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Marsden.Pdf (2.621Mb) SPATIAL AND TEMPORAL VARIATIONS IN THE CONDITION OF AUSTROVENUSSTUTCHBURYIFINLAY, 1927 (BIVALVIA: VENERIDAE) FROM THE AVON-HEATHCOTE ESTUARY, CHRISTCHURCH ISLAY D. MARSDEN & REBECCA M. PILKINGTON Department of Zoology, University of Canterbury, PO Box 4800, Christchurch, New Zealand (Received 16 March 1995; revised and accepted 29 August 1995) ABSTRACT Marsden, LD. & Pilkington, R.M. (1995). Spatial and temporal variations in the condition of Austrovenus stutchburyi Finlay, 1927 (Bivalvia: Veneridae) from the Avon-Heathcote Estuary, Christchurch. New Zealand Natural Sciences 22: 57-67. Weight relationships ofthe cockle Austrovenus stutchburyi were investigated from 9 low tide sites within the Avon- Heathcote estuary in December 1990. The condition index varied significantly between sites. Mean condition index increased with both higher salinity and increased chlorophyll a levels, measured during low tide periods. Results from a 15 month sampling programme showed seasonal patterns in the condition index and dry tissue weights of cockles collected from a marine and a more estuarine site. Highest values were recorded in Spring and lowest values in January and February. Cockles from the more estuarine site showed delayed onset of weight gain and a smaller maximal size than those from the marine site. It is suggested that estuarine cockles show reduced reproductive potential and reduced growth rates compared with the more marine population. The role of reduced salinity as a limiting factor affecting growth in cockles is discussed. Keywords: Austrovenus stutchburyi - dry weight - condition index - bivalve growth - reproduction. INTRODUCTION that inhabit estuaries are not only exposed to tidal variables and a fluctuating salinity regime, but may The New Zealand cockle Austrovenus also be exposed to heavy metals and other pollutants stutchburyi is a dominant bivalve ofthe intertidal that are known to affect growth and reproduction community in estuaries and sheltered bays. Some (Vernberg 1976, Widdows 1985, Marcus et al. ecological studies have described its distribution 1989, Widdows et al. 1990, Roper ef al. 1991). patterns and growth rates from different parts of In a previous study on cockles in the Avon- New Zealand (Larcombe 1971, Stephenson 1981, Heathcote estuary, Stephenson (1981) investigated Blackwell 1984,Dobbinsone/a/. 1989, McArdle & the weight relationships of cockles from 200 stations Blackwell 1989). However, there have been no in the estuary. Results suggested considerable varia­ previous studies investigating the effects of salinity tion in growth rate and annual production within the on the ecology or growth ofthe cockle. Many factors estuary. While substratum type and tidal level ap­ are known to affect the growth rates of bivalves peared to explain much ofthe variability in growth (Bayne 1976) with commercial species such as rates, cockle density or biomass were not correlated mussels and oysters receiving the most attention with any particular environmental factor. However (Buxton et al. 1981, Brown 1988). Although fewer salinity measurements were not recorded as part of studies have investigated growth and reproduction that study. of clams, the major factors thought to affect growth The present study was designed to investigate include: body size; temperature; tidal level; emer­ the weight relationships of cockles exposed to low sion time; density, salinity and food availability tide salinities between 13 and 34 ppt. The health of (Seed&Brown 1975, Broom 1982, Bayne& Newell, cockles was assessed by a condition index (Crosby & 1983, Beukema et al. 1985, Fritz 1991). Bivalves Gale 1990). The sites were restricted to those of 58 New Zealand Natural Sciences 22 (1995) intermediate cockle density (200 to 500 individuals samples were collected using up to 10 random nr2; Stephenson 1981). They were located at the low quadrats (0.25 m2) and 30 individuals chosen ran­ tide level and consisted of similar sediment charac­ domly representing the size range available. For teristics (MacPherson 1978). The study examines each sample, mean CI was calculated together with the variability in the weight relationships within and the SD and coefficient of variation. The effect of between localities in the estuary and compares the cockle size on the weight relationships was investi­ seasonal weight changes at two localities, one with gated by regression analysis of shell length and dry a high and the other with a low salinity regime. tissue weights. Comparisons between sites and be­ tween different time intervals were made first by MATERIALS AND METHODS testing for heterogeneity of the data followed by analyses of Covariance (Snedecor & Cochran 1960). Individuals of A. stutchburyi, were collected At each site, the environmental variables described from 9 sites during December 1990. The sites were earlier were recorded at each time interval and in chosen along an environmental gradient from less addition, a salinity profile was undertaken at