and dominant colour of ingested materials with prevalence for black and blue colours.
... The unusual winter plankton abundance in 2015 may have been responsible for the summer (July) catches of the anchovy Engraulis encrasicolus (Linnaeus, 1758) in the open southern Adriatic that were several times higher than normal (Croatian Ministry of Agriculture, Fisheries and Rural Development). Recent investigations indicate a strong positive correlation of annual anchovy catch with February microzooplankton production (Kraus et al., 2015). This may be related to intensive feeding of adult anchovy on zooplankton during their February prespawning period (Regner, 1996) before attaining commercial length (~9 cm) about 4 and a half months later (Sinovčić, 2000). ...
The paper reports an unusual response of the microzooplankton community to oceanographic conditions observed during the winter of 2015 at the open South Adriatic. Record-breaking nauplii abundance of 13 734 ind. m-3 was sampled for the pen South Adriatic by 50 - μm net sampling. This could be explained by (i) warmer than usual surface and intermediate ocean temperatures, (ii) higher precipitation that freshened and widened surface layer, pushing saline Levantine Intermediate Water below 400 m depth, and (iii) strong wind episodes that transported nutrients from the coastal zone to the open ocean and induced limited vertical mixing. Neritic tintinnids and viable photoautotrophs well below the photic zone support the last possibility. Average seasonal maximum of zooplankton abundance is shifted from spring and early summer to late winter months. Our results are documenting large and fast variations of production conditions, rarely found to occur in oligotrophic waters such as the South Adriatic Sea.
... We identified the following two reasons. Firstly, we showed the example of anchovies (Kraus and Supić, 2011;Kraus et al., 2015) and that understanding the driving forces of phytoplankton production in the ecosystem is important in other fields of marine research, such as fisheries. Other studies following the same approach of combining knowledge about the marine species life cycles on the one hand and the driving pressures of their abundances and distribution patterns on the other could expand our capabilities of a planned exploration of marine resources. ...
Influenced by one of the largest Mediterranean rivers, Po, the northern
Adriatic (NA) production is highly variable seasonally and interannually. The
changes are especially pronounced between winters and seemingly reflect on
total Adriatic bioproduction of certain species (anchovy). We analysed the
long-term changes in the phytoplankton production in the region, as derived
from monthly oceanographic cruises, in relation to concomitant geostrophic
currents distribution in the area and to Po River discharge rates in days
preceding the cruises. In winter and early spring the phytoplankton
abundances depended on existing circulation fields, in summer and autumn
they were related to Po River discharge rates 1–15 days earlier and on
concomitant circulation fields, while in late spring phytoplankton
abundances increased 1–3 days after high Po River discharge rates regardless
of the circulation fields. During the entire year the phytoplankton
abundances were dependent on forcing of the previous 1–12 months of surface
fluxes and/or Po River rates. The role of wind was uncertain but that was partly
due to unmatched sampling time frames between meteorological and sea data.
Low evaporation rates in November reflected significantly on the next
February circulation pattern and, although with somewhat lower significance,
on large phytoplankton blooms in the same month. We showed that the role of
wind in evaporative flux enhancements is not straightforward as
evaporative fluxes are highly dependent on other factors, e.g. air–sea
temperature difference. Wind-induced vertical mixing was only sporadically
related to phytoplankton abundances. From 1990 to 2004 a shift towards large
winter bioproduction induced by circulation changes appeared. The
investigations performed represent the preliminary actions in the construction of an empirical
ecological model of the NA which can be used in the sustainable economy of
the region, as well as for validation of the numerical ecological model of
the region, which is currently being developed.
... However, while they may not yet be sufficient to predict absolute amounts of change and even less successful at indicating the level of error and uncertainty around those predications, modelling tools may be effective in forecasting for different scenarios of change, but such contributions are scarce (e.g. Kraus et al., 2015). Nonetheless, McPhee et al. (2015) developed a model integrating trophic relationships in saltmarsh biological communities. ...
... We identified the following two reasons. Firstly, we showed the example of anchovies (Kraus and Supić, 2011;Kraus et al., 2015) and that understanding the driving forces of phytoplankton production in the ecosystem is important in other fields of marine research, such as fisheries. Other studies following the same approach of combining knowledge about the marine species life cycles on the one hand and the driving pressures of their abundances and distribution patterns on the other could expand our capabilities of a planned exploration of marine resources. ...
