J17_1998.pdf (369.9Kb)

Not to be cited without nrior reference to the authors

. International COuncil for

t.'1e Exploration

of

^{the Sea} C.M: 1998/J: 17

Seasonal changes in distribution and aggregation characteristics of sardinella species in AligolaIl waters

by

Nkosi Luyeye I) and Ole Arve Misund 2)

1) Instituto de Investig~iio Pesqueira, C.P.2OOl, Luanda.Angola 2) Institute of Marine Research, P.O.Box 1870, N-5024 Bergen, Norway

ABSTRACT

Tne distribution and aggregation characteristics of sardinella in Angola from 1985-1996 are analyzed by com.paring. the distribution of acoustic recordings (SA.Values) from surveys conducted byR/V "Dr. Fridtjof Nansen" and by relating distribution area to estimated biomass. These comparisons indicate seasonal changes in distribution of sardinella species.

Th"'Ough the, surv'"eys, -systeuwtically higher estimates nave been recorded in, the northern region during the southern winter and in the central-southern region during the warm season.

There is a southward displacement of the adults as far as Baia <¥>s Tigres for round sadineUa (Sardir.ella aurita) and to Lobito-Bengueia for the fiat sardinella (SardinelIa matlerensis) during the warm season. Northward displacement occurs during the cold season in connection with upwelling.

.----,,-----~--- -.--.--.---.. ~---- - - - - ---.~---.~ ..

2

INTRODUCTION

Thehydrographic regime off Angola is characterized by the cold northward-flowing Benguela current and the warm southward-propating waters of the Angola current. These two currents join (at about 15° S) and form the South Equatorial Current(SEC) as part of the South Atlantic subtropical gyre. The warm Angola current is strong in the summer period, which results in rather stratified water masses and higher sea temperatures. On the contrary, the Benguela current is strong in the winter period causing upwelling and colder surface waters,

These seasonal variations of me. hydrograpipc conditions induce temporal and spatial variability of the distribution oithe peiagic species off Angola (Dias, 1983a, b). There are indications that seasonal changes in the hydrographic conditions cause north - south movement of sardineIIa. During the warm season there may be a southward displacement of adult sardineIIa as far south as Baia dos Tigres for round sardinella and to L.obito for t.he flat sardinella (Anon, 1995). According to Luk~s.hev (1976) the displacement north=south for both sardinella species is linlced to the feeding a..~a as well as t..'1e reproduction. It is knowll that the sardineIIa species feed both on phytoplankton and zooplankton. Seasonal changes in the hydrophic regime influence .thedistribution 6f'the phyta and zooplankton, which lead to a redistribution of the sardineIIa species.

Based on distribution area and .estimated biomass fOllnd by conventional acoustic surveys· by RN «Dr. Fridtjof Nansen» in the period 1985-1996, we analysed how distribution and aggregation behaviour of the sardinella species change with season.

l\-IATERIALS Al'"1) lYiEldODS

In the years 1985, 1986, 1989, 1991, 1994, 1995 and 1996, RN "Dr. FridtjofNan5en"

conducted conventional acoustic surveys off Angola to map the distribution and estimate the biomass of oelalric fish. The surveYS up to 1989 were run with a 38 y_H.z

.

^{... .,}

.

SilI'sad EK400 echo sounder ,connected to all echo int~~~or. Later sUr'/eys were n.L~ with a 38 }uu^lZ-Shru-ad :EKSOO echO: sounder

that

from 1994 was corm.ected to

a

^BEl echo integration unit. _T~e _echo integration units were calibrated according to standard procedure prior to the surveys. ECho recordings during the surveys "Yere scrutinized manually, and echo intensity allOC8tcWto species according to catch compOsition in trawl catches or appearance on the. echogram.

Distribution charts with categories for scattered, den.se,and very dense recordings for the dominating pelagic species 'n'eredrawn for each.survey.The biom8ss of these spe.cies wa~

estima:ted by assUJ:ning a target strength of 2C1 log L - 72, and then using backscatte~g strength. ,distribution area:, and species weight recorded for each survey in the standard calculation method.

