the invasion of sea urchins in tasmania
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7/29/2019 The Invasion of Sea Urchins in Tasmania
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The Invasion of Centrostephanus
Rodgersii in Tasmania MSCI5005: Topics in Marine Science
Kerwin C. Ferrer
Z3444817
20/05/2013
Submitted to:
Prof. Iain Suthers
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Abstract
The strengthening of the East Australian Current has resulted in a range extension of
species towards Tasmania. An example of this is the diadematide sea urchin,
Centrostephanus rodgersii which was historically found in New South Wales only. Aside
from the strengthening of the EAC, the sea urchin was able to successfully migrate into its
new habitat in Tasmania because of overfishing of large predators and thereby decreasing the
resilience of macroalgal habitats. The effects of the invasion of the C. rodgersii is alarming
because of its overgrazing which transforms healthy macroalgal habitats into sea urchin
barrens resulting to a loss of biodiversity. The transformation of macroalgal habitats into
barrens also has a negative effect in the abalone and rock lobster industry which contribute to
around $160 or 94% of the wild catch sector of the Tasmanian fisheries. Looking into
population dynamics such as age, growth and reproduction, it was found out that the
recruitment of C. rodgersii from its historical range is still continuous and that its
reproductive capability in Tasmania is higher compared to that of its counterpart in New
South Wales. It is predicted that the strengthening of the EAC will continue in the future and
that an increase in temperature in the Tasman area is also expected. The strengthening of the
EAC will facilitate in the further transport of larvae from its historical range while the
increase in temperature in the Tasman area will aid in the larval development of the C.
rodgersii which has a temperature threshold of 12°C. Future increase in population of C.
rodgersii is therefore expected and this may lead to wide spread barrens that may cover 50%
of the near shore rocky reef areas as also seen in New South Wales. Management options
include the culling of sea urchins by abalone divers and the introduction and preservation of
large predators such as the rock lobster Jasus edwardsii into Marine Protected Areas. The
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data from the release of large lobsters can then be used to create an ecosystem model that will
identify the minimum population of rock lobsters needed to prevent the sea urchin from
forming barrens. Another possible option is the expansion of the market for sea urchin roe
which will not only help in decreasing the number of sea urchins but also provide a
sustainable source of food.
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Contents
Introduction ...................................................................................................................................... 1
The Centrostephanus Rodgersii....................................................................................................... 1
Discovery of range extension........................................................................................................... 2
Causes of range extension ................................................................................................................ 3
Strengthening and penetration of the East Australian Current into Tasmanian waters ............... 3
Overfishing ................................................................................................................................... 4
Effects of the range extension of C. rodgersii ................................................................................. 5
Overgrazing results to barrens...................................................................................................... 5
Decrease in taxa diversity ............................................................................................................ 6
Effect in the abalone and lobster fishery ...................................................................................... 6
Population dynamics and future implications .................................................................................. 7
Preventive measures and Management options ............................................................................. 10
Conclusion ..................................................................................................................................... 12
References ...................................................................................................................................... 13
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Page | 1
Introduction
Climate change has started to affect the southeastern coast of Australia. This is seen in
the 350 km southwards extension of the EAC towards Tasmania (Ridgway 2007). Several
marine species were observed to have started a poleward range extension into Tasmanian
waters and the sea urchin Centrostephanus rodgersii is one of the controversial species
because of its catastrophic behavior of altering healthy and diverse habitats into sea urchin
barrens (Ling 2008). In this essay, firstly, we will look into the factors that paved way to the
successful invasion of the sea urchin into Tasmania. Secondly, find out the effects of its
migration to the existing marine ecosystem. Thirdly, predict what we may expect in the future
and finally, tackle on the management options that can be used to prevent a catastrophic
invasion of the sea urchin.
The Centrostephanus Rodgersii
The Centrostephanus Rodgersii is a species of sea urchin that belongs to the family
diamatidae. Historically, this species of sea urchin was only native to New South Wales, but
in the past decades, it has started to propagate and increase its population towards Tasmania,
where it was never before seen (Johnson et al. 2005).. C. rodgersii is commonly found in the
rocky subtidal reefs of New South Wales. It is often found in large assemblies that can reach
up to 60 urchins per square meter (Andrew & Byrne 2007). Its diet consists of seaweeds,
algae, byrozoans and sponges and is mostly active and grazes during night time (Miskelly
2002, p.30).