Influenced by one of the largest Mediterranean rivers, Po, the northern Adriatic production is highly variable seasonally and interannually. The changes are especially pronounced between winters and seemingly reflect on total Adriatic bioproduction of certain species (anchovy). We analysed the long-term changes in the phytoplankton production at the transect in the region, as derived from monthly oceanographic cruises, in relation to concomitant geostrophic currents distribution in the area and in the Po River discharge rates in days preceding the cruises. In winter and early spring the phyto-abundances depended on existing circulation fields, in summer and autumn they were related to 1–15 days earlier Po River discharge rates and on concomitant circulation fields, while in late spring phyto-abundances increased 1–3 days after high Po River discharge rates regardless of circulation fields. During the entire year the phyto-abundances were dependant on forcing of the previous 1–12 months of surface fluxes and/or Po River rates. Large February blooms are, as well as February circulation patterns, precondited by low evaporation rates in previous November. From 1990 to 2004 a shift towards large winter bioproduction induced by circulation changes appeared. Performed investigations represent the preliminary actions in building of an empirical ecological model of the northern Adriatic which can be used in the sustainable economy of the region, however also in validation of the numerical ecological model of the region, which is currently being developed.
In the so-called “anomalous” summer in 2017, an increased concentration of DOC was measured in the entire water column at the station SJ107 in the eastern part of the Po River delta-Rovinj transect (Graphical abstract) in the northern Adriatic (NAd). Surface DOC ranged between 1.56 and 3.10 mg l⁻¹, being 2.5 times higher than the long-term (1989–2017) average for the same area. Such enlarged concentration of DOC, typical for eutrophic lagoons or lakes and not for open waters, was most probably a consequence of an intensive bloom of Mnemiopsis leidyi, which was recorded with an exceptionally high population density (up to 76 ind. m⁻³) in the same waters.
The number of jelly combs showed a decrease from the east (SJ107) to the west (SJ101) of the transect (Graphical abstract). Abundance in the central part of the transect was also quite low. Maximum of DOC corresponded with the abundance of the M. leidyi smallest specimens (<3 cm). At the western station of the same profile (SJ101), M. leidyi abundance was insignificant, and DOC concentration appeared to be controlled by seasonal changes in primary production and grazing activities. In contrast, at the eastern station (SJ107), changes in the complex food web interaction shaped the DOC quantity and its quality. A large fraction of DOC at both stations was found to be reactive, significantly contributing to the organic matter pool represented by surface-active substances (SAS).
A large anticyclonic gyre which developed from April to November in 2017 in the area of SJ107, supported calm water column conditions and abundant resources of zooplankton, which could support the residence and accumulation of the invasive ctenophore M. leidyi. For the first time, the presence of a long-lasting gyre (duration of at least 4 months) is documented both with hydrographic measurements and by numerical modelling. Such situation characterised by the production of highly reactive DOC leads to an extreme eutrophic episode.
A dispersion model indicated a high probability of M. leidyi spreading by the end of winter from the Venice lagoon, its hypothetical winter residing area, to the Istrian coast (Croatia).
Cold Air Outbreaks (CAOs) over shallow seas may lead to dense water formation episodes, enhancing water, heat, nutrient and sediment exchanges across the continental margin, with associated seabed reshaping.During winter 2012, a CAO episode characterised by exceptional intensity stroke the northern Adriatic Sea, one of the most effective cool engines driving the Mediterranean circulation, providing a paramount opportunity for an integrated investigation of dense shelf water dynamics.In the present study, we describe this event using a fully coupled modeling approach exploring the effects of mutual interactions among atmosphere, ocean currents and sea surface waves, usually not completely accounted for, in the resulting dense water formation.Whilst atmospheric fields appear to be marginally affected by coupled dynamics in the present case, implications for sea surface elevation and circulation are far from negligible. Measurements collected in the northern Adriatic Sea showed that a physically consistent description of energy exchanges between ocean and atmosphere provides an improved estimate of heat fluxes and of air and sea temperatures. In addition, the explicit inclusion of wave action within the modeling system further enhances the modulation of air-sea exchanges and the propagation of its effect along the water column, resulting in a different intensity of northern Adriatic gyres and in different water fluxes flowing through the formation basin.Through these main controls on the water volume involved in the densification process and on the intensity of momentum input and cooling, a coupled modeling strategy accounting for atmosphere-waves-currents interactions can turn out to be crucial for improving the quantification of thermohaline properties and energy content, newly formed dense water mass, and provide a better description of its migration pathways and rates of off-shelf descent.
