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Written as part of Marine Ecology 5

INVASIVE MARINE SPECIES IN

NEW ZEALAND

 

SURVEY OF THE UPPER WHANGAREI HARBOUR

FOR THE ASIAN DATE MUSSEL(Musculista senhousia)

 

 

Abstract

This is a survey of the Whangarei Harbour concerning the locating of the invasive marine species the Asian date mussel M.senhousia. The mussel was introduced to New Zealand in 1970's and

has spread throughout the Waitemata harbour and is now present in the Whangarei harbour.

The mussel beds were located and core samples taken to estimate population densities, over all population size, and size frequency of the beds. It appeared that some populations were older than others, but no real trend was noted.

 

Acknowledgments:

Thank you to the following people Karen Tricklebank, Simon Hooker(NIWA),

Fred Brooks(DOC), John Holdsworth (MAF) and Glen Mortime(NRC)

 

Contents:

1.0  Introduction

1.1  Mussel ecology

1.2..Method

1.3  Results

1.4  Discussion

1.5  References

1.0   INTRODUCTION

Since New Zealand broke away from Gondwanaland some 60-80 million years ago, species have evolved to co-exist with each other, all being part of the natural eco-system . But with the arrival of the Maori came the Kiore and the dog. With the English many hundreds of other pests arrived, causing havoc on our bush and fresh water streams.

But less well known is the damage that is being done by marine invaders that have arrived aided by man. This is not to say that there was not also gradual arrivals of foreign species in larvae form with the north west oceanic currents. This process has resulted in the introduction of many temperate and subtropical eastern Australian species into northern New Zealand coastal waters. Some have established themselves but many would have perished (Willan 1984).

Other ways in which foreign species may have reached New Zealand over the years are, attached to logs though these rarely cross from Australia, and on the feet of migrational seabirds. As you can imagine the species that could be introduced by these methods is very limiting and a rare occurrence

(Hayward 1997). Therefore, another method of arrival must be involved for the sixty one of New Zealand's invasive marine species to have arrived in the last 150 years (Hayward 1997).

As world trading has opened up and shipping become more regular and faster, invasive marine species have increased (Willan 1997). In the case of the Asian Date Mussel, evidence points to the accidental introduction by shipping from Japanese ports.

Docking records show that between 1970 and 1979 there was a large increase in direct ship movement between Japan and New Zealand. Records indicate the establishment of six Japanese species of marine organism (including the Asian Date Mussel) during this time (Willan 1984). Exactly how they arrived was uncertain. They could be attached to hulls or have arrived in ballast water. New evidence suggests the latter.

 

It is now believed of the sixty-one foreign invasive marine species introduced;

four were introduced deliberately, twenty five as fouling on vessels and three in deck cargo. The remaining 29 arrived in ballast. Of these 10 are from Asia.

Many live in low numbers and have little or no impact on natural biota.

The biggest effects are coming from four bivalves introduced

(Hayward 1997). They are Theora lubrica, Limaria orientalis,

Crassostnea gigas and Musculista senhousia, the Asian Date Mussel

(Cited Willan 85).

This study concentrates on this species and its distribution and effect on the Whangarei Harbour.

Sheltered harbours and estuaries next to major ports such as Whangarei and Tamaki are the worst affected areas(Creese and Hooker 1996).

Musculista senhousia is widely distributed in Eastern Asia where it occurs naturally from Singapore to Siberia (cited Willan 85).

In the last 70 years however Musculista senhousia has been spread from its native land to Western U.S.A. in 1944, Southern France, Western Australia and New Zealand (Creese and Hooker 96).

They were introduced to New Zealand some time in the late 1970's and first recorded in 1980. They were then only present in the Waitemata Harbour and Te Puriri in the Firth of Thames. Up until 1985 they were thought to be confined to the Auckland area, then in August 1985 populations were found in the Whangarei Harbour. These beds were found at Manganese Point, Parua Bay and Takahiwai Point. It is assumed that they came from Auckland by way of shipping (Willan 87).

 

1.1   MUSSEL ECOLOGY

The Mussel is from the subfamily Crinellinae. They are a small mat forming mussel with an elongated shell and a maximum length of around 30 mm. They are covered with a thin olive green periostrciam with wavy purple patterns. These are believed to be growth patterns. (Willan 87). The shell is very thin and soft. It forms beds of byssal threads in which they sit upright and attach themselves to other organisms, smothering them, eventually creating an anaerobic environment beneath them in which very little can survive. It lives in intertidal and subtidal flats, in silt and clay but can also inhabit hard areas such as wharf piles. It only builds a nest when required because of soft substrata but can hold onto wharf piles successfully.

