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What’s happening at Mimiwhangata? (Part 2)

As published in Dive New Zealand magazine, October/November 2002 issue

In 1976 when Lion Breweries asked me to set up a monitoring programme at Mimiwhangata to follow long-term changes in marine life, the concept of monitoring such changes was in its infancy. Basically I had to invent the methods, and decide on just what to monitor.

With the idea of the future marine park in mind, and aware that as soon as the park was opened to the public intense pressure could come on the popular edible species of marine life, I decided to concentrate on those species which people were likely to target, either legally or illegally. I chose the following species: rock oysters, tuatuas, scallops, kina, crayfish, and the range of fishes.

Rock oyster populations declined in the late 1970's and early 1980's due to many large oysters simply dying of old age, and a lack of recruitment of spat to replace them. By the mid 1980's some transects totally lacked oysters where initially considerable numbers occurred. The situation is little changed in 2002. Photographic monitoring has shown that some oysters live to at least 20 years old!

Illegal removal of oysters jumped dramatically shortly after the public gained easy access to the Farm Park which was opened in December 1980. Once the Marine Park was created in 1984, however, removal of oysters dropped again. The following table indicates the illegal removal of oysters in sampling years since 1976. In summer 2002 removal of oysters was similar to that noted back in summer 1977.

 

 Season

 S77

 S78

 S79

 S81

 S82

 S84

 S85

 S86

 S02

 No. taken

 50

 200

 150

 425

 560

 370

 30

 20

 54

 Places

 3

12 

 2

     Table 1. Incidence of illegal removal of oysters at Mimiwhangata since 1976.

In the mid 1970's a large bed of tuatuas persisted for many years in the northern half of Mimiwhangata Beach. They numbered in the millions, roughly 10 million in 1976 at an average length of around 22 millimetres. Slowly their numbers dropped naturally as the individuals grew. By 1986 they were approximately 11 years old, mostly 55 to 60mm long, and numbered approximately 0.8 million individuals. They were a popular food source for visitors to Mimiwhangata for many years, but there has been no sufficiently large settlement of tuatua larvae to replace those taken by people and dying naturally. In recent years tuatua have been difficult to find on Mimiwhangata Beach, although some people still find a few for a feed.

In the early days of monitoring a few very large scallops were present in the channel between the mainland and Rimariki Island. They were never common, and have dropped to even lower numbers over the years. The lack of scallops is probably habitat-related rather than due to human influence.

Kina numbers in rock pools fluctuate fairly widely, and seem to relate to the detailed topography of individual pools. The shape of the pools influences the ease with which urchins migrate into, or out of, particular pools. It seems that recruitment to most pools is through settlement of planktonic larvae. When urchins reach a critical size, they migrate out of the pool and join their subtidal cousins. The only pools in which taking of urchins has influenced numbers are on the intertidal rocks around Okupe Island, where large numbers were removed shortly after the Park was opened, and some pools still show sudden declines consistent with removal by people.

The slow spread of subtidal urchins at the expense of the adjacent kelp forest has continued. It is now known that the spread of urchins, which is a widespread phenomenon from North Cape to East Cape, is related to a broad regional effect of overfishing of crayfish and snapper which eat kina.

There are some spectacular examples around Mimiwhangata of the loss of kelp forests due to excessive kina grazing. For example, at Pa Point in 1976 there was a lush, tall, dense forest of kelp, but by the early 1980's it was noted that kina were beginning to reduce the kelp forest. By summer 2002 this kelp forest had completely gone. The ecological changes brought about by the loss of the kelp have been dramatic. The rock at Pa Point is now covered in a thin layer of silt, urchins and two species of starfish are abundant, and thousands of the invasive parchment worm inhabit every rock crevice. Experience at the marine reserve at Leigh has shown that allowing snapper and crayfish to recover to natural numbers eventually leads to reduction in kina numbers and recovery of the kelp forests and the rich life associated with them.

One of the most immediate effects of total protection noted at the Leigh marine reserve was the rapid rise in both numbers and sizes of crayfish. Within five years the numbers were becoming spectacular. They eventually reached 40 times more abundant inside than outside the reserve.

Interestingly no such effect has occurred at Mimiwhangata, and monitoring shows that there is still too much recreational fishing for crayfish to allow any recovery at all. The following table indicates adult and juvenile crayfish numbers at three monitoring sites for which we have the most complete sequence of data. Apart from a cohort of young crays coming through in the late 1970's which did not translate significantly into a pulse of larger crays, numbers in 2002 are very similar to those in the mid 1970's.

Location

Size

 S77

S78

S79

S81

S82

S84

S85

S86

S02

F3 Lunch Bay

Legal

 3

3

5

13

9

4

16

3

10

 

Small

30

136

141

25

16

23

17

15

28

 

Total

33

139

146

38

25

27

33

18

38

 F6 Porae Point

Legal

8

10

6

7

2

2

10

4

2

 

Small

16

44

81

6

2

6

20

12

5

 

Total

24

54

87

13

4

8

30

16

7

 F9 Taukawau

Legal

-

0

0

12

5

2

1

6

2

 

Small

-

12

34

5

18

18

12

21

31

 

Total

-

12

34

17

23

20

13

27

33


 Table 2. Red crayfish counts at three monitoring sites since 1977.

 

Numbers of fishes have also shown no improvement despite about 17 years of Marine Park status and no commercial fishing since 1994. In fact the large snapper occasionally seen on some transects until about 1980 are very much a thing of the past. Even small snapper are rarely seen on the transects now.

Recent studies by researchers from the Leigh Marine Laboratory have clearly shown no difference in most fishes, particularly snapper, inside the Mimiwhangata Marine Park compared to areas outside the park to the north and south. This is further evidence that the current Marine Park status is having no effect in allowing the marine life at Mimiwhangata to recover towards a more natural state.

The results suggest that a different approach is required. At the Poor Knights, once all fishing was completely stopped about three years ago snapper numbers increased 16 times. Perhaps what is needed at Mimiwhangata is a full no-take marine reserve? Then we would see some real progress.

 

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