In its historical range (New South Wales), the spawning period of the C. rodgersii
starts in Winter, at around June and may last several months depending on location such that
spawning of populations in its northern limits would last a short 1 month period while those
in the southern limit had longer spawning periods (Andrew & Byrne 2007). It was noted by
Andrew & Byrne (2007) that gonad sizes of sea urchins found in the fringe habitat are usually
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higher than those that are living in the urchin barrens which may be caused by the amount of
food available in each habitat. In a study conducted by Huggett et al. (2005) results showed
that after successful fertilization and formation of an embryo, it would take around 55 hours
before it comes to its early pluteus larval stage. During this time, it can be seen as a two-
armed larva because of its prominent long postoral arms while its anterolateral arms are
relatively shorter. After 3 to 4 months, the larva then starts its metamorphosis into a juvenile
urchin, where it becomes radially symmetrical and spines start to form.
Discovery of range extension
The presence of C. rodgersii off Tasmania was first discovered in 1978 in St Helens
and it started to increase and move towards the south along the east coast of Tasmania
(Johnson et al. 2005). Johnson et al. (2005) estimates that the poleward range extension of the
sea urchin C. rodgersii may have started migration to Kent Island (island north of mainland
Tasmania) in 1960 and as years and decades passed, it can now be found even in the
southern parts of mainland Tasmania (see Figure 1). It now has a dense population in Eastern
Tasmania where sea urchin barrens created by its overgrazing are also found in large
numbers. The sea urchin barrens are very numerous in the Eastern side of Tasmania, but it
decreases in number as the latitude increases. In the southern limit of the range extension of
the C. rodgersii, only incipient barrens were observed (Johnson et al. 2005) which is
probably because C. rodgersii has just started to populate the area.
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Figure 1: The History of the migration of the C. rodgersii in Tasmania. From: Ling (2008)
Causes of range extension
Strengthening and penetration of the East Australian Current into Tasmanian waters
Several factors or stressors may have caused the successful invasion of this species
into the Tasman area. First of this is the strengthening of the East Australian Current, which
was observed from an increase in the Sea Surface Temperature in the Tasman area as
observed by Ridgway (2007). Figure 2 shows the penetration of the EAC into the east coast
Tasmania as evidenced by an increased temperature in that area. Because of the extension of
the EAC into the Tasman area, C. rodgersii in its pelagic larval state was able to migrate into
its new habitat (Johnson et al. 2005). C. rodgersii was found to have a larval stage of 3 to 4
months, where it is just floating in the water column for the said duration, enabling it to be
transported to long distances (Huggett et al. 2005). The extension of the EAC also produced
an increase in temperature in the said area which also supported the larval development of the
C. rodgersii, which has a temperature threshold of 12° C (Johnson et al. 2005, Ling et al.
2008). This is an example of a climate change-induced range extension of a species.
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Figure 2: The strengthening of the EAC and its penetration into Eastern Tasmania. From Ridgway (2007).
Overfishing
Another factor that contributed to the success of the migration of C. rodgersii in the
Tasman area is the decrease of resilience of the rocky reef habitats due to overfishing (Ling et
al. 2009a). Ling et al. (2009a) found out that the rock lobster ( Jasus edwardsii), which can be
found in the Tasman area is one of the main species that is able to prey on the C. rodgersii. In
their study, adult sized J. edwardsii with a carapace length of more than 140 mm was able to
prey on C. rodgersii with a test diameter of 60 mm. This predation was observed in the
Marine Protected Areas, where the lobsters were not taken and disturbed. However, habitats
outside of the MPA’s, are experiencing an increase in C. rodgersii population due to the
absence or minimal presence of J. edwardsii because it is being overfished in the said area.
Because of the overfishing of the lobsters in Tasmania, foreign invasion of the C. rodgersii
was not met with resistance.
