Density, size, and mortality patterns of the cockle (Austrovenus stutchburyi) population in Lews Bay, Whangateau: Post-2009’s mass mortality event
Density, size, and mortality patterns of the cockle (Austrovenus stutchburyi) population in Lews Bay, Whangateau: Post-2009’s mass mortality event
1. Introduction
In response to the mass mortality of New Zealand cockles (Austrovenus stutchburyi) observed in 2009 at Lews Bay, Whangateau, a group of Marine 202 students from the University of Auckland have been conducting annual studies on the cockle population. On May 7, 2023, we surveyed multiple stations along the four transects of Whangateau's shore and documented both alive and recently deceased cockles, referred to as "cluckers."
Our objective was to determine the density and size of cockles and cluckers when moving down-shore, enabling comparisons between new findings and previous studies conducted after 2009. Understanding the current status of the cockle population is crucial for assessing the long-term impact of the previous mortality event and determining the species' recovery or decline.2. Methods and Materials
2.1 Study site
The site studied was the intertidal zone of Lews Bay, Whangateau, 75.8km north of Auckland, New Zealand. Lews Bay is a flat environment and serves as a habitat for various species besides cockles. Other organisms that may inhabit the bay include anemones, oystercatchers, and a diverse range of marine life.There were four marked transects (A to D) along the shore. My group observed five of 11 stations along transect A, spaced 50 meters apart, these being, A50, A150, A250, A350, and A450. In previous years, A550 was studied (low tide mark), though was inaccessible due to tidal conditions. Consequently, no data was recorded for A550.
2.2. Sampling process
The method employed was haphazard sampling, using 1/16 m² (0.25 x 0.25 m or 0.0625 m²) quadrats. These quadrats were thrown and excavated to a depth of 10cm, where a sieve (3mm pores) separated the contents from the sand.
The results were documented by calculating the total count of cockles and cluckers at each excavated site and scaling them to an average density per m². A standard deviation was also calculated to show the variability of size and density amongst each site.Their shell length was measured using calipers (mm) and additional observations were made regarding the presence of anemones attached to them. Ten other species were also documented based on their abundance and vitality.3. Results
3.1 Cockle and clucker density observations
Among the 11 examined stations (A0 to A500), there was a significant contrast in the density of cockles and cluckers from A0 to A300 (Fig. 1). Cockle density consistently exceeded clucker density in all locations except for A350, where clucker density surpassed cockles by 106.66 per m² (cockles = 21.33 per m², cluckers = 128 per m²). From A350 to A500, the density of cockles and cluckers became comparable.A noticeable outlier was the cockle density at A300 compared to A250 and A350 (Fig. 1). At A250, the cockle density was 800 per m², which spiked to 1493.33 per m² at A300 before dropping significantly to 21.33 per m² at A350, the lowest value in this dataset. Similarly, the clucker density at A250 was the highest recorded among all sites, with 437.33 per m², compared to an average of 129.46 per m².
In 2023, the recorded density of cockles was 706.88 per m², compared to 875.20 per m² in 2022 (Fig. 2). For cluckers, the density in 2023 was 137.06 per m², while in 2022, it was 332.26 per m². Cockle density peaked in 2020 at 1710.40 per m², coinciding with the lowest clucker density observed at 54.40 per m². However, the density of cockles in 2023 reached the lowest average since 2011, which was 458.66 per m² at the time.
Fig 1. The mean + SE density of cockles and cluckers measured in 2023 (per m²), along the 11 observed transects (m).
Fig 2. The mean + SE density of cockles and cluckers measured (per m²) from 2009 to 2023. No data was recorded for 2010 or 2012 cluckers. 3.2 Cockle and clucker size observations
Among the measured cockles, the largest average shell length was 23.57mm and was found at A450. The smallest was 12.49mm, and was found at A0 (Fig. 4). Cluckers appeared slightly larger than cockles, however, at A500, the average length of cockles was 28.66mm, surpassing cluckers, which measured 28.4mm. The smallest clucker had an average length of 13.50mm, also found at A0.A450 exhibited the most similar size and density range among the cockles among the five sites (Fig. 3e). A350 displayed comparable data to A450, though only four sizes were recorded: 7mm, 8mm, 9mm, and 25mm (Fig. 3d). This limited range of sizes resulted in the largest error bar (Fig. 4).
Excluding A450, A150 showed the steadiest curve, with the most frequent cockle size being 15mm (Fig. 3b). The other three sites exhibited a bell-shaped curve, with A50 having the steepest curve (Fig. 3a). There was a notable decline in cockle size at A350, specifically, the length decreased from 20.26mm at A300 to 12.75mm at A350 before rebounding to 22.60mm at A400.
Fig 3. Length frequency graph showing the frequency of cockles measured at each of the five sites my group studied and their respective length (mm): a = A50, b = A150, c = A250, d = A350, e = A450. 
