Morphological analysis and roe condition of kina (Evechinus chloroticus) and kelp (Ecklonia radiata) abundance in Leigh, New Zealand, following kina removal in 2020
Morphological analysis and roe condition of kina (Evechinus chloroticus) and kelp (Ecklonia radiata) abundance in Leigh, New Zealand, following kina removal in 2020
Abstract
This study investigates the ecological consequences of removing sea urchins (Evechinus chloroticus) from three sites in Leigh, New Zealand, focusing on their impact on kelp forest ecosystems. Sea urchins are vital in shaping coastal ecosystems, particularly kelp abundance. Our research in January 2024 involved morphological analysis of sea urchins and ecological assessments at each site. Results indicate a significant correlation between sea urchin morphology, specifically shell diameter, and habitat conditions. Kina from the kelp forest site exhibited larger shell diameters and high wet weights, suggesting favourable habitat conditions. Despite higher wet weights in sea urchins from the barren site, those from the kelp forest site displayed higher roe weight percentages, underscoring the significant relationship between sea urchin morphology, habitat conditions, and roe quality. The study also explores the impact of sea urchin removal, conducted in 2020, on kelp forest restoration. Our data reveals a substantial increase in kelp abundance in the removal area, transitioning from an urchin barren to an emerging kelp forest. This suggests that targeted sea urchin removal effectively promotes kelp restoration and prevents overgrazing-induced deforestation.
1. Introduction
The protected marine ecosystem surrounding Leigh, New Zealand, is an intricate interplay between biological communities and their surrounding environment. Leigh is a small coastal area on the east coast of New Zealand’s north island and hosts three study sites—the urchin removal area, the urchin barren, and the kelp forest, each harbouring unique habitat characteristics. Sea urchins, known locally as kina (Evechinus chloroticus), play critical roles in shaping the structure of Leigh's coastal ecosystems, specifically kelp abundance. Kelp forests play a crucial role in New Zealand's ecosystem by providing shelter and contributing to secondary production. They support communities of suspension feeders, herbivores, and detrivores (Steneck et al., 2002), making them essential components of coastal biodiversity and ecological stability.
On the 29th of January 2024, sampling was undertaken at Nordic, Leigh, by students of Marine 305 at the University of Auckland. We investigated the morphology of E. chloroticus and their abundance and distribution across each site. Additionally, we analysed how habitat impacts shell diameter, total weight, and roe weight percentages. Through this study, we seek a deeper understanding of the ecological processes shaping Leigh’s coastal ecosystems. We hypothesise that removing E. chloroticus at Leigh has led to kelp (Ecklonia radiata) forest restoration, resulting in lower-quality roe. 2. Methods and Materials
2.1. Sample size, study site and kina processing
A total of 71 kinas were collected (~24 from each of the three sites), these being the kelp forest (site 1), urchin barren (site 2), and urchin removal area (site 3). The urchin removal area covers 1.6-2ha, where divers culled kinas via knife in October 2020.
The shell diameter of each kina was measured to the nearest millimetre (mm) using callipers. Kinas were weighed whole (wet weight) to the nearest gram (g) using scales with tare applied to account for additional weight. Kinas were opened using a knife, and all five gonads (roe) were extracted, blotted dry, and weighed to the nearest gram on a weigh boat. Additional observations, such as colouration, dietary habits, and taste, were recorded for each kina. However, taste observations were subjective and not included in this report.
2.2 Photo analysis of cover and kina numbers
Drop camera photos were taken haphazardly at each of the three sites on the 9th of February 2022, capturing the ecological state of the habitat at depths between 2 and 6 meters. Each student was assigned one photo from each site for analysis. Students classified and recorded the general ecosystem state present in the photos according to predefined categories. The number of individual kina were counted in each photo, specifying an estimated %cover for that site, alongside the %cover of additional species and groups. All collected data was entered into the class worksheet for analysis.3. Results
The samples collected from the three locations revealed a significant linear correlation between E. chloroticus shell diameter and total weight, including roe (Fig. 1). Specimens from site 1 exhibited the highest average shell diameter at 75.19 mm, followed by those from site 2 at 74.4 mm, and site 3 at 69.73 mm. Specimens from site 2 had the highest average wet weight at 154.74 g. In comparison, those from site 1 and site 3 had averages of 145.34 g and 126.66 g (Fig. 2a).