A variety of existing scientific knowledge can be better utilised for improving stock assessments and predictions. In particular, failure to take account of environmental effects, including both biotic and abiotic factors in the term environment, is a serious source of error in fish stock assessments and limits the time horizon of reliable predictions. The report of the EU-funded SAP project and a set of papers resulting from communications presented at the SAP symposium in Bergen in December 2000 emphasis the potential gains by integrating relevant knowledge of processes affecting fish stocks.
Influenced by one of the largest Mediterranean rivers, Po, the northern Adriatic production is highly variable seasonally and interannually. The changes are especially pronounced between winters and seemingly reflect on total Adriatic bioproduction of certain species (anchovy). We analysed the long-term changes in the phytoplankton production at the transect in the region, as derived from monthly oceanographic cruises, in relation to concomitant geostrophic currents distribution in the area and in the Po River discharge rates in days preceding the cruises. In winter and early spring the phyto-abundances depended on existing circulation fields, in summer and autumn they were related to 1–15 days earlier Po River discharge rates and on concomitant circulation fields, while in late spring phyto-abundances increased 1–3 days after high Po River discharge rates regardless of circulation fields. During the entire year the phyto-abundances were dependant on forcing of the previous 1–12 months of surface fluxes and/or Po River rates. Large February blooms are, as well as February circulation patterns, precondited by low evaporation rates in previous November. From 1990 to 2004 a shift towards large winter bioproduction induced by circulation changes appeared. Performed investigations represent the preliminary actions in building of an empirical ecological model of the northern Adriatic which can be used in the sustainable economy of the region, however also in validation of the numerical ecological model of the region, which is currently being developed.
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. AU larvae had empty guts soon after dusk, indicating daytime feeding and rapid gut clearance. With increasing larval length there was a greater percentage of specimens with empty guts, despite suitable food being available 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 Peridinium 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 similar to 50 organisms l(-1). Within the upper 18 m of the water column, there was a reduction 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 composition of the plankton changed. This was reflected in the diet of anchovy larvae, which altered in parallel. There was also an overall 41% 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.
The diet of the larvae of the sciaenid fish Leiostomus xanthurus collected within the Mississippi River plume differs from that of larvae collected in adjacent Gulf of Mexico shelf waters.
Larvae collected in plume water had eaten twice as many food organisms as had larvae collected in shelf waters. Larvae collected
within the plume had eaten mostly small food organisms (tintinnids, copepod nauplii, pelecypod veligers and invertebrate eggs),
whereas larvae collected in shelf waters had eaten mostly larger food items (copepodite and adult copepods). Differences in
the abundance of food organisms in and differences in the photic environments of these water masses may account for the differences
in diet.
Histological criteria were developed to age postovulatory follicles from examination of laboratory- spawned northern anchovy and used to estimate the frequency of spawning of natural populations. One-day-old, postovulatory follicles were the preferred estimator of spawning frequency; 48 hours after spawning postovulatory follicles could not be consistently identified because of their rapid degenera- tion; and the occurrence of postovulatory follicles less than 24 hours old was affected by hour of sampling and sexual composition of the school. The nightly spawning incidence was estimated to be 1624% of the northern anchovy population in February and 12% in March. Thus, females spawned about every 6-8 days during the peak of the breeding season. The rate of oocyte development corres- ponded with this reproductive rate, indicating that a new mode of yolked oocytes matured about once a week. Nearly all eggs in the most advanced mode in the ovary were spawned in one night and the number ofeggs spawnedwas eatimatedtobe389+59egga/goffemale (ovary-free wet weight).The high spawning frequency and the prolonged breeding season of the northern anchovy indicate that total fecundity may be limited by food availability and energy reserves. Ichthyoplankton surveys have become one of the standard methods' for estimating biomass of marine fish populations. At high latitudes many fishes produce a single spawning batch per year (Qasim 1956) and spawning biomass may be di- rectly estimated from total fecundity and produc- tion of eggs and larvae. Multiple (fractional) spawning fishes are characteristic of subtropical and tropical seas (Nikolsky 1963) and estimation of spawning biomass from egg and larval produc- tion is dependent upon the number of spawnings per year and the number of eggs per spawning. At present, no adequate method exists for estimating these two parameters for pelagic, multiple spawn-
The anchovy Engraulis encrasicolus is an important fishery resource in the Adriatic Sea. Fluctuating recruitment of young fish to the stock over time can be related to changes in the environment. The trend of anchovy recruitment in the northern and central Adriatic from 1975 to 2001 was analysed with the aim of identifying possible effects related to 5 environmental factors: surface air temperature, surface atmospheric pressure, quadrant specific wind stresses, Po River runoff and North Atlantic Oscillation (NAO) index. Particular emphasis was placed on 1987, a year of anchovy collapse and fishery crisis. Different types of regression models were applied, both linear and nonlinear (simple and multiple), with predictor variables being environmental factors and parental stock abundance. Positive relationships of number of recruits with autumnal SSE and ESE wind stress and both annual and autumnal Po River runoff were found, with a strength comparable to the relationship between recruits and parental stock. Low levels of these environmental factors were observed just before the 1987 collapse, together with a high frequency of occurrence of NE winds and an extreme positive value of the NAO index in the previous autumn (which may have been unfavourable to recruitment in 1987). All 5 environmental factors could be related to increased or reduced food availability for young stages of anchovy in autumn. © Inter-Research 2006.
