Dive Pacific 175 Dec2020 Jan 2021

Counting fish
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, increases
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.
species are gradually lost.
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 estab-
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
Sea surface temperature range
Sea surface temperature range (°C) at the Poor Knights Islands (PKI) from
1982-2019, with additional long-term monitoring (1967-2019) from
Leigh Marine Reserve. Annual mean temperatures from 1982-2019 were
closely correlated between Leigh and PKI (r = 0.89). Regression lines are
shown for all time series. Regression lines for Leigh show the full time
series as well as the 1982-2019 period only. Regression lines for the PKI
show the 1982-2019 period as mean summer temperature, overall annual
mean temperature, and mean winter temperature.
44 Dive New Zealand | Dive Pacific