DistributiOl1 of fishes in the MhlaIlga estuary in relation to food resources A.K. Whitfield Zoology Department, University of Natal, Pietermaritzburg. Food resources play an important role in determining the Little is known about the distribution of fishes within south­ distribution of fish species in the Mhlanga estuary. Standing ern African estuaries. During 1978 an investigation was crops of detritus and associated micro-organisms, conducted to determine whether the composition of the fish zoobenthos, zooplankton, epiphytic flora and fauna were fauna in the Mhlanga estuary was related to the availability measured in the upper, middle and lower reaches of the of different food resources. The results of the study estuary, and the distribution of 20 fish species related to these (Whitfield 1980) indicated that there was a relationship resources. The distribution of the majority of species correlated with the abundance of preferred food items, except between the diet of the different fish species and food avail­ for the Mugilidae where substrate particle sizes influenced the ability. No attempt was made however to relate the dis­ composition of fish in an area. tribution of the fish species to their food resources. The aim S. Afr. J. Zoot. 1980,15: 159 - 165 of this paper is to present data which show that the dis­ tribUtion of most fish species at Mhlanga is directly related Voedselbronne speel 'n belangrike rol om die verspreiding van to the distribution of particular food categories. visspesies in die Mhlanga estuarium vas te stel. Die voorkoms . ) van detritus en geassosieerde mikro-organismes, soobenthos, 0 Materials and Methods 1 sooplankton, epifitiese flora en fauna is in die boonste, 0 Study area 2 middelste en onderste gedeeltes van die estuarium bepaal, d asook die verspreiding van 20 visspesies verwant aan hierdie The Mhlanga estuary (29°42'S; 31°06'E) is fed by a river e t bronne. Die verspreiding van die meerderheid van die spesies 28 km in length with a catchment area of approximately a 2 d was gekorreleer met die volopheid van voedselitems waaraan 124 km (Chew & Bowen 1971). The mean annual rainfall ( r voorkeur gegee is, behalwe vir die Mugilidae waar within the catchment is about 100 cm, 70% of which falls e h substraatpartikelgroottes die samestelling van 'n vis­ during summer (October -March) (Brand, Kemp, Pretorius s i l gemeenskap in 'n gebied beinvloed het. & Schoonbee 1967). The estuary remains closed to the sea b u S.-Afr. Tydskr. Dierk. 1980, 15: 159 -165 for most of the year because the inflow of freshwater from P e the river is insufficient to counteract the effect of longshore h t drift maintaining the sand bar across the mouth. Heavy y b rains during summer result in a flooding of the estuarine d e lagoon and breaching of the sand bar. The strong outflow of t n turbid water decreases after a few days thus allowing tidal a r penetration of seawater into the estuary. In the absence of g e further rain in the catchment area, the estuary normally c n closes within 10 days. e c i l r Sampling and biomass estimations e d Fish n u Fish were sampled in January, February, March, May, y a July, September and November 1978. The estuary was w e open during March and November and closed during the t a other sampling periods. The following gear was used; a G t large seine net (70 m X 2 m X 12 mm bar mesh), small e n seine net (10 m X 1,5 m X 4 mm bar mesh), fry seine net i b (3 m X I m X 1,5 mm bar mesh), cast net (4 m diameter, a S A.K. Whitfield 12 mm bar mesh) and gill nets (55, 90 & 125 mm stretch y Zoology Department, University of Natal, b mesh). The topography of the upper reaches precluded the d P.O. Bo~ 375, Pietermaritzburg 3200 e use of a large seine net in this region, with the result that c -New address: Institute for Freshwater .studies, u P.O. Box 49, Sedgefield 6573 biomass or numerical comparisons of fishes in the different d o regions on a catch per unit effort basis, were not possible. r p Received 25 September 1979; accepted 29 January 1980 The diet of fish species at Mhlanga was determined using e R 160 S.-Afr. Tydskr. Dierk. 1980, 15 (3) a modified points system (Whitfield 1980). Substrate served in formaldehyde. Samples were collected from the particle sizes from the stomach contents of Mugilidae were lower, middle and upper reaches of the estuary. measured with an eyepiece micrometer in a binocular Each sample was shaken vigorously for exactly 30 sec. microscope. and the freed floc poured through a 1,5 mm mesh sieve to remove any large macrophytic debris, crustaceans and Zoobenthos molluscs. The floc was then concentrated on Whatman Zoo benthos were collected using a Zabolocki-type Eckman number 40 fJlter paper by vacuum fJltration in a Buchner 2 grab (area covered = 236 cm ). Ten grab samples were funnel, scraped from the fJlter and dried to