I recently shot a dream fish, which I will admit was unexpected. Many people get lucky, spearing good fish in their first years of spearfishing. If you’re anything like me, it takes many years and thousands of fruitless hours. I guess the lesson learned is that dedication and persistence will pay off with time. Anyway, this mulloway was in an unusual place. A little gully less than 3 meters deep with no cover from white wash or ecklonia. Fortunately, I don’t think the fish could see me with all of the suspended detritus (mainly seaweed) in the water column, or maybe it was asleep. Either way, I didn’t underestimate the ability of the fishes senses, its wide field of view and the super sensitive lateral line, allowing it to feel vibrations. So with quick thinking, I maneuvered my gun 90 degrees to the left, over my left arm, slowly to avoid vibrating the rubbers or allow it to see me.
I was slow to aim, took my time and stoned the fish. Happy days. I’m not one to weigh fish for competition, records or comparison. I simply don’t get a buzz out of it. A good photo, memory and a good amount of fish to eat is more than enough incentive to go for a dive. We also didn’t weigh this fish because we had no scales on hand, but we took a measurement and compared the length to weight ratio on the DPI website. The fish was 135cm and according to the conversion would have been around 24.9kgs. It got me wondering though, given large fish like this are not overly common, how old was this fish and what is the current status of the fishery? To find out more, I did some research on the age, growth and the status of the mulloway fishery.
If you are interested read on…
Studies from the late 70’s up until 2005 primarily using interpretation of otoliths (ear bones), scales and tag-recapture data have found varying correlation of length and age of individuals. The most recent studies seem to indicate that fish of 60-80 cm (around legal size in QLD and NSW) are between 3-4 years of age, and fish of 120cm are anywhere from 5 to 24 years of age. Currently in NSW, Mulloway is classed as overfished.
Overfished? How and what does this mean?
In 2013 the minimum legal limit (MLL) in NSW was changed from 45cm to 70cm for mulloway (Argyrosomus japonicus) or jew, why? It was estimated that the recreational catch of mulloway is 4 times that of the commercial catch. This is only an estimate, as the sheer volume of recreational fishers is unknown. Unlike commercial fisheries, we are not required to fill out catch logs, and we are much more selective in nature. From the mid 70’s to the mid 2000’s there was a substantial decline in mulloway caught commercially. This, together with the fact that sciaenids worldwide are prone to overfishing was a cause for concern and further research.
So how do fisheries scientists determine whether something is overfished or not? Firstly they utilise a number of mathematical and statistical models. Because fish have variable growth rates and reproductive success given the dynamic environment they live in, there is great difficulty in determining the status of a fish stock with pinpoint accuracy. Moreover, as mentioned with mulloway in the recreational sector, fishing mortality remains unclear as does recruitment (or reproduction and growth to MLL) which is highly variable. The only option so far is to produce mathematical equations from the largest sample size possible and manipulate certain known factors, assuming we have a good portion of the population and that there is a steady state of mortality and recruitment.
How sound is the Fishery Modelling?
Yes, there are a lot of assumptions in the models, but many of them are based off large sample sizes, robust theories and accepted logic. Clearly, it’s the best we have. Due to these limitations, scientists must use conservative estimates. Later, these findings are handed to fisheries managers who then consult many entities and consider a wide range of factors to make decisions on a paricular fishery. The models and figures are used as a foundation to build stock predictions and suggest management actions intended to reduce risks and ensure sustainable fisheries for the future. Obviously, fisheries had varying degrees of exploitation before we have even started to take records, let alone create models. Fisheries science often lacks data that can be difficult and expensive to attain. So the results we do get, must logically lead us to take precautionary approaches such as harvesting fish biomass within a ‘low risk’ range.
What does the science say on Mulloway?
One study conducted between 2002 and 2005 using otolith ageing, lengths and traditional fisheries management models, found that mulloway are being growth overfished. I’ll get to what that means shortly. In that study, 3077 fish where firstly measured and aged by counting opaque zones (growth rings) in sectioned otoliths (ear bones). From this data a mathematical growth curve of length at age is developed. Secondly the condition of the available gonads (sexual organs) were examined to determine the reproductive state of each fish (i.e developing, developed or spent) and combined with the initial growth curve. So with that data at hand, statistical analysis determined that 50% of the females in the sample were sexually mature at 68 cm, and for 50% of males this was 51cm. Now that the growth, age and sex was determined, the mortality, yield per recruit (YPR) and spawning potential of the population was then estimated. The process of attaining the latter three is too mathematically complicated to explain in such few words.
Growth overfishing was indicated by the YPR analysis, depicting a curve; the average mass (kgs) of each fish caught was low and declining. The trajectory of the curve would be higher and more stable (flat) if fish are caught at a larger length at first capture (or MLL). Growth overfishing occurs when fishing pressure is so high that each individual or cohort (age class) of fish is harvested before attaining its maximum or optimal mass. Fish grow rapidly as juveniles and contribute more to growth than adults, because larger fish tend to slow in growth as they get older. Increasing the MLL should therefore increase the YPR. On the flip side, adults contribute more recruits to the fishery due to size being proportional to the number of eggs that are produced. Growth overfishing doesn’t necessarily mean that the population cannot replenish itself through recruitment. However, growth overfishing has led to recruitment overfishing of South African mulloway fisheries in the past. Although not a conclusion of the study, the spawning potential ratio (SPR) has indicated a high risk of recruitment failure. Mulloway fishing mortality range has been estimated at between 22% and 34% of the population per year, nearly twice that of natural mortality. The spawning potential was in a range below 20% of what it would be in an unfished population, indicating that mulloway are at a high risk of recruitment overfishing.
Recruitment overfishing is basically where fish are removed at too small of a size on average to reach sexual maturity and contribute to future generations effectively.
A Brighter Future?
Considering that many fish at 45cm have not sexually matured or grown to an optimum size, size at first capture or the minimum legal lengths needed to be increased to reduce the risk of both growth and recruitment overfishing. Minimum legal lengths applied to a species with high mortality, alone may do little for increased recruitment, unless stock sizes are very low. In some cases, changes in minimum size limits have resulted in small but significant benefits. If successful, there is a lag time between the implementation of a MLL and change in fish abundance and biomass. The impacts of management changes in 2013 will only be determined through long-term monitoring.
Improved research and expanding data collection methods have made the job of a fisheries manager increasing complex, but far more effective. While growing coastal populations will place increased pressures on Mulloway fisheries, better information should allow fisheries managers to become more reactive to potential issues, ensuring the species is sustainably managed into the future.
Let’s hope for the best. Time will tell.
Disclaimer: This article and the information within is only representative of the author’s interpretation of research. Further, this research and interpretation of information is in no way affiliated with any NGO, private, commercial or public entity.