Counting fish at the Poor Knights (and Mokohinaus)

My research based at the Leigh Marine Laboratory focuses on the effects that no-take protection can have on fishes, including unexpected flow-on effects.

At Leigh’s Cape Rodney-Okakari Point Marine Reserve, previous research showed how the recovery of predatory snapper and crayfish under protection impact kina, slowing their grazing and in turn allowing kelp forests recover, an example of the “indirect effects” of reserve protection flowing on from the protection of targeted species. The key aim of my PhD research is to investigate whether similar, indirect effects are common among fish species, and how they might vary at other marine reserves.
The Poor Knights Islands are a star destination of New Zealand diving, previously shown off to television audiences around the world by famous people like Jacques Cousteau and David Attenborough.

Situated in clear oceanic waters 23 km off the Tutukaka coast, these small islands are washed by the East Auckland Current bringing wildlife from warmer locations like Lord Howe Island near Australia which contributes to the Poor Knights’ diverse fish fauna,. Besides coastal staples like snapper and red moki, the Poor Knights are home to a host of colourful wrasses and rarer species as illustrated on these pages. Here, dramatic stone archways, grand caves and sheer sunken cliffs provide surreal settings for some truly special underwater sights.

Recognising just how unique the Poor Knights were, in the 1960s and 70s now-legendary conservationists like the late Kelly Tarlton, Roger Grace, and Wade Doak were instrumental in affording the islands partial protection from fishing in 1981 moving into full no-take protection in 1998.

Snapper Armada

At the Poor Knights, snapper have undergone a population boom, with an armada of large snapper moving in around 1998 and holding fort ever since. But to figure out what other effects protection may have had, I needed to compare what happened at the Poor Knights to another, unprotected location. and for this I selected the Mokohinau Islands which is open to fishing, and has not experienced the same snapper boom.
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In the autumn of 2019, our small team headed out on the University of Auckland’s research vessel Hawere, diving for several days at both locations. We collected data to compare with previous fish monitoring results, often getting bitten by feisty Sandager’s wrasse in the process.

Working on the data, it became clear that at the Poor Knights the snapper biomass (the estimated weight of all the fish combined) was on the rise, indicating a growing number of large, legal-sized fish. In contrast, snapper biomass at the Mokohinau Islands remained low, consistent with a popular target species whose large individuals are frequently fished.

As well there were a few other long-term positive effects of reserve protection, including an increase in the likelihood of observing kingfish at the Poor Knights, and the stable numbers of multiple fished species.

Reverse decline, increase

In the same time period, we observed the decline in numbers of three wrasses (spotty, banded wrasse, and scarlet wrasse) at the Poor Knights, while they increased at the fished Mokohinau Islands. Such changes could be due to competition with (or in the case of the small spotty, predation by) snapper at the Poor Knights.
While the Poor Knights and Mokohinau Islands fish communities were consistently different, the influence of indirect effects of protection is likely to be weak, and likely to apply to just the three of 77 fish species we recorded.

Climate change - tropicalisation

The effects of reserve protection were not the only source of long-term ecological change I was interested in. Our climate is steadily changing, which is leading to a whole litany of problems from freak weather events to melting polar ice caps. And where temperate reefs are exposed to a poleward-flowing current bringing warmer water and its associated species, “tropicalisation” can occur. In this scenario, the warming of the temperate location means that vagrant tropical species can comfortably settle down, while temperate species gradually retreat to cooler waters. For a time, this comingling of tropical and temperate species boosts biodiversity, before the temperate species are gradually lost.
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Tropical fishes show up at both the Poor Knights and Mokohinau Islands, as well as other North Island locales like the Bay of Islands. These occasional arrivals range from the tiny sergeant major to the titanic whale shark, and with all kinds of interesting species recorded at the Poor Knights in warm seasons.

Buffer species

At locations where tropicalisation has occurred (including areas of Tasmania), the recovery of predators in marine reserves can act as a buffer against the process, as the predators are likely to eat or outcompete small newcomers. It follows that if tropicalisation is occurring offshore, it may be more pronounced at the fished Mokohinau Islands.
However tropical species were rare in our data, which spanned from 1998 to 2019, and the numbers of warm water species in general (including subtropical wrasses) did not appear to be gradually increasing. In fact, by looking at past studies by other scientists, it’s evident that most of the subtropical wrasses were far more common in the late 1970s. At that time, north eastern New Zealand experienced a period of particularly warm waters, and subtropical wrasses were able to establish populations at multiple locations. Since then those populations have depended on similar warm water events to top up their numbers. Fittingly, our data showed spikes in the numbers of these species following years with high summer water temperatures.

Sea temperature rise

Sea temperatures for the Poor Knights have been recorded by satellite since 1982, with the average summer temperatures increasing at a worrying 0.2°C per decade in this time. But this is a relatively narrow window to draw conclusions from. At Leigh, sea temperatures have been recorded daily since 1967, and when that whole period is accounted for, there has not been any significant warming in average temperature there.
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Nonetheless the existing Poor Knights temperature records and the Leigh temperature records are closely correlated. And if pre-1982 temperatures were similarly correlated, there may have been a similar lack of long-term warming at the Poor Knights. Though we can’t confidently address pre-1982 temperatures at the Poor Knights Islands, winter temperatures there are still cool, and it’s unclear if new tropical arrivals would fare well enough to establish permanent populations.

"Neither are ... a silver bullet for the problems ... but when carefully designed and properly implemented, marine reserves certainly provide a tool to secure healthier fish populations.

Species comings & goings

Over the years, the Poor Knights have been inhabited by collections of warm water fishes that have waxed and waned with warmer periods. Those early dives that dazzled spearfishers decades ago most likely took place during a bumper season of diversity, but nothing in nature is fixed.
At this point, a group of the subtropical wrasses including combfish were extremely common in 1975 but virtually extinct locally by 1979. However, those populations soon got topped up, and during my 2019 dives I saw those same species again.

Whether the East Auckland Current will continue to supply subtropical fishes to the Poor Knights in the decades to come remains to be seen. Some oceanographic research suggests the effects of climate change on oceanic currents may eventually weaken the connectivity between Australia and New Zealand.

All you can eat buffet?

In the face of long-term changes, the Poor Knights Islands Marine Reserve is working well at maintaining healthy populations of targeted species. Snapper are thriving, and thankfully aren’t treating the reserve as an all-you-can-eat buffet of other species.
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But whether the Poor Knights look something like an ecosystem that was never fished is unclear; the reserve currently extends just 800 m off the islands and is unlikely to help deeper-dwelling target species.

Neither are no-take marine reserves a silver bullet for the problems we’ve lumped on the marine world. But when carefully designed and properly implemented, marine reserves certainly provide a tool to secure healthier fish populations.

I hope that during the next 20-something years, the fishes of the Poor Knights Islands continue to thrive, supporting our marine ecosystems and the humans that rely upon them.