Fig 4. The average length of cockle shells (mm) measured along the 11 sites of transect A. 3.3 Sediment size & anaerobic layer depth
The sediment size showed a consistent pattern across the sites, with an average of 2.42 phi (Table 1). The largest grain size, averaging 2.00 phi, was observed in A50, while the smallest average grain size of 2.67 phi was found in A150 and A250. The average depth of the anaerobic layer was 2.64 cm. A350 had the deepest anaerobic layer, exceeding 10 cm in depth, while A0 and A150 had the shallowest anaerobic layers among the sites (Table 1). 
Table 1. The average sediment grain size and anaerobic layer recorded at each site (distance from the high tide mark in meters). 4. Discussion
4.1 Cockle and clucker densities along transect A
There were notable differences in cockle and clucker density along the transect, particularly from A0 to A300, where cockles consistently outnumbered cluckers.At A250, clucker density peaked, although limited data was available for the surrounding environment. However, the excavation site was approximately 8 meters from the channel, suggesting possible dehydrated conditions. In contrast, cockle density remained moderate but significantly lower than A300.
A300 stood out with the highest cockle density, possibly attributed to favorable water saturation in that area. However, the sandbank at A350 might have impeded cockle migration, potentially resulting in higher densities at that A300. At A350, there is a shift in densities, with cluckers surpassing the density of cockles. A350 exhibited the lowest density due to its limited range of cockles. The site was located on a sandbank with loose and dry sediment. This suggests a potential link between the decreased cockle density and the dehydrated environmentThe survival of cockles relies on a well-saturated habitat for oxygen and food intake, emphasizing the significance of water saturation for their survivorship (Boyden, 1972). An unsaturated environment can induce thermal stress, potentially leading to increased mortality (Zhou et al., 2022).Beyond A350, the cockle density remained relatively low. Predation could play a significant role in this decline, as A500 was identified as an active area for birds, particularly oystercatchers, known to feed on cockles (Sutherland & Polytechnic, 1982).4.2 Cockle and clucker shell length along transect A
In terms of size, cockle and clucker lengths increased near the low tide mark, with the largest average length of cockles observed at A500. This size variation could be due to tidal influences in the high tide zone. The higher intertidal range receives less water and more air exposure and may limit resources and enforce competition. Consequently, cockle growth can be disrupted, hence the steeper curves which were evident at A50, A150, and A250. In contrast, the lower tidal range provides more water access, resulting in sufficient oxygen and nutrient availability, explaining the flatter frequency curve seen at A450. While cluckers appeared slightly larger overall, this is likely a result of natural age-related death.
In 2023, the cockle density initially remained stable but declined as it approached the low tide mark after a spike before the sandbank. Similarly, clucker density remained relatively steady, except for a peak observed at A250. Since 2020, cockle density has consistently declined, reaching its lowest recorded level post-2011.4.3 Potential contributions towards mortality
Regarding sediment characteristics, the sediment size remained relatively consistent across the sites. However, the fine sand can impact permeability, leading to a more compact and less oxygenated environment (Haslett, 2000). While this could potentially contribute to the decline of cockles, a study conducted in 2017 measured the sediment at Whangateau and found similar results to those observed in 2023. The sediment ranged from 76.69µm to 227.2µm or approximately 2.879 phi and 2.122 phi (Jiang, 2017), indicating no substantial change in sediment size since 2017. In both years, the sediment size was classified as medium to fine sand on the Wentworth scale. Therefore, sediment characteristics alone may not be relevant in explaining the decline.Based on the results of a similar study in 2010, cockle size was much larger prior to the mass mortality, at times reaching up to 35mm. This study suggests that cockle mortality likely outweighs recruitment rates and that growth has slowed since 2009 (Tricklebank, Grace & Pilditch, 2021).Although our data cannot provide clear insight into the cause of increased cockle mortality, studies have shown that oystercatchers feed on larger cockles due to greater nutrient intake. They tend to select cockles larger than 20mm and favor those exceeding 30mm (Sanchez-Salazar, 1987), which may contribute to the decline in density near the low tide mark. It is worth noting that human interactions can be ruled out as a cause since Whangateau implemented a harvest ban post-2009.4.4. Conclusions
The study at Lews Bay, Whangateau, offered valuable insights into the present condition of the New Zealand cockle population. It was observed that when moving down-shore, the size of cockles and cluckers increased while their density decreased. However, the cockle population was highest mid-shore. Predation and environmental factors may play a role in the decline of overall cockle density, but cockle mortality may outweigh the recruitment rates.
The population of cockles has reached its lowest point since 2011, and potential causes of mortality were investigated. However, further research is necessary to fully comprehend the factors contributing to the decline and develop effective strategies for the recovery of Whangateau’s cockle population.References
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