THE historic story of black cod (Epinephelus daemelii) in NSW waters is an interesting one.
Also known as saddletail rock cod or black rock cod, black cod were once abundant around headlands, rocky reefs, underwater caves and bommies along NSW’s entire coastline. Juveniles were often encountered in estuaries and rock pools while adult fish were common on near shore reefs from Bundaberg in Queensland south to Bass Strait and west to Kangaroo Island.
However, even as early as 1916, prominent government biologist TC Roughley reported that black cod "had become very scarce in recent years". The species was particularly hard hit by spearfishing in the 1960s and 70s, and because of this, black cod were declared totally protected throughout NSW coastal waters in 1983. Even so, their numbers have been slow to recover, and healthy populations of black cod still exist only in remote locations infrequently visited by humans, such as Middleton and Elizabeth Reefs, and elsewhere, the Kermadec and Three Kings Islands off New Zealand.
Why the black cod have been slow to recover is patially explained by their biology. This species is very slow growing and long lived. Like other members of the family Serranidae, black cod are protogynous hermaphrodites which mature first as females, changing sex to males only at around 100-110cm length, when they may already be 15-20 years old or more. Larger male fish over 120 cm have been aged at 40-50 years old in New Zealand. There also may be social pressures associated with the change in sex, as large male fish often are seen surrounded by a harem of smaller females in places where their populations are still healthy.
Over the past few years there has been increased scientific interest in study of black cod populations in NSW, with much of the study spearheaded by the NSW Marine Parks Authority. The aims of these studies were to “undertake baseline surveys” on the distribution and relative abundance of black cod in marine parks like the Port Stephens Great Lake Marine Park, Solitary Islands Marine Park and other places like Fish Rock where some or all fishing activities were prohibited.
The fact that these “baseline” studies were being undertaken nearly 100 years after problems with black cod populations were initially reported, and in most cases several years after the declaration of the various marine parks concerned, suggests this long overdue research was initiated under a “better late than never” principle.
The results of these surveys are interesting, but from a scientific perspective they have significant limitations which means they should be viewed with caution and interpreted conservatively, considering the methods used. The surveys were done using diver transects which means that sites deeper than around 30 metres depth were impossible to sample properly, because of the inherent limitations of diving at depth. Also, the researchers themselves noted that larger black cod would often attempt to hide, or would swim away rapidly once they saw a diver. How many times these fish see divers before the divers see them and escape undetected is, of course, not known.
Usually, scientists try to circumvent these issues using cautious interpretations and cross referencing other data sets using different survey techniques, such as baited underwater videos, which have been used more recently to try to establish numbers of juvenile black cod in their inshore nursery areas. But again, these other methods bring their own biases and limitations, all of which should be recognised and taken into account.
The results of the diver surveys tended to find relatively few black cod scattered at the various sampling sites both within marine park green zones and outside. For example, only 66 black cod observations were recorded in the Port Stephens Great Lake Marine Park during the two years the surveys were conducted, with the numbers of fish observed at particular sites remaining generally stable between years.
The Marine Parks Authority scientists found there was no significant difference in the number of black cod at sites inside green zones vs sites outside green zones. Indeed, the highest number of black cod ever seen in a single dive survey in the Port Stephens Great Lake Marine Park study was seven fish at the inshore Latitude Rock, an area open to fishing. Black cod in the green zones were larger on average (mean 72 cm) than those in the non sanctuary areas (54 cm average). However, even this could have been an artefact of the study design, as the study sites were not randomly allocated and there was a strong positive relationship between black cod size and distance offshore (larger cod tended to be found further offshore).
The marine parks scientists concluded that the lack of large differences between the green zones and areas open to fishing was because the green zones were only established for two years prior to the study. This is despite the fact that black cod had been protected from fishing for nearly 30 years previous, implying that the scientists considered fishers still mistakenly take protected black cod when they should be returned, or that black cod captured and released do not survive. The marine parks scientists also explained their results by stating “it will probably take considerable time for black cod to benefit from marine park zoning”, and they “hoped that black cod abundance should increase over time, albeit slowly due to their slow growth and reproductive cycle.”
Conducting baseline surveys after marine park establishment is certainly not scientific best practice. Usually to qualify as true baseline studies, they must be done before a certain management action is implemented, not afterwards. Leaving the obvious questions aside, some of the recommendations arising from these studies could have also benefited from more cautious interpretation of the data. For example, from the results of studies of black cod populations in northern NSW from Cook Island to Fish Rock, the marine parks scientists recommended “Removal of all forms of fishing at Fish Rock” to “reduce potential incidental catch (of black cod) at that location.”
Certainly, ongoing education of recreational fishers to reinforce the message that black cod are protected in NSW waters is important, as is information on how to release them with minimal injury if accidentally captured. However, drawing such strong conclusions from studies with such significant limitations is dangerous ground, especially considering other facts such as black cod not tending to be caught on lures trolled for mackerel or marlin, for example.
Furthermore, if the data from the same northern NSW study is treated uncritically and taken at face value, one could also wrongfully conclude that, since the study recorded 67 black cod, but only four estuary cod, that black cod populations are robust in NSW compared to estuary cod, and that estuary cod need more protection. Clearly there is much more to the black cod story than can be gleaned from accepting these baseline studies at face value, and much more information is required before management decisions could be considered possible based on sound science.
In the same report, the authors noted that the numbers of Queensland groper in the Solitary Islands Marine Park appeared very low (two were seen in the dive surveys), despite the fact they have also been protected in NSW waters for over 35 years, which is sufficient time for around two generations of this species to mature and reproduce. The authors noted 40 years ago Queensland groper were relatively common, even though back then they were still being targeted by linefishing and spearfishing. Now they are not, despite 35 years of protection. To me, this indicates that (ranked from least to most likely) either:
- people have been harvesting significant numbers of Queensland groper illegally in NSW over the last 35 years, but without being noticed;
- unseen populations of adult Queensland groper actually exist, but they are not being picked up in dive surveys;
- factors other than fishing are affecting the ability of this species to recruit and replenish its populations.
It is interesting to note that Queensland groper have a similar lifecycle to black cod, in that juveniles live in estuaries and inshore reefs before migrating offshore as they mature. In north Queensland in recent years (where Queensland groper are also fully protected), there have been unprecedented dieoffs of adult Queensland groper due to bacterial infections (Streptococcus agalactiae), which are almost certainly related to reductions in water quality and/or introduced diseases.
Extinction of subtidal oyster reefs in most Queensland estuaries since the early 1900s is just one part of a marked reduction in the extent and quality of estuarine nursery habitat caused by extensive land clearing and coastal development. Degradation of their nursery habitat is likely to have significantly reduced recruitment for Queensland groper and represents an important bottleneck that would explain the failure of this iconic species to recover, despite full protection from fishing, including inside the green zones that make up 1/3 of the Great Barrier Reef Marine Park. Could this be the same story for black cod?
So when scientists from the NSW Marine Parks Authority state in their 2012 report that “adequate protection of intertidal habitats should be considered to assist with the recovery of juvenile black cod”, do they mean they want more green zones in rock pools or other inshore waters? Or is it possible that they are beginning to acknowledge there are likely to be other critical bottlenecks besides fishing in the recovery process for a species that has already been “fully protected” from fishing for 30 years?
The story of black cod in NSW waters is indeed an interesting one, but the next few chapters in the story are likely to be the most interesting yet!