As NSW fishers should know, changes to the recreational fishing regulations came into effect on 3 November 2014. These changes included the creation of a slot size limit for the Murray cod. The former minimum size limit of 60 centimetres was reduced to 55 centimetres; and a maximum size limit of 75 centimetres was introduced.
While slot sizes are nothing new in fisheries management, they haven’t been seen too often in an Australian context until recently. Even so, a lot of fishers are already aware of the idea behind slot limits with many voluntarily releasing the ‘big breeders’ in favour of keeping a few smaller fish.
For many fishers, then, it appears that we accept the hoped for benefits that selective harvest of our sportfish will bring. But for anyone that was wondering how these numbers come up, here is an explanation of why a slot limit can help conserve our legendary Murray cod.
It may or may not come as a surprise that the Murray cod formed the backbone of a commercial inland fishery during the late 1800’s and early 1900’s. I’d never really thought about there being a commercial inland fishery in NSW until I started researching for this article (see Rowland 2004 here) – but it only closed in 2001!
In the southern waters of NSW and in Victoria, commercial fishing for Murray cod was a pretty significant factor in the decline of the species particularly from the late 1800’s to the 1930’s. Murray cod remained the major commercial fish species on inland waters until the early 1950’s when stocks appear to have reduced to such a level that golden perch took its place as the most heavily commercially fished species in the Murray-Darling Basin.
Commercial fishing was not the only cause of decline in NSW cod stocks. In northern rivers where there was no commercial fishery, it is likely that the release of toxic chemicals from sheep dips in the early 1900’s was a major cause of fish mortality.
The impacts of the introduced redfin perch peaked in the 1950’s. The redfin, having a diet exactly the same as cod and golden perch, competed directly for food, while their earlier spawning season would have enabled juvenile redfin to prey on Murray cod larvae.
Changes to river flows particularly through the creation of dams, weirs and levees since the 1950’s has significantly altered the waterways that cod inhabit. Cold water pollution (releasing cold water from dams during summer) and flood mitigation all but remove the conditions required for optimal spawning and survival of larvae and juvenile fish.
Recreational fishing began to take a toll after the 1950’s as the cod was still important in the diet of many inland communities. With the development of better vehicles and increased access to the bush, fishing pressure increased and the use of illegal fishing methods continued to exploit the species.
Fishing regulations were introduced for the Murray cod in 1992 including a closed season, a size limit and bag limit. The implementation of these regulations since then has played a significant role in the continued and apparent recovery of the species across the state.
The current recreational fishery for Murray cod in NSW is heavily geared towards catch and release, with a survey conducted in 2005 indicating that on average, anglers were releasing around 77 per cent of the cod they caught.
The Theory behind Slot Limits
Minimum size limits are aimed at protecting fish until they have reached sexual maturity and have theoretically had the chance to spawn at least once (see some relevant articles – Hill here and Hixon et al. 2014 here).
Traditional management practices have focussed mainly on raising the minimum size limit. This practice means that more fish have had a chance to breed on one or more occasions before they can be harvested. The downside of this is that it doesn’t necessarily protect the best breeders in the population.
For many species, large females are ‘more’ important members of the breeding population because they produce more eggs (bigger fish equals bigger ovaries). In addition to this, bigger females are known to produce higher quality eggs – eggs that are bigger and better provisioned – meaning the resulting larvae grow faster and are less likely to starve. Bigger fish are also themselves more capable of surviving bad years, therefore being ready to spawn again when conditions improve.
It is also likely that older fish are more competent breeders as they have more experience under their belts. This relates not only to selecting good spawning sites; male Murray cod guard the nest (potentially until the juveniles leave it), keeping the eggs aerated and keeping potential predators at bay. An older fish might be better at the job through experience. Seeing as they don’t eat during this period, large healthy males may also be better equipped to withstand the energetic demands of such a task.
How this fits the Murray cod
Firstly, it’s handy to understand a few numbers relative to the Murray cod. Research shows some variability in the length to age relationship (Kearney & Kildea 2001 here); but generally the principles behind management remain the same.
Minimum Size Limit
At the age of 4 years the average female cod is around 48 centimetres or longer. At the same age an average male is 53 centimetres or longer. Correspondingly, 77 per cent of females and 72 per cent of males are sexually mature by this age/length.
By the time a cod is 5 years old it is most likely sexually mature. The simplest description for us is that all cod of 59 centimetres or greater in length are sexually mature.
So at a Length of 55 cm (the new minimum size limit) we should see the vast majority of cod as sexually mature, with most legal sized females having had at least one breeding season under their belts.
Maximum Size Limit
When it comes to reproductive potential, not all females are created equal. The relative fecundity (the reproductive rate; or the number of eggs produced) of Murray cod ranges from 3.2 to 7.6 eggs per gram of body weight. But importantly, the absolute fecundity has been shown to be from 6800 eggs for a newly mature female of 48 centimetres in length; to 86 600 eggs in an old female measuring 105 centimetres.
From these numbers it is easy to see that a few big females are far more productive than a lot of little ones. It would take more than 12 young, small females to produce as many eggs as one old, big female; and the eggs produced by the little girls would be on average smaller with less yolk provisions for the larvae.
The Victorian Department of Environment and Primary Industries conducted extensive research and modelling of varying slot size limits and their effects on Murray cod populations. Some of the results of their work can be viewed here.
Given a basic minimum size limit of 60 centimetres, a cod would be legally protected from harvest for at most 6 to 7 years of its life; and during most of that time the fish would not be contributing to the breeding population. Considering that cod have the potential to live well in excess of 50 years; that’s a long time that an adult fish is vulnerable to harvest.
Putting in place a maximum size limit of 100 centimetres, such as that which was in place in Victoria, leaves a cod vulnerable to harvest for about 10 years.
A minimum size of 50 centimetres and maximum of 70 centimetres, on the other hand, reduces that vulnerability window to just three years of a cod’s lifespan. The vulnerable period would coincide with early sexual maturity when fish are less productive breeders; while protecting both immature fish and highly productive older ones.
The new 55 – 75 centimetre slot keeps the vulnerable period to around three years, while ensuring a high proportion of fish are sexually mature beforehand.
Protection for Ponde
Fisheries management is always going to be a compromise between the interests of a variety of stakeholders with all sorts of priorities. For the Murray cod in NSW, the introduction of the new slot limit provides legal protection for our young up-and-comers as well as the experienced veterans of the rivers – to some the ‘trophy’ fish. The protection of these vitally important members of the population will in time lead to more fish at either end of the spectrum, with a healthy supply of middle-size fish that we can continue to enjoy eating (come on, no one can deny how tasty a cod is!).
According to the aboriginal Dreamtime story, after Nepelle and Ngurunderi had speared the great ponde (Murray cod); it having carved the Murray River during its trip down the infant stream to Lake Alexandria; and after they had cut and thrown pieces of it into the water to create all the other fish of the river; they threw the remainder back saying “you keep on being ponde” (Rowland 2004 p.40).
As long as ponde keeps swimming in the waters of the Murray-Darling Basin, our waters will continue to be the stuff of legend.