three- saline areas close to the two river mouths, to fully monthly intervals from the time of low tide through saline conditions close to the estuary mouth (Fig. 1). to high tide. The bivalves were collected at high tide by a SCUBA Age structure of A. stutchburyi from the two diver sampling at low tide level (approximately sites was compared by counting the annual shell 0.2 m above chart datum). Preliminary sampling growth rings using the techniques described in showed that cockle densities at these locations were Stephenson 1981.Larcome(1971)andCoutts(1974) below 500 nr2 (see also Stephenson 1981). For each have confirmed the formation of annual rings in site, 30 bivalves of a representative size range were populations of cockles from southern New Zealand. used for weight relationships. Shell length, the Not all individuals showed clear external annual maximum distance between the anterior and poste­ rings but confirmation of a winter ring was seen at rior margins ofthe shell, was measured to the nearest both sites between July and August ofthe monthly 0.1 mm using vernier calipers. samplingprogramme. One hundred individuals from The condition index (CI) dry tissue weight/shell each site were collected between November 1991 weight X100 was chosen because this index has been and February 1992. Shell length, dry weight and shown previously to correspond with increases in number of rings were recorded for each individual. somatic and reproductive tissues, in other bivalves. Length-age plots were constructed for cockles col­ It can also be measured with precision (Crosby & lected from both sites and the mean age calculated Gale 1990, Pridmore et al. 1990, Rainer & Mann for each2.5 mm length group. The density of cockles 1992). Surface water temperature (°C) was recorded from the study areas had remained similar over this 2 at the time of cockle collection and water samples (3 time with a mean density ± SE of 357 ± 37.6 rn* at 2 replicates) were taken through a 0.1 mm plankton the estuarine site and 279 ± 31.3 nr at the more net, from the surface water and water from the saline marine site. sediment surface. Chlorophyll a levels were esti­ mated using a Turner fluorimeter calibrated using RESULTS known concentrations of chlorophyll a (\ig\l) stan­ dards (Yentsch & Menzel 1963). At each site, Mean condition indices for cockles collected at salinity measurements (ppt) were recorded using a different sites in the estuary are shown in Table 1. No hand held refractometer, during both high and low cockles were found at Site 8 which is a region of tide periods. strong water currents close to the mouth of the The two sites selected for a temporal study were estuary. The highest individual condition index, 9, an estuarine site close to the Heathcote River mouth and highest mean condition index, 7, were found in (Site 2) and a more saline marine site close to front of the oxidation pond (Site 3). The lowest Beachville Road (Site 9). Monthly samples of cock­ indices were from Beachville Road (Site 9). The les were taken at spring tides from November 1990 condition index varied significantly between sites to February 1992. Unfortunately, the cockle samples (ANOVA F[7228] = 29.3, P = 0.001) and there was a collected during March 1991 were lost. At both sites, high degree of within site variation along the eastern LD. Marsden & R.M. Pilkington: Variation in the condition of Austrovenus stutchburyi 59 PEGASUS 34ppt HEATHCOTE RIVER Moncks Bay Figure 1. Location of sampling sites in the Avon-Heathcote Estuary and low tide salinity values (ppt). 1. Humphries Drive; 2. Heathcote River Mouth; 3. Oxidation ponds; 4. Avon River mouth; 5. Jellicoe Park; 6. Heron Street; 7. Estuary Channel; 8. Moncks Bay, 9. Beachville Road. 60 New Zealand Natural Sciences 22 (1995) Table 1. Mean condition index (CI) of A. stutchburyi within the Avon-Heathcote estuary and chlorophyll a levels (ugl"1 ). SD, standard deviation; CV, coefficient of variation corrected for bias. Site Mean SD CV Chlorophyll a (ugl1) surface benthic 5.14 0.90 17.6 5.3 5.0 4.33 0.77 18.0 3.7 3.0 7.07 1.27 18.2 7.1 6.8 4.21 0.98 23.3 4.8 4.1 4.32 0.89 20.9 4.1 5.3 4.37 1.06 24.6 3.0 3.0 4.65 1.34 29.0 4.4 5.3 4.12 0.84 18.2 3.7 3.0 margin of the estuary. Multiple comparison tests enous then the regression lines were tested for (Newman Keuls) comparing mean condition indices differences in the slope or elevation of the lines. between sites, separated out Site 3 (oxidation ponds); Differences in the elevation ofthe regression lines grouped sites 1 (Humphries Drive) and 7 (estuary were considered only if the slope values were simi­ channel), but found no significant differences be­ lar. These comparisons are shown in full in Table 3. tween the remaining sites (PO.05). They show significant differences between sites, At all sites, cockle tissue dry weight (y) in­ either due to heterogeneity ofthe data (Sites 9 & 2), creased significantly with shell length (x) following differences in slope values (Sites 1, 3, 4, 5) or the equation y = ax*.
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