Roe weight percentages varied among sites, with specimens from site 1 exhibiting the highest percentage at 1.80% (2.62 g), followed by those from site 3 at 1.10% (1.39 g), and site 2 at 1.07% (1.65 g) (Fig. 2b).
Analysis of species and group cover (Fig. 3) revealed variability among sites. Kina were most abundant at site 2 (8.38%) and less so at sites 1 (2%) and 3 (0.89%). Kelp dominated across the study area, particularly at site 1 (78.36%), with lower coverage at site 3 (31.22%) and minimal presence at site 2 (0.19%). Turfing algae, crustose coralline algae (CCA), sand/sediment, and other species varied coverage among sites, with relatively low coverage at site 1 (<10%) but higher presence at sites 2 and 3, reaching up to 34.41% (Fig. 3). Notably, site 2 exhibited the highest percentage of other groups (10.5%), which were predominantly rocks, though site 1 had a small coverage of five green-lipped mussels (Perna canaliculus).
Fig 1. The correlation of E. chloroticus shell diameter (mm) and total weight, including roe (g), shown across the 3 sample sites at Leigh, recorded in January 2024.
Fig 2. The weight of E. chloroticus across the 3 sample sites at Leigh, recorded in January 2024. A = Wet weight (g), B = Roe weight (g), C = Weight distribution (%).
Fig 3. Average percentage cover (%) of species/groups on the seafloor observed through photographic samples taken at Leigh in November 2022.4. Discussion
4.1. Correlation of site cover and kina morphology
The higher averages of shell diameter and wet weight observed at site 1 suggest favourable habitat conditions relative to kina growth. Despite kina from site 2 exhibiting a higher average wet weight, those from site 1 had higher roe weight percentages. This discrepancy indicates that shell diameter and roe weight of kina may not necessarily correlate, highlighting the influence of habitat on roe condition.Kina from site 1 generally exhibited fattier roe with light yellow colouration and minimal excrement, suggesting healthier conditions (Sun & Chiang, 2015). In contrast, kina from site 2 appeared thinner, with roe displaying brown and black colours and higher excrement presence. This suggests that site 1's high kelp abundance contributes to improved roe quality, possibly due to the nutritional benefits derived from kelp grazing (Lourenço et al., 2018). Alternatively, it is plausible that roe quality is influenced by the protective habitat provided by the kelp forest, allowing kina to invest less energy towards defensive mechanisms (Van Buskirk & Schmidt, 2000), thus directing more energy towards enhancing roe quality. Specimens from site 3 exhibited smaller shell diameters and lower weights. This observation strengthens the correlation between roe quality and habitat conditions, aligning with findings from similar studies (Claisse et al., 2013).4.2. Kelp forest restoration
During the removal of kina in 2020 through culling (1.6-2 ha), the area (site 3) was classified as an urchin barren, with little kelp abundance (Miller & Shears, 2022). Following the removal, our data indicated a substantial increase in kelp abundance within the removal area compared to the adjacent barren (site 2). During photo analysis, it was noted that site 3 showed evidence of intermediate states, transitioning from a barren to a kelp forest. This suggests that kina removal is effective in kelp restoration, while those unremoved will overgraze, causing kelp deforestation (Claisse et al., 2013)..4.3. Conclusions
Our investigation showed a significant correlation between roe quality and habitat characteristics, with specimens from the kelp forest exhibiting the highest average roe weight. Our findings underscore the importance of habitat conditions, particularly kelp abundance, in influencing kina morphology and roe quality. Furthermore, our study supports the hypothesis that kelp forest restoration occurs following kina removal, as evidenced by the transitioning stages of kelp coverage observed in the removal area. This highlights the ecological dynamics of coastal ecosystems and the potential benefits of targeted management interventions. Overall, our findings emphasise the importance of preserving and restoring kelp habitats through kina removal to maintain the biodiversity of New Zealand’s coastal ecosystems.References
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