Catch length-frequency data of Otolithes ruber (Schneider, 1801) obtained from the "Projecto de Pesca Experimental da RDA" on Sofala Bank, Mozambique, in 1987 were used to estimate growth parameters, mortalities and exploitation rates. The yield-and biomass-per-recruit analyses based thereon suggest that the stock is overfished

Schultz, N.

Aquatic Commons

Revista de Investiga9ao Pesqueira 
Maputo No. 21 (1992):41-49 
Preliminary Investigations on the Population 
Dynamics of Otolithes ruber (Sciaenidae) 
on Sofala Bank, Mozambique 
by 
NORBERT SCHULTZ 
ABSTRACT 
Catch length-frequency data of Otolithes ruber (Schneider, 1801) obtained from the "Projecto de 
Pesca Experimental da RDA" on Sofala Bank, Mozambique, in 1987 were used to estimate growth 
parameters, mortalities and exploitation rates. The yield- and biomass-per-recruit analyses based 
thereon suggest that the stock is overfished. 
RESUMO 
Foram estimados os parametros de crescimento, os coeficientes de mortalidade e as taxas de 
explora9ao do especie Otolithes ruber (Schneider 1801), atraves das distribui9oes de comprimentos 
das capturas do "Projecto de Pesca Experimental da RDA", no Banco de Sofala, Mo9ambique, em 
1987. A analise das curvas de captura e de rendimento e biomassa por recruta leva a sugerir que o 
stock desta especie se encontra intensamente explorado. 
41 
42 
INTRODUCTION 
Data on the fish stocks of Sofala Bank were obtained in the past through surveys aimed at the shrimp 
stocks. In 1985, studies of the landings of one segment of the industrial fishery were initiated. It is 
only in 1987 that the Instituto de Investigacao Pesqueira initiated studies aimed specifically at the 
coastal fish resources of Sofala Bank. 
The croaker Otolithes ruber (Sciaenidae) represents one of the most important resources in the coastal 
part of Sofala Bank. It is caught by all fisheries in this area: the industrial, semi-industrial and the 
traditional fisheries. The present contribution is an attempt to assess the status of this resource. 
MATERIALS AND METHODS 
The basic data used for this preliminary study of the population dynamics of 0. ruber are given in 
Table 1. 
The data were sampled from the landings of the "Projecto de Pesca Experimental da RD A" in Beira 
in 1987, and originated from four 26.5-m sidetrawlers operating as part of the industrial bottom trawl 
fishery in the coastal area of the Sofala Bank (Fig. 1) at water depths from 7 to 20m. 
16 
17 
Cii 19 !, 
Cll 
"0 
z 
;:: 
19 0 
...J 
Mozambique 
eo~' \(. 
Longitude (0 E) 
The fish caught by that fishery are 
graded into three classes and 0. 
ruber is not abundant in the first of 
these. The second class, generally 
medium-sized fish, represents 6% 
of the total catch; of this, 0. ruber 
represented 71% in weight. The 
Fig. 1. Fishing area of the "Projecto 
de Pesca Experimental da RDA", on 
Sofala Bank, with schematic 
representation of Otolithes ruber. 
Table 1. Otolithes ruber - length distribution in the landings of the second and third class fish, Projecto de Pesca Experimental da RDA, Sofala Bank, 1987. 
