Effects of zone and wave exposure on population structure and recruitment of the mussel (Perna perna) in South Africa

• Main Author: Lindsay, Justin Robert
• Format: Others
• Language: English
• Published: Rhodes University 1999
• Subjects: Perna > South Africa; Perna > Growth; Mussels > South Africa
• Online Access: http://hdl.handle.net/10962/d1005371

Certain aspects of the population dynamics of the brown mussel, Perna perna, were examined at 18 sites along the south coast of South African. Specifically the effects of wave exposure and tidal height were examined in relation to mussel size, biomass and density. A single set of samples was removed from each of the 18 sites, over three spring tide cycles. Sites were classified as exposed or sheltered prior to sampling. Principal component analysis (PCA) (based on mussel length data) and length frequency histograms revealed that there was a general decrease in the modal size of the adult mussel cohort with an increase in tidal height. The effects of exposure on mussel size decreased higher on the shore. On the exposed low shore the maximum size of mussels had a mean length of 102.3mm and was significantly larger (ANOVA, p<0.0001) than that for mussels on sheltered shores (86.7mm). The difference between mean maximum lengths of mussels on the mid shore was not so great, exposed sites had a average mean maximum length of 79.9, while on the sheltered shores it was 68.4mm. On the high shore the difference between the average mean maximum lengths at exposed and sheltered sites was only 3.9mm. The fact that the effects of exposure were greatest on the low shore was also borne out in the PCA. In this analysis low shore exposed and sheltered zones separated into two groups with little overlap, mid shore exposed and sheltered zones were positioned next to each other, and exposed and sheltered high shore zones were clumped together. Densities of adult mussels (>l5mm) were calculated as real densities from randomly placed quads i.e. not from areas of 100% cover. Density decreased up the shore; low, mid and high shore zones were significantly different from each other (ANOVA , p<0.0001; followed by multiple range tests). There was no significant difference between the densities of mussels at exposed and sheltered sites within each zone (ANOVA, p=0.7155). Recruit (<l5mm) densities increased with an increase in adult mussel densities, and this relationship was significant at all zones and for both degrees of exposure (regression analysis, p<0.05 in all cases). The regression between recruits and adults was strongest on the mid and high shore exposed sites. There was a general trend towards stronger regressions and greater predictability with an increase in shore height. The presence of free space within the mussel beds and significant regressions between recruit and adult densities indicates that mussel populations are recruit limited. Mean biomass decreased with an increase in shore height and was probably related to the decrease in size and density of mussels at higher shore levels. Exposure did not affect the average biomass within each zone. A fine scale study of the effects of wave exposure, tidal height and substratum type on recruit densities was undertaken at two sites, viz. Diaz Cross and High Rocks. Two shores, one exposed and one sheltered were identified at each of the sites. All shores were classified prior to sampling. Sampling was completed over a 30 day period during peak recruitment, and samples were removed on as many days as sea and tide conditions permitted i.e. daily when possible. The total density of early plantigrades was greater at Diaz Cross than it was at the High Rocks, and this may be related to the local hydrodynamic patterns adjacent to the two sites. Exposure affected the densities of early and late plantigrades on algae on the low shore sites, where greater numbers of recruits were recorded on exposed low shore zones. Densities of plantigrades on the mussel bed and on algae on the mid and high shore were not affected by exposure. Low and mid shore zones usually had greater densities (at 100% cover of substratum) of plantigrades than the high shore zones, this was probably related to lower settlement rates on the high shore as a result of reduced submergence time. Generally greater plantigrade densities were recorded on algal substrata than on the mussel bed. In only one of the 20 comparisons completed was the density of plantigrades greater on mussels than it was on algae. However when the area of the substratum within a zone was taken into account the number of plantigrades in the mussel bed at a zone was often greater than the number on algae within the same zone. In close to half of these comparisons the total numbers of plantigrades were greater on the mussels than on the algae. This was due to the greater area of mussel bed available to recruits. There was no evidence supporting the suggestion of secondary settlement of plantigrades from algae to the mussel bed. The results of this study demonstrate the importance of wave exposure, tidal height and substratum on certain aspects of the ecology of Perna perna. The importance of these factors is demonstrated at both the adult and early recruit stages of this mussel.