It is classified as being a great opportunist because of its fast growth, taking only nine months to reach maturity, and because it can survive in areas of low oxygen levels and has a high reproduction capability. The amount of their larvae in the planktonic layer has been recorded as being as much as

17,000per sq.metre and their spat settles in greater numbers than any other bivalves. (William 85).

It lives in patches from 5 sq.metres to 10hectares in size. Populations of this size have the potential to have adverse affects on the ecology of a harbour

(Hayward 97). When Musculista senhousia makes its nest, the colony rapidly changes the habitat from sand into mud flats through the stabilisation of sand and silt grains caught in byssal threads (Willan 85).

Density of population can be very high, as many as 28,650 per sq.m. With densities this high, they can dominate the local fauna.

The mats that are formed are 20-40mm deep which means that other bivalves such as pipi are excluded from this area because they can't project their siphons through the compact mussel colonies (Willan 85). Only those animals which can move freely through the mat can live in it, such as the polychaete worm.

 

The environmental affects on the area are short lived as the beds live for only 2 years maximum before dying out. However accumulation of mud and silt, may take some time to be washed away and effects on the fauna may be irreversible (Creese & Hooker 96). It has been noted that a bed can be killed suddenly when several extreme low tides coincide with extremely hot weather, drying out the beds(Willan 87).

All harbours in the North Island possess extensive sand/mud flats which are capable of hosting Musculista senhousia (Willan 85). Since being introduced to New Zealand, it has spread north to Parengarenga Harbour and south to East Cape. This was in 1987 and so far it has not being recorded else where (cited Hayward 97).

So far there have been no reports of this mussel having any affects on commercial oyster and mussel farms. This is probably because they like different habitat areas. However, there is the possibility that Musculista senhousia could compete with oyster spat for food (Willan 87).

The introduction of this species has highlighted the need for stricter monitoring and regulations on ship discharge as it is done in Canada, U.S.A. Britain and France (Willan 87).

 

1.2   METHOD

A detailed search was carried out on the northern side of the upper Whangarei harbour. Only unvegetated areas (areas without mangrove) above the low water mark were surveyed.

Once a bed of Asian Date Mussels was located, three random samples were taken using a 66mm diameter tin to a depth of 30 mm.

Each sample was washed through a 1mm mesh sieve. The number of M.senhousai. per sample were converted into densities per square metre, this allowed information to be compared with previous experiments.

The length of each mussel shell was measured using callipers and analysis of the resulting lengths plotted on a frequency histogram.

This quantitative measure method is time consuming because every mussel has to be removed from the byssal bed and measured. It is also hard to determine whether a mussel is dead or alive as they do not open when they die.

Bed size and area was measured and recorded on a map of the harbour. Any interesting observations were recorded. The density (estimated) per sq.metre, was worked out.

 

1.3   RESULTS

The area searched covered virtually all available inter-tidal habitat that was not vegetated. My inter-tidal survey of the harbour in 1997 showed many populations of Musculista senhousia in the harbour. The areas most densely populated were further up the harbour, the most dense being along the Onerahi foreshore and on Limestone Island. In areas where M.senhousia were first reported, no beds could be found only empty shells. This indicates that the beds have either died or are below the low water mark.

 

The two beds on the Onerahi foreshore were both very densely populated having a total population of around 4.4 million mussels

(10,000-15,000 per sq.metre) and covered an area of around 400 sq.m. These populations are as large as dense populations found in a previously published study by Creese & Hooker in 1996. By looking at their size as an indicator to their age it would appear that they were between 18 months to 1 year old.

This suggests, if information from previous studies is true, that the population will only last another year before dying.

It appears as if site two is already starting to die off as the number of live mussels is decreasing and the number of dead shells is increasing. The average size of live mussels in this mat was 19.4mm, compared to the average size in Creese's study on the Waitemata Harbour which were 21.4 - 22.4mm.

It was while at this bed that I noticed fragments of mats breaking off with the wash of boats and being shifted along the beach by the out going tide. It occurred to me that this may be a way that the mussel spreads. Similar observations were made by Creese & Hooker in 1996. To test this observation of spread they carried out an experiment in which a section of mat was transplanted further down the beach. However it did not survive but it could do subtidally and more tests should be done.

 

 

Site number five showed an unusual trend. It showed that all the live mussels were small(average of 16.4mm) and all the dead mussels were much bigger (average of 21.6). This suggests that young mussels are colonising an older mat. This contradicts results from Creese & Hooker where it was found

that once a mat is established, there is no recruitment of juveniles.

Site six was a small to mid size population covering 160 sq.m with a density of 4,500 per sq.m. It appears from looking at this data, that the population of this site is dying. This conclusion is drawn from the large amounts of empty shells and their apparent age.