Tnebiomass estimates of sardineila from 1985 ⁱ to ·1996 were used for comparisonslletween the warm and the cold season. For each survey the fraction. of biomass was estimated for the

three

regions:Cunene-BeIlguel<i (south), 8enguela-Luanda (centre) and Luanda-Cabinda (north) ). Similarly the distribution areas of scattered, dense and very dense recoidingsof sardinella were estimated for each survey by use of a planimeter. Statistical compa..risons ef the· biomass between the three areas were rnade by V'/ilcoxon 2-sa..uple test based on both t..1e biomass and the biomass fraction. Correlation coefficients (r,) between the two seasons, in the

3

categories scattered, dense and very dense per regi~n.'were calculated on the basis of the acoustic recordings.

!lRSULTS

Generally, the sardineHa species were found near the coast (Figs. I - 7) over bottom depths from 20 - 200 m along the entire shelf of Angola. In 1985, the sardineHa species were consentrated in the Benguela - Luanda region in the warm season, but with a clear northward shift in the distribution in the cold season. In 1986 the distribution of the sardinella species was more similar both in the warm and the cold season. In the later years, especi"l1y in 1996, a clear tendency to a northward shift in the distribution of the sardinella species from the warm to the cold season was apparent.

Figure 8 shows u'ie bio",8SS distribution of sardinella species in the summer and winter periods respectively, as observed through the surveys with the RV "Dr. FridtjofNansen". The figure shows clearly that in the summer period, characterized by more stratified water masses and higher temperatures in the water column, more than 50% of the biomass were found in the central region. On the contrary, the highest biomass was found in the northern region in the winter period~ characterized by upwe!!ing ~nd c'o!der su.1face waters. Tt-Js pattern was probably due to north-south migrations' t..~at most likely apply to bou.1. species.

In general, the biomass fraction in the Cabinda region varied from 0.14 - 0.42 in the warm season to 0.30 - 0.73 in the cold season. For the Luanda region the biomassfraction varied from 0.54 - 0.85 in the warm season, to 0.27 - 0.50 in the cold season. Both the biomass and the biomass fraction of sardinella that were recorded in the Cabinda region were sigpificantly higher in the cold season than in the W~nn seJ:1~on (p < 0.05 in beth cases, Wilcoxon 2-sat"Uple test). Thebiomass fraction in this region increased by minimum 0.06 to 0.48 from the warm to the cold season. This indicates that there was a movement of sardinella into the Cabinda region from the warm to the cold season. For the Luanda region, the biomass was not significantiy different between the warm and the cold season (p > 0.05), but the biomass fraction was significantly higher in the warm than in the cold season (p < 0.05). In general. the biomass fraction in the Luanda region decreased by minimum 0.05 (1986) to maximum 0.42 (1985)from the warm to the cold season. This indicates that there was a movement of . sardinella out of the Luanda region from the warm to the cold season. Altogether, these results illustrate seasonal spjft..s in the biomass distribution of sardinella alung the Angoian coast with a north.ward movement- in- the fu-st

pari

of the year (from the warnr to the coid season) and a southward movement in the iast part of the year (from the cold to the warm season).

When the total biomass of sardineHa increased, the distribution area of sardinella in .scattered aggregations increased accordingly (r

=

^{0.55. P < 0.05, n}

=

28).1'here was a similar tendency for the distribution area of sardi~ella in dense ~md very denSe aggregations also, but the correlations between these parameters and the total biomass were not significant (r = 0.22, P >

0.05, n == 26, r = 0.39, P > 0.05, n = 21, respectively). There was a seasonal dependence in the correlation between the distribution area of sardinella in scattered recordings and the total biomass of sardinella, because the correlation was significant only for the cold season (Table

0.

The. distribution area of sardinella in dense and very dense recordings were not correlated to the total biomass, neither for the warm nor the cold season. None of the aggregation

4

fractions (scattered, dense, very dense) was correlated to the biomass, neither forthew~, nor the cold season (Table 1).

Tabie l. Correlations between acoustic recording categories and biomass of sardinella (r=

correlation coefficient, n = number of observations, *: p < 0.05) .