M<mth I an Feb Mar Apr MAy lun lui Aug Sep Oct Nov De., 
n.-.:tan• '1:"{3" '1:"/3" '1:"/3" '1:"/3" '1:"/3" '1:"/3" '1:"/3" '1:"/3" '1:"/3" '1:"/3" '1:"/3" '1:"/3" 
2.5 4 I 0 0/ 3 
3.5 o I 7 o I 3 0 I 5 0 I 4 o I I o I 2 
4.5 0 I 2 0 I 10 o I I 3 I 16 o I 9 o I 7 o I 13 0 I 3 
s.s o I 2 0 I 21 0 I 13 I I II 2 I 8 o I 17 o I 20 o I 4 o I 7 o I 2 o I 9 
6.5 o I 5 o I 8 I I 20 o I 31 2 I 49 I I 13 I I 27 o I 38 o I 12 1 I 19 o I 30 o I 5 
1.5 0 I 2 o I 3 Of 23 I I 30 3 I 55 o I 24 o I 24 o I 51 o I 27 2 I 37 I I 15 I I 17 
8.5 0 I s o I 3 I I 23 0 I 27 8 I 47 o I 39 o I 28 I I 26 I I 27 I I 31 S I 109 I I 52 
9.5 o I 14 0 I 5 o I 14 0 I 39 6 I 44 0/ 45 I I 23 o I 12 I I 19 o I 26 I I 6S 0 I 80 
IO.S 0 I 2.S o I 6 2 I 14 o I 31 2 I 33 o I 2.S o I 21 o I 4 o I IS o I 27 3 I 45 3 I 72 
II.S 3 I 41 o I 7 0 I 20 o I 26 o I 14 o I 9 o I 13 o I 5 o I 7 o I 2.S 1 I 45 o I 76 
12.5 o I 22 1 I 6 o I 13 0 I 20 o I 9 01 12. o I II o I 17 o I 6 o I 16 o I 41 2 I 67 
13.5 2 I 7 I I 17 o I 12 o I 29 o I 14 I I 10 o I 20 0 I 20 o I 9 o I 12 I I 26 o I 44 
14.5 2. I 13 I I 6 o I 9 o I 19 o I 19 I I 14 0 I IS o I 28 o I 14 o I 3 I I 2.9 I I 44 
u.s I I 5 3 I 10 3 I 17 o I 14 o I 10 o I 10 o I 14 o I 30 o I 26 o I 8 o I 31 o I S6 
16.5 I I 2 2 I II 5 I 19 I I 22 I I 4 I I 12 o I 2.S 2. I 32 o I 28 2 I 10 0 I 30 o I 31 
17.5 1 I 7 6 I 8 21 18 5 I 19 2 I 10 I I 8 2 I 36 13 I 36 7 I 44 10 I II 1 I 2.S 0 I 2.9 
18.5 2/ 10 201 10 to I 12 14 I 23 s I 17 4 I 13 21 23 23/ 37 20/ 2.9 33 I 14 10 I 39 6 I 48 
19.5 1 I 5 30 I s 33 I 17 17 I 24 6 I IS 16 I 4 17 I 33 41 I 32 so I 30 34/ 6 28/ 38 13 I 51 
20.5 14 I 7 40/ 7 36/ 24 321 17 261 18 271 15 271 41 74 I 30 65 I 9 38/ 14 36/ 35 2.51 22 
21.5 30 I s 42/ 1 70 I 2.S 55 I 16 53 I 13 30/ 12 44/ 27 901 16 661 7 39 I s 38/ 20 48/ 35 
22.5 38 I 2 39 I 1 78 I 12 661 6 73 I 6 56/ II 56 I II 08/ 5 81 I 2 241 4 47 I 12 6SI 22 
23.5 541 1 2.91 1 78 I II 63 I 1 73 I 8 691 7 68/ 10 891 9 541 1 23/ 3 33 I 2 55 I 12 
24.5 ss I 4 14 I 0 49 I 4 47 I 0 601 1 52 I 7 64/ 4 76 I 6 67 I 0 21 I 3 30/ 3 38/ s 
23.5 so I 0 18 I 0 38 I 4 53 I 1 42 I 0 62 I 2 79 I 3 71 I 2. 521 0 17 I 0 271 0 38/ 5 
26.5 441 0 14 I 0 2.91 0 33 I 0 33 I 1 34/ 0 32/ 0 361 1 261 0 4 I 1 U/ 1 26/ 4 
27.5 35 I 0 6 I 0 29 I 0 2.91 1 201 0 39 I 0 461 2 27/ 0 22/ 0 3 I 0 16 I 1 17 I 0 
28.5 37 I 0 7 I 0 :u I 0 27/ 2 271 0 291 0 44/ 0 22/ 0 IS I 0 2 I 0 S I 1 19 I 2 
29.5 201 0 4 I 0 II/ 0 17 I 0 IS I 0 14 I 0 271 0 31 I 0 9 I 0 1 I 0 5 I 0 II I 0 
30.5 7 I 0 4 I 0 7 I 0 II I 0 12. I 0 14 I 0 19 I 0 IS I I 17 I 0 7 I 0 12 I I 
31.5 4 I 0 1 I 0 8 I 0 7 I 0 4 I 0 8 I 0 18 I 0 6 I 0 61 0 s I 0 21 0 
32.5 1 I 0 6 I 0 2 I 0 2 I 0 3 I 0 7 I 0 7 I 0 6/ 0 1 I 0 1 I 0 8 I 0 
33.5 8 I 0 2. I 0 S I 0 1 I 0 I I 0 5I 0 9 I 0 1 I 0 2/ 0 3 I 0 
34.5 2 I 0 4 I 0 1 I 0 1 I 0 6 I 0 1 I 0 '1. I 0 3 I 0 
35.5 3 I 0 1 I 0 3 I 0 1 I 0 7 I 0 2/ 0 3 I 0 3 I 0 3 I 0 
36.5 I I 0 1 I 0 S I 0 2 I 0 1 I 0 1 I 0 
37.5 1 I 0 2 I 0 I I 0 
38.5 I 0 1 I 0 1 I 0 1 I 0 1 I 0 
39.5 1 I 0 
40.5 2 I 0 1 I 0 
41.5 
42.5 1 I 0 01 