Ontogenetic variation in the diet of Engraulis encrasicolus (L.) was examined in the northern Adriatic Sea, off the River Po delta, by stomach contents analysis. Anchovy (10 to 130 mm total length) were collected during a 1 wk cruise in October 2002. Feeding activity was mainly diurnal in all the length classes: stomach fullness, calculated for each 10 mm size class, was always higher during the day than at night, with maximal values measured for the 40 to 49 and 50 to 59 mm. size classes. All analysed life stages of E. encrasicolus had a strictly zooplanktivorous diet based on a few species of copepods. The principal prey of E. encrasicolus were small-sized copepods, measuring about 0.2 to 0.3 or 0.5 to 0.6 mm in prosoma length. The copepods Euterpina acutifrons and Oncaea spp. dominated anchovy diet during the day, both in terms of frequency, number and biomass, in all anchovy size classes. During the night, bivalve larvae were also important dietary items, with Oncaea spp. and E. acutifrons making up >69% of total prey number for all anchovy size classes. Within the considered size range, the dimensions of particles does not seem to be a limiting factor in food selection, and no relationship was found between anchovy length and the maximal dimension of ingested prey. Ivlev's electivity index confirmed that European anchovy was able to select specific food items and preferred small copepods such as E. acutifrons and Oncaea spp., and bivalve and decapod larvae.
The results of the updated and quality-checked data base of field observations on chlorophyll a (Chl a) collected in the period 1970–2007 in the Northern Adriatic Sea are presented. From the last decade, SeaWiFS satellite information
was also considered. Results demonstrate a global tendency towards Chl a reduction in the period of investigation, which is more marked in the eutrophic area under the influence of the Po River.
In the rest of the basin, which presents meso- or oligotrophic characteristics, long-term changes are more difficult to detect.
The long-term field dataset can be divided into two periods: the last decade characterized by the strong decrease observed
in the whole northern Adriatic and the earlier period with no or slight increase. The recent substantial reduction of Chl
a concentrations is confirmed all over the basin (−0.11mgm−3year−1) from satellite-derived information. Results are consistent with recently evidenced decrease in concentrations of phosphate
and ammonia and point to the existence of oligotrophication in the Northern Adriatic. Results indicate forcefully that the
still common perception of the Adriatic Sea as a very eutrophic basin is no longer appropriate, at least for its northern
part and in recent years.
