Rapp. Comm. int. Mer Médit., 36,2001
251
The Daily Egg Production Method (D.E.P.M.) is applied to estimate
the spawning biomass. It was developed in the 80s by the USA
National Marine Fisheries to estimate the small pelagic biomass. Both
the method and its applications are widely described in the NOAA
Technical Report NMFS 36 text (1).The biomass estimate is based on
the following equation:
whereB= the spawning biomass in metric tons, P
o
= the number of
eggs per sampling unit, (m
2
per day), A = total survey area (in m
2
per
sampling units), W= average weight of mature females (grams), R=
sex ratio (fraction of mature females by weight), F= batch fecundity
(average number of eggs per mature female), S= fraction of mature
females spawning per day,k= conversion factor from grams to metric
tons.
This survey has been carried out within the program “Estimation of
the Mediterranean anchovy (Engraulis encrasicolus L.) biomass by
the daily egg production method in Tracian Sea (Greece) and South
Western Adriatic Sea (Italy)”, contract n. 98/040, financed by the EEC,
in the Lower Adriatic Sea.
The survey was carried out between 30 August and 18 September
1999, both during the day and at night, covering most of the repro-
ductive zone in the eastern arc of the lower Adriatic. Both the egg sur-
veys and adult surveys were carried out using chartered research ships
equipped with midwater otter trawls. In order to catch more adult sam-
ples were additionally used commercial vessels equipped with ?ying
midwater pair trawls and purse seine. The sampling and data process-
ing methods as regards the egg and adult campaigns has been
described by Casavola (2, 3).
A total of 122 ichthyoplancton samples were collected by means of
a CalVET net (0.250 mm mesh), raised vertically from a depth, when
possible, of 100 m, covering a sea surface of 12,862 km
2
.The egg
sampling procedure was based on a 5 nautical mile path (stations and
transepts), with 20 transepts. In the laboratory, teleost eggs were sep-
arated in plankton samples caught in a CalVET net and fixed in 4%
formalin . Anchovy eggs were then counted and staged according to
the degree of embryonic development (4) in order to calculate mortal-
ity rates using specific software “Stageage” and “Eggreg” (5, 6), start-
ing from the concentration of embryonic phases (divided by area) and
water temperature at a depth of –20 mt., recorded in each sampling
station. The elaboration of the data relative to the positive stratum,
obtained using a weighted nonlinear least squares regression (7), has
made it possible to define a mortality curve using the classic model,
P
(t)
= P
(0)
x e-
zt
,wheret= the age in days,T
(t)
= the number of
eggs/0.05m
2
of age t, P
(0)
= daily egg production/0.05m
2
,z= daily
instantaneous death rate.
From a total of 43 trawls, 22 adult anchovies were sampled. The
parameters relative to the adults (W, F, SandR), as the number of indi-
viduals sampled was not always equal in every trawl, were estimated
by means of average weighted and variances using the equations of
Picquelle and Stauffer (8). The average weight of females, W, was cal-
culated as the average weight of mature females per haul, all females
in the sample were active spawners. 1078 females were used in the
estimate of this parameter. It was necessary to adjust the weight of
females which were in the hydrated condition, due to the water reten-
tion during hydration, using the reg
r
e
s
s
i
o n: W = -0,1343 +
1,0607xW*whereW*, is the ovary-free weight of females with no
hydrated eggs.
The batch fecundity,F, was estimated using the regression between
batch fecundity and weight (without ovaries,W
h
*), of 97 hydrated
females, carefully chosen from among the 133 caught during the cam-
paign. The weight of the hydrated females used ranged from 6.7 to
33.0 g, the length from 11.1 to 17 cm, with an average weight of 17.2g
and average length of 13.9 cm. The resulting linear regression is
F=58.973+372.38xW
h
*(R
2
= 0.79).