lol!!ln 417 I 182 284 I 119 540 I 349 492 I 412 489 I 417 466 I 317 582 I 439 750 I 412 572/ 316 2561 282 32.S I 70S 401 I 195 
nnmp!es 3 I 3 3 I 3 4 I 6 4 I 6 4 I 7 3 I 6 3 I 6 4 I 8 4/ s 1 I 3 3 I 6 3 I 7 
W(ks)of 
samploa 75.780 I 5.580 27.590 I 3.115 79.673 I 13.770 78AIO I 21A3S 77.269 I 10.915 77.917 I 10.517 96A3S I 20.280 111.945 I 18.150 100.7S9 I 12.995 28.610 I 1.98S 46.718 I 20.710 63.673 I 31.605 
cat<h (kg) 4440 I 6499 1124 I 4211 2.S2.S I 5967 2804 I 3397 6134 1 4763 6351 I 3277 ISS! I 1619 6983 I 7389 9470 /13850 4990 110117 2772 /11802 7366 110706 
.p. 
CN 
44 
third class, which consists of small-sized fish and the juveniles oflarger fishes, contributes 87% of 
the landings; 0. ruber represented 7% of the landings. Only the landings of second and third class 
fish were sampled and compiled per month. 
Total lengths (TL) were measured to 1 em below and the total weight of each sample was noted. To 
obtain the total length distribution of 0. ruber in the catch, the sample frequencies were raised to total 
catch. 
The calculations were all performed using the Compleat ELEFAN software (Pauly, this vol.). 
As a first step, estimates ofTLoo and Z/K were obtained using the modified W etherall plot (W etherall, 
1986; Pauly, 1986). Then, response surface analyses were performed, using ELEFAN I, to obtain 
an initial estimate ofK. The initial estimates ofTLco and K were then refined using the search routine 
of ELEF AN I. This analysis suggl:sted that the data should be regrouped into classes of 2 em. 
Subsequently, a standard length-converted catch curve was constructed to estimate probabilities of 
capture, which were smoothed using a moving average. The original length distributions were then 
corrected for incomplete selection/recruitment and new growth parameter estimates were obtained; 
these were used to construct a second catch curve, using the method of Pauly ( 1990), and to estimate 
M, forT= 2YC, using the empirical equation of Pauly (1980). 
The yield- and biomass-per-recruit analyses wereperformedassuming knife-edge selection (Beverton 
& Holt, 1966), then using a realistic selection ogive (Pauly, this vol.). A length-structured VP A, i.e., 
the VPA version of Jones' length cohort analysis (Jones, 1984) was performed to complement the 
catch curve and Y/R analyses. 
RESULTS AND DISCUSSION 
The Wetherall plot provided estimates ofTL= = 45.9 em and Z/K = 6.7 (Fig. 2). The initial estimate 
ofK (left panel) used to estimate the probabilities of capture was 0.32 year-1; Fig. 3 shows the catch 
curve of the derived probabilities, used to correct the original length distribution for the effects of 
selection and/or incomplete recruitment. The subsequent analyses, using ELEF ANI, of the corrected 
data confirmed the first estimates of TLoo and K, and generated associated values of C = 0.45 and 
.... 
~ 
<I 
' z 
c:: 
-E 
0 15 
-
10 
- 5 
0 
........ 
0 
0 
0 
0 
0 
o not used 
• used 
OCloo 
Ooo 
00 
0~~ 
I I I I I ' 
2 9 ·rs 23 30 37 
Cutoff length CTL' ,em) 
...... 
.ll! .5 !'1 I ... 
oOooao l. 