Site seven shows no really obvious trends. The average size here was 18.4mm for live mussels and 19.7mm. for dead mussels. The average density was around 4,312persq.m., similar to the previous one except it covered a much smaller area of only 25 sq.m.

Site nine had the largest population of mussels, 3.5 million, and covered

a large 400 sq.m area. It had a reasonable density of 8,722per sq.m. Mortality appears to be spread over the range of ages.

At sights 8, 10, 11, 12 and 13, no live mussels were found only empty shells. This may indicate the presence of a mat in the sub-tidal zone or that a mat was there but has since died. If shells are found it can be assumed that a mat is near because the mussels have very fragile shells and when the dead valves of M.senhousia are transported more than a few metres from the site where they lived they are likely to disintegrate (Willan 85).

At site 13 in Parua Bay where the first mussel beds were identified in 1985, shells found were of considerable size.

Here the average size of the mussels was 26.9mm and the maximum size was 31 mm. These mussels were obviously of considerable age and could be the remains of one of the first beds in the Whangarei Harbour.

It appeared as if it had been some time since the bed had die as the periostrciam had peeled off and many were crushed. Here I carried out an extensive search below the low tide mark, as the water was only shallow but found nothing.

1.4   DISCUSSION

M.Senhousia were mainly found at mid to low tide levels in

semi-exposed/sheltered localities. It was noted that fragments of cockles or other shells were always present under a bed. These may aid in the initial attachment of recruiting juveniles(Creese and hooker 96)

Under the mussels, the substrata was anaerobic black with very little life apart from polychaete worms of which there appeared to be perhaps higher numbers than usual. Other experiments have proven that M.senhousia prevents other marine organisms from being able to live or become established. These were done by Creese & Hooker on the Waitemata Harbour and it could be considered that results will be similar in the Whangarei harbour.

The Waitemata Harbour hosted 35 different species of macro fauna. Inside the M.senhousia mats it was found there was a 38% decrease in the species numbers. The main species affected by mussels were pipi (Paphus australis) and cockle (Austrovenus stutchbury).

It was observed that birds appeared to be feeding in these areas and in some cases even defended them against invaders (including myself). Whether they were eating the mussels or other fauna present is unsure. Bird species included the red bill gull, black back gull, oyster catcher and even ducks. As the shells are very soft it may be possible for birds to crush or open shells easily. Similar observations were made by Hayward in his 1997 study on the ecology of the mussel.

The ducks, oyster catchers and other sea birds seen feeding on mat edges, have also been observed in America, where they were seen feeding on the similar Zetna mussel. This may have an effect on the shape of the mat but have very little affect on the population densities.

M.senhousia colonises an area and forms a thick byssal thread mat. The life span of this mat is determined by the long livity of the mussel, which is 1-2 years. Because they have high mortality rates and a short life span they require large numbers in each population (cited Creese & Hooker 96).

Densities found in my experiments were similar to those found in Asia where densities vary from 500-28,000 per sq.m. in the intertidal zones.

A mussel is classed as being a juvenile when it is under 10 mm in size. As spawning is in May no mussels under 10mm were found. Between July and November the mussels do not grow much (Creese & Hooker) this would explain the size of mussels found to be in the 10-14mm size bracket.

The populations were all dominated by mussels of 18-20mm in size with the largest being 31 mm long and the smallest only 11mm.

It was noted that seven out of the thirteen sites found, were next to a fresh water stream, rivulets or run off area. For example mat number six was located at the outlet of Waikaraka stream. Similar observations were made by Creese & Hooker in their survey of the Tamaki Estuary. Just what attracts the larva to these areas is unknown, the outlets may supply food or the fresh water may enhance colonisation.

By continually monitoring the populations identified, over a long period of time(as done by Creese)it would provide information on the Asian Date Mussel in the Whangarei harbour. The total intertidal population in the area mapped is over 12 million and covers a total area of 1,280 square metres.

1.5   REFERENCES

Hooker .S and Creese .R(1996)Ecology and environmental Impacts of Musculista senhousia(mollusca: Bivalvia mytilidae) in Tamaki Estuary Auckland, New Zealand. New Zealand Journal of Marine And Fresh water research 31:225-236.

Willan .R.C (1987): The Mussel Musculista senhousia in Australasia, another aggressive alien highlights the need for quarantine at ports. Bulletin of marine science 41:475-478.

Hayward .B.N (1996) Introduced marine organisms in New Zealand and their impact in the Waitemata Harbour, Auckland. 36:197-223.

Fisher.E- National Geographical, Mussel Power Pg 104-121

Willan R.C (1985): successful establishment of the Asian Date Mussel Musculista senhousia(Benson in Cantor,1842) in New Zealand. Records of the Auckland Institute and Museum 22:85-96

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