The fraction of mature females spawning per day, (S), was deter-
mined by hystological analysis of post-ovulatory follicles (PO) classi-
fied in three age bands (9): day-0 PO = 0-8 hours, day-1 PO = 9-32
hours and day-2 PO = > 32 hours. The S, given that the day-0 PO
females are oversampled, is expressed as a fraction of females with
day-1 post-ovulatory follicles.
The relationship between the sexes, (R), was calculated as a pro-
portion of the mature females by weight.
Table 1 shows the D.E.P.M. parameters and the estimated anchovy
biomass. From 1994 to 1995, in the Lower Adriatic sea, there has been
an increase of anchovy biomass from 8,129 metric tons to 14,307 (2,
3). In 1999 a value of 10.361 metric tons has been found, showing a
reduction by 25% of the stock in these later years. The daily fecundi-
ty value of the population of E. encrasicolusobserved in 1999 in the
Southern Adriatic sea (20.17 egg/day g of spawner) is lower than that
reported by García et al., (10) and Garcia and Palomera (11) (69.8:
Catalan sea and the Gulf of Lions; 69.6: Ligurian and N. Tyrrhenian
sea), and significantly lower than results obtained by Palomera and
Pertierra (12) (100.1: Catalan sea). In the latter case, the higher values
of daily fecundity in the anchovy population is due to higher values
regarding both egg production of the sample and laying frequency.The
daily mortality rate (0.65) is similar to the values reported by Palomera
and Pertierra (12) for the Catalan sea (0.56) and it is, instead, lower
than the value reported by Garcia et al., (10) for the Catalan sea and
for the Gulf of Lions (1.09) and in the Ligurian sea and the N.
Tyrrhenian (1.55).
Table I. - D.E.P.M. parameters estimate of Engraulis encrasico-
luspopulation from the Apulian Adriatic Sea; coefficient of
variation in parentheses. Survey 30 August – 18 September
1999.
References
1 - Lasker R. (Editor) 1985. NOAATech. Rep. NMFS, 36: 1-99.
2 - Casavola N., 1998. Rapp. Comm. Int. Mer Medit., 35: 394-395.
3 - Casavola N., 1999. Biol. Mar. Medit.6(1): 553-555.
4 - Moser H.G., Ahlstrom E.H., 1985. In : Lasker R. ed. NOAA Tech.
Rep. NMFS 36:37-41.
5 - Lo N.C.H., 1985. In: Lasker R. ed. NOAA Tech. Rep. NMFS 36: 43-
50.
6 - Motos L., 1996. Sci. Mar., 60(2):195-207.
7 - Dixon W.J., Brown M.B., eds. 1979. University of California Press,
Berkeley.
8 - Picquelle S., Stauffer G., 1985. In: Lasker R. ed. NOAA Tech. Rep.
NMFS 36: 7-16.
9 - Hunter J.R., Macewicz B.J., 1985. In: Lasker R. ed. NOAA Tech.
Rep. NMFS 36: 79-94.
10 - García A., Palomera I., Rubín J.P., Pérez N., Giovanardi O., Rubiés
P., 1995. Rapp. Comm. int. Mer Médit., 34: 243.
11 - García A., Palomera I., 1996. Sci. Mar., 60(2): 155-166.
12 - Palomera I., Pertierra J.P., 1993. Sci. Mar., 57(2-3): 243-251.
ANCHOVY BIOMASS ESTIMATE BY THE DAILY EGG PRODUCTION METHOD
IN THE SOUTH-WESTERN ADRIATIC DURING 1999
N. Casavola*, E. Rizzi, E. Hajderi and A. Aprea
Servizio Ittico Ambientale, Bari, Italy
kP
o
AW
B=RFS
D.E.P.M. Parameters
Po1
1.875 (0.23)
Po
0.812 (0.10)
z
0.648 (0.27)
A
Stratum 0 (km
2
)
7,288
A
Stratum 1 (km
2
)
5,574
W
16.97(0.039)
F
6,379 (0.039)
S
0.102 (0.098)
R
0.529 (0.042)
DEPM Biomass (MT)
10,361 (0.642)
