• Abstract and Figures
• Public Full-text

Rebecca Malloy 18/02/20 Measuring Shellfish Quantities on Opoutere Beach in the Wharekawa Harbor Introduction Opoutere Beach, located within the eastern side of the Coromandel Peninsula, is one of the few relatively undeveloped coastal landscapes remaining in the area. This beach and its surrounding environments provide a range of benefits to humans and wildlife, serving as a place for recreation, culture, education, and as habitat for many marine and terrestrial species. Although this particular part of the coast has not experienced much housing and residential development, the catchment as a whole has been impacted by surrounding land use practices in the last two centuries (Gibberd, 2015). Environmental degradation, in combination with overharvesting of marine animals, has had considerable impacts on species populations in the Hauraki Gulf. Shellfish stocks, specifically cockles (Austrovenus​ stutchburyi), wedge-shells (Macomona​ liliana), and pipis ​ ​ (Paphies​ australis), have seen significant depletion in abundance during the last several decades. ​ These shellfish are not only regarded as highly important cultural and recreational harvest species, but can be seen as indicators of environmental health due to their sensitivity to sediments and pollutants (Hewitt, 2013). In coordination with the Community Shellfish Monitoring program, we participated in an ongoing shellfish survey in the intertidal area of the Wharekawa Harbor, in order to quantify the abundances and sizes of A.​ stutchburyi and M.​ ​ liliana within monitoring plots in the area. ​ Equipment ● 25 cm x 25 cm Sieve (depth of 8 cm) ● Bucket ● Hand trowel ● Shellfish measuring gauge ● Waterproof data sheets ● Guide to shellfish identification Methods On February 12th and 13th, we set out to collect the abundances of shellfish in the intertidal zone of Opoutere Beach, and specifically measured the shell lengths of A.​ stutchburyi and M.​ liliana. Prior to starting the data collection process, transects lettered A through J were ​ placed throughout the monitoring site 50 meters apart, according to predetermined coordinates used since 2010. From these transects, a total of 210 plots marked with flags were placed 25 meters apart from each other, also selected by using predetermined coordinates. To begin the monitoring, a sieve was randomly placed within two meters of each flag, which was then traced with a hand trowel around the outside to denote the spot prior to digging. After moving the sieve, a hole was dug 10 centimeters deep to collect the sample for that site. The contents from the hole were then placed into the sieve until it was filled to the opening. Using water from the channel, the sand was rinsed from the sieve as water was slowly poured from a bucket, leaving only shells behind. Because we only needed to record the amounts of live shellfish present in the sample, all empty and broken shells were discarded from the rinsed sieve. With the exception of A.​ stutchburyi and M.​ liliana, each individual present was identified using a species identification ​ ​ guide and tallied categorically by species on a data sheet. The individuals were then returned to the channel outside of the sample site. Once the tallying of those species was completed, the shellfish measurement gauge was used to identify the lengths of the shellfish that remained in the sieve, A.​ stutchburyi and M.​ liliana. The gauge used for monitoring was developed with a ​ ​ predetermined categorical system, each correlating to a range of millimeters in size (i.e. category A included shell lengths from 1 to 5 mm, category B from 5 to 10 mm). After measuring the lengths of all A.​ stutchburyi and M.​ liliana present in the sample, these too were returned to the ​ ​ environment. This monitoring was repeated at every plot over two days until the data collection was completed. Results All data collected over the two monitoring days was then entered into an ongoing Excel spreadsheet that included data from previous years, dating back to 2010. The data from 2020 will be submitted to the regional coordinator of Waikato Regional Council for this project. The summary data and overall findings will be reviewed. Figure 1. Wedge shell densities in transects A through J (per square meter) at Wharekawa Estuary, 2020. Figure 2. Wedge shell densities in transects A through J (per square meter) at Wharekawa Estuary, 2019. In Figure 1, wedge shell densities are shown for transects A through J. At transect A, there were a total of 193 wedge shells present in the plot samples. In each transect following transect A (with the exceptions of transects H and