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



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



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.



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



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



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.



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.



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.



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.



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



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



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



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)



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



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.



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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the invasion of sea urchins in tasmania

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