For the love of cod

KEEPING TABS: A blue cod is caught, measured and released in the Marlborough Sounds during a fish survey in 2010.
KEEPING TABS: A blue cod is caught, measured and released in the Marlborough Sounds during a fish survey in 2010.

Fisheries scientist Glen Carbines offers some advice on improving the survival rates of blue cod in the Marlborough Sounds.

What's going on in the Marlborough Sounds blue cod fishery?

Over the past 20 years this fishery has struggled to accommodate increasing effort from more fishers, better boats, improved GPS and sonar technology. The odds seem to be progressively staked against this iconic fish.

It is becoming more difficult to ensure the stocks' sustainability while satisfying the insatiable demand for the South Island's favourite fish. Here are a few essential things to consider.

Small scale management

Most blue cod move less than one kilometre, which makes them vulnerable to local depletion.

This species can really benefit from smaller scale localised management in areas of high fishing pressure. Take a dive at the Long Island Marine Reserve in the Marlborough Sounds and you will see this for yourself.

Or see how Fiordland blue cod populations within Dusky Sound have recovered since commercial fishing was removed and recreational bag limits slashed.

Blue cod are quite resilient and can often recover quite quickly when excessive fishing pressure is removed, as seen from the previous closures of the Marlborough Sounds blue cod fishery.

Preserve habitat

Young blue cod need somewhere to both keep out of sight of predators and to find food. Consequently, they are found mainly among rubble or living "biogenic" structures such as tube worms, bryozoan corals, sponges, shellfish beds, etc.

It is therefore important to protect at least some of these types of habitats from the physical damage of dredges and trawlers, or from degradation such as sedimentation from land use or marine farming.

Large breeding females

Blue cod are a moderately slow growing fish, taking more than seven years to reach the 30cm size limit in the Marlborough Sounds.

While they can reach sexual maturity at 21-26cm, the reproductive output of female blue cod increases considerably with size. So it is important to ensure that the population maintains a healthy number of large females for breeding.

Balanced sex ratios

Most blue cod are born female and are able to change sex to male, but this is a complex process that seems to be influenced by size, age and social interactions.

The aggressive and territorial behaviour of large males seems to be an important factor suppressing sex change in smaller females. This explains why catches from heavily fished areas such as the Marlborough Sounds tend to be dominated by small males (because there are few large males remaining to suppress female sex change).

Unbalanced sex ratios can limit egg production and risk reproductive failure. A shift to smaller sized females becoming males earlier can also decrease the overall female biomass and potentially further limit egg production because smaller fish have fewer eggs.

Ensuring large blue cod remain in a heavily fished population requires good survival rates of the fish returned, using either a larger minimum size or a maximum size limit. A network of closed areas throughout the Marlborough Sounds would further ensure the availability of large blue cod for reproductive success. But currently less than 1 per cent of the Marlborough Sounds coastline is protected by marine reserves.

More closed areas could provide a safeguard for heavily fished blue cod populations, helping to balance sex ratios and ensuring the presence of large females for reproductive success throughout the Marlborough Sounds.

Survival of returned fish

While blue cod are a robust fish that don't suffer barotrauma (burst swim bladder), those that swallow hooks often bleed to death after being released. To improve the survival of returned blue cod it is important to lip or mouth hook fish.

Using hooks that are not swallowed (eg, size 6 and above) will significantly increase the survival chances of returned blue cod.

More effort could be spent regulating the types of hooks used in the Marlborough Sounds, approving only hooks that are scientifically shown to catch fewer small fish, avoid being swallowed and are biodegradable if they cannot be safely removed. Handling time should also be kept to a minimum.

Unfortunately, many blue cod returned in good condition are still taken by predators such as shags and barracouta.

The current regulations requiring that fish are immediately returned to the water causes a build up of these predators around boats, increasing the likelihood that fish released one by one are successfully caught by these predators. This could be avoided if blue cod were returned directly to the seabed, which can easily be done using an inverted barbless hook and a heavy weight to rapidly descend returned fish.

For larger boats, a change in legislation could allow a cage to be hung off back of boats giving blue cod time to recover from capture and handing. Prior to leaving an area, the cage would be lowered to the sea bed and opened for a single mass release which would greatly increase the survival chances of returned blue cod.

The proposal to take the first three fish caught will likely encourage irresponsible fishing behaviour such as high grading much like the failed experiment of the 28cm size limit of the mid 1990s.

What blue cod need now are sensible and innovative ideas that work with what we know about the biology of blue cod, rather than repeating the mistakes of the past.

The future of the Marlborough Sounds blue cod fishery is not only in the hands of politicians and fisheries managers, it is equally in the hands of those handling the fish. It is a shared responsibility.

  • Glen Carbines is director of independent research company Saltwater Science and has studied blue cod for more than 20 years.

The Marlborough Express