. COI1'elations

Scatter-vs .. Biomass Dense ,vs. Biomass VerY dense- vs. Bio!'!1.ass Scatter fraction vs. Biomass

Dense fraction vs. Biomass Very dense fraction vs. Biomass

*' :;;; COi"relatiun

D~CUSSION

Wannseason

r= 0.25. n = 15 r =0.31.0 = 14 r= 0.48. n = 14 r= ·0.15. 0';' \3 r= 0.20. n = \3 r- ·0.01. 0 -13

Cold season

r=79*.0=13 r= 0.08. n= 12

_ _ "'''''''' _ _ 0 1 ~v.,:,,,,.u ... o

r=0.01.n=7 r=0.01.n=7 r-·O.l2.0=7

The survey results show that. there were seasonal changes in the distribution of sardinella species along the coast of Angola. Seasonal variability of the hydro graphic regime and distribution of phyto- and zooplan ^ktOD probably cause a clear movefl1~nt north-sout.l-t~

Oue to the higher temperatures in 1995 (Benguela-El niiio) in the first part of the year (from the warm to the cold season), sardinella were redistributed in the northern region oVer depths more than 700 m (Anon, i995).

As already pointed out by Dias (1983a, b), a greater activity of the Angola current,. with which the species are connected, may cause a seasonal (March-May) movement of single schools,as far as the waters off northern Namibia, as observed in 1983 (Thomas, 1984a). The increase and

rf""~",,o::II(.'j3I _~ nf th".. h-inmas'" I'lIocot-i"YVlI1'""It" ... ".<lO ... ...I .f..-... lQQICO: ... 10nJ:: :_A:,.. ... ^{....I ..} L.. .... - ... :\.:1:_._ -...

... L ... "" ... ~ .... .u. ... u. ... u." ... vu ... ^{~.L 'I'~} .U.V:U.L ,.L7UJ t.v .L77V .Luu.u .• au;;u ^_u~ ... pY;:)i:UV1.11Ll" ,VA.

:nort..'1-south "movements in ~onnection with the _ upwelling ;aIid

-me

Angoia -current.

Baptism (1977) showed. that the movement of ~e sardinella stock results in seasonalityof fishing of individual areas of the coast (Table.3). As may be seen from this table, the same pattern was observed with the time series of survey estimates. from the "Dr. FridtiofNansen"

~urveys.

With regard to the sardinella species, it is

~ommon

lrJlow!edge that theirbeh"viour is

~trnnO'lv lnf111pnrprl hu th~ CP!licnn~l '!:IInl'l thp pnu;1"'I"'\nTn ... nt'.:l1 .... n.n,...;'ti ... nC' ... " .... ,..ut"tA_A,i"_tJ..A,~_'!Io, I

"'-~.&O"'"J &~~& __ &._--"'J - -...-.""":&-~ ... - _ ... 9 ... ~_ ... " ' ... 'lo6.L ... v ... ....:II " ' ... " ' ... u. ... 'Uo ... Wo ... 1I,Wo- \

Anon, 1991).

Acoustic SlIIllplingbas the advantage of covering the fish resourc~ in a relative short time, i.e.

to give a snapshot of the abundance of pelagic ~pecies, However. the method suffers from a series of biases aridsources of variance which are.difficult to. evaluate and reduce (MacI..ennan

& Simmonds 1992). Of the main problems are those related to fish behaviour,as the day-night variations in fish distribution and density, near-surface distribution and vessel avoidance.

These factors may have influenced the acoustic biomass estimates significantly, !>ut,. our comparisons of acoustic recordings and distribution area for different seasons .are:; probably

~egliabie ~fected.

5

At the boundary between Congo and Angola (Maytimbe area,S S), there is a spawning area for the sardinella species. Juveniles start moving southward to Angola when about 12-13 cm in length at an age of approximately 2 years. Generally, the adults are found in Angola and a small percentage of the juveniles is found in the southern part of the Luanda region (Pta das Palmeirinhas). The flat sardinella are generally more coastal and sedentary and live in areas at a sea surface temperature of about 26°C as well as with lower salinity (34 p.s.u). On the contrary, the round sardinella are more offshore and prefer a sea surface temperature of about 24°C as well and higher salinity (35 p.su.) (Gheno and Campos Rosado, 1972; Wysokinski,

1986).

T'ne disiribution of sardineila species is very dynamic, and these species are fast-swimming fish with significant changes in behaviour during the day. The most common feature for the sardinella in this region is the concentration in schools at daytime, often close to the surface (Anon, 1985/86). At night the schools tend to dissolve and the fish are spread in somewhat deeper layers, however, still in the upper part of the water masses.