0 2 0 
0 
0 
0 
I I I 
2 3 4 5 6 7 
Relative age (years- r 0 } 
...... 
-<I 
' z 
c:: 
45 
Fig. 2. Modified Wetherall plot for Otolithes 
ruber, "Projecto de Pesca Experimental da 
RDA", Sofala Bank; the regression equation 
is Y = 5.95-0.129 X, with r = 0.989. This leads 
to TL_ = 45.9 em and ZIK = 6.7. 
• used 
o not used 
15 0 o 0 0 
0 
0 
5 oo 0 
0 
0 '~-+--~,--~,~~~--~~--~~--~~~~~ 0 2 3 4 56 7 8 
Relative age C years -r 0 } 
Fig. 3. Catch curves for Otolitbes ruber at Sofala Bank, Mozambique. 
WP = 0.65 (Fig. 4). This figures suggests that one strong cohort dominated the fishery in 1987, and 
leaves only a faint possibility of a second, weaker cohort. 
Table 2 compares the growth parameters obtained here with those of other authors. As might be seen, 
the value ofTL.,.. obtained here compares well with Gislason's (1985) value, although not with his 
values of K which appears to be too low. 
46 
35 
30 
25 
20 
15 
E 10 
C) 5 
.£: 0 
... 
0 
c 
~ 
0 
35 
- 30 {!. 
25 
20 
15 
10 
5 
0 
J F M A M J J A s 0 N D 
Month 
Fig. 4. Length distribution ofOtolithesruber with estimated growth curves, "Projecto 
de Pesca Experimental da RDA", Sofala Bank, 1987. Above: Restructured length 
distribution. Below: Original length distribution; the parameters are TLoo = 45.9 em; 
K = ), 32 year-1, C = 0.45 and WP = 0.650.65. 
Table 2. Comparison of growth parameter estimates in Otolithes ruber. 
Area 
San Miguel Bay, 
Philippines 
Sofala Bank, 
Mozambique 
Sofala Bank, 
Mozambique 
Source 
Naval una 
(1982) 
Gislason 
(1985) 
this study 
a<!>'= 1og10K + 21ogLoo (Pauly & Munro, 1984). 
TLOO 
(em) 
29.5 
33.5 
35.6 
42.9 
45.9 
K 
(year-1) <!>'a 
0.4555 2.60 
0.43 2.73 
0.43 2.74 
0.144 2.42 
0.32 2.84 
Navaluna's (1982) growth parameter estimates are also different from those estimated here, which 
can be attributed to the extremely strong fishing pressure in San MiguelBay, Philippines, which led 
to the absence of fish above 28.5 em from the bay (Pauly, 1982). 
47 
The values of TLoo and K obtained here, although preliminary, thus appear credible, both because 
of the representativeness of the data analyzed here, and because seasonal growth oscillations were 
explicitly considered, something never done before with 0. ruber. 
The catch curve analysis based on the method of Pauly (1990), i.e., accounting for seasonal growth 
oscillation (Fig. 3, right panel) led to an estimated value of Z = 1.95 year-1. This is higher than the 
estimate of Z that can be obtained by multiplying the Wetherall estimate of Z/K with the estimate 
of K, i.e., 6.5 x 0.32 = 1.34 year-1. Natural mortality (M) was 0.7 year-1. This leads to a fishing 
mortality ofF= 1.25 year-1 and an exploitation rate of E = 0.64 for the fully exploited part of the 
stock. 
A trial run with length-structured VP A (not shown) suggested that fishing mortality is relatively 
constant in the older fish. This validates the results of the catch curve analysis, which assumes 
constant mortality for the larger sizes. 
Fig. 5 presents the results of the yield- and biomass-per-
recruit analyses. As might be seen, the results obtained 
based on the knife-edge assumption differ greatly from 
those using the estimated probabilities of capture, with 
the latter being more conservative than the fonner. 
Thus, the value ofE giving maximum yield-per-recruit 
(Emax) is 0.64 in the knife-edge case, and only 0.44 in 
the other case. The corresponding values of E0_1 are 
0.55 and 0.43, respectively, while the values of E 
corresponding to half the unexploited stock (E0_5) are 
0.32 and 0.28, respectively. 
Fig. 5. Yield- and biomass per recruit of Otolithes ruber, 
SofalaBank, 1987 (full dots refer to present 1988 conditions). 
A: Assuming knife-edge selection.B: Using estimated probabilities 
of capture. 
