Roger Grace’s research 12 years ago showed crays in big trouble in the north


Male and female crays held by diver

By Roger Grace

In 2007, Dive magazine ran a feature by Dr Roger Grace (to celebrate our 100th edition) on the parlous state of crayfish in the north of New Zealand. Roger’s work proved prophetic. As part of the deep respect we have for Roger and in his honour at this time of his passing we are reprinting here that article slightly abridged under the original title.

Remember this was written 12 years ago. What have we done to address this dire situation?

Talk to any OLD diver (any diving in the 1960’s) and you will hear stories of crayfish feelers bristling out of every crevice at Tiritiri Island, 7-pound crays at Ponui Island, plenty around Waiheke Island, and giant 20-pound plus packhorse crawling around in the kelp at the Cavalli Islands. What has happened to them? If the Quota Management System is so good at sustaining our fisheries, why aren’t there still lots of big crays out there?

These days most divers are pleased if they can catch a couple of crays around 2 or 3 pounds in weight. Each generation of divers thinks what they are seeing is “normal”, unaware that what was “normal” 30 years ago is far different from what you see now.

Lonely sublegal cray at Mimiwhangata

Well, we now have a reality check. We have some Marine Reserves which have been established for long enough to show what crayfish populations would have been like about 50 years ago. And the results are a bit of a shock.

I have been studying crayfish at Mimiwhangata and Tawharanui in a long-term monitoring programme which began in the mid 1970’s. Mimiwhangata and Tawharanui are Marine Parks on the northeast coast which have been in existence since the early 1980’s. At Mimiwhangata, commercial fishing ceased in 1994, but recreational fishing has continued. At Tawharanui, all fishing ceased in 1983, and the area has been totally protected since that time. My studies covered several years… “before and after” protection.

A full analysis of results was published in 2006 in the scientific journal Biological Conservation (Shears, 2006 Long-term trends in lobster populations in a partially protected vs. no-take Marine Park, Biological Conservation 132:222-231). All rather technical and necessary.

Pictures tell the story

To summarise the data, I have presented the information in three histograms (reproduced on these pages).

Decadal trends of legal-sized red crays. Partial protection (red) and open fishing areas (blue). Only in the no-take area (white) have crayfish numbers recovered from the hammering all crays had in the 1960’s and 70’s.

The trends are pretty obvious. In the unprotected areas at Tawharanui, legal-sized red crays quickly dropped away to nothing! At Mimiwhangata, although numbers were the highest of the three areas in the 1970’s, over time they have dropped away to very small numbers despite the lack of commercial fishing since 1994. In contrast, in the no-take zone at Tawharanui crayfish numbers have increased dramatically with very high numbers in the current decade.

Not only are there high numbers, but there are many large crays too, with 10-pounders not uncommon! The results represent 800 kilogrammes of legal-sized red crays per hectare of reef in the protected area, whereas outside the protected area on the transects there are none!

20 minute counts

I did a simple 20-minute count and size estimate of crays inside and outside the western boundary of Tawharanui Marine Park in January 2006. The map shows where I counted, the two sites being only one kilometre apart, on the same reef type and in a depth of eight metres. Anyone can do this. Just anchor your boat and search around on the bottom for 20 minutes. The graph shows the results in terms of pounds weight (I can estimate cray sizes in pounds, but have difficulty with kilos!).

20-minute counts of red crayfish at Tawharanui. Graph shows a few undersized crays in the open fishing area (blue), but good numbers of small and large crays in the no-take zone (red).

The difference is again very obvious. There are far more crays in the no-take zone, and a good size range up to three at 6 to 7 pounds. There were NO legal-sized crays seen in the open fishing area. Notably there were far fewer sublegal crays in the unprotected area which raises an interesting point. If people don’t take sublegal crays, why are there far more in the protected area than outside? The answer lies in crayfish behaviour. Crayfish are gregarious, meaning they like to be together in groups. And little crayfish like to be where there are already lots of other crayfish including big ones.

10 day window for breeding

Crays on rock at Tawharanui

Recent research by Dr Alison MacDiarmid has brought to light some very interesting facts about crayfish breeding. Not only do large crayfish produce far greater numbers of eggs than small ones, but apparently when a female cray is ready to mate, it seeks large males to breed with because the fertilisation success is much greater than if she breeds with a small male. She has only a 10-day time window, however, in which to find a mate, after which time if not successful the eggs are resorbed into the ovaries which causes partial sterilisation for the rest of her life.

Because of overfishing, there are now very few large males in the general population, so the chances of a female cray finding a suitable mate are greatly reduced. In no-take marine reserves and parks, however, there is the full size and age range of crays including big males, so only in these areas is breeding success assured.

Alarm bells screaming

All this should be ringing serious alarm bells for the crayfishing industry and fisheries managers. If I was a commercial crayfisherman, I would be screaming out for more marine reserves! I would be demanding at least 10 kilometres of coastline protected in every 100 kilometres. The current way the fishery is managed has led to very serious depletion of crayfish stocks. We may be approaching a point where the few marine reserves we have may be starting to prop up the crayfish stocks outside the reserves.

Past management has led to grossly depleted numbers, wrecking of the natural size and age structure of the population, disruption of natural behaviour and social organisation, erosion of natural levels of breeding success, and probably loss of genetic diversity. Not to mention difficulty for fishermen catching their quota. In CRA 1&2 (Northland and Auckland east coasts) catch per unit effort is down to 0.5kg per pot haul. That is on average each time a fisherman hauls a pot he gets one just-legal cray! That would hardly cover his fuel costs.

It is time for a complete overhaul of philosophy of management of our crayfish populations. If you are concerned write to the Minister of Fisheries, Parliament Buildings, Freepost, Wellington, and express your views.

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