constant weight. taken every two months from the lower, middle and upper The dried sample was then ground to a powder and a reaches of the estuary. Each sample was initially sieved with subsample removed for energy determinations. a 1 mm mesh and all organisms retained by the sieve pre­ served in 10% neutralized formaldehyde. Material which Energy determinations passed through the 1 mm mesh was then washed vigorously A Phillipson micro bomb was used to obtain energy values in 4% formaldehyde which caused the burrowing forms to for benthic floc, zooplankton, zoobenthos, epiphytic flora emerge and become suspended. The supernatant was and fauna. To produce complete combustion of floc and passed through a 0,5 mm mesh sieve and the organisms epiphytic flora samples it was necessary to add l.ul of combined with those retained by the 1 mm mesh. The grab mineral oil to each pellet. The energy content of the oil was sampler did not pentrate below 15 cm depth and deep then subtracted from the total Joules measured, to burrowing zoobenthos such as the sand prawn Callianassa determine the energy content of the sample. All animals kraussi was not captured. The distribution of C. kraussi were washed, dried to constant weight at 50°C in a was determined using a prawn pump (72 cm length, 6 cm vacuum oven and weighed on a Cahn gram electrobalance. diameter). All samples had to be preserved in formaldehyde and there­ Each zoobenthic sample was washed in the laboratory, fore energy determinations were restricted to fixed material. causing the animal material to become suspended, and However values obtained from the above material should enabling it to be poured through a 0,2 mm mesh sieve. The not differ significantly from those of fresh material since animals were then identified and counted using a binocular formaldehyde is very soluble in water and is a non­ 2 microscope and expressed as Joules m- • coagulent fixative which preserves most lipids (Baker 1966). Zooplankton Zooplankton in the water column were collected every two Physical environment . ) months using a Clarke-Bumpus plankton sampler fitted Salinities were measured using a Goldberg optical 0 1 with a flow meter and a 70.um mesh net. The sampler was salinometer and temperatures with a standard thermometer. 0 2 Water transparency was recorded with a light meter linked held just below the water surface and towed for three d I to a submersible photo-electric cell, mounted in the centre of e minutes at approximately 5 km h- • Day and night samples t a were collected from the lower, middle and upper reaches of a Secchi disc. d ( The benthic floc sampling apparatus was used to remove the estuary. All samples were preserved immediately in 4% r e the upper 5 cm of substrate from sampling stations in the h formaldehyde (Analar grade). s i lower, middle and upper reaches of the estuary. The particle l Zooplankton samples were analysed using standard sub­ b size distribution was obtained by washing the substrate u sampling procedures. Subsamples constituting 10-15% by P through a series of standard sieves with mesh sizes of 4, 2, volume of the parent sample were used. The number of e 1, 0,5, 0,25, 0,125 and 0,063 mm. The contents of each h t organisms in each taxon present was obtained and ex­ sieve were measured by volume and expressed as y pressed as Joules m-3• b proportions of the whole sample. d e Epiphyticflora andfauna t n A minimum number of 30 10cm lengths of submerged a Results r g Phragmites stems were collected using secateurs and pre­ e Fish distribution c served in formaldehyde. The reed depths between the water n 2 The catch composition of fishes in the lower, middle and e surface and substratum, and densities per m were mea­ c i upper reaches of the estuary is shown in Table 1. Each l sured for the lower, middle and upper reaches of the r species has been assigned a trophic category based on a de­ e estuary. d tailed assessment of the diet of fishes from Mhlanga (Whit­ n In the laboratory epiphytic algae were scraped from the u stems using a scalpel, dried, weighed and milled. A sub­ field 1980). Only species comprising more than 10 indivi­ y a sample of the powder was used for energy determinations. duals were considered for analysis. A biomass assessment w e Epifauna were identified, counted and expressed as Joules revealed that Ambassis commersoni and Sarotherodon t a m-2• mossambicus showed a 'preference for the upper reaches; G Glossogobius giuris and Myxus capensis for the upper and t e Benthicfloc n middle reaches; Gerres rappi, Liza alata, Pomadasys i b Benthic floc is the term used here to describe the mixture of commersonni and Valamugil cunnesius for the middle and a S detritus and microorganisms which is found on benthic lower reaches; Crenimugil crenilabis, Leiognathus equulus, y b substrates in the Mhlanga estuary.