Predators in our midst

A dead possum on a main road (near my house) in suburban Auckland.
A dead possum on a main road (near my house) in suburban Auckland.

When I first started trapping I was amazed by the quantity of Possums I caught. I worked multiple trapping lines in the city that were surrounded by houses, roads and waterways. Where were the possums coming from?

There were two articles about local rat ‘attacks’ Rat chews ear of sleeping 9-year-old & School warns students over rats after teen bitten by dying rodent recently. And they got me thinking again… where do they come from? Do the rats really come from the bush like the article claims or are the rats just as dense in suburbia? And what of the possums? Maybe it’s time we put some tracking tunnels and wax tags up in our backyards because if we really want a Predator Free NZ we are going to have to start at home.

Squandered

SQUANDERED: THE DEGRADATION OF NEW ZEALAND’S FRESHWATER by Dr Mike Joy
SQUANDERED: The degradation of New Zealand’s freshwater by Dr Mike Joy

I was stoked to have helped design this eBook for Mike who is a kiwi ledgend.

BioBlitz

My son found this albino cockroach on a night walk through Kepa Bush.
My son found this albino cockroach on a night walk through Kepa Bush.

BioBlitz is awesome. I had a great time learning about fungus:
http://naturewatch.org.nz/observations/1342538
http://naturewatch.org.nz/observations/1342527
http://naturewatch.org.nz/observations/1342521
http://naturewatch.org.nz/observations/1342520

And had a great time exploring the bush at night:
http://naturewatch.org.nz/observations/1342536
http://naturewatch.org.nz/observations/1342534
http://naturewatch.org.nz/observations/1342531
http://naturewatch.org.nz/observations/1342530
http://naturewatch.org.nz/observations/1342529
http://naturewatch.org.nz/observations/1342516
http://naturewatch.org.nz/observations/1342515

Spat

An ASAP experiment designed to generate hunches not scientific facts.


This experiment is designed to answer:

  • Is settlement attempted on filamentous structures around the Bastion reef?
  • What kind of organic substrates do spat like?
  • What size mesh best protects the spat from predators?
  • Does raising the bed help settlement? (Presumably from sediment effects).
  • How many holes can I put in my wetsuit in one outing?

Where
Bastion reef is ideal location because divers do not go there. The mussel population does not look to be harvested presumably because it is too close to city runoff. There are blue mussels present but there numbers are relativly low. It is also easily accessed by Kayak so I will not have to scratch anyones nice boat.

Design
Three cages 300x300x900mm (joined for stability) enclose 3 mixed bird netting wrapped bundels of test substrate at two different levels. The top level bundels (above 200mm) should be wire tied to the inside cage lids and should be at least Xmm above the sediment. The 3 mesh sizes test spotty predation the two heights test sediment effects.

“A 40mm spotty could eat a lot of spat”

  • 5mm x 5mm plastic mesh. A 7mm tall spotty could get through on an angle.
  • 12mm x 12mm square mesh. A 16mm tall spotty could get through on an angle.
  • 40mm x 40mm square mesh. A 52mm tall spotty could get through on an angle.

A cow tag with a note about the experiment with my contact details was attached for curious divers (or if it gets washed up on the beach). Each bundle was clearly labled (cow tags) just incase they get mixed up.

Bricks(2) will be added inside the cages to compensate for the floatation provided by the plywood. Four large concrete block anchors attached by rope to the plywood and 20mm pegs should hold the cage still.

Materials:

  • Galvanised 18G 1.25mm wire
  • Countersunk stainless steel 316 grade 10g x 50mm decking screws
  • Buttoned needle point 8g 13mm galvanised screws
  • 23cm galvanised ground staples
  • Square welded wire netting galvanised 12mm
  • Square welded wire netting galvanised 25mm
  • PE Gutter Guard 5mm
  • 12mm CD Grade treated plywood
  • 24mm treated plywood
  • My wifes old metal shelving
  • Shelves from my mother in laws old sun house
  • Some staples and Nylon
  • 4 concrete blocks

Materials

Settlement substrates

A. Coconut fibre (a door matt, welded with latex)

B. Cabbage tree leaves (long dead)

C. Manuka (fresh)

D. Gorse (seeding at collection time)

E. Mussel shell (fresh and old)

Hyroids (dry, collected on beach, a small amount of mussel beard in the centre of each bundle
Hyroids (dry, collected on beach, a small amount of mussel beard in the centre of each bundle

Substrates to try in the future:

  1. Manila rope
  2. Filamentous seaweed
  3. Feathers
Bundled
Bundled

Each bundle was labeled:
BH BL SH SL CH CL
B=big S=small C=control H=high L=low

The note and 1 of 20 pegs going through the base into the seafloor to hold the cages in place.
The note and 1 of 20 pegs going through the base into the seafloor to hold the cages in place.
Ready to deploy
Ready to deploy

Deployed: 13 Dec 2014

location Just off the edge of the reef, on the southern side.