KeywordsNorthern Adriatic-Chlorophyll a
-Time series-Phytoplankton-Trends
The biological and population dynamics parameters, population structure, length and age distribution, sex ratio, reproduction, age and growth, mortality, migration, estimation of population size and maximum sustainable yield as well as exploitation of the anchovy, Engraulis enerasicolus (n = 20 910) were studied from sampling undertaken in the Middle and partly in the North Adriatic Sea (Dalmatia) during 1974-1990 period. The overall anchovy length from both the coastal and the open sea water samples ranged between 7.5 and 18.7 cm. Samples of catches from the open sea waters comprised the anchovy of greater mean lengths than those from the coastal waters. The differences in mean lengths between sexes were small. The over- all ratio of male and female was almost I : I. Spring months represent prematurity and summer months the period of anchovy maximum sexual activity. Winter is the inactivity period in its sexual cycle. In the prematuritv stage male gonads were found to be in slightly more advanced stages than those of females. Anchovy started to spawn earlier in the open sea waters and reached the spawning maximum sooner than in the coastal waters. Female gonad weight values as well as the values of gonosomatic index were greater than in males. Sexual evolution in bigger anchovy specimens from the open sea waters took place earlier than in smaller ones from the coastal water- areas. The length-weight relationship for all fish was described bv the following parameters: a = 0.0040 : b = 3.0. Anchovy aged 0 to 4+ years old were present in the samples. Age groups 1+ and 2+ were the most prominent. They reach the asymptotic length of 19.4 cm. The fastest growth in anchovy occurs during the first three years of life. The von Bertalanffy growth parameters for all fish were L p =19.4 cm: Wp = 34.8 g : K = 17.57 yr r and t0 = - 0.5 yr-1. Overlapping of lengths are most evident in age groups 2+ and 3+. The highest total mortality rate (Z) was found in the youngest anchovy individuals, whereas their survival r-ate was the lowest. The average annual mortality (A) was 61 %. The relatively high total mortality coefficient value Z = 1.183 is evident. Natural mortality of anchovy population is M = 0.65. Fishing mortality (F= 0.53) has a lesser influence on total mortality. The exploitation rate was E = 0.45. The estimation of anchovy population size and maximum sustainable yield (MSY) shows that this species could have been exploited more than they had been exploited in some years of the investigation period.
The spatial-temporal distribution of anchovy planktonic stages during the spawning season was affected more by the dynamics of biotic environmental factors (primary production and zooplankton quantity) than by abiotic ones (temperature, salinity). -from Bull. Sci. Sect. A Yougosl.
The survival from hatching to day 8 of larval grouper, Epinephelus septemfasciatus, was examined in two rearing experiments. In Experiment 1, the larvae were fed with aloricate ciliates, Euplotes sp. 1 or Euplotes sp. 2, at different concentrations. Significantly higher survival of the larvae was observed in the treatment with the highest density of Euplotes sp. 1 (1.4 x 104 cells l-1) on days 5 and 6 after hatching. Experiment 2 consisted of different prey concentrations of a loricate ciliate, Favella taraikaensis. Significantly higher larval survival was observed in the treatment having the highest density of F. taraikaensis (4.8 x 103 cells l-1) on days 4-6 after hatching. Larval gut content analyses failed to show any ciliates in Experiment 1, whereas hard body parts of F. taraikaensis were detected in Experiment 2. However, feeding on Euplotes by grouper larvae was confirmed by labelling the former with fluorescent microspheres. The lack of ciliate remains in Experiment 1 might be due to complete digestion by the larvae. The present experiments demonstrated that the higher ciliate densities enhanced larval survival until 4-6 days after hatching, suggesting the importance of ciliates as initial food for first-feeding E. septemfasciatus. It is likely that ciliates in the marine ecosystem may bridge the gap until grouper larvae can encounter foods such as copepod nauplii.
This paper is a review of the knowledge on the anchovy planktonic stages in Adriatic sea. The area of reproduction of this species is very wide in the Adriatic since the anchovy will spawn anywhere down to the depth of 200 m. However the main spawning area is in the eutrophic waters of the western part of the shallow northern Adriatic and along the Italian coast to the peninsula of Gargano. Anchovy spawn in the Adriatic generally from April to October, in the temperature and salinity ranges of 11.6 - 27.6°C and 9.1 - 39.6 ppt, respectively. Developmental times of Adriatic anchovy eggs and larvae, as well as the growth curves of larvae and postlarvae, were estimated under experimental conditions. The growth curves of postlarvae were also estimated from otolith readings. Biomass assessments of Adriatic anchovy by means of egg production method was made possible by the known temperature-development relationships, as well as the knowledge of the residence times of particular developmental stages. Spawning biomass was estimated in the northern and middle Adriatic during the period from 1976 to 1990. Detailed investigations of long-term fluctuations of anchovy egg production, quantity of larvae and postlarvae, as well as of instantaneous mortality rates of postlarvae, were performed in the middle Adriatic for the period from 1962 to 1976. Fluctuations of egg production and larval abundance were found positively correlated with fluctuations of temperature, salinity, primary production and zooplankton quantity, with a lag of one year. As eutrophication particularly affected the shallow northern Adriatic and the zone along the Italian coast as far as the peninsula of Gargano, it seems that this phenomenon negatively affected the reproduction of the anchovy in its main spawning areas.
