Design of Artificial reef in Chile
Design
of Artificial reef in Chile
Masatoshi FUTAGAWA / CORDUNAP
May 22, 2012
1. Functions of Artificial reefs (AR)
Formation of flora and fauna
Algae larvae will attach to AR which was carried by
current after installation of artificial reef (AR), and the larvae grow with
nutrition in water and sunlight. Water current stagnated by AR and grown algae,
and sediment attach or settle on AR, the sediment gives more nutrition to
algae, and flora is formed around AR.
The algae harbor fish larvae and juveniles and feed annelids
(worms), arthropods (copepoda, crabs, shrimps), herbivorous and omnivorous fishes.
Those animals feed carnivorous fish including pelagic fish that stays above AR
then form fauna. Moreover, some of fish and finfish use AR for spawning ground and
close life-cycle at AR.
High productivity
Productivity depends on production of algae which
relate to nutrition of water, sunlight, current and design of AR (height,
width, thickness, surface area and material) according
to study. Thus, algae buds are covered by cage to protect
from predators until grown sometime. Post
type AR makes algae growth well due to Echinodermata (sea urchin,
starfish) that are hard to climb high.
Selective production
ARs are designed to aggregate and settle particular or
variable fish which consider as a habit of targeted fish. AR has two designs,
for juvenile and adult fish. As for juvenile, the function is protection from
predators until it grow strong and big enough. Particularly, the AR has several
openings which is narrower than predator size, around 7 cm height. AR for adult
requires more space for resting and evacuation in the night, and opening is about
10 cm width. Possibly, AR for adult equipped spawning ground for reproduction.
Easy to transport, install and harvesting
Prefabricated AR consist
several sections, is ideal to transporting, can be assembled at sea bottom and
dismantle during harvest which is easy to fabricate and minimize transportation
cost.
2. Materials
Material of AR should be durable which can work over
30 years to keep fish and minimize cost, and it should be heavy and thick in
order not to move or broken by current and waves. Natural materials are ideal, not synthetics,
which dissolve or decompose in water without chemical contamination in a long
term such as wood, stones, shells, iron and concrete are recommendable. AR cost, especially material cost, occupies
much in Marine ranching expenses. Thus, discarded or unnecessary materials are ideal
materials to minimize AR cost. The followings show several kinds of materials.
Abandon boats, tanks, trains
are minimum cost but doubtful
at points of water contamination about oils and paints. Also, it works for aggregation of fish but not
for convenient harvest like fishing net hooks easily and possibly water
contaminates due to very complicated structure.
Iron steel has advantages
which is easy, quick and any forms to fabrication, and secure wide living
space. Especially, cast iron
releases iron ion cleaning water and oxidation slowdown due to cover a filament
at surface after few month. Disadvantage is shorter life compare to concrete.
Porous concrete
is ideal material which has porosity structure like sponge and alga larvae attach
to surface easily and other small animals as well. Also, using gravel
substitute discarded
concrete from construction excess to minimize cost.
Moreover, it is lighter than solid concrete due to porosity
structure and which transport and install easily at sea
bottom. However, it is attached to solid concrete base for use due to not
strong material as solid concrete.
Fly ash
(coal
ash)
is by-product of thermal power station by coal, blast furnace slag is by-products of iron works and crushed stone are mix with porous concrete
to minimize cost. Fly ash shows minimum compressive strength among materials according
to study.
Timbers reef, made of thinning
of a forest or pine sawyer tree (Pinus
taeda or P. thunberg), quickly aggregate
fish, seabream and rockfish, which are observed a month after installation, and
has more biodiversity compare to concrete one due to rough surface. However, the
timbers are eaten by ship-worm (Teredo navalis japonica) and it last
long until 6 years. Even so, the concrete base functions for aggregation
continually. Thus, it installs with concrete AR and aggregate fish ahead until
concrete AR function at the start of marine ranching.
Scallop shells
assists settlement of alga larvae compare to solid concrete. It is
recommendable to put shell pieces at surface of AR to enhance growth of algae.
Artificial macroalga,
made of plastics or nylon, that aggregate arthropods instantly and it use for
substitute of macroalga until real algae grow.
3. Location and installation
Key factors of Marine ranching are production of
Algae, Seeds quality of releasing juvenile, Aggregation of fish, Protection
from predation and poaching and Reproduction of fish (ASAPR). Among those
factors, location and installation method are very important issue in order to produce
fish efficiently.
AR for macroalge should be installed at a shallow
area (around 15 m depth) where it reaches sunlight and not harm the structure by
strong current and waves. It should be install at hard bottom, not muddy bottom.
Minimum volume of AR is 400 m3 to aggregate
fish efficiently according to the
paper and high fish aggregation occurred that installing
AR at wide distribution and face to current on the line. The fish stay at upper
reaches of a current to taking feeds, organic matters and copepods, ahead. The
study says mixture of different height of ARs give more biodiversity.
4. AR design for Chile
Chilean coastal fish resources are exhausted due to
overfishing even the ocean has high productivity. I believe that marine
ranching can quickly recover or even enhance the resources with very nutritious
water at Chile coast. Chile has advantage which has the availability of scallop
shell from the culture industry, thinning of a forest timbers, excess pebbles
from a mines and local made fish meal.
AR for nursery (ARJ)
The reef shapes pyramid (1.8 x 1.8 x 0.9 m, 0.35 m3
for fish) which consists base (1.8 x 1.8 x 0.1 m with 4 feet) and four layers plates
(1.5 - 0.83 x 0.07 m) with a hole (50 m/m) at center. The base made of solid concrete
with scallop shell fragments at surface and the layers made of porous concrete.
Ideally, the edge has wing which protect sea urchin to climb up and eat algae
buds. Also artificial algae are attached to layers which aggregate fish. The
reef has 7 cm height and it arrow fish to grow size 20 cm or 150 g
approximately.
ARJ aggregate fish, taken care then released to hatchery
made juvenile (5 – 8 cm) and hatched larva until juvenile with protection from
predators. Additionally, feeding, low cost simple formulation, enhances growth
and survival, and minimizes diffusion of the fish if necessary.
AR for adult (ARA)
The
reef consist the Cubic (1.6 x 1.6 x 1.6 m, 3.6 m3 for fish) which is
made of porous concrete and the Base (1.8 x 1.8 x 0.1 with 4 feet) which is
made of solid concrete with scallop shell fragments at surface. The cubic
consist four prefabricated panels (1.37 x 1.71 x 0.07 m) which has nine windows
(10 x 20 cm), and opening (8 cm) between panels. Possibly, some concrete blocks
are installed inside the cubic to increase capacity. ARA take care released
fish and aggregated fish until harvest and feeding is effective as ARJ.
Marine ranching
design
The aim of Marine ranching (MR) in Chile is to
recover and enhancement of coastal fish resources with hatchery made juvenile,
AR and fishing management. MR consist of the Cells which form 2 units of ARJ
and 10 units of ARA with 4 m distance installation, total area 582 m2
and volume 37 m3 for fish, and require 11 cells to meet 400 m3
that shows well aggregation. From the beginning, install few cells to evaluate
functioning of AR and expand to large scale after modification of AR. Timber
reef (post type), iron reef and high structure of AR are mixed in the cells and
increase aggregation.
FAD (Fish Aggregating Device) aggregate pelagic fish,
jacks and tunas, very well. Possibly, install FAD at MR area with releasing of
pelagic fish juvenile with control fishing, not over fishing as “ecological
trap”.
Originally, FADs consists several floaters, rugs, bamboos, buoys, rope or chain
and anchor. Recently, we use large or intermediate type, install middle of
water, and increase aggregation.
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