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Effects of the range extension of C. rodgersii
Overgrazing results to barrens
One of the main issues that is of most concern regarding the invasion of C. rodgersii
in Tasman area is its formation of barrens. Johnson et al. (2011) emphasizes on this saying
“The critical issue associated with the advent of C. rodgersii in Tasmania is the formation of
barrens habitat caused by their overgrazing of macroalgae and intense predation on a wide
range of sessile benthic invertebrates” (p. 27). In New South Wales, it was observed by
Andrew & O’Neill (2000) that barrens cover around 50 per cent of the near-shore rocky reef
area which is dominantly due to the overgrazing of the C. rodgersii. Johnson et al. (2011)
predicts the possibility that the same can happen in the rocky reef areas of the east coast of
Tasmania. Widespread barrens were found at the northeastern parts of Tasmania where the C.
rodgersii have been grazing for years and they were positively correlated to the presence of
C. rodgersii (Johnson et al. 2005). In the southern limit where C. rodgersii was relatively
new, a few incipient barrens are starting to occur (Johnson et al. 2005). Trasformation into
barrens does not necessarily occur right after the arrival of the C. rodgersii but would usually
take a decade or two. Ling and Johnson (2009) suggests that barrens formation is
“…associated with relatively mature sea urchin populations, as widespread barrens are only
currently observed at sites where the average population age is >18 yr ” (p. 122). They further
add that an eventual growth in population of the sea urchin in the said area would also lead to
the transformation from a macroalgal habitat to a sea urchin barren. Another interesting note
is that to transform a macroalgal habitat into a sea urchin barren would require a higher
population of C. rodgersii, however, to maintain a sea urchin barren would require less sea
urchin density (Hill et al. 2003). This means that to be able to rehabilitate a sea urchin barren
back to a macroalgal habitat would require a regular and massive harvesting or culling of sea
urchins which Johnson et al. (2005) points out as not an easy task.
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Decrease in taxa diversity
Another effect of the invasion of the C. rodgersii is the loss of biodiversity. A study
by Ling (2008) looked into the effect of removing the C. rodgersii from incipient barrens in
Tasmania. He found out that in incipient barrens where C. rodgersii was extracted, canopy-
forming microalgae such as Ecklonia radiata and Phylospora comosa started to occur in
greater numbers than in incipient barrens where the C. rodgersii were undisturbed. Ling
(2008) also states that “The removal of C. rodgersii clearly increased taxonomic richness,
total abundance and Shannon diversity of benthic fauna…” (p. 888). It was found out that
barrens produced by C. rodgersii grazing resulted to a loss of around 150 taxa. Recovery of
the macroalgal habitat from an incipient barrens state after removal of C. rodgersii resulted
15 months after the start of the study which is in contrast to what was observed in New South
Wales experiments that resulted to slow recovery. The author further notes that patching up
incipient barrens in Tasmania resulted to increased diversity because of the addition of grazer
resistant taxa which was rarely observed in healthy macroalgal beds. The author concludes
that range extension of habitat-modifying organisms would highly affect marine diversity.
Effect in the abalone and lobster fishery
The Tasmanian fisheries contribute the largest share in the fisheries production of
Australia at 27 per cent amounting to $596.7 million in 2010-11 (ABARE 2012). This
comprises of the aquaculture sector and wild-catch sector. The Abalone Fishery accounts for
58 per cent ($97 million) of the wild-catch sector while the rock lobster accounts for 36 per
cent ($59.5 million). The abalone and rock lobster fisheries therefore are very important to
the Tasmanian Fisheries production. However, due to the invasion of the C. rodgersii, the
abalone ( Haliotis rubra) and rock lobster ( Jasus edwardsii), which are mainly found in
macroalgal habitats, are in danger of being affected by the overgrazing activities of the
newcomer sea urchin. In New South Wales, a negative relationship was observed between the
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population of sea urchin to that of the abalone (Andrew et al. 1998, p.61). Similarly, this
relationship was also seen in East Tasmania, where an increase in density of C. rodgersii led
to a decrease in abundance not only of abalone, but also of lobsters (Johnson et al. 2011).
This maybe because lobsters and abalone are not commonly found in barrens created by C.
rodgersii, but in addition to that, Strain & Johnson (2009) found out that the abalone
population in intact algal beds was also negatively affected by the presence of C. rodgersii
even when barrens have not yet occurred. Widespread barrens have started to increase in
number in the East coast of Tasmania which could lead to negative consequences on the
lobster, abalone and other species that rely on the macroalgal habitat (Johnson et al. 2005).