We analyzed oceanographic, hydrologic and atmospheric conditions in the northern Adriatic (section between the Po River delta and Rovinj) from 1990 to 2004 in which extreme phytoplankton blooms appeared. The largest blooms occurred in February, March, July and October, and were higher in surface layers and in the western, more eutrophic part of the region. They often appeared when the Istrian Coastal Countercurrent (ICCC; strong southward current indicating a “closure” of the northern Adriatic circulation system) was pronounced. They seem to be related to the intensities of surface fluxes and Po River discharge rates. The blooms can float over the northern Adriatic for a long period (up to 30 days; typical for March) and can appear simultaneously in separate circulation systems with a different species distribution. Finally, we present a hypothesis based on comparisons of geostrophic currents and fish stock that specific February northern Adriatic oceanographic conditions play the key role in the Adriatic anchovy stock.
New approaches are needed for investigating planktonic prey-predator interactions in situ, free from enclosures and long term incubations. Lengthy incubations can lead to several artifacts, including selective mortality of plank- tonic organisms (1) lysis of some taxa upon fixation (2), alteration of prey-predator encounter rates through per- turbations of the turbulent flow field (3) and modification of natural search and avoidance behaviors of predators and prey (4). Microscopic analysis of the gut contents of predators is a feasible alternative only if prey leave diges- tion-resistant hard parts. Here we describe a different ap- proach involving immunochemical methods. We report the development and first application of an immunoassay that permits detection of predation on soft-bodied, non- loricate ciliates (Strombidium sp.). Polyclonal antibodies raised against Strombidium sp. recognize both intact cil- iates and partially assimilated ciliate antigens occurring in a predator's gut. We demonstrate, by both immuno- chemical and conventional methods, unequivocal pre- dation by first-feeding larvae of the northern anchovy (Engraulis mordax) on nonloricate ciliates. The intensity of the immunochemical reaction, quantified by enzyme- linked dot blots and reflection densitometry, is propor- tional to prey density and to the predator's ingestion rate. Polyclonal antisera were produced in New Zealand white rabbits (5) against Strombidium sp. cultured on a diet of the bacterium Vibrio natriegens (6). Immunoglob- ulin G (anti-Strombidium IgG) was isolated from the antiserum by precipitation in 45% (w/v) ammonium sul- fate, desalted by gel filtration, then fractionated by dieth- ylaminoethyl ion exchange chromatography. Detailed
The abundance of anchovy Engraulisjaponicus larvae, >20 μm ciliates, copepod eggs and nauplii, and microzooplankton herbivorous activity were studied in the Yellow Sea in June 2000. Anchovy juveniles and larvae were found in only 6 of the 19 stations sampled. The ciliate communities were dominated by 2 species: Laboea strobila and Strombidium compressum. In the surface waters, the abundance of L. strobila ranged between 0 and 560 ind. 1-1. S. compressum only appeared at Stns 15 to 18 (20 to 3300 ind. 1-1). L. strobila was found mainly in the top 20 m. The abundance of L. strobila was less than 50 ind. 1-1 in waters deeper than 25 m. S. compressum showed subsurface abundance peaks at the salinity abnormality. Tintinnids occurred occasionally with abundance lower than 100 ind. 1-1. The total ciliate abundance fell in the range of 40 to 3420 ind. 1-1. The ciliate biomass in the surface water and the water column ranged between 0.15 and 6.76 μg C 1-1 and 0.4 and 134.4 mg C m-2, respectively. In the surface waters, the abundance of copepod eggs and nauplii ranged from 0.3 to 3.1 and 1.1 to 15.6 ind. 1-1, respectively. The average abundance of copepod eggs and nauplii in 4 depth (0, 5, 10 and 20 m) fell in the range of 0.2 to 2.8 and 1.0 to 29.4 ind. 1-1, respectively. As a food item of the E. japonicus post-larvae, the abundance of copepod nauplii and eggs appeared to be low. The abundance peaks of ciliate and E. japonicus post-larvae coincided. Although not found in the gut of E. japonicus post-larvae, aloricate ciliates might be ingested by first-feeding anchovy larvae, preventing initial starvation and prolonging the time to irreversible starvation. On the basis of dilution experiments with positive microzooplankton grazing rates, microzooplankton grazed at rates of 0 to 0.61 d-1. Grazing pressure of microzooplankton on chlorophyll a standing stock (Pi) and potential chlorophyll a primary production (Pp) were 17 to 46 % and 35 to 109 % d-1, respectively.
