Abstract
In the present study, the biology and population dynamics of Macedonian Shad (Alosa macedonica), an endemic fish species of Lake Volvi (Macedonia, Greece), were examined. Fish were taken monthly from October 1995 to October 1996 using a series of vertical gill nets with mesh sizes of 14, 18, 22, 26, and 30 mm. In addition, four 24 h samplings were conducted seasonally in order to study the diel variation of feeding activity and composition. To assess prey selectivity for A. macedonica, zooplankton samples were also collected by vertical haul using nets with mesh sizes of 106 and 48 μm, for the collection of crustaceans and rotifers, respectively.Macedonian shad exhibits sexual dimorphism. Females were characterized by (1)higher lengths-at-ages for ages >2 year, (2) higher mean length/weight, (3) highermaximum length/weight, (4) lower condition and (5) lower natural mortality. High correlations were obtained between total (TL), standard (SL) and fork (FL) length. The slopes of the total ...
In the present study, the biology and population dynamics of Macedonian Shad (Alosa macedonica), an endemic fish species of Lake Volvi (Macedonia, Greece), were examined. Fish were taken monthly from October 1995 to October 1996 using a series of vertical gill nets with mesh sizes of 14, 18, 22, 26, and 30 mm. In addition, four 24 h samplings were conducted seasonally in order to study the diel variation of feeding activity and composition. To assess prey selectivity for A. macedonica, zooplankton samples were also collected by vertical haul using nets with mesh sizes of 106 and 48 μm, for the collection of crustaceans and rotifers, respectively.Macedonian shad exhibits sexual dimorphism. Females were characterized by (1)higher lengths-at-ages for ages >2 year, (2) higher mean length/weight, (3) highermaximum length/weight, (4) lower condition and (5) lower natural mortality. High correlations were obtained between total (TL), standard (SL) and fork (FL) length. The slopes of the total-standard and total-fork length regressions differed significantly between sexes, whereas for the fork-standard length regression nosignificant differences were found between sexes. The slopes of the total length-netweight regressions did not differ significantly between sexes. A. macedonica growsallometrically (the values of the slope of the total length-net weight regression were lower than 3 for both sexes). Scales were used for age determination and back-calculation of length for age. The frequent absence of the first annulus, mainly in the older fish, the accumulation of closely spaced annuli near the edge of the scales of older fish and the presence of false annuli are the major problems in the age determination. The formation of the annulus took place from February to April. The greatest annual growth in length occurred during the first year of life. Although the annual growth rate decreased during the second year, it was relatively rapid until the age of three years, remaining comparatively low thereafter. Females had a higher growth rate than males. At all ages the mean back-calculated lengths of females were significantly higher (t-test, p<0.05), except for age 1 where males grew faster. Mean back-calculated lengths did not display Rosa Lee's phenomenon. The mean back-calculated lengths of males and females were applied to the Von Bertalanffy equation and growth parameters were calculated. For 330 males they were L∞=178.9 mm, K=0.371/yr, t₀=—1.230 yr and for females L∞=216.7 mm, K=0.293/yr, t₀=—1.098 yr. The largest individual caught was a 6-year-old, 213.9 mm(total length) and 116.4 g (total weight) female. Growth of A. macedonica in Lake Volvi can be characterized as low, most probably due to the high population density and the intense intraspecific trophic competition. The reproductive cycle of A. macedonica has been described for both sexes on a macroscopic scale in terms of the gonadosomatic (GSI) and hepatosomatic (HSI) index, and in addition for females on a microscopic scale in terms of histological changes in the ovary and changes in the oocyte size frequency distribution. The reproduction of Macedonian shad lasted from the middle of June until the middle of August. The spawning is multiple and the eggs are released at least in two batches. Macedonian shad attains sexual maturity at the age of one year. The absolute fecundity varied between 5000 to 61250 eggs/female with an average of 24462 eggs/female. Hydrated-eggs diameter averaged 0.66 mm and ranged from 0.32 to 0.66 mm. Mean absolute fecundity and mean hydrated-egg diameter of A. macedonica was higher and lower, respectivelythan in other freshwater stocks of Alosa. In this study, a total of 28 planktonic invertebrate species were identified (19 Rotifera, 5 Cladocera, three Copepoda and one planktonic Mollusc larva), four of which (Ascomorpha ecaudis, Ascomorpha sp., Gastropus stylifer, Kellicottia longispina) were recorded for the first time in Lake Volvi. The total zooplankton abundance and biomass ranged from 102 to 1327 ind/1 and 167 to 3415 μg/l, respectively. Generally, rotifers dominated in the total abundance and copepods in the total biomass. Observed phenomena in the zooplankton community of Lake Volvi, such as the domination of rotifers and the lack of large-sized cladocerans, are probably attributed to the predation by the planktivorous fish, especially A. macedonica. Macedonian shad in Lake Volvi feeds mainly on zooplankton. Copepods (47.7% in number) represented the major part of the food, but according to electivity indices, cladocerans (18.5% in number) were the most preferred. Rotifers (8.9% in number), were less abundant. Insects (mainly Chaoborus flavicans larvae) and fish (Knipowitschia caucasico and Rhodeus amarus) were also present. The diet composition changed with season and body size. In summer-autumn the diversity of food increased. The older fish (170-200 mm) were characterized by a consumption of larger prey, such as insect larvae and fish. At the beginning of summer and during autumn feeding was more intense whereas reduced during winter and in May and August. Feeding was found to be directly dependent on water temperature. Diel feeding activity peaked at the mid-day, whereas reduced at night. Diet composition varied throughout the diel period. A greater consumption of crustaceans was observed during the day and rotifers at night. Gill net selectivity for A. macedonica was estimated for samples collected with nets of 14, 18, 22, 26, and 30 mm mesh sizes (nominal bar length). The individual selection factors (SF) ranged between 6.72 and 9.03, whereas the total selection factor (TSF) was 8.21. The optimum selection total lengths increased from 114.9 mm for the 14 mm net to 246.2 mm for the 30 mm net. The modal lengths of the length frequency distributions of A. macedonica caught in gill nets increased from 120 mm TL, for the 14 mm net, to 170 mm TL, for the 22 mm net, but decreased for 26 and 30 mm nets. The decreased modal lengths for the larger mesh sizes (26 and 30 mm) was due to the entanglement of A. macedonica caught. The combined catch of the five different mesh sizes was dominated numerically by Rutilus rutilus (46.1%), A. macedonica (45.3%) and Abramis brama (5.0%), whereas all other remaining species each contributed less than 5%. The catch of the 14 mm gill net was dominated by R. rutilus (74%), of the 18 and 22 mm net by A. macedonica (71 % and 75 %, respectively), while the catch of the 26 and 30 mm net by A. brama (52% and 46%, respectively). The age-frequency distribution for each monthly sample of A. macedonica showed that age-groups II and ΙΠ were dominant in both sexes for all months, except in July and August where age-group I constituted the most numerous group for females and males, respectively. Age-group 0 was poorly represented, possibly owing to the selectivity of the gill nets. Five and six-year old individuals were rare and mainly females. The overall sex ratio (male: female, 1:1.78) was significantly biased in favour of females (χ²=377.71, p<0.05). The mean values of total mortality (Z), estimated using five different methods, were 0.73 for males, 1.01 for females and 0.99 for sexes combined. The mean values of natural mortality (M), estimated using two different methods, were higher for males (M=0.73) than females (M=0.58). The mean value of natural mortality for sexes combined was estimated to be M=0.57. The exploitation rate (E) was <0.50, a fact that must be attributed to the low commercial value of Macedonian shad and the underfishing of its population in Lake Volvi.
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