Table 2. Seasonality of sardinella fishing on in dividual sectors of the Angolan coast (after BaEtista, 1977).

Season

Area Feb Mar- May-Jun Jul-Aug Sep-Oct Nov Dec-Jan

ADr

.

Northern Angola good highest good weak increasing good weak Central Angola highest good decreasing weak increasing good weak Southern Angola average average weak weak increasing average weak

REFERENCES

Anon. 1985-96. Surveys of the fish resources of Angola (Cruise report "Dr. FridtjofNansen").

Tnc.titlltP nfM!:tnn.:>o l),p,eoPo":lo'l' .... h ll.:. ... "" .... "'T"" ... n ...

... _ ... ....,-. ...... ~ .... u.L ... u., .LI ... O .... .u., .L'V.lwa].

Baptisia, S.R da Fonseca, i 977. The distribution and movements of the sardinellas

(SardineIla aurita Val. and Sardinella maderensis Val.) off Angola Coast. Collect Sci. Pap.

ICESAF/recl. Doe. Sci. CIPASFlCo1ecc. Doe. Cient CIPASO, 4: 21-24.

Dias, C.A. 1983. Note on the evidene-e of a perm~nent southwa.rd flow of upper oceanic

trn-:nhpri,... ---r--,a_-w~tprc. ^~~-~-... ... nff Anrrn.1o!:1 & ~ .. O"" ... ...,. _ ... "2ot 1,)0<::' £"' ... 11 ... uu ... - . ~f"':,,.-u •• D, -,,-1' . ... ';nL f"'-umml! - ... .-. C::~ &... A"u' ~r··I·",LII· ~ • • . , , , QCL1_1Vn,) ...

Gheno and Campos Rosado, J.M. 1972. Distribution and seasonal migrations of Sardinella maderensis off coasts of Gabon and northern Angola

Lukashev, V.N. 1976. Distribution of Sardinella maderensis and Sardinellaaurita off the coast of Angola. Trudy VNIRO. T 71. Ser 281-293.[ In Russian

1

McLennan, D.N. and Simmonds, EJ. 1992. Fisheries acoustics. Chapman & Hall, London.

Thomas. RM. i984. Catches of Sardinelia aurita off South West in 1983. Collect. Sci. Pap.

ICSEAFlReci. Doe. sci. CIPASEJColecc. Doe. Cient. CIPASO. 11(2):87-90.

6

Wysokinski,A. 1986. The living marine resources of the Southeast Atlantic.FAO Fish.Tech., (178) Rev. 1, 120 p.

,.

loc::

^t

1^{0 ..,)-.}

I

~ 1-9

m

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• >100

I

11"

I

14" 11" 14"

Figure 1. Distribution of acoustic recordings of sardinella off Angola in the summer (0

and winter periode (Ill) in 1985.

' I ---c-\---,I~"

1 86-i ^{A \ U}

~ 1-10

B

^11.;..50

.51-100

• >100

-

186 - 11

14"

Figure 2. Distribution of acoustic recordings of sardinella off Angola in the summer

<n

and winter periode (ll) in 1986.

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Figure 3. Distribution of acoustic recordings of sa..""dinella off;~"'lgola in the SUIiijT.er (l) and winter periode

cm

^{in. 1989.}

191-!

I

~ ^1-10

~ 1.1-50

• >100

I

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191-!!

I

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I

Figure 4. Distribution of acoustic recordings of sardineila off Angoia in the summer (l) and winter periode (II) in 1991.

194-1 \

~s·

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~7

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Figure 5. Distribution of acoustic recordings of sardinella off Angola in the summer (I) and winter periode (IT) in 1994.

95-1

SA (m2 /nm2)

~ 1-100 1/1101-500 .501-1000 .>1000

)

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cm

^{in 1995.}

"

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Figu ... ~ 7. Distribution of acoustic recor-dings of sardinella off Angoia in the summer (r) and winterperiode

(m

^{in 1996.}

~O~---~~---~---~

- : - A~

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1

=

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2 =

winter period

Survey

Figure 8. Survey estimates for sardinella off Angola 1985-1996.

.Cunene-Bengusla(S)

! I

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