The Black Sea anchovy E. encrasicolus ponticus is omnivorous. Zooplankton (mainly Copepoda and Cladocera) is the food preferred by anchovy. When zooplankton biomass is low, anchovy can switch to con- sume any other available food: mysids, fish or polychaete larvae, and phytoplankton (it comprised about 80% of food items in some cases, when zooplankton biomass in the sea decreased.) This fact demonstrates the high feeding plasticity of anchovy that allows for intensive food ingestion even when zooplankton concentration decreases (Bulgakova, 1993a). Experimental studies of E. encrasicolus behaviour show that it is capable of two feeding strategies: fil- ter feeding and biting, as it is known for other anchovy species (E. mordax and E. capensis). Choice of feed- ing mode is determined by three factors: prey concentration, prey size and fish size. With increasing food concentration, fish switch from biting to filter feeding. The threshold concentration (when one half of the school begins to filter feed and the other half bites) declines with increasing prey size and increases with increasing predator size. The diel dynamics of Black Sea anchovy feeding is not constant. It varied at the different 24-h stations and was related to food composition. When feeding on copepods, anchovy stops the foraging activity at night; while feeding on phytoplankton or mysids results in an increase in night foraging activity. No diel feeding rhythm was found in experimental conditions with constant food concentration, the feeding intensity was high and continuous during the whole period of the experiment. Thus, anchovy can feed continuously day and night if there is available prey. On the base of experimental studies on food digestion and evacuation from the stomach, the exponen- tial model was preferred for estimating the evacuation rate. The values of anchovy daily rations reach 32% of body wet weight, calculated on the basis of field data on anchovy mean fullness indices during the spawning season and experimental data on digestion rates (Bulgakova, 1993b). The experiments for the direct calculation of ingested preys were undertaken in holding tanks to test the data obtained on anchovy daily rations. The food ingested during 24 h was 13-55% of body wet weight. The calculation of the anchovy energy budget during the spawning season on the basis of new data on reproductive intensity (Lisovenko and Andrianov, 1996) showed that one female consumed about 146 Cal per gram daily, which is equal to 25% of body wet weight. The anchovy population needs to consume up to 80,000 t of plankton per day in the summer season (about 20% of the daily fodder plankton production in the Black Sea), and it plays a major role in the Black Sea ecosystem dynamics.
Presented results indicate that a long-term (several months ahead) forecast of the winter northern Adriatic conditions is possible and that it can be based on the analysis of meteorological conditions and geostrophic circulation fields of the previous autumn. Using 1981–2007 February data we show that in winters of the type A, salinity in the northern Adriatic is lower and production of phytoplankton higher than in the B type winters. This indicates that the impact of the Po River waters on the northern Adriatic is more pronounced during the A type winters. The two types, A and B, have already been identified on the basis of the bottom density differences between eastern and western part of the northern Adriatic but more precise definition is given here. Based on the data collected during hydrographic cruises in 2000 (type B conditions) and 2001 (type A conditions), we found that the two winters differed in geostrophic circulation patterns as well. Eastward of the Po River delta there was a large cyclonic gyre in 2000 and a large anticyclonic gyre in 2001. Circulation patterns were highly dependent on autumn conditions, with bottom density changes as the most likely triggering cause. Strong surface heat losses and many bora episodes preceded the winter of 2000, while moderate cooling and sirocco events preceded the winter of 2001.Highlights► There are A and B types of northern Adriatic winter conditions. ► In winters of type A waters of Po River are drawn offshore, into northern Adriatic. ► In winters of type B waters of Po River stay close to the coast. ► Forecast of A and B conditions seems to be possible several months in advance.
Zooplankton was sampled during 39 cruises, from 1990 to 1993, at four fixed stations in the open northern Adriatic. Hydrographic
factors were important in determining the abundance of the smallest and largest components of the northern Adriatic food chain
during this period. Nauplii—especially those of the smallest size fractions—were the major mediators of material transfer
between primary producers and higher trophic levels. There was a significant difference in the vertical distribution of nauplii
size fractions between the eastern and western parts of the northern Adriatic, but not in their population density. According
to multiple correlation analyses, the abundance of naupliar size fractions in the western area correlated strongly with temperature
and with certain biological factors. This study confirms the important influence of the Po River and of mid-Adriatic waters
on the planktonic ecosystem of the northern Adriatic.