-::I 
L. 
0 
Ql 
.... 
.... 
Ql 
a. 
"' 
"' 0 
E 
0 
:0 
Ql 
> 
+= 
0 
Ql 
a: 
-·:; 
,_ 
0 
Ql 
.... 
... 
Ql 
a. 
"0 
Ql 
>. 
Ql 
-~ 
-
..2 
Ql 
a: 
1.0 
0.9 
0.8 
0.7 
0.6 
0.5 
0.4 
0.3 
0.2 
0.1 
0 
1.0 
0.9 
0.8 
0.7 
0.6 
0.5 
0.4 
0.3 
0.2 
0.1 
0 
8 
• Eo.5 Emax ~·~I I tl rt I 
·~ 
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 
Exploitation rate ( E = F/Z) 
48 
The actual value of E = 0.64 is well above Eo.s and even above Emax and hence suggests that the 
stock of 0. ruber of Sofala Bank is overfished. Note that this conclusion stands even if one relies on 
the lower estimate of Z = 1.34 year1 presented above. 
The available length-frequency data cover a wide length range which includes both juveniles and 
adults. Thus problems of size selectivity are minimized, although some juveniles may have been 
missed, as well as some large adults, because they were consumed by the crews. The absence of strong 
size selection may be the reason why correcting the data for selection effect did not change the growth 
parameter estimates. 
It may also be noted that the two length-converted catch curves used here led to the same estimate 
of Z ( = 1.95 year 1 ), in spite of the fact that only one of them accounted explicitly for seasonal growth. 
This is attributed to the fact that the samples analyzed here included numerous annual cohorts, a 
feature which tends to reduce the impact of growth oscillations on standard catch curve estimates of 
Z (Pauly, 1990). 
Further investigations will have to be performed to verify the results presented here, notably to assess 
the quality of the growth and mortality estimates, and to test whether our assessment of the state of 
the demersal fishery of Sofala Bank, presently based on 0. ruber only, can be confirmed for other 
species. 
ACKNOWLEDGEMENTS 
The author wishes to express his gratitude to Dr. D. Pauly for his help using the ELEF AN programs 
and for editing this paper. 
REFERENCES 
BEVER TON, R.J.H. AND S.J. HOLT (1966) -Manual of methods for fish stock assessment. Part 
2. Tables of yield functions. FAO Fish. Tech. Pap. 38. Rev. 1: 67 pp. 
GISLASON, H.; (1985)- A short note on the available information about demersal fish on the 
shallow part of Sofala Bank. Rev. Invest. Pesq. (Maputo) 13: 83-95. 
49 
JONES, R.; (1984)- Assessing the effects of changes in exploitation pattern using length composition 
data. FAO Fish. Tech. Pap. 256. 
NA V ALUNA, N.A.; (1982)- Morphometries, biology and population dynamics of the croaker fish, 
Otolithes ruber, pp. 38-55./n D. Pauly and A.N. Mines (eds.) Small-scalefisheries of San 
Miguel Bay, Philippines: biology and stock assessment. ICLARM Technical Reports 7: 124 
pp. 
PAULY, D.; (1980)- On the interrelationships between natural mortality, growth parameters and 
mean environmental temperature in 175 fish stocks. J. Cons. ClEM 39(3): 175-192. 
PAULY, D.; (1982)- The fishes and their ecology, pp. 15-33./n D. Pauly and A.N. Mines (eds.) 
Small-scale fisheries of San Miguel Bay, Philippines: biology and stock assessment. 
ICLARM Technical Reports 7: 124 pp. 
PAULY, D.; (1986)- On improving operation and use of the ELEFAN programs. Part II. Improving 
the estimation ofL_. Fishbyte 4(1): 18-20. 
PAULY, D.; (1990) - Length-converted catch curves and the seasonal growth of fishes. Fishbyte 
8(3): 33-38. 
PAULY, D. AND J.L. MUNRO (1984)- Once more on growth comparison in fish and invertebrates. 
Fishbyte 2(1): 21. 
WETHERALL, J.A. (1986) - A new method for estimating growth parameters from length-
frequency data. Fishbyte 4(1): 12-14. 


Preliminary investigations on the population dynamics of Otolithes ruber (Sciaenidae) on Sofala Bank, Mozambique

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Preliminary investigations on the population dynamics of Otolithes ruber (Sciaenidae) on Sofala Bank, Mozambique

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