I only got 19 of my 20 pegs in but they all went in without too much effort. It did not sink that fast so I put two of the anchor blocks on top of the cage.
I only got 19 of my 20 pegs in but they all went in without too much effort. It did not sink that fast so I put two of the anchor blocks on top of the cage.

No holes in my wetsuit! (yet)

When

“Most spawning occurs in late spring to early autumn, but larvae can be present all year. Sexual maturity has been observed in some populations to begin from 27mm shell length, with most individuals sexually mature by 40mm shell length. Sexual maturity is reached in the first year, and females can produce up to 100 million eggs per season. Fertilisation is largely dependent on the proximity of adults.”

– http://fs.fish.govt.nz/Doc/5449/GLM_FINAL%2008.pdf.ashx

Monitoring
Check it once a month (take a brush to clean muck of cage surfaces), after 3 months or before the cage rusts too much the 6 bundels of substrate should be opened up on the beach. Each substrate photographed with a macro lense. Ideally it is then relocated somewhere with no mussels and no reef (for spotty to hide in) to see if transporting spat only to seed locations generates a mussel reef.

The photographs should be shown to marine scientist who can show me how identify spat. I can then count spat and work out spat per square cm or something.

results matrix

Prediction/ Prevention

  1. It will get washed away/ use bricks, peg it down & design it so wave action is likely to make it spin.
  2. It will sink into the mud/ wooden floor.
  3. It will get damaged by people/ add a note saying its an experiment and give them a URL.
  4. It will rust to bits/ check on it every month.
  5. Spotty still eat all the spat/ I have used the smallest mesh possible.
  6. Nothing/ leave it untill we have spat or it rusts away?

UPDATE: 14 Jan 2015

No movement of the cage. Lots of marine growth and sediment, several small spotty (I didn’t see any on installation). Cage cleaned off (a bit). I took this video pre-clean.

Cleaning off the smallest mesh was not that effective.
Cleaning off the smallest mesh was not that effective. I need scuba gear.

UPDATE: 14 Feb 2015

Cleaned again (same method). Visibility less than 1m.
Cleaned again (same method). Visibility less than 1m.

RESULTS: 05 March 2015

Cages opened, substrates well documented with macro lens. Not enough spat settlement for useful substrate observations.

OBSERVATIONS

Coir and cabbage tree leaves had the most suspect spat (I could not confirm without a microscope). Use a microscope to look for spat as you need to look at anything suspect from more than one angle.

The manuaka had the most hydroids (or fine branching structures) growing on it. This is significant as Hydroids are know settlement structures.

Several starfish including 11 arms in the control cage. Shrimps and all sorts of crabs but mostly half crabs and decorator crabs ,all sorts of young shells from whelks to turbans. Lots of other marine growth and invertebrates I could not identify.

Surprising lack of holes in my wetsuit! The hardest thing was lifting the bundles in trash bags back onto the boat by myself.

RECOMENDATIONS
The lack of settlement in all cages suggests spotty are not the main problem at this site. A cage placed higher up in the exposed tidal area would be interesting, as there must be settlement near the existing adult mussels (2 meters above where I had my cages).

UPDATE: Nov 2016
Worth noting nearly 2 years later that parts of the cages are still intact. There are beautiful hyrdroids and finger sponges growing on them and some not so welcome invasive clubbed tunicates. I noticed Mediterranean fanworm nearby and the entire green-lipped mussel reef has been devastated by human harvesting.

Hyroids
Hyroids
Clubbed Tunicate (Styela clava)
Clubbed Tunicate (Styela clava)
Finger sponge
Finger sponge

Things Aucklanders can do to help the Hauraki Gulf

Inspired by some recent conversations, I put together this list (in no particular order).

Things Aucklanders can do to help the Hauraki Gulf

Invertebrates

Nudibranch

If I really care about biodiversity and 95% of described species are invertebrates then I am spending too much time with birds. Tho based on a talk I heard at the Zoological Society we don’t know how to look after them yet. The things we do to protect birds also help the insects (removing rodents and wasps).