Winter thermohaline properties of the northern Adriatic are analysed here with the aim of getting a better insight into dense water formation on the shelf. The hydrographic parameters collected in February in the 1967–2000 interval at two stations, the first located close to the eastern shore (station 1), and the second positioned near the Po river mouth (station 2), are compared. Two types of winter hydrographic conditions are distinguished: type A when bottom salinity and density are higher at station 1 than at station 2 and type B when these parameters are higher at station 2 than at station 1. Type A is more likely to occur in warmer and type B in colder winters. Both A and B distribution types can occur in periods when the Adriatic is under the influence of very saline waters of Mediterranean origin. Interannual changes in density are, at both stations, more dependant on haline than on thermal variations. At both stations temperature was somewhat higher in the early seventies than during the eighties and nineties, while salinity and density were lower in the early seventies and early nineties than in other years of the analysed period. By comparing the 1967–2000 changes in hydrographic conditions in February to monthly values of northern Adriatic surface fluxes and Po river discharge rates, it is shown that winter thermohaline characteristics in the region depend on processes which occur much earlier, i.e. during the previous autumn and late in spring of the preceding year, and even during the previous winter, 12 months before.
The dominant prey of small larvae of two pleuronectid flounder species, greenback flounder Rhombosolea tapirina Günther and long-snouted flounder Ammotretis rostratus Günther, were bivalve veligers, which were also the most abundant zooplankter in the environment. When large larvae of both species were collected together, the dominant prey of Rhombosolea tapirina was the cladoceran Evadne nordmanni Loven, while paracalanid copepodids were the dominant food of Ammotretis rostratus. The relationship between predator and prey size was similar for both species, with size of prey consumed increasing with larval growth. Smallest feeding larvae consumed larger prey ( width) than most larvae of marine fish at this stage. Comparison of diets of flounder larvae with abundances of microplankton in the environment revealed strong selection for prey type as well as prey size. Similar distributions of Evadne nordmanni and paracalanid copepodids in microplankton samples indicated that divergence in prey selection by larger larvae was due to innate preference, rather than fine-scale co-occurrence with prey. From a knowledge of feeding period, average gut contents, and gut evacuation time, I estimated that flexion-stage Rhombosolea tapirina would consume ≈ 75 prey per day. I conclude that at levels of larval fish and prey abundance observed in Port Phillip Bay, Australia, flounder larvae would have negligible impact on prey populations.
To evaluate the role of fish larvae as a link between the microbial loop and higher trophic levels, predation of protozoan zooplankton by young larvae was investigated. More than 400 individual fish larvae with total lengths of less than ca 10 mm in 52 different taxonomic groups were collected at different sampling times from several coastal regions, and the gut contents of larvae were examined under epifluorescence microscopy after staining with DAPI. Among numerous fragments of copepod nauplii, many flagellate-like cells with a size of 5 μm and ciliate-like cells with a size of 20 to 30 μm were frequently recognized. The number of protozoan cells varied significantly from one larva to another. Some individuals had more than 60 protozoa, while others contained none at all. The amount of protists contained in the gut of larvae depended on the fish species and did not show any trend with the body or mouth sizes of larvae, nor the sampling site or season. Fish taxa were divided into 3 groups depending on the amount of protists in the gut: 'abundant', 'moderate', and 'none'. The Acanthopterygli group contained the highest concentration of protozoa. Results of the present study suggested that fish larvae of some taxonomical groups were important predators of protozoa and may be an important link between the microbial loop and the grazing food chain.
Anchovy (Engraulis Encrasicolus, L.) Stock Assessment in the Adriatic Sea
- N Cingolani
- A Santojanni
- E Arneri
- A Berlardinelli
- S Colella
- F Donato
- G Giannetti
- G Sinov Ci C
- B Zorica
- B Mar Ceta
Cingolani, N., Santojanni, A., Arneri, E., Berlardinelli, A., Colella, S., Donato, F.,
Giannetti, G., Sinov ci c, G., Zorica, B., Mar ceta, B., 2005. Anchovy (Engraulis
Encrasicolus, L.) Stock Assessment in the Adriatic Sea: 1975-2004. General
Fisheries Commission for the Mediterranean (GFCM). Scientific Advisory
Committee (SAC), Subcommittee on Stock Assessment (SCSA), Rome,
pp. 26e30. September 2005.