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.