Population dynamics and future implications
Predicting future developments of the range extension of C. rodgersii in Tasmania
would be helpful in managing the and preparing solutions for the said problem. By looking
into the population dynamics of the invading urchin we can infer possible future scenarios
(Ling et al. 2009b) and therefore help in planning future management actions. Ling and
Johnson (2009) studied the age and growth of the C. rodgersii from the macroalagal
boundary habitat and recently formed barrens in northeastern Tasmania. They tagged around
300 C. rodgersii at each site by injecting tetracycline-HCL in the peristomial membrane (the
oral area) of the urchin. The tetracycline creates stains in the jaw of the sea urchin, and after a
year, the growth of the jaw can be measured by distinguishing the stained and the non-stained
area of the jaw under UV light. An indeterminate growth pattern was observed on the change
in jaw length and the initial jaw length of the sea urchin. This implies that as the jaw length
grows longer, its growth rate becomes slower. A growth model is then created for each
habitat (barrens and macroalgal boundary) and from this, the age of the sea urchins were then
determined. In their study, sea urchins in the macroalgal habitat demonstrated a faster growth
rate (in terms of test diameter, thickness and weight) than their contemporaries in the barrens
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habitat. However, Ling & Johnson (2009) observed that the reproductive power of the sea
urchin living in barrens did not decrease significantly as opposed to studies of C. rodgersii in
New South Wales by Byrne et al. (1998).
The same method was used by Ling et al. (2009b) where they developed a growth
model for the C. rodgersii in eastern Tasmania. In their study, a wide variety of ages which
include plenty of old aged C. rodgersii were observed in the northern parts of eastern
Tasmania while younger populations were seen towards the south. Ling et al (2009b) states
“the broad range of C. rodgersii ages in NE Tasmania indicates that the range extension has
not occurred as the result of a single, massive recruitment event” (p. 725). This, they say
shows that multiple recruitment events (with larvae coming from the historical range)
contributed to the establishment of C. rodgersii in Tasmania and these migration events may
be continually ongoing. Banks et al. (2010) also confirms this with the result of their genetic
analysis on the C. rodgersii showing minimal genetic differences of the urchin population in
Tasmania from that of mainland Australia (NSW) which dismisses the probability of a
founder effect or a single massive migration.
Ling et al. (2008) studied the reproductive cycle of C. rodgersii in Tasmania.
Collecting samples every two months for a period of 18 months, they were able to identify
the month of July and August as the spawning season of the C. rodgersii. This was inferred
from a decrease in Gonad Index (GI) and an increase in the number of successful induced
spawning in those months. The study also showed that the GI of C. rodgersii in Tasmania
was higher compared to those in New South Wales which indicate a higher reproductive
potential for the Tasmanian inhabitants. They inferred that this maybe due to the higher
availability of food in macroalgal beds and the lower population density resulting to less food
competition between urchins. Moreover, Ling et al. (2008) also observed that the temperature
threshold for the development of the C. rodgersii embryo to its two-arm larval stage was
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12°C which also corresponds to the result of the study of King (1992), as cited by Ling et al.
(2008), for the C. rodgersii found in New South Wales. This implies that the development of
the C. rodgersii requires temperatures above 12°C during winter time (time of spawning)
which in recent years has shown exceedance of that value. The development of the C.
rodgersii to a larva was also found to be faster with an increase in temperature.
The continuous larval recruitment of the C. rodgersii from mainland Australia is
evident from the studies of Ling et al. (2009b) and Banks et al. (2010). The reproductive
potential of Tasmanian C. rodgersii seems to be high according to Ling & Johnson (2009)
and Ling et al. (2008). It is also found that the C. rodgersii is able to complete its
reproductive cycle when the temperature is above 12°C during the time of spawning (Ling et
al. 2008). Years of temperature data during winter time (Figure 3) shows an increasing trend
which is primarily due to the penetration of the EAC into Tasmanian waters (Ridgway
2007). If this trend continues, an increase in temperature in Tasmanian waters will result in
higher and faster reproduction capability of the C. rodgersii and a continuous supply of
larvae from mainland Australia is expected.
Figure 3: Winter temperatures in eastern Tasmania since 1946 -2007. From Ling et al. (2008)
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Preventive measures and Management options
The catastrophic shift from healthy macroalgal kelp beds into sea urchin barrens is
one of the priorities that management must focus on. It seems that the range extension of the
C. rodgersii towards the south will still continue especially that climate change is imminent.
Therefore, to be able to prevent further damage to the marine ecosystem of Tasmania, several
mitigation measures should be done.
