Paddle Crab Aquaculture

Introduction

There is limited understanding of paddle crab resources and how best to manage and aquaculture them in many countries, particularly where fisheries management resources and enforcement capabilities are limited. The growth of paddle crab aquaculture is likely to lead to economical and socioeconomic success within crab fisheries.

Currently there is a high market demand for soft shelled paddle crab products. There is a potential in aquaculture to develop holding and control methods for producing soft shell crab products from paddle crabs taken from the coastline or culturing the crabs through their life cycle.

It is in the paddle crabs 'soft shell' state, immediately after moulting that the market demand for paddle crab lies. Without its hard exterior the crab can be cooked and eaten whole without the difficulty of removing the shell therefore with minimal effort a high quality product is ready for serving at top restaurants, markets and supermarkets.

Significant growth of paddle crab aquaculture is only going to occur from hatchery sourced stock due to the current moratorium on issuing further paddle crab fishing permits and large numbers being heavily over fished due to being taken as by catch in other fishing operations such as trawling and set netting.

Paddle crab biology

The paddle crab Ovalipes Catharus is a species of crab of the family Portunidae. "There are seven species of swimming crab in New Zealand waters" (Osborne 1997) Ovalipes Catharus is the only species supported by capture fisheries and aquaculture. In countries where swimming crabs are farmed or fished, they are an important source of income from both export and local sales, and are utilized by local fishers.

The paddle crab Ovalipes Catharus is "found on sandy beaches, and in estuaries and harbors throughout New Zealand, the Chatham Islands, and east and south Australia." (Wear 1988.)

They are abundant from the inter tidal zone to at least 10m depth, although it is not uncommon for them to be found in deeper water.

Paddle crabs are mainly active in early evening or at night, when they move into the shallow inter tidal zone to feed.

Paddle crabs feed mainly on mollusc's or crustaceans and "also on polychaetes, several fish species, cumacens, and occasionally on algae" (Ministry of Fisheries 2009)

Mating generally occurs during Winter and Spring (May to November). Female crabs then migrate to deeper water to spawn during warmer months. The basic life cycle consists of two planktonic larval stages "the zoea and a (crab-like) megalopa (Ministry of Fisheries 2009). After spawning the eggs are incubated until they hatch. The larvae inhabit offshore deeper waters until reaching the megalopa stage of their life cycle when they migrate inshore to settle.

Current Aquaculture

Currently there is a high market demand for soft shelled paddle crab products. There is a potential in aquaculture to develop holding and control methods for producing soft shell crab products from paddle crabs taken from the coastline or culturing the crabs through their life cycle. Attempts have been made at growing these crabs but in order for farming to be successful further research into their biology, life cycle and growing requirements needs to be undertaken to develop a reliable method of inducing soft shell stage in this species of crab.

Fresh crab meat is a specialist item fetching a high price both in New Zealand and overseas."Paddle crab sustains a fishery ranging from 45,000 to 89,000 per annum"(Jeffs, 2003) and produces four separate product lines; whole fresh crab, crab meat, crab paste and soft shell crab all of which are sold fresh, canned and pasturised.

"Commerical interest in paddle crabs was first realised in New Zealand in 1977-1978 when Napier fishers caught good numbers of large crabs in baited lift and set pots" (Wear 1988). Since then the fishery has expanded rapidly although annual catches vary as currently targeted fishing operations are restricted to only permit holders, Ministry of fisheries have stated that there is currently a moratorium on issuing further paddle crab fishing permits. Amendments to the current commerical regulations to allow crabs to be a target fishery may need to be made in order to establish a viably successful fishery in New Zealand. Regardless, the majority of catch caught by permit holders is likely to be hard shell paddle crabs and methods for holding, feeding and identifying or inducing moulting still need to be determined before the higher value soft shell crab product can be developed.

Jeffs (2003) states that " Currently the soft shell crab market demand outstrips supply and the price is high ex-factory FOB New Zealand $30 per kilogramme"

In order to grow, crabs must shed their shell. The crab does this by developing a new skin under the shell and when ready to moult will pump itself full of water and split open its shell. Once the shell is split the crab continues to pump itself with water in order to stretch the new skin. This skin is soft but according to research done by Wear (1988a) "will quickly harden over the next 6-24 hours through the deposition of calcium to form a new shell"

Without the crabs hard exterior the crab can be cooked and eaten whole without the difficulty of removing the shell therefore with minimal effort a high quality product is ready for serving at top restaurants and for sale at markets and supermarkets.

Previous efforts to aquaculture paddle crab overseas involve "targeting crabs in pre-moult condition" (Jeffs, 2003) and holding the crabs in a culture system. Farmers educated in the life cycle and biology of the paddle crab are able to read the signs in the crab and determine when the crabs will moult and enter into their soft shell stage.

The blue crab also a swimmer crab comes from the same family Portunidae as the paddle crab, many similarities are drawn between the two crabs and research between the two species is often linked. In the Blue crab fishery "crabs are held in shedding trays and the newly moulted crab is removed from the water and killed" (Blue crab Florida seafood and aquaculture) . Once out of the water it is impossible