I), there is a continual decrease in wedge shell density: where transect B had 185 wedge shells present; C contained 161 wedge shells, transect D contained 123 wedge shells, transect E contained 110 wedge shells, and in transect F, 78 wedge shells were present. The least amount of wedge shells were present in transect J. When comparing this data from 2019 in Figure 2, there is a high density of wedge shells present at transect A of 139 individuals, where there is a steeper downward trend in density for transects B through D. Transect B contained 105 wedge shells; transect C contained a total of 88 wedge shells, and Transect D contained 79 wedge shells. When transects E and F are reached, a spike in wedge shell density occurs, with 104 wedge shells present at transect E and 129 wedge shells in transect F. This is an interesting observation because as the data shows in Figures 1 and 2, there is a significantly higher density of wedge shells present at transects E and F in 2019 than when compared to densities for 2020. A similarity found in the data collected from 2019 and 2020 is that transect J contained the least amount of wedge shells. Figure 3. Cockle size distribution (by 5 mm increments) in percentages at Wharekawa Estuary, 2020. Figure 4. Cockle size distribution (by 5 mm increments) in percentages at Wharekawa Estuary, 2010. In Figure 3, the cockle size distribution is divided by size class, which was determined by the shellfish measurement gauge with letters A (>5 mm) through J (45-50 mm). There is a trend in the data which depicts a curve, increasing from class A until size class C, where a downward trend begins to size class G. The size class with the highest amount of cockles was size class C, being 10-15 mm. When comparing these trends with Figure 4, the same curve trend is present, but size class D contained the most number of cockles in the distribution. This shows that in the data collected, the cockles present in the samples were generally smaller in 2020 data collection than they were in 2010. Discussion When comparing wedge shell densities from 2020 to 2019, there are significant differences between both data sets. Wedge shell densities have been the highest at transect A in both years. Although there was a downward trend in the amount of wedge shells moving from transects A to D, there were significant differences in wedge shell densities in transects E through when comparing 2020 to 2019. This data suggests that there has been a decrease in the overall densities of wedge shells present in these particular monitoring sites, and the density of wedge shells was generally higher throughout transects A through D in 2020 compared to 2019. Shellfish densities can be impacted by a number of environmental gradients, such as temperature change, water depth, sedimentation levels, and excess nutrient deposition into water systems (Hewitt, 2013). Because shellfish are particularly sensitive to contaminants such as sediments and pollution, it is very possible that densities of wedge shells present in certain transects could be impacted by these cumulative effects from land use changes around the Wharekawa Harbor. Long term studies on wedge shell densities in these transects are required to support this theory. When comparing data on cockle sizes from 2020 to 2010, there is a clear difference in size class distribution. Cockles that were in class D (15-20 mm) were more abundant in 2010 than any other size class, whereas the most abundant size class in 2020 was class C (10-15 mm). This suggests that larger-sized cockles may have decreased in abundance in the monitoring sites since 2010. The growth rate of cockles can be affected by many factors, including nutrient concentrations and sediment load that occurs in the water column (Wells, 1970). The data collected in 2010 was during the month of September, whereas the 2020 data was taken in February, so this could also be a potential factor in the differences in size distribution. Bibliography Gibberd, Bronwen, 2015: Hauraki Gulf Forum Community Monitoring Programme Annual Report 2014/15. Hauraki Gulf Forum Committee Report. June 2015. Hewitt, Judi, and Niwa. “Effect of Increased Suspended Sediment on Suspension-Feeding Shellfish.” NIWA, 26 Nov. 2013, niwa.co.nz/publications/wa/vol10-no4-december-2002/effect-of-increased-suspended-sediment-o n-suspension-feeding-shellfish. Wells, Rebecca. “Changes to Austrovenus Stutchburyi Growth Rate since Early Human Settlement in New Zealand: an Indication of the Extent of Human Impact on Estuarine Health.” OUR Archive Home, University of Otago, 1 Jan. 1970, ourarchive.otago.ac.nz/handle/10523/8215. .

Load more

  7

Copy Link