The Institute of Marine and Aquatic Studies (IMAS) has partnered with the Fisheries
Research and Development Corporation (FRDC) and the Natural Resource Management
(NRM) Program in a project called Centrostephanus project (Institute of Marine and Aquatic
Studies 2011). Its aim is to evaluate possible options to prevent further barrens formation due
to C. rodgersii overgrazing. One of the plans being implemented by the Centrostephanus
project is the Culling of the C. rodgersii by divers which they call as “Protect your patch”. By
this, professional divers while harvesting abalone as a livelihood are tasked to cull C.
rodgersii urchins. They are assigned to specific areas where incipient barrens occur and have
to take records every time they finish culling sea urchins and harvesting abalone. This area is
then compared to another area where no culling took place and then find out if kelp bed
recovery is observed.
Another plan of the Centrostephanus project is the release of large lobsters in marine
protected areas and evaluating the effect of the addition of lobsters through ecosystem
modeling (IMAS 2011). Ling et al. (2009a) reported that the rock lobster J. edwardsii with
>140mm carapace length is effective in preying on sea urchins. Furthermore, Ling and
Johnson (2012) have pointed out the effectiveness of Marine Protected Areas in increasing
resilience and decreasing the survival rate of C. rodgersii because of the presence of large
predators such as J. edwardsii. If large number of lobsters is released in an area, it may
facilitate in the control of the population of the C. rodgersii and may reduce the future
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occurrence of sea urchin barrens. In 2008, before the creation of the Centrostephanus project
and IMAS, the Tasmanian Aquaculture and Fisheries Institute already piloted the release of
large lobsters in sea urchin barrens and found out that these lobsters were effectively eating
the sea urchins (Ling et al. 2009c). Ling and Johnson (2012) further adds that an increase in
size and population of rock lobsters and other predators of the C. rodgersii can also be
attained by “rotational spatial closures, managed predator relocation, upper size limits and/or
catch reductions, allowing accumulation of large predator biomass on rocky reefs” (p. 1242).
Data from the release of large lobsters is then used to construct an ecosystem model to
identify the minimum population of lobsters needed to control sea urchin populations and
prevent barrens formation (IMAS 2011).
Another option is the creation or expansion of the market for C. rodgersii roe. In New
South Wales, Andrew et al. (1998) suggested the creation of a market for sea urchin roe
(gonads) to be able to help decrease the C. rodgersii population and increase the abundance
of abalone in the area. Not only does this help in minimizing the effects of the C. rodgersii to
the marine ecosystem, it also creates a sustainable source of food. However, to be able to
produce profitable and marketable sea urchins, the quality of roe should also be considered.
Ling et al. (2008) and Andrew et al. (2007) pointed out that gonad sizes of C. rodgersii living
in macroalgal beds or incipient barrens are relatively larger (better quality roe) than those
living in urchin barrens. A seafood company in Tasmania has recently started harvesting and
processing sea urchins roe quality is good (Norwood 2012). They are already supplying to the
local Australian market and are planning to export their sea urchin roe to Japan, where
demand for this product is large. The company will also start to supply a rock lobster
fishermen’s association to look into the possibility of including the sea urchin roe into baits
used to catch rock lobsters.
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Certainly, the issue of the invasion of the C. rodgersii should be listed in the priority
list for marine ecosystem managers in Tasmania. Ling et al. (2009a) further emphasizes on
the importance of addressing this issue: “Management must therefore aim to prevent further
phase shifts to sea urchin barrens because the strong hysteresis effect makes rehabilitation of
existing barrens to kelp beds exceedingly difficult” (p. 22344).
Conclusion
It is evident that the C. rodgersii was able to successfully invade the East coast of
Tasmania as seen in its continuous and unopposed migration. With a projected climate
change, through the strengthening and extension of the EAC in East Tasmania(Ridgway
2007), the sea urchin will be able to continuously migrate from its historical range and into
Tasmania. If the increase in temperature in Tasmania continues, the sea urchin will also be
able to successfully reproduce and complete its life cycle and probably will be able to
propagate southwards. The barrens formation of the sea urchin affects the marine ecosystem
supported by the macroalgal beds, especially the valued abalone and the rock lobster fisheries
(Johnson et al. 2005). If the invasion of the sea urchin continues resulting to widespread
barrens in Eastern Tasmania, the fishing industry would suffer. Therefore, to be able to
prevent this from happening, management action should be done such as culling of sea
urchins, increasing the size and population of rock lobsters that are able to prey on the sea
urchin and develop a market for sea urchin roe.
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